Vitamin D is crucial for musculoskeletal health, being required for the adequate absorption of calcium from the gastrointestinal tract. Vitamin D is a secosteroid synthesised via the action of UVB light on the skin, forming cholecalciferol (vitamin D3) (Fig. 1)(Reference Gibney, Lanham-New and Cassidy1). While this is the predominant physiological source of vitamin D, it can also be obtained from the diet in animal and plant foods (ergocalciferol or vitamin D2) and from fortified foods(Reference Laird, Ward and McSorley2,Reference Holick3) . Its role in bone health is well established(4), with deficiency increasing the risk of rickets in childhood and osteomalacia in adults(Reference Gibney, Lanham-New and Cassidy1). Secondary hyperparathyroidism due to vitamin D deficiency can result in musculoskeletal pains and muscle weakness(Reference Saggese, Vierucci and Boot5). Peak bone mass, which may determine up to 60 % of osteoporosis risk in later life can only be achieved with sufficient vitamin D and calcium intake(Reference Gibney, Lanham-New and Cassidy1). The role of vitamin D may also extend beyond bone health. For example, vitamin D receptors are found in numerous cells including immunological (T- and B-cells), osteoblasts, β cells and mononuclear cells, and in many organs such as the brain, heart, reproductive and the gut(Reference Marino and Misra6). Interaction of transcription factors [1,25-hydroxyvitamin D2 (1,25(OH)D2)] with the vitamin D receptors modulates gene expression, influencing numerous physiological functions including anti-cancer, immunological and anti-inflammatory effects(Reference Wacker and Holick7,Reference Zhang, Miller and Li8) . Thus, it may be involved in the pathogenesis of hypertension, stroke and CVD and may also play a role in immunity, autoimmune diseases, type I and II diabetes, multiple sclerosis, cancer, depression and dementia(Reference Marino and Misra6,Reference Zhang, Miller and Li8–Reference Föcker, Antel and Ring12) . While plausible physiological mechanisms exist for these potential extra-skeletal effects, evidence from robust randomised controlled trials is limited and causality has not been established(4,Reference Marino and Misra6) . However, maintaining adequate vitamin D status (>50 nm/l) has been associated with decrease in all-cause mortality in a recent large prospective cohort study(Reference Sutherland, Zhou and Hyppönen13).
Fig. 1. Vitamin D metabolism. 25(OH)D, 25-hydroxyvitamin D; Ca, calcium; FGF-23, fibroblast growth factor 23; PTH, parathyroid hormone; SPF, sun protection factor.
Vitamin D status is determined by a number of intrinsic, environmental and lifestyle factors. Biosynthesis of vitamin D from UVB sunlight (290–315 nm) is dependent on the correct latitude, and for countries above 40°N such as Ireland (52–55°N) this is negligible between October and March(Reference Holick3,Reference Mithal, Wahl and Bonjour14) . Cloud cover, time of day, altitude and air pollution can also affect production and give rise to regional variations in status(Reference O'Sullivan, Laird and Kelly15). Factors including age, skin type, sunscreen use and clothing cover also determines dermal synthesis(Reference Lips, van Schoor and de Jongh16). Finally, the absorption and bioavailability of vitamin D is affected by malabsorption conditions (e.g. Crohn's/coeliac disease), medication, smoking and obesity(Reference Holick3,Reference Yang, Zhao and Liu17) .
As a result of limited sun exposure, the Irish population is dependent on dietary sources of vitamin D, though intakes remain low, and most do not meet the RDA(Reference Cashman, Muldowney and McNulty18). The RDA as set by the Food Safety Authority of Ireland varies by age as shown in Fig. 2(19–22). In addition, the proportion at risk of deficiency is rising due to demographic and other changes(20). For example, the population is ageing, with the proportion over 65 years set to double by 2050(Reference Sheehan and O'Sullivan23) and there has also been an increase in those of ‘non-white’ ethnicity(24). Levels of overweight and obesity are also on the rise, increasing from 55 and 19 % in 2006 to 61 and 25 %, respectively, in 2016(Reference Sheehan and O'Sullivan23). While there are relatively few cases of rickets, the number reported has increased with twenty-three cases recently described in two Dublin hospitals(20). For these reasons, knowledge of both trends and current vitamin D status and intake is important in several subgroups of the population.
Fig. 2. Irish vitamin D supplement recommendations by age group.
There is no universal agreement on definitions of deficiency and sufficiency by professional bodies (Table 1). Vitamin D > 125 nm/l is suspected by the National Academy of Medicine as being harmful to health with potential negative effects on falls, depression and possibly other outcomes including cancer and all-cause mortality in some studies(25–Reference Zittermann, Prokop and Gummert J27). However, the National Academy of Medicine takes a precautionary approach that also factors in ethnic/genetic differences so as to maximise public health protection(Reference Zittermann, Prokop and Gummert J27). Despite this, overt vitamin D toxicity causing hypercalcaemia is rare and usually occurs at levels above 375 nm/l(4,Reference Jones28) .
Table 1. Serum 25-hydroxyvitamin D recommendations
Due to the limited half-life of the biologically active 1,25(OH)D (4–6 h) and its tight feedback control, vitamin D status is assessed by monitoring concentrations of 25(OH)D (half-life 3–4 weeks) which is under no feedback regulation(Reference Holick29). There are several types of vitamin D analytical techniques, with varying sensitivities and specifications(Reference Atef30). These include binding assays; RIA, chemiluminescence immunoassay, protein-binding assay, and bioanalytical assays such as HPLC and liquid chromatography tandem mass-spectrometry (LC-MS/MS). Binding assays are relatively quick and inexpensive but are subject to interference from other vitamin D metabolites and may overestimate 25-hydroxyvitamin D [25(OH)D] concentration(Reference Holick29). HPLC and LC-MS/MS allows for the quantification of a large number of samples, but require more technical skill(Reference Atef30). LC-MS/MS is now considered the gold standard of vitamin D assessment and allows for the measurement of both 25(OH)D2 and 25(OH)D3(Reference Holick29). The vitamin D standardisation programme and vitamin D external quality assessment scheme were developed to improve the accuracy and repeatability of vitamin D assessments(Reference Sempos, Vesper and Phinney31,Reference Carter, Berry and Durazo-Arvizu32) . Vitamin D intake is typically assessed with FFQ that include 24-h re-calls and 3–7-d food diaries. Weighed food diaries are most commonly utilised in Irish national nutrition surveys and calculate vitamin D intake using nutrient composition databases(Reference Timon, Blain and McNulty33).
To date, no study has comprehensively reviewed vitamin D research with regards to vitamin D status and intakes on the island of Ireland. We aim to summarise the peer-reviewed studies and official reports published since 2010 or earlier for specific sub-groups where no other data was available. For the purpose of this review, we defined deficiency as <30 nm/l and excess as >125 nm/l unless otherwise stated(25).
Vitamin D status and intake by population categories
Pregnancy and fertility
The largest Irish study (n 1768) in pregnant women found a prevalence of deficiency of 17 %(Reference Kiely, Zhang and Kinsella34), which was similar to other large studies where it ranged between 15 and 17 %(Reference McCarthy, Murray and Malvisi35,Reference Hemmingway, Kenny and Malvisi36) . In other studies, it varied between 13 and 65 % but sample sizes were small and were not likely to be representative(Reference Toher, Lindsay and McKenna37–Reference Onwuneme, Martin and McCarthy40). In general, deficiency was less prevalent in early pregnancy (13–29 %)(Reference Kiely, Zhang and Kinsella34,Reference Hemmingway, Kenny and Malvisi36–Reference O'Callaghan, Hennessy and Hull38,Reference Zhang, Lucey and Horgan41–Reference Smith, O'Brien and Alberdi47) with rates increasing with gestation in most studies(Reference Zhang, Lucey and Horgan41,Reference Walsh, McGowan and Kilbane42,Reference O'Brien, O'Sullivan and Kilbane45,Reference Smith, O'Brien and Alberdi47) . A high proportion of mothers (25–65 %) were also found to be deficient at delivery(Reference Onwuneme, Diya and Uduma39,Reference Onwuneme, Martin and McCarthy40) . As expected, a seasonal variation in vitamin D deficiency in pregnancy was found(Reference Kiely, Zhang and Kinsella34,Reference O'Callaghan, Hennessy and Hull38,Reference O'Riordan, Kiely and Higgins43,Reference O'Brien, O'Sullivan and Kilbane45,Reference Holmes, Barnes and Alexander48–Reference O'Brien, Kilbane and McKenna50) , with the lowest prevalence of 3–7 % detected in summer/autumn(Reference Kiely, Zhang and Kinsella34,Reference O'Callaghan, Hennessy and Hull38) .
The prevalence of levels <50 nm/l in the largest studies were between 42 and 44 %(Reference Kiely, Zhang and Kinsella34–Reference Hemmingway, Kenny and Malvisi36), while in smaller studies variance was pronounced (44–91 %)(Reference O'Callaghan, Hennessy and Hull38,Reference Onwuneme, Martin and McCarthy40) . A seasonal effect was also evident, with prevalence generally higher in winter v. summer(Reference Kiely, Zhang and Kinsella34,Reference O'Callaghan, Hennessy and Hull38,Reference O'Brien, O'Sullivan and Kilbane45,Reference Holmes, Barnes and Alexander48) . Prevalence of levels below <50 nm/l is similar to some pregnancy studies in Northern Europe where it affected approximately 50 % in the UK(Reference Gale, Robinson and Harvey51) and Belgium(Reference Vandevijvere, Amsalkhir and Van Oyen52). Only one Irish study (n 138) looked at men and women undergoing fertility treatments and found that one out of five was deficient(Reference Neville, Martyn and Kilbane53).
Dietary and supplement intakes in pregnancy
Dietary intakes in pregnant women ranged between 1⋅9 and 10⋅7 μg/d(Reference Hemmingway, O'Callaghan and Hennessy46,Reference McGowan and McAuliffe54) , with 80–99 % not meeting the recommendations(Reference McGowan and McAuliffe54–Reference Lindsay, Heneghan and McNulty56). By comparison, in the UK, 98–100 % were found not to meeting the advised intake of 10 μg/d(Reference Calame, Street and Hulshof57). In Ireland the RDA in pregnancy is no different from the general adult population at 15 μg/d(22). However, evidence suggests that pregnant women require 20 μg/d to meet sufficiency (>50 nm/l)(Reference Alhomaid, Mulhern and Strain58) with 10–15 μg/d advised by a European consortium(Reference Lips, Cashman and Lamberg-Allardt59). In Finland, the 2003 food fortification of liquid milk resulted in a significant increase in vitamin D intakes in pregnant women(Reference Prasad, Lumia and Erkkola60), and as such should be considered as a public health measure to improve vitamin D status in Ireland.
Nearly 40 % of pregnant Irish women reported taking a specific vitamin D supplement in a Dublin study (n 175) in 2016, though the sample was from a confined area(Reference Windrim, Crosby and Mitchell61). However, when including a multivitamin containing vitamin D, 74⋅3 % were actually taking some form of supplementary vitamin D(Reference Windrim, Crosby and Mitchell61). Importantly, less supplement use in pregnancy has been associated with an increased risk of low vitamin D status in Irish infants(Reference Kiely, O'Donovan and Kenny49). Supplementation in Irish pregnant women was also the strongest predictor of 25(OH)D > 30 nm/l with Caucasian females more likely to supplement than those of other ethnicities(Reference Toher, Lindsay and McKenna37).
Children/adolescents
In the largest study to date (n 5524), 15 % of children (5–19 years) were deficient(Reference McKenna, Lyons and Flynn62), while the second largest (n 1226) found a deficiency rate of 23 %(Reference Scully, Laird and Healy63), though both were conducted in the Dublin area. In most studies deficiency prevalence varied between 5 and 23 %(Reference McKenna, Lyons and Flynn62–Reference Glatt, McSorley and Pourshahidi68). The most recent nationally representative study that measured vitamin D levels in teenagers (aged 13-18 years) found that 21.7% were deficient though sample size was small (n=246)(Reference Cashman, Kehoe and Kearney69). The highest prevalence of deficiency (63–68 %) was reported in adolescents (aged 12 or 15 years) in Northern Ireland(Reference Carson, Pourshahidi and Hill70). However, vitamin D was assessed over 20 years ago, was not checked in the months of July or August and the study population was derived after stratified sampling so may not be more broadly representative. Furthermore, the results are discordant with other studies. As expected, the prevalence of deficiency was lower in the summer(Reference Ní Chaoimh, McCarthy and Hourihane66,Reference McCarthy, Collins and O'Brien71,Reference Hill, Flynn and Kiely72) and higher in winter when it affected 18–30 % of teens(Reference McCarthy, Collins and O'Brien71,Reference Hill, Flynn and Kiely72) , and 26 % of children aged 1–17(Reference Scully, Laird and Healy63). In general, female children also had lower vitamin D status(Reference Scully, Laird and Healy63,Reference Carroll, Onwuneme and McKenna65,Reference Carson, Pourshahidi and Hill70,Reference Hill, Cotter and Mitchell73,Reference Cashman, Hill and Cotter74) , but not in all studies(Reference McVey, Geraghty and O'Brien64).
Notably, there was a ‘U-shaped’ relationship between deficiency prevalence and age, being lower in younger children (1–12 years) and greatest in adolescents and older children (>12 years)(Reference McKenna, Lyons and Flynn62,Reference Scully, Laird and Healy63,Reference Carroll, Onwuneme and McKenna65) . For example, a recent large study found a greater prevalence of deficiency in over v. under 12s (24 v. 16 %)(Reference Scully, Laird and Healy63). Lower rates of deficiency (2 %) have been found in toddlers (aged 2 years) and children under 5 (13 %)(Reference McVey, Geraghty and O'Brien64,Reference Ní Chaoimh, McCarthy and Hourihane66) , with higher 25(OH)D also reported in those under v. over 4 years (61⋅0 v. 46⋅1 nm/l, P < 0⋅001)(Reference Carroll, Onwuneme and McKenna65). Similarly, in a recent large study, deficiency was lower (5 %) in toddlers (1–4 years) but much higher (15⋅4 %) in older children (5–19 years)(Reference McKenna, Lyons and Flynn62). Better vitamin D status in younger children (<5 years) may relate to Ireland's infant supplementation policy(Reference Andersson, Swolin-Eide and Magnusson75). Conversely, lower rates of supplements and fortified food consumption has also been found in Irish teens compared to younger children(Reference Black, Walton and Flynn76). One study also reported that older teens (15–18 years) had lower mean 25(OH)D and were more likely to be deficient than younger teens (13–15 years)(Reference Cashman, Kehoe and Kearney69). Greater screen time/sedentary behaviour and increased obesity rates in this age group may be important factors(77–Reference Karalius, Zinn and Wu79). Socioeconomic status may also explain some of this variation as it has been associated with a higher prevalence of deficiency in Irish children(Reference Scully, Laird and Healy63). Overall, reports are broadly consistent with findings in northern European countries (47–69°N)(Reference Sioen, Mouratidou and Kaufman80–Reference Nälsén, Becker and Pearson83) and in the UK 19 % of 11–18-year-olds were deficient, compared to just 2 % of those aged 4–10(Reference Beverley, David and Kerry84).
Despite in general, less deficiency in younger children, this was not apparent for those aged <1 year. For example, deficiency in new-borns was high ranging from 34 to 63 % (based on cord blood samples)(Reference McCarthy, Murray and Malvisi35,Reference Onwuneme, Diya and Uduma39,Reference Walsh, McGowan and Kilbane42,Reference Kiely, O'Donovan and Kenny49) while in preterm infants 64 % were deficient at delivery(Reference Onwuneme, Martin and McCarthy40).
Overall, approximately half of children aged 1–17 years had levels <50 nm/l(Reference McKenna, Lyons and Flynn62,Reference Scully, Laird and Healy63,Reference Carroll, Onwuneme and McKenna65,Reference Glatt, McSorley and Pourshahidi68,Reference Cashman, Kehoe and Kearney69) , with a seasonal variation identified(Reference Scully, Laird and Healy63,Reference Ní Chaoimh, McCarthy and Hourihane66,Reference Hill, Cotter and Mitchell73) , similar to findings in the European Union (EU)(Reference Andersson, Swolin-Eide and Magnusson75,Reference Sioen, Mouratidou and Kaufman80–Reference Heimbeck, Wjst and Apfelbacher85) . Similar to deficiency, prevalence was lower in younger children (1–5 years), at 21–39 %(Reference McKenna, Lyons and Flynn62,Reference McVey, Geraghty and O'Brien64,Reference Ní Chaoimh, McCarthy and Hourihane66) , and higher in teens at 36–89 % as in UK studies(Reference Cashman, Dowling and Škrabáková81). The prevalence of 25(OH)D < 50 nm/l in new-born cord blood was particularly high (between 80 and 92 %)(Reference McCarthy, Murray and Malvisi35,Reference Onwuneme, Diya and Uduma39,Reference Walsh, McGowan and Kilbane42,Reference Kiely, O'Donovan and Kenny49) , and similar (79–92 %) in preterm and term infants(Reference McCarthy, Murray and Malvisi35,Reference Onwuneme, Martin and McCarthy40,Reference Walsh, McGowan and Kilbane42,Reference McCarthy, McKenna and Oyefeso44,Reference Kiely, O'Donovan and Kenny49) .
Dietary and supplement intakes in children
The first nationally representative dietary survey in 2010/2011 in Irish children aged 12–59 months found that 70–84 % had intakes <5 μg/d (mean of 3⋅2 μg/d)(86). In a small study (n 97) of 5-year-olds in 2019, intake remained low with just 6⋅2 % having consuming above 5 μg/d(Reference McVey, Geraghty and O'Brien64). Recent nationally representative surveys of older children (5–12 years) and teens (13–17 years) found the majority (94 %) had intakes <10 μg/d, with little improvement between 2003/2004 and 2017/2018(78,87,Reference Kehoe, Buffini and McNulty88) . In fact, comparing surveys, intakes had improved only a little, from 2⋅7 to 3⋅7 μg/d for teens and from 2⋅3 to 4⋅2 μg/d for children(87,89–91) . Similarly, in a recent nationally representative study of teenagers (aged 13-18) median intake was 2.9 μg/d(Reference Cashman, Kehoe and Kearney69).
In Irish infants, milk/formula comprised of 29 % of total intakes(86), similar to that in UK children aged 12–18 months(Reference Lennox, Sommerville and Ong92). Milk/products were also the greatest source of vitamin D in children <4 years in Ireland followed by meat and its products as found in the UK(86,Reference Lennox, Sommerville and Ong92) . However, in Irish children aged 5–12 years, fortified cereals were the largest contributor, followed by meat and then milk products(87) as also identified in Belgium and the UK(Reference Cashman, Dowling and Škrabáková81,Reference Huybrechts, Lin and De Keyzer93) . Meat and its products also account for the primary source of vitamin D in Irish children over 13(78,94) , despite its relatively low content. This is also reflected in the UK, where it comprised of 35 % of dietary vitamin D intake(4).
Children aged 1–5 years in Ireland are recommended to receive 5 μg/d, supplementing if necessary, with older children (aged 6-11 years) advised to consume 10 μg/d(20,22) . Overall, supplements are an important contributor to vitamin D status in children and adolescents in Ireland(Reference Carroll, Onwuneme and McKenna65,Reference Hennessy, Browne and Kiely95) , as found elsewhere in Europe(Reference Absoud, Cummins and Lim96–Reference Oberg, Jorde and Almas98). The most recent national dietary survey (2017/2018) of Irish children aged 5–12 years indicated that just 10 % consume a vitamin D supplement(87) which compares to 17 % in a representative sample (aged 1–4 years)(Reference Hennessy, Browne and Kiely95). Supplement use has also been found to decrease with age, with 21 % of 5–8, 16 % of 9–12 and 15 % of 13–17-year-olds consuming a vitamin D containing supplement(Reference Carroll, Onwuneme and McKenna65,Reference Ní Chaoimh, McCarthy and Hourihane66,Reference Black, Walton and Flynn76,Reference Hennessy, Browne and Kiely95) . Similar findings have been reported in the UK, with higher supplementation rates in younger children (14–16 %) compared to teens (5–6 %)(Reference Calame, Street and Hulshof57). Indeed, since the introduction of an infant supplementation policy in Ireland, initiation of a 5 μg/d supplement from birth increased to 92 %, with 30 % of parents compliant during the first year(Reference Hemmingway, Fisher and Berkery99). In one study, supplement use was reported in 23 % of Irish children (aged 1–17) attending hospital though this could be due to underlying medical reasons(Reference Carroll, Onwuneme and McKenna65).
Adults (<50 years)
The largest study (n 63 290) revealed that 13 % of Irish adults (<50 years) were deficient between 2020 and 2021(Reference McKenna, Lyons and Flynn62). In a nationally representative study, deficiency affected 7 % whereas it was much more common (38 %) in hospital-based inpatients with COVID(Reference Cashman, Muldowney and McNulty18,Reference Barrett, Youssef and Shah100) . Deficiency was also more prevalent in younger adults and in winter(Reference Scully, Laird and Healy101–Reference O'Sullivan, Nic Suibhne and Cox103). In Dublin (53°N), between 25 and 30 % were deficient(Reference Kilbane, O'Keane and Morrin104,Reference McKenna, Murray and Crowley105) , though there was a lower prevalence of 10–11 % (<25 nm/l) at a similar latitude in the west of Ireland(Reference Delos Reyes, Smyth and Griffin106). In Coleraine (55°N, Northern Ireland) there was a wide variation in deficiency (2–23 %) and in two studies it affected only 0–2 %, though sample sizes in all were small (n < 100)(Reference Hill, Flynn and Kiely72,Reference Todd, McSorley and Pourshahidi107–Reference Todd, Madigan and Pourshahidi109) . Overall, deficiency in Ireland appears similar to other European countries(4,Reference Lips, Cashman and Lamberg-Allardt59,Reference Cashman, Dowling and Škrabáková81,Reference Spiro and Buttriss110–Reference Lamberg-Allardt, Outila and Kärkkäinen112) .
Despite not generally being considered an ‘at-risk’ group, studies indicate that deficiency is more prevalent in younger v. older adults (>50 years)(Reference McKenna, Lyons and Flynn62,Reference Barrett, Youssef and Shah100–Reference Kilbane, O'Keane and Morrin104,Reference Griffin, Wall and Blake113) . For example, 18 % of younger adults (aged 18–39) v. 15 % (aged 40–49) were deficient(Reference Scully, Laird and Healy101,Reference Griffin, Wall and Blake114) . In addition, urban dwelling younger adults had higher levels of deficiency (<25 nm/l) than rural ones(Reference Griffin, Wall and Blake113). This nadir in younger adults has also been reported in Romania(Reference Chirita-Emandi, Socolov and Haivas115), Canada(Reference Naugler, Zhang and Henne116), the USA(Reference Herrick, Storandt and Afful117) and Brazil(Reference Martini, Verly and Marchioni118). Lack of sunshine exposure due to time spent indoors in a working environment and reduced dietary and supplementary intakes of vitamin D may be factors(Reference Cashman, Muldowney and McNulty18,94,Reference Sowah, Fan and Dennett119) .
Levels <50 nm/l were found in more than half of adults living in Dublin(Reference Kilbane, O'Keane and Morrin104,Reference McKenna, Murray and Crowley105) and 44 % in those living in the West(Reference Delos Reyes, Smyth and Griffin106). In several locations across Ireland, a similar prevalence (40–55 %) was also identified(Reference Cashman, Muldowney and McNulty18,Reference Magee, Pourshahidi and Wallace120) . In younger Irish adults (aged 18–39) nearly half (45 %) had levels <50 nm/l(Reference Scully, Laird and Healy101,Reference Griffin, Wall and Blake114) and up to 58 % had <50 nm/l in winter(Reference Hill, Flynn and Kiely72,Reference Scully, Laird and Healy101–Reference O'Sullivan, Nic Suibhne and Cox103) , with this proportion being higher in Cork(Reference Hill, Flynn and Kiely72) and Dublin(Reference Scully, Laird and Healy101–Reference O'Sullivan, Nic Suibhne and Cox103). Indeed, studies showed higher rates of deficiency and levels <50 nm/l in winter in Irish adults aged under 50(Reference Scully, Laird and Healy101,Reference Laird, Shannon and Crowley102,Reference Griffin, Wall and Blake113) consistent with findings in the UK(4). By comparison, in the EU, 34–64 % of adults under 65 had 25(OH)D < 50 nm/l(Reference Cashman, Dowling and Škrabáková81,Reference Tønnesen, Hovind and Jensen111,Reference Hoge, Donneau and Streel121,Reference Muldowney, Lucey and Paschos122) .
In general, Irish females had higher 25(OH)D than males(Reference McKenna, Lyons and Flynn62,Reference Scully, Laird and Healy101,Reference Laird, Shannon and Crowley102,Reference Delos Reyes, Smyth and Griffin106,Reference Griffin, Wall and Blake113,Reference Griffin, Wall and Blake114) though are twice as likely to consume a supplement and are more likely to be taking vitamin D which might explain this(Reference Cashman, Muldowney and McNulty18,Reference Kiely, Flynn and Harrington123,Reference Peklar, Henman and Richardson124) . Large global meta-analyses have also found that females had a borderline increased vitamin D status compared to males(Reference Manios, Moschonis and Hulshof125,Reference Hagenau, Vest and Gissel126) .
Older adults
Vitamin D deficiency is prevalent in older Irish adults (>50 years) and in large studies ranged between 11 and 13 %(Reference McKenna, Lyons and Flynn62,Reference Scully, Laird and Healy101,Reference Laird, O'Halloran and Carey127) . In the largest nationally representative TILDA study (The Irish longitudinal study on ageing) of 5356 older adults, 13 % were deficient, similar to the findings of an EU meta-analysis(Reference Laird, McNulty and Ward9,Reference Cashman, Dowling and Škrabáková81) . In the large Trinity, Ulster, Department of Agriculture cohort study, prevalence of deficiency ranged from 13⋅8 and 27⋅3 % in older unsupplemented adults, and up to 43⋅6 % in those who were frail and cognitively impaired(Reference McCarroll, Beirne and Casey128). However, participants were in disease-defined cohorts, included hospital outpatients and were not representative of the wider population. As expected, deficiency was more prevalent during winter(Reference McCarthy, Collins and O'Brien71,Reference Hill, Flynn and Kiely72,Reference Laird, O'Halloran and Carey127,Reference Hill, Collins and O'Brien129) .
Regional variation in the prevalence of deficiency has been found in the nationally representative TILDA study where it was lower in Leinster compared to other provinces, likely reflecting in part variances in UVB exposure(Reference Laird, O'Halloran and Carey127). However, prevalence rates have also varied within the same areas with widely varying deficiency rates of 11–86 % in Dublin(Reference Scully, Laird and Healy101,Reference Freaney, McBrinn and McKenna130) , 2–17 % in Cork(Reference Hill, Flynn and Kiely72), 10–42 % in Galway and 14–35 % in Northern Ireland(Reference Laird, McNulty and Ward9,Reference Feehan, Armstrong and Magee131) . Socioeconomic factors may play a role as suggested by the TILDA study and one large investigation in the Dublin and surrounding areas(Reference Griffin, Wall and Blake114,Reference Lardner, Fitzgibbon and Wilson132) . Urban residing older adults also had increased rates of deficiency(Reference Griffin, Wall and Blake113), in keeping with UK findings(Reference Aspell, Laird and Healy133), as did community dwelling older adults and nursing home residents(Reference Griffin, Wall and Blake113) where it ranged between 35 and 42 %(Reference Griffin, Wall and Blake114,Reference Feehan, Armstrong and Magee131) . Hospital in-patients also had lower vitamin D status than those in primary care(Reference McKenna, Lyons and Flynn62). Similarly, non-community dwelling adults had lower vitamin D status in the UK(Reference Hirani and Primatesta134) and in Europe at similar latitudes(Reference Pilz, Dobnig and Tomaschitz135–Reference Bruyere, Decock and Delhez137). This is likely due to reduced physical activity, lack of sun exposure and poor adherence to supplementation(Reference Bruyere, Decock and Delhez137,Reference Mortensen, Tetens and Kristensen138) .
In recent studies of Irish adults (>50 years), between 30 and 50 % had levels <50 nm/l, which was more prevalent in those living in northern locations and urban areas(Reference Scully, Laird and Healy101,Reference Griffin, Wall and Blake114,Reference Laird, O'Halloran and Carey127) . Similar findings were reported in the UK English longitudinal study of ageing and in other EU countries where it ranged from 50 to 59 %(Reference Lips, Cashman and Lamberg-Allardt59,Reference Cashman, Dowling and Škrabáková81,Reference Aspell, Laird and Healy133,Reference Andersen, Mølgaard and Skovgaard139) . Seasonal decline in status was also greater with increasing age, with levels <50 nm/l in winter occurring in 64 % (aged 70–75) v. 34 % (aged 51–69) in 2006(Reference Hill, Flynn and Kiely72). This was replicated by the TILDA study in 2018 which found 43 % with <50 nm/l, with higher rates in the winter in those aged 70+ v. 50+(Reference Laird, O'Halloran and Carey127,Reference Laird and Kenny140) . Despite a reduced capacity for dermal synthesis, UVB light and sun enjoyment were still identified as an important contributor to vitamin D status in older Irish adults(Reference O'Sullivan, Laird and Kelly15).
Dietary and supplement intakes in adults
The most recent (2011) dietary survey in adults found that vitamin D intake in Ireland was 6⋅9 μg/d in older adults (>65 years), and 4⋅3 μg/d in those aged 18–64 years(94). The majority (90 %) of adults aged 18–64 had intakes <10 μg/d, as did 87 % of men and 77 % of women over 65 years(94). This indicates little improvement since the first dietary survey in 1997/1999. In fact, overall mean intake of vitamin D then was 3⋅4 μg/d(Reference O'Brien, Kiely and Harrington141,142) while 74 % of 18–64-year-olds had intakes <5 μg/d(Reference Hill, O'brien and Cashman143) and 93 % had <10 μg/d(Reference Black, Walton and Flynn144). Lowest intakes were identified in younger adults (18–35 years) (mean 2⋅8 μg) compared to those aged 36–50 years (mean 3⋅4 μg) or 51–64 years (mean 5⋅8 μg) in 2001(Reference O'Brien, Kiely and Harrington141). Recent findings are similar to those reported in the UK, Germany, Denmark, and the Netherlands though higher than that in Portugal, Spain and Italy(Reference Lips, Cashman and Lamberg-Allardt59). By comparison, dietary intakes have been found to be higher in northern European countries such as Iceland, Norway, Sweden and Finland(Reference Lips, Cashman and Lamberg-Allardt59,Reference Jenab, Salvini and van Gils145) .
Meat, fish and supplements were the greatest contributors of vitamin D in the diet of Irish adults(Reference Cashman, Muldowney and McNulty18) similar to some EU countries such as the Netherlands. However, in countries such as France and Spain, fish and eggs are the primary and secondary sources(Reference Spiro and Buttriss110,Reference Van Rossum, Buurma-Rethans and Dinnissen146,Reference Olza, Aranceta-Bartrina and González-Gross147) . Overall, fortified cereal/products found to contribute to 10 % of intake in the Irish diet(Reference Galvin, Kiely and Flynn148), compared to 13–20 % in the UK(4).
Adolescents and adults (12–65 years) are recommended to consume 15 μg of vitamin D daily, with older adults (>65 years) who are housebound with little access to sunlight advised to take 20 μg/d(19,22) . However, only 10–17 % of Irish adults were found to be taking vitamin D supplements(Reference Cashman, Muldowney and McNulty18,Reference O'Sullivan, Nic Suibhne and Cox103,Reference Laird and Kenny140) , but received more of their intake this way than from dietary sources(Reference Cashman, Muldowney and McNulty18,Reference Kiely, Flynn and Harrington123) . In a recent (2019) TILDA report, just over 10 % of over 70s reported consuming a vitamin D supplement(Reference Laird and Kenny140). Supplement use was also a predictor of vitamin D status in adults(Reference Cashman, Muldowney and McNulty18,Reference Todd, Madigan and Pourshahidi109,Reference Magee, Pourshahidi and Wallace120) and older adults(Reference O'Sullivan, Laird and Kelly15,Reference Laird, O'Halloran and Carey127,Reference McCarroll, Beirne and Casey128,Reference Andersen, Mølgaard and Skovgaard139,Reference Laird and Kenny140) . In fact, supplement use has been found to be the strongest determinant of vitamin D status in the Trinity, Ulster, Department of Agriculture(Reference McCarroll, Beirne and Casey128) and TILDA(Reference Laird and Kenny140) cohorts of older adults, with a mean increase of 21⋅4–35⋅4 nm/l detected(Reference McCarroll, Beirne and Casey128). Supplementation was also found to increase with age(Reference Cashman, Muldowney and McNulty18,Reference Hill, O'brien and Cashman143) , with its contribution to dietary intake nearly twice as high in older adults aged 50+ (17 %) v. younger adults (9 %)(Reference Cashman, Muldowney and McNulty18). This has been confirmed elsewhere(Reference Calvo, Whiting and Barton149), with higher rates in older adults (24–32 %) in the UK(Reference Calame, Street and Hulshof57). Supplements relating to bone health (calcium with/without vitamin D) are the most consumed in older adults(Reference Peklar, Henman and Richardson124). Irish women are more likely to take supplements than men(Reference O'Sullivan, Laird and Kelly15,Reference Kiely, Flynn and Harrington123,Reference Peklar, Henman and Richardson124,Reference Laird, O'Halloran and Carey127) , where they contributed more to total vitamin D intake(Reference Hill, O'brien and Cashman143) as also found in the UK(Reference Henderson, Gregory and Swan150).
At-risk populations
Ethnic populations
The largest study to focus on an ethnic population discovered that more than two-thirds (67 %) of Southeast Asians were vitamin D deficient, but only included 186 patients who lived in the Dublin area(Reference Laird, Walsh and Lanham-New151). There were only three other studies that reported vitamin D status in those of non-white ethnicity, with small sample sizes and the largest only having eighty-one adults(Reference Kiely, Zhang and Kinsella34,Reference Toher, Lindsay and McKenna37,Reference Carroll, Onwuneme and McKenna65) . Non-white pregnant women had greater deficiency (59–88 %) compared to Caucasians (36 %)(Reference Toher, Lindsay and McKenna37), while another study identified a 19 nm/l difference in mean 25(OH)D between white and non-white pregnant women(Reference Kiely, Zhang and Kinsella34). Mean 25(OH)D was also lowest in children of African ethnicity living in Ireland(Reference Carroll, Onwuneme and McKenna65). Ethnic minorities living in northern locations are known to be at increased risk of low vitamin D status due to reduced cutaneous synthesis(Reference Lips, Cashman and Lamberg-Allardt59,Reference Spiro and Buttriss110) . By comparison, in the UK, 96 % of Southeast Asian women had levels <50 nm/l in winter and had lower serum 25(OH)D compared to white women(Reference Darling, Hart and Macdonald152). Similarly, non-European populations, particularly pregnant women, living in Europe were at greater risk compared to their indigenous counterparts(Reference van der Meer, Middelkoop and Boeke153–Reference Eggemoen, Falk and Knutsen156). Despite recommendations by the European Calcified Tissue Society, there are currently no specific guidelines for higher vitamin D intake in ethnic populations in Ireland(Reference Lips, Cashman and Lamberg-Allardt59).
Medical conditions
Malabsorption disorders: There were five studies of adults with Crohn's disease(Reference Kelly, Suibhne and O'Morain157–Reference Gilman, Shanahan and Cashman161) though sample sizes were small (n < 100). Prevalence of levels <50 nm/l were 50–64 %(Reference Kelly, Suibhne and O'Morain157,Reference Nic Suibhne, Cox and Healy158,Reference Raftery, Martineau and Greiller160) in keeping with a global meta-analysis where half had levels <50 nm/l(Reference Sadeghian, Saneei and Siassi162). A strong seasonal effect was also found, with about 20 % having levels <50 nm/l post-summer v. 50 % post-winter(Reference McCarthy, Duggan and O'Brien159,Reference Gilman, Shanahan and Cashman161) and with up to 90 % with levels <80 nm/l(Reference Nic Suibhne, Cox and Healy158). Furthermore, wintertime levels <50 nm/l were twice as common (50 %) compared to healthy controls (25 %)(Reference McCarthy, Duggan and O'Brien159). Crohn's patients in Ireland who had bowel surgery were also three times more likely to have levels <50 nm/l compared to non-Caucasians(Reference Chatu, Chhaya and Holmes163). Comparatively in the UK, 66 % of adults with Crohn's disease had levels <50 nm/l, with significantly lower status in the winter(Reference Chatu, Chhaya and Holmes163). The vast majority (88 %) of Irish patients with refractory coeliac disease had levels <50 nm/l, as did those with a recent diagnoses (58 %) compared to patients with controlled disease(Reference Keaveny, Freaney and McKenna164).
Other disorders
In Irish patients with multiple sclerosis, significantly greater deficiency (<25 nm/l) was found compared to age-/sex-matched controls (28⋅3 v. 19⋅2 %)(Reference Lonergan, Kinsella and Fitzpatrick165). Mean 25(OH)D levels were also higher in areas with a lower prevalence of multiple sclerosis(Reference Lonergan, Kinsella and Fitzpatrick165). Nearly two-thirds (65 %) of patients with systemic lupus erythematosus had levels <75 nm/l after the summer in one 2008 study(Reference Cusack, Danby and Fallon166). In psoriasis patients, 75 % were found to have wintertime levels <50 nm/l(Reference Ryan, Moran and McKenna167) and, in individuals attending a rheumatology clinic 26 % were vitamin D deficient (<25 nm/l) and 70 % had levels <53 nm/l(Reference Haroon, Bond and Quillinan168). Deficiency prevalence was 41 % in patients with total knee arthroplasty (41 %)(Reference Kelly, Campbell and Sheahan169).
In adults with chronic obstructive pulmonary disease, vitamin D status was low (<50 nm/l) in 47 %, particularly in winter (75 %) and in house-bound patients(Reference Carson, Pourshahidi and Madigan170). In patients with obstructive sleep apnoea, 72–89 % had levels <50 nm/l, and 98 % had <75 nm/l(Reference Kerley, Hutchinson and Bramham171,Reference Kerley, Hutchinson and Bolger172) . This is similar to the findings of a global meta-analysis, which identified that disease severity was associated with lower vitamin D status(Reference Neighbors, Noller and Song173). Prevalence of levels <50 nm/l was also high in renal patients (69 %)(Reference Cronin, Byrne and Doyle174) and those who had thyroidectomy (75 %)(Reference Griffin, Murphy and Sheahan175). Between 35 and 50 % of Irish asthmatic children had vitamin D levels <50 nm/l(Reference Kerley, Elnazir and Greally176,Reference Kerley, Hutchinson and Cormican177) , in keeping with a recent global meta-analysis(Reference Wang, Ying and Zhu178). Up to 40 % of children with autism had levels <50 nm/l and 75 % had <75 nm/l(Reference Kerley, Elnazir and Greally176,Reference Kerley, Power and Gallagher179) , consistent with a meta-analysis in 2016 that attributed lower status to factors such as increased dietary restriction and lack of time outdoors(Reference Wang, Shan and Du180).
Vitamin D excess
The most recent and largest (n 100 505) cross-sectional study of adults in 2022 found a prevalence of 25(OH)D > 125 nm/l of 1⋅7–2⋅3 % though included patients mainly in the Dublin area(Reference McKenna, Lyons and Flynn62). It also identified that excess levels were higher during v. before the COVID pandemic (2⋅1 v. 1⋅7 %, P < 0⋅001) which could be due to increased dosage of new-to-market vitamin D supplements(Reference McKenna, Lyons and Flynn62). Previously, it was estimated that up to 5 % of the Irish adults in the population may be at-risk of levels >125 nm/l, with apparent increases between 1994 and 2013(Reference Kilbane, O'Keane and Morrin104,Reference McKenna, Murray and Crowley105) . However, most studies have found a prevalence of vitamin D excess of between <1 and 3 %(Reference Cashman, Muldowney and McNulty18,Reference Neville, Martyn and Kilbane53,Reference Scully, Laird and Healy101,Reference McKenna, Murray and Crowley105,Reference Griffin, Wall and Blake113,Reference Laird, Walsh and Lanham-New151) . Importantly though, all of these studies are subject to significant bias as they include patients who had their vitamin D checked by request of their doctor. Higher levels of excess has been identified in Irish females (4 %) and older adults (4 %)(Reference Scully, Laird and Healy101) as found elsewhere(Reference Dudenkov, Yawn and Oberhelman181,Reference Batman, Saygili and Yildiz182) . This is likely due to females being twice as likely to use supplements, especially those over 50(Reference Kiely, Flynn and Harrington123,Reference Peklar, Henman and Richardson124) . A particularly high prevalence of excess of 9 % was identified in Irish pregnant women in a randomised controlled trial though they were supplemented with up to 20 μg/d of vitamin D(Reference O'Callaghan, Hennessy and Hull38). Vitamin D toxicity (25(OH)D level of 1617 nm/l) resulting in severe hypercalcaemia was also reported in one patient, though was explained by high-dose supplementation (250 μg/d for 2 years)(Reference Duffy and Brassill183). Conversely, lower levels of excess (0⋅3–0⋅9%) has been identified in nursing homes, hospital outpatient clinics, in ethnic minorities and in pregnant women(Reference Kiely, Zhang and Kinsella34,Reference McCarthy, Murray and Malvisi35,Reference Griffin, Wall and Blake113,Reference Griffin, Wall and Blake114,Reference Laird, Walsh and Lanham-New151) , groups that are already at greater risk of deficiency. In the largest study of children aged over 4 (n 5524) prevalence of vitamin D excess was 0⋅5 % but was in higher in toddlers (4⋅6 %) and infants (12⋅1 %)(Reference McKenna, Lyons and Flynn62). A similar prevalence (0⋅4–0⋅6 %) has been found in children (>4 years) in other studies(Reference Scully, Laird and Healy63,Reference Ní Chaoimh, McCarthy and Hourihane66) or has not been detected at all(Reference McCarthy, Murray and Malvisi35).
Vitamin D status over time
There have only been two studies that have specifically examined changes in vitamin D status over time. In one that included individuals (n 43 782) in the Dublin area over 20 years (1993–2013) an average increase in 25(OH)D of 0⋅68 nm/l per year was estimated(Reference McKenna, Murray and O'Keane184). However, it is possible that the reason for testing may have changed over time and it did not account for potential variation in factors affecting vitamin D. More recently when comparing annual change in vitamin D status prior to v. during the pandemic, a 3-fold increase was noticed with a higher annual rise of 2⋅8 nm/l. This result however, may not be generalisable as it was based on vitamin D results from a Dublin hospital, though was attributed to a greater availability of high-dose supplements and increased public awareness(Reference McKenna, Lyons and Flynn62). While vitamin D status may have increased, particularly in some sections of the population, nearly all of the most recent studies still identify a significant of level of deficiency and levels <50 nm/l(Reference McKenna, Lyons and Flynn62,Reference Scully, Laird and Healy101,Reference Griffin, Wall and Blake113,Reference Laird and Kenny140) .
Discussion
This is the first review of vitamin D status in Ireland and identifies that deficiency is commonly affecting 15–23 % of children, 13 % of adults and 15–17 % of pregnant women in the largest and most recent (<5 years) studies(Reference Kiely, Zhang and Kinsella34–Reference Hemmingway, Kenny and Malvisi36,Reference McKenna, Lyons and Flynn62–Reference McVey, Geraghty and O'Brien64,Reference Ní Chaoimh, McCarthy and Hourihane66–Reference Glatt, McSorley and Pourshahidi68,Reference Laird, O'Halloran and Carey127) . Deficiency was more prevalent in adolescents v. younger children (1–12 years), and in younger (<50 years) v. older adults (>50 years). There was also a particularly high prevalence in infants (<1 year) and it was also more common with increasing gestation in pregnancy. A very high rate of deficiency (67 %) was identified in Southeast Asians, though other studies of non-white ethnicity are sparse. Similarly, those with medical conditions had increased prevalence of vitamin D inadequacy, with more than half of those with respiratory conditions and the majority of those with malabsorption conditions having levels <50 nm/l.
The seasonal variation of vitamin D status was also evident with higher levels of deficiency and prevalence of levels <50 nm/l in winter. The overall prevalence of deficiency remains significant though there is some evidence to suggest a small increase in 25(OH)D levels in the past decade particularly during the COVID pandemic. The lowest vitamin D intakes were found in children (2⋅3 μg/d)(Reference Hennessy, Browne and Kiely95) and pregnant women (1⋅9 μg/d)(Reference McGowan, Byrne and Walsh55), with the highest (6⋅9 μg/d) in older adults(94). This review indicates that low vitamin D status is widespread in the population among several groups who also have inadequate vitamin D intake.
Implications for public health
Fortification/supplementation
Guidelines for vitamin D intake in those aged 12-65 years, including at-risk groups such as pregnant women, adolescents and dark skinned ethnicities, were recently published by the FSAI and advise 15 μg/d(22). These were based on minimising the risk of deficiency and were similar to a previously calculated 12 μg/d to avoid deficiency in most of the population(Reference Cashman, Kiely and Andersen185). However preventing winter deficiency in 97⋅5 % of individuals of South Asian and Black ethnicity at Irelands’ latitude may require an even higher respective daily intake of 27⋅3 μg (1092 IU) and 33⋅2 μg (1328 IU). Furthermore, the guidelines do not cover a proportion of the population who have levels between 30–50 nm/l and may still be at risk of vitamin D inadequacy. For example, an intake of 25–28 μg/d may be needed to maintain wintertime sufficiency in the Irish population (>50 nm/l)(Reference Cashman, Wallace and Horigan186,Reference Cashman, Hill and Lucey187) though achieving this via diet alone is not possible, so supplementation and a multi-food fortification strategy may be necessary.
Currently, fortified foods provide 11 % of total dietary vitamin D intake in adults, where they have the potential to reduce inadequacy(Reference Hannon, Kiely and Flynn188). In Irish children age 1–4 years, fortifying cow's milk and a 5 μg/d supplement in modelling studies were estimated to reduce inadequate intakes (<10 μg/d) from 95 to 12–36 %(Reference Kehoe, Walton and McNulty189). However, this would be insufficient for meeting the European Food Safety Authority (EFSA) adequate intake level (15 μg/d)(Reference Lyons, Kerr and McNulty190). Fortification of milk and bread was reported as having the potential to ensure that 70 % of older Irish adults (>50 years) meet a daily allowance of 10 μg/d(Reference McCourt, McNulty and Walton191). In older adults, an association has been found between fortified milk and better vitamin D status(Reference McCarroll, Beirne and Casey128). Fortifying food staples such as milk and bio-enriched eggs was also estimated to reduce the wintertime decline in serum vitamin D Irish adults(Reference McKenna, Freaney and Byrne192,Reference Hayes, Duffy and O'Grady193) . However, in a modelling study, numerous food items would need fortification to ensure vitamin D (>50 nm/l) all year around in Irish adults(Reference Cashman, Kazantzidis and Webb194). A more novel way using ‘biofortification of foods’ with vitamin D via feed modification and UV radiation in Ireland has shown potential, particularly enriched meat and could be further explored(Reference Neill, Gill and McDonald195).
In 2003, mandatory fortification of butter/spreads and milk products in Finland enabled 91 % of the population to reach sufficiency by 2011(Reference Jääskeläinen, Itkonen and Lundqvist196), a public health measure that could be considered in Ireland. Nonetheless, some groups of the population consume less fortified foods and are less likely to benefit(Reference Lehtonen-Veromaa, Möttönen and Leino197). Exploring a multi-food system fortification approach that includes bread as well as dairy products could be considered to target a wider population(Reference Spiro and Buttriss110). Hence, care is required to ensure that excessive vitamin D consumption is avoided(Reference Cashman and Kiely198) as a small percentage of the population consuming high-dose supplements could be at risk of vitamin D toxicity(Reference McKenna, Murray and Crowley105,Reference McKenna, Murray and O'Keane184) . Reassuringly, a national monitoring survey in Finland after food fortification concluded that levels above 125 nm/l were rare, though ongoing surveillance was advised(Reference Raulio, Erlund and Männistö199). Given the higher prevalence of vitamin D levels >125 nm/l recently reported in Ireland and the increased proportion of new-to-market supplements above the tolerable upper-intake level, monitoring would seem prudent(Reference McKenna, Lyons and Flynn62). Additionally, a code of practice for food business to control the level of fortification and limit its addition to designated food vehicles would be useful(Reference Buttriss and Lanham-New200).
We acknowledge there are a number of factors that may result in a variation in vitamin D status between studies. Some were small in size, were in different geographical locations, included non-representative populations (e.g. clinical or hospital outpatient setting) and used different vitamin D assays. There is also likely to be differences in supplementation rates and other factors affecting vitamin D status between studies. While some nationally representative studies were small, the review includes several very large and recent studies.
Future research
There are limited studies of vitamin D status in non-white ethnic individuals that now comprise 5 % of the Irish population(24). Additionally, there are no studies in minority groups such as Irish Travellers and institutionalised younger adults, all of which are groups where research is required. Furthermore, in light of low vitamin D intakes in the homeless population and nearly 9000 people in emergency accommodation in Ireland, attention could also be focused here(Reference Darmon, Coupel and Deheeger201,202) . Finally, meat is the primary source of dietary vitamin D in Irish adults though nearly one in five are vegetarian or vegan and are at-risk of deficiency, though no studies have specifically examined their vitamin D status(203).
Conclusions
Prevalence of vitamin D deficiency in Ireland was 15–17 % in pregnancy, 15–23 % in children and 13 % in adults and remains high despite some increase after the pandemic. Those at increased risk include infants (below 1 year), adolescents (12–18 years), adults (<50 years), those in the third trimester of pregnancy and non-white minorities. There is limited data in institutionalised adults, the Travelling community and those of non-European ethnicity. Given the prevalence of widespread deficiency, an updated public health policy to increase vitamin D intake, including a vitamin D awareness campaign and the careful fortification of key food groups frequently consumed by the population may be required.
Financial Support
This research is partially funded by Mercers' Institute and Tirlán (formerly Glanbia Ireland). Tirlán has no role in study design, data collection and analysis, or interpretation of data; in the writing of the manuscript or in the decision to publish the results.
Conflict of Interest
None.
Authorship
Conceptualisation: E. L., J. B. W. and K. McC.; formal analysis: H. S.; funding acquisition: E. L., J. B. W. and K. McC.; investigation: H. S.; methodology: H. S.; project administration: H. S.; supervision: K. McC. and E. L.; writing – original draft: H. S.; writing – review and editing: H. S., E. L., M. H., J. B. W. and K. McC. All authors have read and agreed to the published version of the manuscript.
Open access
