A multi-disciplinary expert group met to discuss vitamin D deficiency in the UK and strategies for improving population intakes and status. Changes to UK Government advice since the 1st Rank Forum on Vitamin D (2009) were discussed, including rationale for setting a reference nutrient intake (10 µg/d; 400 IU/d) for adults and children (4+ years). Current UK data show inadequate intakes among all age groups and high prevalence of low vitamin D status among specific groups (e.g. pregnant women and adolescent males/females). Evidence of widespread deficiency within some minority ethnic groups, resulting in nutritional rickets (particularly among Black and South Asian infants), raised particular concern. Latest data indicate that UK population vitamin D intakes and status reamain relatively unchanged since Government recommendations changed in 2016. Vitamin D food fortification was discussed as a potential strategy to increase population intakes. Data from dose–response and dietary modelling studies indicate dairy products, bread, hens’ eggs and some meats as potential fortification vehicles. Vitamin D3 appears more effective than vitamin D2 for raising serum 25-hydroxyvitamin D concentration, which has implications for choice of fortificant. Other considerations for successful fortification strategies include: (i) need for ‘real-world’ cost information for use in modelling work; (ii) supportive food legislation; (iii) improved consumer and health professional understanding of vitamin D’s importance; (iv) clinical consequences of inadequate vitamin D status and (v) consistent communication of Government advice across health/social care professions, and via the food industry. These areas urgently require further research to enable universal improvement in vitamin D intakes and status in the UK population.

The role of vitamin D in supporting the growth and maintenance of the skeleton is robust; with recent research also suggesting a beneficial link between vitamin D and other non-skeletal health outcomes, including immune function, cardiovascular health and cancer. Despite this, vitamin D deficiency remains a global public health issue, with a renewed focus in the UK following the publication of Public Health England's new Dietary Vitamin D Requirements. Natural sources of vitamin D (dietary and UVB exposure) are limited, and thus mechanisms are needed to allow individuals to achieve the new dietary recommendations. Mandatory or voluntary vitamin D food fortification may be one of the mechanisms to increase dietary vitamin D intakes and subsequently improve vitamin D status. However, for the food industry and public to make informed decisions, clarity is needed as to whether vitamins D2 and D3 are equally effective at raising total 25-hydroxyvitamin D (25(OH)D) concentrations as the evidence thus far is inconsistent. This review summarises the evidence to date behind the comparative efficacy of vitamins D2 and D3 at raising 25(OH)D concentrations, and the potential role of vitamin D food fortification as a public health policy to support attainment of dietary recommendations in the UK. The comparative efficacy of vitamins D2 and D3 has been investigated in several intervention trials, with most indicating that vitamin D3 is more effective at raising 25(OH)D concentrations. However, flaws in study designs (predominantly under powering) mean there remains a need for a large, robust randomised-controlled trial to provide conclusive evidence, which the future publication of the D2–D3 Study should provide (BBSRC DRINC funded: BB/I006192/1). This review also highlights outstanding questions and gaps in the research that need to be addressed to ensure the most efficacious and safe vitamin D food fortification practices are put in place. This further research, alongside cost, availability and ethical considerations (vitamin D3 is not suitable for vegans), will be instrumental in supporting government, decision-makers, industry and consumers in making informed choices about potential future vitamin D policy and practice.

There is a need for food-based solutions for preventing vitamin D deficiency. Vitamin D3 (D3) is mainly used in fortified food products, although the production of vitamin D2 (D2) is more cost-effective, and thus may hold opportunities. We investigated the bioavailability of D2 from UV-irradiated yeast present in bread in an 8-week randomised-controlled trial in healthy 20–37-year-old women (n 33) in Helsinki (60°N) during winter (February–April) 2014. Four study groups were given different study products (placebo pill and regular bread=0 µg D2 or D3/d; D2 supplement and regular bread=25 µg D2/d; D3 supplement and regular bread=25 µg D3/d; and placebo pill and D2-biofortified bread=25 µg D2/d). Serum 25-hydroxyvitamin D2 (S-25(OH)D2) and serum 25-hydroxyvitamin D3 (S-25(OH)D3) concentrations were measured at baseline, midpoint and end point. The mean baseline total serum 25-hydroxyvitamin D (S-25(OH)D=S-25(OH)D2+S-25(OH)D3) concentration was 65·1 nmol/l. In repeated-measures ANCOVA (adjusted for baseline S-25(OH)D as total/D2/D3), D2-bread did not affect total S-25(OH)D (P=0·707) or S-25(OH)D3 (P=0·490), but increased S-25(OH)D2 compared with placebo (P<0·001). However, the D2 supplement was more effective than bread in increasing S-25(OH)D2 (P<0·001). Both D2 and D3 supplementation increased total S-25(OH)D compared with placebo (P=0·030 and P=0·001, respectively), but D2 supplementation resulted in lower S-25(OH)D3 (P<0·001). Thus, D2 from UV-irradiated yeast in bread was not bioavailable in humans. Our results support the evidence that D2 is less potent in increasing total S-25(OH)D concentrations than D3, also indicating a decrease in the percentage contribution of S-25(OH)D3 to the total vitamin D pool.

Vitamin D has become one of the most discussed nutrients in human nutrition, which has led to an increased interest in milk as a vitamin D source. Problems related to fortifying milk with synthetic vitamin D can be avoided by securing a high content of natural vitamin D in the milk by supplying dairy cows with sufficient vitamin D. However, choosing the most efficient route and form of supplementation requires insight into how different vitamin D metabolites are transported in the body of cattle. There are two forms of vitamin D: vitamin D2 (D2) and vitamin D3 (D3). Vitamin D2 originates from fungi on roughage. Vitamin D3 originates either from endogenous synthesis in the skin or from feed supplements. Vitamin D2 is chemically different from, and less physiologically active than, D3. Endogenous and dietary D3 is chemically similar but dietary D3 is toxic, whereas endogenous D3 appears well regulated in the body.

It has been suggested that vitamin D2 is not very prevalent in the human food chain. However, data from a number of recent intervention studies suggest that the majority of subjects had measurable serum 25-hydroxyvitamin D2 (25(OH)D2) concentrations. Serum 25(OH)D2, unlike 25(OH)D3, is not directly influenced by exposure of skin to sun and thus has dietary origins; however, quantifying dietary vitamin D2 is difficult due to the limitations of food composition data. Therefore, the present study aimed to characterise serum 25(OH)D2 concentrations in the participants of the National Adult Nutrition Survey (NANS) in Ireland, and to use these serum concentrations to estimate the intake of vitamin D2 using a mathematical modelling approach. Serum 25(OH)D2 concentration was measured by a liquid chromatography–tandem MS method, and information on diet as well as subject characteristics was obtained from the NANS. Of these participants, 78·7 % (n 884) had serum 25(OH)D2 concentrations above the limit of quantification, and the mean, maximum, 10th, 50th (median) and 90th percentile values of serum 25(OH)D2 concentrations were 3·69, 27·6, 1·71, 2·96 and 6·36 nmol/l, respectively. To approximate the intake of vitamin D2 from these serum 25(OH)D2 concentrations, we used recently published data on the relationship between vitamin D intake and the responses of serum 25(OH)D concentrations. The projected 5th to 95th percentile intakes of vitamin D2 for adults were in the range of 0·9–1·2 and 5–6 μg/d, respectively, and the median intake ranged from 1·7 to 2·3 μg/d. In conclusion, the present data demonstrate that 25(OH)D2 concentrations are present in the sera of adults from this nationally representative sample. Vitamin D2 may have an impact on nutritional adequacy at a population level and thus warrants further investigation.

Public health recommendations do not distinguish between vitamin D2 and vitamin D3, yet disagreement exists on whether these two forms should be considered equivalent. The objective of the present study was to evaluate the effect of a daily physiological dose of vitamin D2 or vitamin D3 on 25-hydroxyvitamin D (25(OH)D) status over the winter months in healthy adults living in Dunedin, New Zealand (latitude 46°S). Participants aged 18–50 years were randomly assigned to 25 μg (1000 IU) vitamin D3 (n 32), 25 μg (1000 IU) vitamin D2 (n 31) or placebo (n 32) daily for 25 weeks beginning at the end of summer. A per-protocol approach, which included ≥ 90 % supplement compliance, was used for all analyses. Serum 25-hydroxyvitamin D3 (25(OH)D3), 25-hydroxyvitamin D2 (25(OH)D2) and parathyroid hormone (PTH) were measured at baseline and at 4, 8, 13 and 25 weeks. Geometric mean total serum 25(OH)D concentrations (sum of 25(OH)D2 and 25(OH)D3) at baseline was 80 nmol/l. After 25 weeks, participants randomised to D2 and placebo had a significant reduction in serum 25(OH)D3 concentrations over the winter months compared with vitamin D3-supplemented participants (both P< 0·001). Supplementation with vitamin D2 increased serum 25(OH)D2 but produced a 9 (95 % CI 1, 17) nmol/l greater decline in the 25(OH)D3 metabolite compared with placebo (P< 0·036). Overall, total serum 25(OH)D concentrations were 21 (95 % CI 14, 30) nmol/l lower in participants receiving vitamin D2 compared with those receiving D3 (P< 0·001), among whom total serum 25(OH)D concentrations remained unchanged. No intervention-related changes in PTH were observed. Daily supplementation of vitamin D3 was more effective than D2; however, the functional consequence of the differing metabolic response warrants further investigation.

Vitamin D2 from irradiated edible mushrooms might present a possible dietary source of this vitamin, subject to its bioavailability. Having previously optimized a method for the conversion of ergosterol in mushrooms to vitamin D2, this paper examines the vitamin D-enriched mushrooms (Lentinula edodes) for their bioavailability of the vitamin, using an animal model. Thirty male Wistar rats were fed for 1 week with a diet deficient in vitamin D. After this 1-week period, six rats were randomly selected and killed for analysis of initial bone mineral density, and serum level of 25-hydroxyvitamin D. A group of twelve rats of the test animals received 1 μg of vitamin D2 from irradiated mushrooms for a period of 4 weeks until being killed. The remaining twelve rats were fed un-irradiated mushrooms at the same level to act as controls. At the end of a 4-week period, the mean serum 25-hydroxyvitamin D level of the experimental group was 129·42 (sd 22·00) nmol/l whereas it was only 6·06 (sd 1·09) nmol/l in the control group. Femur bone mineral density of the experimental group of animals was significantly higher (P<0·01) than the control group. In addition, serum Ca concentrations among groups were shown to be significantly higher (P<0·01). It may be concluded from the results that vitamin D2 from UV-irradiated mushrooms is well absorbed and metabolized in this model animal system. Significant increase in femur bone mineralization (P<0·01) was shown in the presence of vitamin D2 from irradiated mushrooms compared with the controls.