Vitamin D metabolism

Vitamin D is a precursor of the active form 1,25-dihydroxyvitamin D (calcitriol) and is present in two forms; vitamin D₃ (cholecalciferol) and vitamin D₂ (ergocalciferol). The main source in human subjects is via action of solar ultraviolet-B (UV-B) radiation (270–300 nm) on skin, converting 7-dehydrocholesterol (provitamin D₃) into pre-vitamin D₃, which is then rapidly converted to vitamin D₃. Vitamin D₂ is produced via UV-B radiation on plant sources such as mushrooms and yeast. Natural dietary sources of vitamin D₃ are few and include fish liver oils, oily fish, egg yolks and some fortified foods (e.g. some dairy and breakfast cereals) and supplements. Irrespective of how it is acquired, vitamin D offers limited function until it has been activated, a process which requires two hydroxylation steps⁽Reference Bouillon, Carmeliet and Daci^14, Reference DeLuca¹⁵⁾. Firstly in the liver by a number of enzymes but primarily vitamin D₃25-hydroxylase (CYP2RA)⁽Reference Schuster¹⁶⁾, which converts it to the inactive precursor, 25-hydroxyvitamin D (25(OH)D), the prominent circulating form used to determine vitamin D status in human subjects. The second hydroxylation step occurs in the tubular cells of the kidney, by action of the enzyme 25vitamin D₃-1α-hydroxylase (CYP27B1), to the biologically active metabolite, 1,25-dihydroxyvitamin D₃. This active metabolite is an important modulator of calcium and phosphate homeostasis, importantly; however, both the activating enzyme CYP27B1 and the active form of vitamin D, 1,25-dihydroxyvitamin D₃, are present in many non-renal tissues including the human brain, immune cells, cardiovascular systems and pancreatic islets. Due to the ability to be synthesised endogenously, vitamin D is widely accepted not as a classical vitamin but as a steroid hormone, furthermore, its synthesis in the human brain has led to it being commonly referred to as a neurosteroid⁽Reference Holick¹⁷⁾.

Factors influencing vitamin D status in ageing

Several factors influence vitamin D status including skin pigmentation, use of sunscreen or concealing clothing, season, latitude, being older or institutionalised, obesity, malabsorption, renal and liver disease and medication use. In general, UV-B radiation on skin is the main source of vitamin D for human subjects. In countries north of the equator (40–60°N), vitamin D UV-B doses are inadequate for 6 months of the year (October–March)⁽Reference Webb, Kline and Holick¹⁸⁾. Many other environmental factors implicate vitamin D UV-B dose availability including the ozone layer, cloud cover, air pollution, surface reflection and altitude⁽Reference Engelsen¹⁹⁾. With advancing age skin integrity decreases; skin thinness and a reduction in transdermal cholesterol reduce the efficiency of UV-B vitamin D production, as much as 50 % in older adults compared with younger adults⁽Reference MacLaughlin and Holick²⁰⁾. Behavioural and social changes seen with advancing age further limit cutaneous vitamin D production. Less time spent outdoors due to ill health or limited mobility (institutionalised or homebound), medication use (loop diuretics, statins, glucocorticoids), changes in body composition (increase in fat and decrease in muscle), sun avoidance (melanoma risk), reduced skin exposure (clothing and colder temperatures) and sunscreen use are all significant barriers that contribute to inadequate UV-B vitamin D production in older adults⁽Reference de Jongh, van Schoor and Lips^21–Reference Wortsman, Matsuoka and Chen²⁵⁾.

The UK Scientific Advisory Committee on Nutrition recommends an intake of 10 µg/d (400 IU/d) vitamin D for older adults, a target hard to achieve by dietary contribution alone unless oily fish is eaten daily. Experts in the UK and Ireland recommend that during the winter months and for those at risk of deficiency, vitamin D supplementation should be considered^{(26, 27)}. Evidence suggests vitamin D supplement uptake is typically low in the population, including older adults⁽Reference Cashman, Muldowney and McNulty²⁸⁾. For the most part, dietary sources are limited and not generally consumed in adequate amounts by older adults⁽Reference Cashman, Muldowney and McNulty^28, Reference Cashman, Hill and Lucey²⁹⁾, with an exception for those residing in countries where vitamin D food fortification is in place (United States, Canada, Sweden and Finland). The effects of age on intestinal absorption and decreased renal function may hinder vitamin D metabolism and availability⁽Reference Kinyamu, Gallagher and Balhorn^30, Reference Kinyamu, Gallagher and Petranick³¹⁾. It has been hypothesised that the metabolism of vitamin D may be reduced due to a decline of intestinal VDR distribution in older adults; however, few studies have been conducted in human subjects, with small samples and conflicting findings⁽Reference Ebeling, Sandgren and DiMagno^32, Reference Kinyamu, Gallagher and Prahl³³⁾.

Vitamin D deficiency and ageing

Serum 25(OH)D is accepted as the most accurate measure of vitamin D status; however, there is less agreement on how to define deficiency in the general population or specifically to older adults. The Institute of Medicine advocate serum 25(OH)D concentrations ≥50 nmol/l (≥20 ng/ml) for bone health, and with levels <30 nmol/l (<12·5 ng/ml) considered to be deficient⁽Reference Ashwell, Stone and Stolte³⁴⁾. The term vitamin D insufficiency is used to describe serum levels of ≥30–50 nmol/l (≥12·5–20 ng/ml)⁽³⁵⁾. In contrast, the endocrine society recommends serum 25(OH)D levels >75 nmol/l to maximise an effect on bone and muscle metabolism. Suboptimal levels of vitamin D are much more common than clinical toxicity which is rare⁽Reference Vieth³⁶⁾. Reported vitamin D deficiency rates in published studies vary widely, with considerable methodological differences including the definition of serum 25(OH)D status, sample size, population, sex-specific cohorts, latitude, season and mode of vitamin D assay. Prevalence estimates for vitamin D deficiency in community-dwelling older adults across the European Union and the UK are summarised in (Table 1), along with the specific 25(OH)D cut-off criteria applied to define deficiency. As illustrated in (Table 1), deficiency ranged from 0 to 100 %. Overall, however, it is apparent that suboptimal vitamin D status is widespread in older adults, irrespective of the definition applied. For example, the Health Survey in England reported that 13 % of females and 8 % of males were vitamin D deficient, using the more conservative cut-off of <20 nmol/l⁽Reference Hirani and Primatesta³⁷⁾, at 5 years follow-up deficiency rates increased to 15 and 9·6 %, respectively⁽Reference Hirani, Tull and Ali³⁸⁾. When defining criteria is set at <50 nmol/l, 62·5 % of females and 50 % of males were classed as vitamin D deficient in this community-dwelling cohort. However, deficiency rates were almost double in cohorts defined as cognitively impaired⁽Reference McCarroll, Beirne and Casey³⁹⁾. In a recent study of Irish adults, a 10 % higher prevalence of vitamin D deficiency was reported in adults aged 65 years and older compared to middle-aged adults, 35·7 and 44·0 %, respectively⁽Reference Cashman, Muldowney and McNulty²⁸⁾.

Table 1. Prevalence of 25-hydroxyvitamin D (25(OH)D) deficiency in community-dwelling adults, aged over 50 years in Europe

D-def, serum 25(OH)D deficient; T, total; S, summer; W, winter; m, male; f, female.

* Deficiency defined <22·5 nmol/l.

† Deficiency defined <37 nmol/l.

Other factors related to cognitive assessments

Each domain is highly influenced by other mental factors, a psychological concept called central arousal which describes internal states such as mental fatigue and needs to be considered when assessing cognitive performance⁽Reference Eysenck, Derakshan and Santos⁵⁰⁾. Mood, sleep, anxiety and depression are highly correlated with cognitive performance and can significantly alter the outcome⁽Reference Conroy, Golden and Jeffares^51, Reference Simpson, Maylor and McConville⁵²⁾.

The assessment of cognitive function is further complicated in defining the cognitive status of study participants, a distinct difference must be acknowledged when considering cognitive changes which are age-related and those which are a result of disease. Furthermore, inter-individual variability in the form and severity of cognitive decline is largely influenced by education and health-related factors. To accurately assess changes in cognitive status, these factors must also be considered, whilst acknowledging that older adults vary widely in the extent to which they are affected by these changes⁽Reference Kanai and Rees⁴⁸⁾. For example, to our knowledge, no studies of vitamin D status and cognitive performance considered assessment of pre-morbid IQ, which is highly correlated with most cognitive tests, since decline is essentially relative to pre-morbid ability⁽Reference Barker-Collo, Bartle and Clarke⁵³⁾, a score within the ‘normal’ range can, in fact, represent cognitive decline for an individual with high levels of pre-morbid functioning.

Types of outcome measures commonly used in vitamin D and cognition research

Neuropsychological performance tests are the methods most commonly used in vitamin D and cognition studies. Cognitive function can be assessed by a number of validated pen-and-paper or computer-based tests. Direct measures of brain location and effectiveness of many cognitive processes can be assessed using electroencephalograms, or assessment of brain activation associated with specific domains of cognitive function through functional MRI, often considered the gold standard, however, like all tests is subject to limitations. The relationship between neuronal activity and functional ability is not fully understood⁽Reference Poldrack and Wagner⁵⁴⁾ and their use in measuring nutritional effects should be interpreted carefully. Emerging mobile technologies which obtain real-time information regarding abilities to perform activities of daily living, which are highly correlated with neuropsychological tests, may change the way cognitive research is conducted, in way of preventative strategies for dementia⁽Reference Allard, Husky and Catheline⁵⁵⁾.

As the evidence for an underlying link between vitamin D and cognitive performance remains inconclusive, the best approach is to include a battery of cognitive tests that cover a variety of domains; this may help identify the specific cognitive processes involved. Several large-scale ageing studies incorporate a comprehensive battery of validated cognitive tests⁽Reference Hannigan, Coen and Lawlor^56, Reference Kenny, Coen and Frewen⁵⁷⁾; their use in cognitive studies on vitamin D may be helpful. However, the logistics, time and practicalities of applying comprehensive cognitive batteries in vitamin D studies may be a barrier. Recently we reported preliminary findings that this was feasible and acceptable in adults aged 60 years and older, at three time points over a 6-month period⁽Reference Aspell, Lawlor and O'Suillivan⁵⁸⁾.

Vitamin D and Alzheimer's disease

Other interventions have been conducted in participants with mild–moderate AD and institutionalised older adults; however both showed no effect, were of short duration and add little in terms of preventable, risk factors for AD. One reported an improvement in attention and reaction times in 139 ambulatory subjects who were supplemented with vitamin D for 6 months⁽Reference Dhesi, Jackson and Bearne⁷³⁾. The other study was retrospective in design, non-blinded and involved only thirty-two participants presenting with subjective memory complaint, had a short duration of treatment of 28 d and used only one limited test of cognition⁽Reference Przybelski and Binkley⁷⁴⁾. Several studies report low levels of 25(OH)D in the community in patients with AD⁽Reference Buell, Dawson-Hughes and Scott^75–Reference Sato, Asoh and Oizumi⁷⁷⁾.

Vitamin D metabolism and the central nervous system

The discovery of 1α-hydroxylase and metabolic pathways for vitamin D in the human brain⁽Reference Eyles, Smith and Kinobe⁸⁷⁾ and cerebrospinal fluid support a localised catabolic pathway for vitamin D in the central nervous system. The three enzymes necessary for complete synthesis and catabolism of active vitamin D has also been expressed⁽Reference Balabanova, Richter and Antoniadis^88, Reference Reinhardt and Horst⁸⁹⁾. Serum 25(OH)D has also been evidenced to cross the blood–brain barrier a characteristic shared by other neurosteroids⁽Reference Kalueff, Minasyan and Keisala⁹⁰⁾.

Vitamin D receptor and the central nervous system

The nuclear functions of 1,25-dihydroxyvitamin D₃ are mediated through the VDR, a member of the nuclear receptor family identified in over 2700 genomic sites⁽Reference Ramagopalan, Heger and Berlanga^82, Reference Petkovich, Brand and Krust⁹¹⁾. Experimental scientists have demonstrated the extensive mapping of the VDR in the rat brain⁽Reference Feron, Burne and Brown^92–Reference Walbert, Jirikowski and Prufer⁹⁵⁾, showing a similar distribution in human brain tissues⁽Reference Eyles, Smith and Kinobe⁸⁷⁾, with the earliest evidence in post-mortem brains of AD and Huntington's disease patients⁽Reference Eyles, Smith and Kinobe^87, Reference Sutherland, Somerville and Yoong⁹⁶⁾. The identification of its distribution in neuronal and glial cells in the human brain followed thereafter, supporting a functional role in light of local production of active vitamin D in the human brain⁽Reference Eyles, Smith and Kinobe⁸⁷⁾. Areas identified in both animal and human brain tissue show similar patterns⁽Reference Stumpf, Sar and Clark⁹⁴⁾, in peripheral neurons,⁽Reference Tague and Smith⁹⁷⁾ and in several cell types of the central nervous system⁽Reference Prufer, Veenstra and Jirikowski^93, Reference Walbert, Jirikowski and Prufer^95, Reference Cornet, Baudet and Neveu⁹⁸⁾. The action of 1,25-dihydroxyvitamin D₃ upon binding to the VDR has been linked with a diverse range of biological systems such as immune modulation, cell growth and cell differentiation all of which impact brain function via interaction with target genes⁽Reference Gronemeyer, Gustafsson and Laudet^99, Reference Omdahl, Morris and May¹⁰⁰⁾.

Vitamin D and neuroprotection

Animal and cell culture evidence suggest that vitamin D may protect the structure and integrity of neurons through detoxification pathways and neurotrophin synthesis⁽Reference Wang, Wu and Cherng¹⁰¹⁾. Vitamin D is known to affect the expression of three of the four neurotrophins; nerve growth factor, glial-derived nerve factor and neurotrophin 3⁽Reference Neveu, Naveilhan and Baudet¹⁰²⁾. Treatment of vitamin D in adult rats resulted in increased glial-derived nerve factor expression and immunoreactivity after dopamine toxicity damage⁽Reference Sanchez, Lopez-Martin and Segura¹⁰³⁾. Furthermore, vitamin D₃ may attenuate neurotoxicity; administration of calcitriol has been shown to increase levels of antioxidants, such as glutathione in the rat brain which protects oligodendrocytes and the integrity of nerve conduction pathways critical to mental processing⁽Reference Chen, Lin and Chiu¹⁰⁴⁾. The in vitro analysis shows vitamin D treatment inhibits production TNF-α and IL 6, suggesting an anti-inflammatory role⁽Reference Lefebvre d'Hellencourt, Montero-Menei and Bernard¹⁰⁵⁾.

Vitamin D and regulation of intraneuronal calcium

Elevated levels of calcium in the brain leads to neurotoxicity. Three calcium-binding proteins have been shown to be modulated by vitamin D in brain tissues, calbindin, parvalbumin and calretinin⁽Reference Alexianu, Robbins and Carswell¹⁰⁶⁾. All three are widely and uniquely distributed in the adult brain and are believed to serve a neuroprotective role as calcium buffers⁽Reference Fierro and Llano¹⁰⁷⁾ as well as being involved in critical brain functions. In vitamin D-deficient mice, certain calcium channels are shown to be unregulated leading to an increase in Ca²⁺⁽Reference Zhu, Zhou and Yang¹⁰⁸⁾ and in vitro evidence has shown that vitamin D can down-regulate calcium channels⁽Reference Brewer, Thibault and Chen¹⁰⁹⁾.

Vitamin D and secondary vascular protection

Vascular dementia is the second most commonly occurring type of dementia after AD, though markedly different can also co-exist as ‘mixed dementia’. Vascular dementia is characterised by cognitive dysfunction secondary to ischemic or haemorrhagic brain lesions due to cerebrovascular disease or CVD. This type of brain damage may result from an influx of excitatory amino acids, inflammatory responses and changes in cell function, which result in excessive calcium entry⁽Reference Román, Tatemichi and Erkinjuntti¹¹⁰⁾. With these changes presents an increase in intracellular nitric oxide production and increased oxidative stress.

Vitamin D may help improve vascular-related brain disease by mediating harmful effects of inflammation, calcium dysregulation and increased oxidative stress. During transient ischemic events, transforming growth factor and glial-derived nerve factor are upregulated in hippocampal cells to promote survival⁽Reference Garcion, Sindji and Leblondel¹¹¹⁾. As described earlier, vitamin D enhances innate antioxidative defences by increasing glutathione and glial-derived nerve factor concentrations⁽Reference Garcion, Sindji and Leblondel^111–Reference Wion, MacGrogan and Neveu¹¹³⁾. These particular changes were shown to attenuate ischemic brain disease in rodents⁽Reference Wang, Wu and Cherng¹⁰¹⁾. Furthermore, 1,25 dihydroxyvitamin D inhibits inducible nitric oxide synthetase⁽Reference Garcion, Nataf and Berod¹¹⁴⁾, an enzyme that is up-regulated during ischaemic events and in patients with AD⁽Reference Vodovotz, Lucia and Flanders¹¹⁵⁾. It is responsible for generating nitric oxide which is known to cause damage to neurons and oligodendrocytes at high concentrations⁽Reference Law, Gauthier and Quirion¹¹⁶⁾.

It is plausible that vitamin D may influence vascular-related dementia via indirect mechanisms. Therapeutic intervention with vitamin D regulates blood pressure⁽Reference Lind, Wengle and Wide^117, Reference Pfeifer, Begerow and Minne¹¹⁸⁾, cardiac hypertrophy⁽Reference O'Connell, Berry and Jarvis¹¹⁹⁾ and plasma rennin activity⁽Reference Burgess, Hawkins and Watanabe^120, Reference Li, Kong and Wei¹²¹⁾. There is an inverse relationship between vitamin D levels and congestive heart failure⁽Reference Zittermann, Schleithoff and Tenderich¹²²⁾ and vitamin D insufficiency is associated with incident CVD⁽Reference Wang, Pencina and Booth⁸⁶⁾.