December may seem a strange time to discuss the sunshine vitamin D. However, 2013 has not only seen a plethora of studies heralding its potential role in the prevention of non-skeletal disorders such as cancer, dementia and cardiovascular disease, but conversely has ended with media reports that have questioned the role of vitamin D supplementation in prevention of disease. So what has 2013 taught us about Vitamin D and what’s in store in the future?
Vitamin D status and deficiency
There is no standard UK definition of an ‘optimal’ level of vitamin D. Currently serum levels of 25(OH)D below 25nmol/L qualify as deficient, this being the threshold associated with increased risk of rickets/osteomalacia. Higher thresholds though have been proposed by the 2013 National Osteoporosis Society clinical guideline for patient management, which suggests adoption of a threshold of serum 25(OH)D < 30 nmol/L as deficient.
There is recent evidence of low vitamin D status in the UK. The proportion of children aged 11 to 18 years and adults aged 19 to 64 years who had 25(OH)D concentrations below the lower threshold for vitamin D adequacy was 19.9%, and 18.6% respectively. Last month, the FSA survey of vitamin D status of the Scottish adult population reported an average Vitamin D level of 37.5nmol/L, but also that more than a third of participants had a level below 25nmol/L.
Vitamin D and non-skeletal diseases
Whilst we have known for some time about Vitamin D and its role in bone health, in a systematic review published this month in the Lancet Diabetes and Endocrinology, Autier et al discuss a large number of observational studies that suggest high concentrations of Vitamin D may be protective in many non-skeletal diseases. For example, those with high vitamin D status had a decreased risk of cardiovascular events (by up to 58%), diabetes (by up to 38%), colorectal cancer (by up to 33%) and all-cause mortality (by up to 29%).
However randomised controlled intervention studies revealed a different picture, with supplementation trials of Vitamin D typically not reducing risk. The results may suggest that although low vitamin D status is associated with a number of disorders, supplementation does not reduce risk and the authors conclude that it may be possible that low vitamin D is not a cause but rather a consequence of ill health.
However, the lack of benefit in supplementation trials may be attributed to study design including inadequate supplementation, using populations that are not sufficiently vitamin D deficient at baseline and using the incorrect formulation of vitamin D. Earlier studies have suggested that vitamin D2, ergocalciferol is less biologically active than vitamin D3, cholecalciferol.
Vitamin D and the elderly
Trials included in the above review consistently showed that supplementation can reduce all-cause mortality (relative risks ranging from 0.93 to 0.96). Most of these trials included elderly women, living in institutions, likely to have habitually low vitamin D status.
It is postulated that ageing leads to a decreased capacity to synthesise vitamin D in the skin, and this alongside changes to nutrition and lifestyle, such as reduction in sun exposure, lower food intake and changes in dietary habits, might contribute to a deficiency that could have a negative effect on disease course and survival.
Vitamin D deficiency is also associated with increased risk of falls. After examination of the evidence base, including randomised controlled trials and meta-analyses, the European Food Safety Authority Scientific Panel concluded that a cause and effect relationship has been established between the intake of vitamin D and a reduction in the risk of falling.
Sources of Vitamin D
The action of sunlight on skin is an important source of Vitamin D. However many environmental, physical and personal factors affect this including skin melanin pigments and latitude. Typically, throughout the UK, regular short sunlight exposure (avoiding sunburn) between 11am and 3pm, during the months of April to October should be sufficient to achieve adequate Vitamin D levels. According to one study, during the summer and autumn, 3%, of British adults have 25(OH)D levels under 25 nmol/L, but during the winter and spring, this rises to 16%.
Vitamin D is found in only a few foods, with fatty fish and fish oils, liver, meat and eggs being the main natural sources. Powdered milks, fat spreads, and breakfast cereals are often fortified with vitamin D in the UK.
Recommended nutrition intakes (RNI) for vitamin D are set only for those aged up to 4 years, 65 years and over, and for pregnant and breastfeeding women. Recent dietary survey data shows mean intakes from food sources was 26% of RNI in children aged 1.5 – 3 years and 33% for adults aged 65 years and over. Inclusion of intakes from dietary supplements brought this mean intake up to 32% and 47% respectively, but mean intakes remained well below RNI.
Supplement recommendations
The majority of the people should be able to get the vitamin D they require by eating a healthy balance diet and by getting some summer sun.
The Department of Health recommends vitamin D supplements of 10 micrograms a day for all pregnant and breastfeeding women and people aged 65 years and over for people who are not exposed to much sun including people who cover up their skin when outdoors, or those that are housebound or confined indoors for long period. Children aged 6 months to five years and over should take a supplement in the form of vitamin drops to help them meet their requirements. Risk groups also include the obese who typically have lower 25(OH)D levels. This may be due to less sun exposure or greater storage of vitamin D by fat tissue.
Vitamin D – what next?
Low levels are clearly linked to bone conditions although evidence of a causal link to other chronic diseases is still inconclusive.
A SACN working group on vitamin D has been established to update its 2007 report and review dietary reference values and make recommendations. They are due to publish a final report in the near future. Large trials are also now underway testing whether supplementation can reduce the risk of cancer, cardiovascular disease, diabetes, infection and declining cognitive functions with the first results not available until 2017.
In the meantime it is premature to recommend widespread vitamin D supplementation for the general population, given the lack of evidence around possible risks of raising levels of vitamin D in healthy people for a long time. For those involved in care of any of the groups that require supplements, it is important to ensure that these are given. Dietary surveys show take up of supplementation remains low. For those specifically involved in residential care of the elderly ensure that they have frequent short exposure to sunlight in the summer months. And for caterers make sure vitamin D rich foods are on the menu.
