Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
While our bodies do manufacture vitamin D on exposure to sunshine, the levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
And the science supports maintaining adequate levels, with vitamin D deficiency in adults reported to precipitate or exacerbate osteopenia, osteoporosis, muscle weakness, fractures, common cancers, autoimmune diseases, infectious diseases and cardiovascular diseases. There is also some evidence that the vitamin may reduce the incidence of several types of cancer and type-1 diabetes.
EFSA gives D the nod
The science supporting the muscle function of vitamin D, as well as the vitamin’s role in immune health, is sufficiently robust to have merited a positive opinion from the European Food Safety Authority’s (EFSA) Panel on Dietetic Products, Nutrition and Allergies (NDA).
The Panel concluded that “a cause and effect relationship has been established between the dietary intake of vitamin D and contribution to the normal function of the immune system and healthy inflammatory response, and maintenance of normal muscle function”.
Only recently Danish scientists reported that vitamin D is necessary to trigger T cells – the immune system’s killer cells – into action, and insufficient levels of the vitamin mean the cells remain dormant and inactive (Nature Immunology, doi: 10.1038/ni.1851).
“Scientists have known for a long time that vitamin D is important for calcium absorption and the vitamin has also been implicated in diseases such as cancer and multiple sclerosis, but what we didn't realize is how crucial vitamin D is for actually activating the immune system – which we know now,” said scientists from the University of Copenhagen.
Emerging science to support the muscle-benefits of the vitamin include a recent study from McGill University in Canada found that insufficient blood levels of vitamin D may be associated with the accumulation of fat in muscle tissue, leading to lower muscle strength.
The findings were said to be the first to show a clear link between vitamin D levels and the accumulation of fat in muscle tissue, and were published in the Journal of Clinical Endocrinology and Metabolism (doi:10.1210/jc.2009-2309).
The vitamin’s role in calcium absorption has also led to a wide range of studies supporting a bone health and anti-osteoporosis role for vitamin D, but this is not covered by the EFSA opinion.
A growing area of potential is cardiovascular health, and while no positive health claim opinions have been announced in this area, some leading vitamin D figures believe it will arrive sooner or later. DSM Nutritional Products’ senior marketing manager in human nutrition and health, Wouter Claerhout, said clinical trials in this area would provide interesting new data in the heart area. “It’s only a matter of time,” he said.
Epidemiological studies are supportive of the heart health benefits of the sunshine vitamin, with data from big US studies, including the Third National Health and Nutrition Examination Survey.
A study from the Harvard School of Public Health has also reported that increasing vitamin D levels may reduce the risk of heart attacks. A study published in the Archives of Internal Medicine (Vol. 168, pp. 1174-1180) found that a doubling of blood levels of vitamin D was associated with a halving of the risk of a heart attack (myocardial infarction).
The vitamin has been linked to a range of cardiovascular benefits including a reduced risk of peripheral arterial disease (PAD) (Arteriosclerosis, Thrombosis, and Vascular Biology, 2008, Vol. 28, pp. 1179-1185).
A number of studies have also reported that increased levels of the vitamin may reduce the risk of dying from heart disease. An Austrian study, also published in the Archives of Internal Medicine (Vol. 168, pp. 1340-1349), found that people with low blood levels of vitamin D were more than twice as likely to die from cardiovascular disease as those with high levels.
Scientists from the Netherlands, Austria, and the US reported in Clinical Endocrinology (Vol. 71, pp. 666-672), that low blood levels of the sunshine vitamin were associated with increased risk of all-cause mortality, and mortality from heart disease.
This research followed hot on the heels of similar findings published in Nutrition Research by scientists led by Richard Semba from the Johns Hopkins University. Semba and his co-workers noted that several biologic mechanisms could explain a causal relationship between vitamin D deficiency and mortality, with the vitamin’s active form (1,25-dihydroxyvitamin D) linked to a range of effects including control of inflammatory compounds, regulating immune health and blood pressure, or reducing arterial hardening.
No health claims have been granted in the area of vitamin D’s anti-cancer potential, however. The link between vitamin D intake and protection from cancer dates from the 1940s when Frank Apperly reported in Cancer Research a link between latitude and deaths from cancer, and suggested that sunlight gave "a relative cancer immunity".
Since then a raft of studies has floated down the science river reporting risk reductions for a range of cancers, including colorectal, breast, oral, stomach, and pancreatic.
The largest body of science is for colorectal cancer, but this area is controversial. While some studies have reported significant reductions in risk, others have reported null results. Indeed, back in 2006 results from the Women's Health Initiative (WHI) stated that daily supplements of vitamin D and calcium 'had no effect' on the risk of colorectal cancer in women.
However, the WHI was widely criticised due to the study limitations, including the free-living population being allowed to take supplements on their own. This resulted in the placebo group having similar vitamin D and calcium intake as the intervention group.
Research is ongoing in this area.
While the list of potential health benefits may already be relatively long and impressive, data indicate that vitamin D may also provide other health benefits, including reducing the risk of diabetes and conditions such a Crohn’s disease. However, the science in these areas is in its infancy.
To read the other articles in this series, please follow the links below.
Part 1. Markets: Global vitamin D boom remains elusive