Higher vitamin D levels and/or increased exposure to sunlight has been associated with a reduced risk of a wide variety of conditions including cardiovascular disease, auto-immune disease (disease where the immune system reacts against the body’s own tissues such as in multiple sclerosis, rheumatoid arthritis and type 1 diabetes) and several forms of cancer. However, associations do not prove causality. It is possible, for instance, that individuals with higher vitamin D levels that get themselves into the sun more often are essentially healthier and more likely to be disease-free than those who don’t get out so much.
It was essentially this point that was made in an editorial in the Archives of Internal Medicine [1]. The authors of this study wrote:
“…it seems intuitively unlikely that a single hormone could play a substantial role in preventing or ameliorating the diverse range of diseases that have been linked to low levels of vitamin D. A more plausible and prosaic explanation for the findings of the observational vitamin D studies is the presence of common confounders. Vitamin D levels are directly related to sunlight exposure and physical activity, and inversely related to adiposity. It is likely that less healthy individuals, who are more likely to subsequently experience morbid events, will be heavier, less active, and more sunlight-deprived than healthier ones and therefore have lower levels of 25(OH)D [vitamin D].”
This may or may not be true. However, one fact that supports the idea that vitamin D may directly affect disease risk is that vitamin D receptors exist widely around the body including fat tissue, adrenal gland, bone, brain, breast, cartilage, colon, hair follicles, intestine, skeletal muscle (muscles connected to bones such as those in the legs and arms), cardiac (heart) muscle, kidney, liver, ovary, pituitary gland, retina, sperm, thyroid and uterus (womb). The relevance of this? Well, the presence of a receptor for a substance in a tissue is usually a pretty good clue to the fact that this substance exerts some influence over the structure and/or function of that tissue.
A study published this week adds to the evidence that vitamin D might directly affect health and risk of disease [2]. This research looked at the binding of vitamin D to the DNA in the cells. The study found that vitamin D had the capacity to bind to getting on for 3000 sites on DNA. It also had the ability to affect the activity of 229 genes, including those associated with multiple sclerosis, Crohn’s disease and type 1 diabetes – all conditions the risk of which has been linked to vitamin D levels and/or sunlight exposure.
Interestingly, the study found instances where vitamin D binds to genes more commonly found in those of European or Asian (as opposed to, say, African) decent. Why might this be? Some have suggested an evolutionary explanation. When our ancestors migrated out of Africa to other parts of the globe, sunlight exposure would reduce, and so would vitamin D levels. Lower vitamin D levels might have lead to bone issues, including a narrower pelvis and birth canal in women, which reduced the chances of successful reproduction. However, those with a genetic make-up that made better use of any available vitamin D might be less prone to this, and be more likely to pass their genes on to the next generation.
This latest research, along with our knowledge that vitamin D binds to practically every tissue in the body, provides pretty compelling evidence that this substance has a direct and important role in health and disease. The links between higher vitamin D levels and/or sunlight exposure appear might, at least in part, be down to the fact that healthier, fitter individuals are more likely to get out in the sun. However, this appears to be far from the full story.
References:
1. Grey A, et al. Vitamin D – A Place in the Sun? Arch Intern Med. 2010;170(13):1099-1100
2. Ramagopalan SV, et al. A ChIP-seq defined genome-wide map of vitamin D receptor binding: Associations with disease and evolution. Genome Res. 2010 Aug 24. [Epub ahead of print]
