Vitamin D: Ubiquitous and Essential
There is a reason why vitamin D made it to the top of our “most researched vitamin of the year” list for the third year in a row. Roughly one quarter of all research into the 12 vitamins was performed with vitamin D in 2013. This week in the International Journal for Vitamin and Nutrition Research, Stöcklin and Eggersdorfer summarize the current situation in vitamin D research.
In the past, vitamin D was only seen as a nutrient needed for bone health. Centuries before vitamin D was discovered, the bone deformities that vitamin D deficiency caused in early childhood were described in seminal medical texts (see Rajakumar for an interesting historical perspective). We now know that the active form of vitamin D, 1,25 hydroxyvitamin D, regulates serum calcium levels and enables calcium absorption in the intestines, therefore adequate levels of both calcium and vitamin D are needed to avoid bone-related vitamin D deficiency diseases such as rickets in children, osteopenia in adults, and osteoporosis in adults, particularly the elderly.
Non-bone related functions arise from the vitamin D receptor, a protein in cells that responds to the active form of vitamin D. Norman and Bouillon list 38 different types of tissue that contain the vitamin D receptor. This receptor means that cells react to levels of the active form of vitamin D, thus broadening the range of different organ systems that are affected by vitamin D beyond hone. Many cells can also produce the 1-alpha-hydroxylation enzyme, which can convert the circulating form of vitamin D, 25-hydroxyvitamin D, to the active form, therefore allowing organs to produce their own, local supply of active vitamin D, independent of the kidneys.
There is developing interest in the role that vitamin D plays in muscle function. In particular, the slow-twitch muscle fibers that can contract without tiring for longer periods of time to maintain posture and balance, appear to require adequate vitamin D levels to function normally. This means that general muscle fatigue, including both poor athletic performance and frailty in the elderly, can both be linked to vitamin D deficiency. See an open-access review by Ceglia for more information.
There is some continuing discussion of the “optimal” intake of vitamin D needed to maintain good health amongst vitamin D experts, and TalkingNutrition has covered the subject often over the past few years, including here, here, here and here. Observational evidence seems to point to maintaining vitamin D levels above 75 nmol/L, which in the absence of a sun-loving lifestyle can only be achieved with vitamin D supplementation above recommended minimum intake levels. Some large scale and long term clinical studies are currently underway to determine whether vitamin D supplements can influence diseases such as heart disease, cancer, and general health status in the elderly.
The problem of low levels of vitamin D is discussed in the article. Despite a paucity of data for many countries, it seems that a large proportion of the world’s population is likely to be vitamin D deficient. This map shows that average vitamin D levels are less than 50 nmol/L for populous countries such as China, India and Russia. Levels are also low in sunny countries such as Iran, Lebanon and Jordan. These high levels of vitamin D deficiency are due to lifestyle changes that involve spending more time indoors, and behaviors that aim to reduce the incidence of skin cancer, which involve avoiding sun exposure.
Stöcklin and Eggersdorfer summarize the evidence as such:
- Observational studies link vitamin D deficiency with a host of diseases in addition to bone disease, such as cardiovascular disease, cancer, immune-related diseases and general poor health.
- Intervention studies have found that vitamin D intakes above 700 IU help prevent fractures in elderly subjects, who are at risk of falls.
- There is currently a lack of consensus on which vitamin D intakes achieve “optimal” levels for health.
- Vitamin D deficiency appears to be widespread throughout the world.
- When exposure to sunlight is low, vitamin D intakes from food are unlikely to meet requirements, therefore supplements should be considered.
Stöcklin E, Eggersdorfer M. Vitamin D, an Essential Nutrient with Versatile Functions in Nearly all Organs. Int J Vitam Nutr Res. 2013 Apr;83(2):92-100. doi: 10.1024/0300-9831/a000151. http://www.ncbi.nlm.nih.gov/pubmed/24491882
Adams JS, Hewison M. Extrarenal expression of the 25-hydroxyvitamin D-1-hydroxylase. Arch Biochem Biophys. 2012 Jul 1;523(1):95-102. doi: 10.1016/j.abb.2012.02.016. Epub 2012 Mar 14. http://www.ncbi.nlm.nih.gov/pubmed/22446158
Ceglia L. Vitamin D and its role in skeletal muscle. Curr Opin Clin Nutr Metab Care. 2009 Nov;12(6):628-33. doi: 10.1097/MCO.0b013e328331c707. http://www.ncbi.nlm.nih.gov/pubmed/19770647
Norman AW, Bouillon R. Vitamin D nutritional policy needs a vision for the future. Exp Biol Med (Maywood). 2010 Sep;235(9):1034-45. doi: 10.1258/ebm.2010.010014. Epub 2010 Jul 28. http://www.ncbi.nlm.nih.gov/pubmed/20667908
Ogan D, Pritchett K. Vitamin D and the athlete: risks, recommendations, and benefits. Nutrients. 2013 May 28;5(6):1856-68. doi: 10.3390/nu5061856. http://www.ncbi.nlm.nih.gov/pubmed/23760056
Rajakumar K. Vitamin D, Cod-Liver Oil, Sunlight, and Rickets: A Historical Perspective. Pediatrics 2003; 112:2 e132-e135 http://pediatrics.aappublications.org/content/112/2/e132.abstract