Nina Laidlaw Rumler, B.A.
Headlines about vitamin D abound. Is it as good as they say? How does it impact exercise and athletes?
Advocates claim a wide range of proven or implied benefits, such as:
- Improved physical performance
- Bone strengthening (rickets, osteoporosis, osteomalacia, gum/tooth diseases)
- Strengthening of the immune system –fewer colds, flu, pneumonia, allergies.
- Inhibition/prevention of hypertension, diabetes, rheumatoid arthritis, lupus, multiple sclerosis, cancer, heart disease.
- Improvement in many other conditions including acne, Alzheimer's disease, asthma, autism, autoimmune disorders, infections, Crohn's disease, chronic fatigue, cystic fibrosis, depression, kidney disease, muscle weakness and pain, obesity, osteoarthritis, Parkinson's disease, peripheral artery disease, pelvic floor disorders, psoriasis, sports injuries, tuberculosis, urinary incontinence, and others.
Increasing research about the vitamin’s functions gives a logical basis for at least some of these claims. But what has stood the test of time about vitamin D? How can we weed out the profit-agenda claims to identify the solid benefits? What amount should we take, and how? Is there a risk of too much?
Vitamin D is produced through UV rays from sunlight triggering vitamin D3 synthesis in the skin
and through diet (D2) or supplements. The total is measured in the body as serum 25(OH)D level. Genetic and disease factors affect absorption, so a blood test is the clearest measure whether levels are deficient or healthy, and whether a supplement is needed. D3 is believed to be the most effective form of supplement.
Sunlight as a source of vitamin D has its drawbacks. Besides the risk of cancer, sunlight varies with cloud cover, pollution, latitude, season, and time of day. Some experts say that those living north of Atlanta, GA, should not rely on sunlight as a source. Dark skin pigmentation, skin aging, and sunscreen reduce skin production of vitamin D. However, sunlight does not lead to vitamin D toxicity, due to various built-in protective mechanisms.
Toxicity typically results from high intakes of dietary supplements. Vitamin D is “fat-soluble” – excess is stored in fat tissue, rather than being flushed out by water like vitamin C. Serum 25(OH)D levels do not indicate the accumulation in tissue, which can become toxic. Toxicity can lead to anorexia, nausea, vomiting, and heart arrhythmias, followed by weakness and calcification, with subsequent damage to the heart, blood vessels, and kidneys. Kidney damage may be irreversible.
The NIH Fact Sheet listed at the end gives recommended intakes, at-risk groups, dietary sources, etc. The “Vitamin D and Health” section summarizes findings on health claims.
- Bone conditions (bone strength, rickets, osteomalacia, osteoporosis): Definite benefit, helpful in reduction of fractures.
- Cancer: Inconsistent – protective against some cancers in some studies; increased risk or no effect in others.
- Diabetes, hypertension, glucose intolerance, multiple sclerosis: Mixed and inconclusive evidence.
In conclusion, in addition to anecdotal reports of improved physical performance, there is strong support for certain benefits of healthy vitamin D levels, as shown in blood tests. But little is heard about the dangers of too much, so a physician should be consulted before loading up on supplements or believing all the claims.
Further reading:
- National Institutes of Health Fact Sheet: Vitamin D. http://ods.od.nih.gov/factsheets/vitamind/ (accessed July 21, 2011)
- Chung M, Balk EM, Brendel M, Ip S, Lau J, Lee J, et al. Vitamin D and calcium: a systematic review of health outcomes. Evidence Report/Technology Assessment No. 183, 2009. http://www.ahrq.gov/downloads/pub/evidence/pdf/vitadcal/vitadcal.pdf
- Vitamin D2 is much less effective than vitamin D3 in humans. Armas LA, Hollis BW, Heaney RP. J Clin Endocrinol Metab. 2004;89(11):5387-91. http://press.endocrine.org/doi/pdf/10.1210/jc.2004-0360