Courtesy of Dr. Kirsty Fairbairn, Invigorate Nutrition, www.invigoratenutrition.com
There are not many vitamins where food is NOT the body’s best source. In fact, there is only one: vitamin D.
Vitamin D is a fat-soluble vitamin that is best obtained from sunlight exposure onto our skin. Vitamin D is made in our skin from UVB light exposure and warmth. Being outside in the fresh air is important; UVB light does NOT pass through window glass.
Over winter, our blood vitamin D levels decline slowly, typically reaching their lowest levels in late winter/early spring. In Northern latitudes (above 37 degrees North for example) barely enough UVB rays reach earth for us to make enough vitamin D over winter. With less sunlight exposure, we need to be more proactive in getting sun exposure when we can, or ensuring that healthy food sources of vitamin D feature in our diet. There are some natural sources of vitamin D, including fatty fish. In the US, many foods may be fortified on a voluntary basis, including milk and milk alternatives, yogurt and yogurt alternatives, enriched flour, breakfast cereals, soy beverages, margarine, cheese and fruit juice.
Vitamin D metabolism is closely linked to that of calcium and phosphate, and is important for good bone health1. Deficiencies in vitamin D lead to poor mineralization of bones, seen as rickets in childhood, and osteomalacia and osteoporosis in adults, reflecting the loss of calcium from bones. Adolescence is a critical time for bone growth and development; almost half our adult skeleton will be laid down during adolescence. This is why consuming enough calcium and vitamin D, alongside regular exercise, is so important in this age group. Vitamin D is so closely linked to calcium, that if you didn’t have any vitamin D, you would only absorb 10-15% of the dietary calcium you eat into your body! This is because vitamin D increases the absorption of calcium in the intestines.
Vitamin D acts like a steroid hormone – sounds exciting doesn’t it! There are vitamin D receptors all around the body, particularly in the small intestine, bone, liver, kidneys and (of interest to a sports dietitian like me) in skeletal muscle. In some studies, vitamin D status is associated with strength or performance2,3,4 , and in others it is not3,5. However, an association does not always indicate a cause and effect. A colleague of mine once used the wonderful analogy that eating more ice-cream in summer does not affect your ability to swim, although there are also more drownings (unfortunately) in summer. This does not mean that eating icecream causes drowning! Associations need to be tested out in a randomised controlled trial, where one group gets the vitamin and one group gets a placebo that looks and tastes the same, for a reasonable length of time to see an impact (which can be a long time in nutrition research!). Then the researcher measures performance before and after the trial to see whether the two groups respond differently over that time. The research seems to be showing that any performance issues might be limited to athletes who have low vitamin D status at the start of the study. Indeed, a vitamin D supplementation trial in Israeli swimmers showed no impact on performance6, and we found limited impact of vitamin D supplementation on rugby union performance here in Southern New Zealand7.
At this stage, it appears that supplementing with vitamin D only seems justified if you have low blood vitamin D levels to start with. People who are at higher risk of vitamin D deficiency include people who do not get outdoors much, people who prefer to wear lots of clothing to cover their skin, the elderly, pregnant women and people with darker skin (it takes more UVB exposure to make an equivalent amount of vitamin D in darker skin).
I would strongly recommend discussing vitamin D status with your physician or sports dietitian before considering a vitamin D supplement, as there is a risk of vitamin D “intoxication” and calcification in soft tissues if blood vitamin D levels get too high. This because vitamin D is a fat-soluble vitamin, it can accumulate in the body and is not easily excreted from the body via urine. This is quite different to water-soluble vitamins like B vitamins or Vitamin C – it is easy to just pee out the excess of those! On the other hand, we do not make excess vitamin D in our skin, as there are mechanisms in place in our body to limit vitamin D formation when we already have enough.
Most of us make plenty of vitamin D in our skin over summer, assuming that we get outdoors. Of course, this sun exposure needs to be carefully balanced with sun-smart behaviours (using sunscreen, hats and clothing for sun protection) to ensure we don’t increase our risk of developing skin cancer. Here is a link to the guidelines on Sun Safety from the Centers for Disease Control and Prevention: https://www.cdc.gov/cancer/skin/basic_info/sun-safety.htm. Season and latitude both affect the amount of UVB light that we are exposed to, in that in winter, and at higher northern (or southern) latitudes, we are further from the sun, so UVB light that has further to travel and is weaker, and we spend less time outside in winter. Thus, now would be a good time to get your vitamin D levels checked if you are curious about your status – talk to your physician about it. Then get on with looking forward to summer!
Dr. Kirsty Fairbairn
Advanced Sports Dietitian, www.invigoratenutrition.com
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1 Institute of Medicine, Vitamin D and Calcium Dietary Recommendations report,
2 Hildebrand et al. Compromised vitamin D status negatively affects muscular strength and power of collegiate athletes. Int J Sports Nutr Ex Metab 2016; 26: 558-564.
3 Gieker et al. Vitamin D status and muscle function among adolescent and young swimmers. Int J Sports Nutr Ex Metabo 2017; 27: 399-407.
4 Fitzgerald et al. Association between vitamin-D status and maximal-intensity exercise performance in junior and collegiate hockey players. J Strength Cond Res 2015; 29(9): 2513-2521.
5 Dubnov-Raz et al. Vitamin D concentrations and physical performance in competitive adolescent swimmers. Ped Exerc Science 2014; 26: 64-70.
6 Dubnov-Raz et al. Vitamin D supplementation and physical performance in adolescent swimmers. Int J Sports Nutr Ex Metab 2015; 25: 317-325.
7 Fairbairn et al. Vitamin D3 supplementation does not improve sprint performance in professional rugby players: A randomised, placebo-controlled double blind intervention study. Int J Sports Nutr Ex Metab 2017 (in press); https://doi.org/10.1123/ijsnem.2017-0157