Sound Bites! Vol.3 #3 Vitamin D: To D or Not to D? That is the Question.
Now that it’s winter, a nutrition topic that I think particularly germane is vitamin D. Why? Because much of the vitamin D we need is produced through the interaction of the sun’s ultraviolet (UVB) rays with substances under the skin. And winter, besides yielding less than ideal rowing conditions, results in a shortage of vitamin D producing UVB light. In fact, in Seattle and other 47 degree-ish latitudes, some researchers purport UVB may be insufficient in the winter to meet our body’s demand.
Other factors which may decrease levels of vitamin D include pollution, use of sunscreen, little time spent outdoors, use of full body clothing, darker skin color, aging, intestinal malabsorption, kidney or liver dysfunction, and limited dietary intake.
Vitamin D is required for the absorption of minerals like calcium and phosphorus which are needed to build strong bones. Vitamin D plays an important role in preventing osteoporosis and other bone disorders. Vitamin D deficiency causes rickets in children, resulting in bowed legs and other abnormalities. Rickets was quite prevalent in the United States before the 1930’s, when cod liver oil and eventually fortification of milk became common.
Over the past decade, there have also been many intriguing epidemiological and observational studies suggesting that vitamin D plays an important role in prevention of diseases such as breast, colo-rectal and prostate cancers, type 1 diabetes, heart disease, multiple sclerosis, depression, schizophrenia, and many other conditions.
Recommended intakes for vitamin D are based on a person getting very little vitamin D from the sun. Guidelines from the US Food and Nutrition Board (also see FNB) are:
Life Stage Recommended Amount
Birth to 12 months 400 IU
Children 1–13 years 600 IU
Teens 14–18 years 600 IU
Adults 19–70 years 600 IU
Adults 71 years and older 800 IU
Pregnant and breastfeeding women 600 IU
Of course, as rowers, a relevant question might be whether vitamin D enhances performance and do athletes need greater amounts of D compared to the average person. In a 2009 review of the world literature reported in the official journal of the American College of Sports Medicine (ACSM), Cannel and colleagues identified five areas in which vitamin D appeared to have a positive effect on physical characteristics or performance in a variety of populations, including athletes, patients, children, rats, and the elderly.
Observations included 1) the use of UVB light increased the performance of athletes, 2) seasonal correlation can be seen between peak performance and peak vitamin D levels, wherein performance is better in the late summer and drops precipitously in the fall, 3) vitamin D increased the size and number of fast twitch muscle fibers in vitamin D deficient patients as well as increased muscle mass in rats, 4) neuromuscular function improved in subjects with marginal to deficient vitamin D status with supplementation, particularly in relation to reaction time, balance, timed performance tests, and/or muscle strength, and 5) many world records were set in outdoor events during the 1968 Olympics in Mexico City, a location close to the equator during the long days of summer, whereas few were set at indoor events. As the authors noted, these were primarily observational studies and, though persuasive, were not conclusive in determining efficacy of vitamin D in athletics.
A United Kingdom 2013 study reported in Journal of Sports Science concluded that supplementation with vitamin D resulted in a significant improvement in sprint time and vertical jump compared to placebo in the control group.
A comprehensive review of the literature in 2013 by Ogan and Pritchett of the Department of Nutrition, Exercise and Health Science at Central WA University in Ellensburg concluded that, although there are limited studies on the effect of vitamin D on performance, there is sufficient data to recommend an optimal goal of >40 ng/mL and that athletes be tested yearly to assure they are meeting this optimal blood level. (Note: blood levels of vitamin D are measured in nanograms per milliliter or millimoles per liter).
So, to paraphrase Shakespeare, “To D or Not To D? That is the Question.” My answer is, based on the reviews and studies highlighted above, you should at least get the recommended intake of vitamin D through a combination of sun, food, and if indicated, supplements. Get out in the sun without sunscreen at least 15 minutes 2-3 times per week when possible and consume foods that are naturally high or fortified with vitamin D, for example:
Salmon, sockeye, 3 ounces 447 IUs
Shrimp, 4 ounces 162 IUs
Orange juice, vitamin D-fortified, 1 cup 137 IUs
Milk, vitamin D-fortified, 1 cup 100 IUs
Egg, with yolk, 1 large 41 IUs
Shiitake mushrooms, 1 cup 29 IUs
Since 77% of the general population is estimated to be vitamin D insufficient (20-32 ng/mL), it may make sense to have your vitamin D levels evaluated by your health care provider.
That being said, the US Preventive Services Task Force (USPSTF) in November 2014, citing insufficient data to support otherwise, recommended against routine vitamin D testing in healthy people, saying only people at risk for deficiency need be tested. This includes people with osteoporosis, weight loss surgery, celiac disease, and those on medications that interfere with vitamin D (eg, anticonvulsants). And it makes sense to discuss your risk level with your health care provider before dosing above the recommended levels.