Are you “D-ficient”*?

By Candace Chemtob, B.S. and M.S. in Human Nutrition

Vitamin D is a hot topic these days. For decades our knowledge of vitamin D was limited. Mistakenly, many health professionals believed that the function of vitamin D was limited to bone strength and calcium/phosphorus metabolism. Recently, there has been a flurry of interest and research regarding this vitamin. And rightly so, for vitamin D has been found to have a strong relationship to cancer, heart disease, and autoimmune diseases. Emerging from recent research, vitamin D has also been found to have an influence on immunity and muscle strength, both of which may effect your athletic performance.

The likelihood that you are vitamin D deficient is high. More than 40% of adult Americans are vitamin D deficient, with some groups having much higher rates of deficiency including African Americans, and those who are overweight, aging, and living in northern climates (NHANES, 2005-6). Vitamin D deficiency is common world wide as well, with an estimated half of the world population lacking adequate levels of this vitamin.

Given vitamin D’s importance to overall health, you might assume that we know a lot about vitamin D. This assumption would be wrong. In fact, vitamin D was not even identified until 1969 and it’s history is interesting. Most intriguing is that vitamin D (the active form, 1,25 -OH) is NOT a vitamin. Vitamin D is a hormone. This hormone had already been designated a vitamin, and the name was never changed to more accurately reflect it’s physiologic role in the body. Hormones are “chemical substances produced in the body that control and regulate the activity of certain cells or organs.” And as with other hormones, the effects of vitamin D are far reaching. Vitamin D receptors have been found on immune, skeletal, breast, colon, and fat cells just to name a few. With receptors throughout the body, it should come as no surprise that we are discovering that vitamin D has a wide range of physiologic effects.

Muscle cells have vitamin D receptors. In a study of post menopausal women (up to 65 years old), vitamin D deficiency was associated with a decrease in muscle strength (as measured by walking tests and handgrip strength), along with weaker knee extensor and hip abductor muscles. Vitamin D status has also been shown to reduce respiratory muscle strength. Using data from a large health survey, a strong relationship between respiratory muscle strength and vitamin D status was revealed. In fact, vitamin D status had a stronger association with respiratory muscle strength than a history of smoking (Chest, 2005). To date, however, the effect of vitamin D on athletic performance per se has not been studied. However, given the convincing data on the relationship between this vitamin and muscle strength in general, one could make an argument that athletes should take their vitamin D status seriously.

Vitamin D receptors have also been found on immune cells (B and T cells) and studies have shown that low vitamin D levels are associated with immune suppression. Initially, this relationship was proposed due to the commonplace observations that the highest occurrence of the flu is in winter months when vitamin D levels are lowest. Studies have backed up these casual observations showing that adults with low vitamin D levels are more likely to report a recent cold, cough, or upper respiratory tract infection. Additionally, a 2010 study of Japanese school aged children found that children who took the vitamin D supplements had a 40% lower incidence of type A influenza. Although this was a small study of 340 children, it highlights the strong influence vitamin D has on the immune system and prevention of influenza (Am J Clin Nutr, 2010). As discussed in a previous article, athletes who stay well perform better, so this may be of particular interest to athletes.

Beyond the potential roles of vitamin D status on athletic performance and it’s well known role in maintaining bone strength, vi- tamin D is critical to your overall health. Vita- min D has been found to reduce many other health risks. For example, the heart is a large muscle, and like skeletal muscle, it also has vitamin D receptors. Fifty thousand healthy men were followed for 10 years and men with low levels of vitamin D were TWICE as likely to have a heart attack than men with adequate levels (Arch Intern Med, 2008). As if that is not enough of a reason to be concerned about your vitamin D levels, there is also a strong relationship between low vitamin D levels and increased risk of colon and other cancers.

The recommended laboratory test for vitamin D status is serum 25-hydroxyvitamin D levels. There is controversy regarding the optimal levels of serum vitamin D. As it stands, a serum level of 20 ng/mL and above is considered adequate (IOM). From meta analysis of dozens of studies, new evidence is emerging that higher levels of serum vitamin D may be necessary to protect against cancer, heart disease, and other disease conditions. The Harvard School of Public Health in May 2012 recommended a serum level of 30 ng/mL 25-hydroxyvitamin D.

If you are beginning to appreciate the importance of vitamin D, and want to boost your levels, you may be surprised to find that very few foods are naturally rich in vitamin D. Vitamin D containing foods include fatty fish (swordfish, salmon and tuna), liver, eggs, mushrooms, and sardines. The primary dietary sources of vitamin D are fortified foods, such as dairy products, and supplements. The suggested daily intake of vitamin D for ages 1 to 70 years old is 600 IU per day (RDA, 2010). Vitamin D can also be synthesized by our bodies with sun exposure. Experts believe that 5 to 30 minutes of sun exposure twice a week without sunscreen is sufficient to maintain vitamin D levels (NIH fact sheet June 2011). However, using sunscreen (SPF>8) will block the needed UV rays from penetrating the skin and prevent the synthesis of vitamin D.

Considering that squash is an indoor sport and many of it’s enthusiasts live in more northern climates, I urge you to have your vitamin D level checked. Putting it’s potential effect on your athletic performance aside, the importance of vitamin D on your overall health cannot be overstated. Be sure you are not “D-ficient”.