Vitamin D: Is Sunlight Enough?

Sun exposure is one way to get vitamin D, but is it the best way? A new study suggests you might need to supplement even if you spend a lot of time outdoors in natural sunlight.

If you get some sun each day then you probably don’t need to supplement vitamin D, right? Wrong, says a forthcoming study in the Journal of Strength and Conditioning Research.

Vitamin D affects your fitness, body composition, and even testosterone levels, making it a popular supplement. Direct sun exposure causes the body to create its own vitamin D, which leads most people to believe that supplementation isn’t necessary if you simply get a small dose of sunlight each day. However, the new study showed that sunlight may not be enough to keep vitamin D at optimum levels.

Researchers examined forty male and female college students who did not use vitamin D supplements. The students were tested for a variety of factors including vitamin D levels, aerobic capacity, anaerobic power, strength, and body composition. Each student also reported his or her level of daily sun exposure, and none of the students used sunscreen.

At least half the students were clinically deficient in vitamin D, and sun exposure didn’t make any difference whatsoever. That’s right, many of the students with low vitamin D reported daily sun exposure in Louisiana – and in the summer, no less.

Nevertheless, while vitamin D levels were unrelated to sun exposure, they were definitely related to other measurements. Higher vitamin D levels correlated with better body composition and greater aerobic capacity. However, strength and anaerobic power were unaffected.

This study leads to several important conclusions. First, it’s a safe assumption that about half of normal, active, seemingly healthy people are vitamin D deficient. Second, sun exposure isn’t adequate to correct this deficiency. Vitamin D obtained through diet appears to be more important. Finally, vitamin D is important to athletes. I don’t think anyone will turn down fat loss or better aerobic performance, and vitamin D enables both.

Unfortunately, the study stops short of making a recommendation for vitamin D consumption. The Institute of Medicine recommends 600 IU per day. Only one of the students in this study met that level. Many still criticize that recommendation as too low, but nobody will make a firm recommendation because we know very little about how much vitamin D the body can tolerate. Personally, I take 4000 IUs per day and I haven’t died from it yet.

So examine your diet and determine if you need to supplement with vitamin D. It’s cheap, easy, and vital to athletic performance.


1. Laura Forney, et al. Vitamin D Status, Body Composition and Fitness Measures in Younger, Physically Active Individuals. Journal of Strength & Conditioning Research (forthcoming): POST ACCEPTANCE, 26 July 201. doi: 10.1519/JSC.0b013e3182a35ed0

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