Now that daylight savings time has ended, we say goodbye to summer and goodbye to sunshine. Unfortunately, we also need to bid adieu to our "sunshine vitamin" – vitamin D. Vitamin D is an important vitamin we make when ultraviolet light hits our skin, then is processed in our kidney and liver. The trouble is that when the sunlight is limited during the winter months, so are ultraviolet rays.

Vitamin D is a fat-soluble vitamin, which means it is stored in adipose (fat) tissue; however, most individuals' stores will be reduced by January without supplements.

Vitamin D deficiency and insufficiency are common in athletes, especially for those athletes participating in indoor sports such as basketball, dance, wrestling, swimming, ice hockey, and squash. Stella Lucia Volpe, PhD, RD, LDN, a professor of nutrition science at Drexel University, says, "I think athletes should be analyzed for vitamin D levels once a year or at least every other year, especially if you are an athlete who does not get much sun exposure or who uses a lot of sunscreen."

Not all is known about the impact of vitamin D deficiency on athletic performance. We know that it affects bone health. But there are also vitamin D receptors in muscle cells, and that it affects muscle function as well. A study of Finnish military recruits showed that recruits with low vitamin D levels had higher rates of stress fractures during basic training.

One review examined the vitamin D studies and found that athletes with vitamin D levels <25 ng·mL−1, but not those with >25 ng·mL−1, had improved strength with supplementation. Dr. Volpe published a review study that reports many benefits from supplementing vitamin D in athletes with low values. Not all studies have found positive effects from vitamin D, however. For instance, a recent study found no correlation between vitamin D levels and swimmers' times.

The Institute of Medicine recently raised the recommendation of vitamin D to 600 IU/day from 200 IU/day. Dietary sources of vitamin D include cold water fish (halibut, salmon, tuna, and cod), egg yolks and fortified foods.

"Most of the vitamin D rich foods are derived from animal products,"says Nyree Dardarian, MS, RD, LDN, director of the Center for Integrated Nutrition & Performance at Drexel University. "That's because only animals synthesize vitamin D, with one exception – the mushroom."

Dardarian notes that mushrooms synthesize vitamin D from a plant sterol called ergosterol, that converts to vitamin D the same way humans do. "This makes mushrooms a unique plant option for vegans or vegetarians," she says. "However, there is a catch. All mushrooms are not created equal. Mushrooms that have been exposed to UV light produce much more vitamin D than their non-UV exposed counterparts."

Vitamin D insufficiency is common and not just in athletes. Some people with low levels may benefit from replacement. Current opinion recommends vitamin D levels in athletes to be between 40 and 100 ng·mL−1.

Taking extra vitamin D above the recommended amount when you're not actually deficient likely won't help your performance, and there is evidence that getting too much D may actually be harmful. Monitoring of vitamin D levels is needed when supplementing vitamin D in athletes since elevated values above 125 nM can be found in athletes taking replacement, raising concerns about adverse effects. These include weakness, fatigue, sleepiness, headache, loss of appetite, dry mouth, and metallic taste. That's why it's important to be tested by your doctor before boosting your daily regimen.

Read more Sports Doc for Sports Medicine and Fitness.