Vitamin D3 supplements provide a form of vitamin D that is found in the human body. Vitamin D3 is produced when the skin is exposed to ultraviolet light (like sunlight). You can also get it from certain foods, such as wild salmon, egg yolks, and fortified milk. Of course, Vitamin D3 can also be taken in supplement form. Since it can be difficult to get enough of it through sunlight and diet, doctors sometimes recommend such supplements.
Vitamin D3 has many roles in the body, including facilitating calcium absorption by the intestines, promoting bone growth and boosting the immune system. (Learn more about the Why Athletes Need Vitamin D.) The usual supplement dosage varies among adults, based on body mass (weight), amount of sun exposure, and level of deficiency. Consult your doctor before taking vitamin D3.
A few years ago, during a routine physical exam, a blood test revealed that I had a vitamin D deficiency. As you might have guessed, the exam was administered during the middle of winter, and I live in northeast Ohio. My doctor suggested a 1,000 IU vitamin D3 supplement, in addition to my daily multivitamin (which contains 600 IU vitamin D). Prior to my exam, I felt that my energy level had been a bit lower than normal, and I was experiencing somewhat of a plateau in my strength and conditioning (keep in mind, I’m very active). (Learn 5 ways to break through a plateau.) Although I realize this is anecdotal, within a few weeks after starting the supplementation, I felt more energized and stronger (I didn’t really change any other component of my nutrition or training). Since then, I have shared my experience with scores of clients, of all ages, most of them athletes. Of those who have consulted with a physician and started taking supplemental D3, the vast majority have reported similar results, to some degree.
Here are more good reasons to consider a vitamin D3 supplement.

Health Benefits of Vitamin D3

• Promotes calcium absorption
• Helps build bones
• Enhances muscle strength and function
• Improves immune function
• Boosts mood, energy level, and motivation
• Contributes to nervous system function
• May help defend against certain cancers and high blood pressure

Vitamin D3 and Athletic Performance

Considerable research links vitamin D3 to improved athletic performance, specifically in the areas of strength, speed, and balance. How much it might improve your performance depends on how deficient you are to begin with (but a surprising number of athletes have been found to be vitamin D deficient).
Vitamin D3 acts directly to increase protein synthesis in the muscles. Studies have found that vitamin D administration enhances cross-sectional growth of Type II muscle fibers and results in strength gains. (Find out more about muscle fibers.)
Other studies have found that vitamin D3 administration improves neuromuscular functioning, including balance, muscle strength, and reaction time.
Research has demonstrated an association between seasonal vitamin D3 level variations and athletic performance, with peak performance and trainability occurring during the summer months, when D levels are highest, followed by sharp declines in autumn and a low point in winter.
Finally, athletes who consumed calcium and vitamin D3 (in fat-free milk) after working out became stronger, gained more lean muscle mass, and lost more body fat than those who drank a carbohydrate drink after their workouts.