Sun's out, guns out.

It's not just about being jacked and tan.

Athletic Performance and Vitamin D

Results: Numerous studies, particularly in the German literature in the 1950s, show vitamin D-producing ultraviolet light improves athletic performance. Furthermore, a consistent literature indicates physical and athletic performance is seasonal; it peaks when 25-hydroxy-vitamin D [25(OH)D] levels peak, declines as they decline, and reaches its nadir when 25(OH)D levels are at their lowest. Vitamin D also increases the size and number of Type II (fast twitch) muscle fibers. Most cross-sectional studies show that 25(OH)D levels are directly associated with musculoskeletal performance in older individuals. Most randomized controlled trials, again mostly in older individuals, show that vitamin D improves physical performance.

Conclusions: Vitamin D may improve athletic performance in vitamin D-deficient athletes. Peak athletic performance may occur when 25(OH)D levels approach those obtained by natural, full-body, summer sun exposure, which is at least 50 ng·mL-1. Such 25(OH)D levels may also protect the athlete from several acute and chronic medical conditions.

A few more articles:

• Can Vitamin D Improve Your Athletic Performance?
  
• "D" is for Doping
• Vitamin D Prevents Swine Flu
• Vitamin D Science

In the winter, it's basically impossible to get enough vitamin D. In Tacoma in December, you need to be out in the sun for over two hours per day to get the necessary amount. I take 6000 IU per day during the winter and on days I don't get out in the sun.