2,000 IU of vitamin D3 per day for 2 weeks increased exercise performance by 30% while lowering physical exertion.

It also decreased cortisol (stress hormone) levels and lower blood pressure. While this study was very small (13 people), other studies have shown that vitamin D3 improves exercise performance and increase muscle strength. It has also been shown that vitamin D3 inhibits the activity of the enzyme needed to make cortisol.