Taking 2000 IU of vitamin D daily for 12 weeks raises vitamin D levels in the blood of young, healthy men who were initially deficient, but this change is not linked to changes in testosterone, estradiol, cortisol, or SHBG levels.

Taking a daily vitamin D pill of 8000 IU for 3 months can boost vitamin D levels in young men from dangerously low to healthy levels, while those who take a fake pill (placebo) stay low.

In men taking 2000 IU of vitamin D daily for 12 weeks, blood levels of vitamin D rise more when their diet is high in fat than when it is high in carbohydrates.

Taking a high-dose vitamin D pill every day for three months won't help young men who are low on vitamin D build more muscle or gain more lean body mass from weight training, even though their vitamin D levels go up to normal.

If young guys who don't have enough vitamin D lift weights and also take a combo of whey protein and a high dose of vitamin D every day for six weeks, their body's muscle-building hormones go up and muscle-limiting hormones go down—better than just lifting weights alone.

If young guys who don’t have enough vitamin D take 4000 IU of vitamin D and 25 grams of whey protein every day while doing strength training for six weeks, their vitamin D levels go up enough to get into a healthy range.