You know that vitamin D is a critical supplement, not just for your health but also for your muscle size and strength, and for burning bodyfat. That’s right, D can also help you get stronger, increase muscle protein synthesis, reduce bodyfat and raise testosterone levels. FLEX recommends getting in a good 2,000–5,000 international units of vitamin D per day, but not just any ol’ D will do. You need to make sure you are taking vitamin D3, known as cholecalciferol. In a recent study from Creighton University (Omaha, Nebraska) subjects took approximately 7,000 IU of vitamin D2 or vitamin D3 per day for 12 weeks. They reported in a 2011 issue of the Journal of Clinical Endocrinology & Metabolism that D3 was about 90% more potent at raising levels of 25-hydroxyvitamin D, the storage form of vitamin D in the body, than vitamin D2. It also increased stored levels of the vitamin three times more than D2 did.Find the right supplements for your diet and fitness goals at GNC Live Well.