“This is the only vitamin supplement I recommend to my clients,” says Bridger. “Most of us don’t get enough exposure to the UV rays our bodies need to synthesize it. Vitamin D deficiency is linked to a list of health issues as long as my arm and while it won’t help you lift more, it will keep your immune system strong and ensure your body is in the best position possible to build and maintain muscle.”

Go for the liquid version in capsules. It’s more easily absorbed by the body, so you get more bang for your buck.