Our bodies can make vitamin D from the sun’s ultraviolet rays. But sunlight is so limited in northern climates during the winter months, your body might not produce any vitamin D at all. Here’s another issue: people with more body fat typically have lower levels of vitamin D because one form is stored in fat. A study published in the journal Clinical Nutritionlooked at what a daily dose of 4,000 IU did for overweight and obese resistance training subjects.

These 23 volunteers received either vitamin D or a placebo daily for 12 weeks. During this period, all subjects trained with weights. In the end, those supplementing with 4,000 IU of vitamin D lost more inches from around their waistline and gained a greater degree of strength than the placebo group.

True Strength Moment: The recommended daily dose of vitamin D was increased in 1997 to between 400 and 600 IU, depending on age. Some experts suggest increasing it even more to between 800 and 1,000 IU a day. While there are upper limits on what would be considered sensible, the nutrient needs of active adults outweigh those of less active individuals. That goes for micronutrients as well as the macronutrients protein, carbohydrates and fats. This study seems to back up that theory.