In a recent study at Wake Forest University School of Medicine, people who had high levels of vitamin D in their blood were significantly stronger during a grip-strength test than those with lower amounts. "Vitamin D deficiencies have been associated with muscle weakness," says Denise Houston, the study's author. "It may be because the nutrient regulates calcium-which is crucial for protein synthesis-and a hormone that helps build muscle." For maximum strengthening and added protection from many diseases, including several forms of cancer, you need at least 400 international units (IU) of vitamin D a day. One cup of fortified yogurt or milk will give you 100 IU; a serving of fortified cereal such as Total, the whole shebang. Stronger bones and muscles? We'll drink (milk) to that.