Just three miles into the Olympic marathon in Beijing last summer, American record-holder Deena Kastor, 36, felt a pop in her right foot and dropped to the ground in pain from a broken metatarsal. The cause: A deficiency in vitamin D had weakened her bones.

Three out of four Americans today aren’t getting enough vitamin D, and the ramifications could be severe. A deficit has been linked to everything from cancer to diabetes, osteoporosis, and heart disease. Numerous studies have shown the benefits of D: It aids calcium absorption, keeps us agile as we age, and may fend off colds. It could even make us better athletes, improving reaction time and building muscles.