Sunlight is strong! Vitamin D may be the secret to big muscle, says a study in Medicine & Science in Sports & Exercise.

Scientists found that people with higher blood levels of D 
tend to have stronger upper- and lower-body muscles. “The vitamin acts like a hormone. It might affect testosterone,” says Dr Paul 
Thompson, lead author of the study. Another study found that vitamin D can lead to bone and muscle weakness.

Most adults need 600 IU of vitamin D daily.