Overweight adults who cut their daily calorie intake by nearly a third saw a 50 percent boost in immunity, according to a six-month study out of Tufts University. (Those who cut calories by 10 percent had smaller improvements.) Restricting calories may reduce levels of compounds in the body that depress your immune response, says Tufts nutritional immunologist Simin Meydani, D.V.M., Ph.D. Animal studies suggest that calorie restriction could work in normal-weight individuals too. “Try to maintain your body weight at what is considered ideal,” Meydani says, because eating more than what you need drags the immune system down. And remember: when you cut back on quantity, you need to be even more vigilant about the quality of your diet. Aim to eat more fruits and vegetables—and choose lean protein sources, such as fish, chicken and low-fat dairy.