My sister is doing a 3-day juice fast to detox her body and lose weight.

The idea that a juice-only diet will cleanse your system of toxins and help you shed pounds is absolutely wrong, says Janet Helm, R.D., a spokesperson for the American Dietetic Association. "It's not wise to have severe diet restrictions like that; it deprives you of nutrients." In fact, says Helm, this type of liquid fast can actually slow down your metabolism. "Your body begins to think it is in starvation mode, and it starts to conserve energy." If you're simply looking to improve or cleanse out your digestive system, "fiber is the most important thing," says Helm. To lose weight, look for ways to cut fatty extras from your diet, add more fruit and vegetables in their whole form, and increase your exercise and activity level.