Both hot and sweet peppers may enhance weight-loss efforts. Research has shown that capsaicin—the substance that gives hot red peppers (or chilies) their kick, and boosts our metabolism—keeps immature fat cells from developing into full-fledged ones. And a study presented in April found that a compound in some sweet peppers (called CH-19 Sweet), which resembles capsaicin, provides similar positive metabolic effects—minus the burning mouth and lips.