While probiotics encourage the growth of good bacteria, prebiotics feed the ones you already have. When you eat prebiotic foods like onions and leeks, they produce butyrate, a metabolic wonder drug that improves insulin sensitivity and increases the amount of fat you burn, says Rafael Kellman, M.D., author of The Microbiome Diet. Now that it's summer, pop either on the grill to bring out their sweetness. Drizzle with olive oil, salt, and pepper and cook until golden.
While many people turn to artificial sweeteners in a misguided attempt to whittle their waistlines, those fake sugars are likely to have the opposite effect. According to researchers at Yale, artificial sweeteners are actually linked with an increased risk of abdominal obesity and weight gain, possibly because they can trigger cravings for the real stuff and spike insulin levels in a similar fashion to real sugar.