A study published in Nature found that naturally lean women have more bacteria called bacteroidetes in their gut, and women who carry more fat tend to have more gut bacteria known as firmicutes. "The good news about this is that you can influence your personal bacteria by eating more fibre," says Knapp. "Fibre breaks down into short chain fatty acids that feeds the bacteria bacteroidetes," she adds. Aim for five cups of fruits and vegetables (combined) a day to help boost this lean bacteria and ward off belly fat. Start with these 10 high-fibre foods, and try to eat them every week.