Belly fat isn't just a nuisance that makes your clothes feel tight; it's also seriously harmful. This type of fat, known as visceral fat, is a major risk factor for type 2 diabetes, heart disease, and other conditions. Many health organizations use BMI (body mass index) to classify weight and predict the risk of metabolic disease. However, this is misleading, as people with excess belly fat are at an increased risk even if they look thin.