Hormonal imbalances involving insulin and stress are two primary causes of stomach weight gain, according to For Dummies. These changes can trigger a physiological increase in visceral fat, which is fat that accumulates in the upper abdomen, and subcutaneous fat, which accumulates in the lower abdomen.
According to For Dummies, food is digested and broken into smaller particles that the body uses for energy. Carbohydrates consumed turn into glucose, which is absorbed into the bloodstream and turned into insulin. The body stores excess sugar it receives from insulin in tissue cells, especially in the abdominal area. Therefore, excess fat is a result of sugar buildup that has no where else to go. Foods that contain higher levels of carbohydrates and sugar, such as white bread, white rice and desserts, are more likely to lead to belly fat.
Stress can also play a role in causing excess stomach fat. According to Women's Health, a research study found that having continuous negative relationships with people is more likely to lead to weight gain. The reason given is that people develop adaptive behaviors, such as eating more and exercising less, to cope with the stress. Harvard Medical School notes that abdominal fat can be brought under control by doing moderate exercise for 30 minutes a day.