Poor diet. Sugary food such as cakes and candy, and drinks such as soda and fruit juice, can: ...Too much alcohol. Consuming excess alcohol can cause a variety of health problems, including liver disease and inflammation. ...Lack of exercise. ...Stress. ...Genetics. ...Poor sleep. ...Smoking.There are many reasons why people gain belly fat, including poor diet, lack of exercise, and stress. Improving nutrition, increasing activity, and making other lifestyle changes can all help.