This document outlines several potential causes of weight gain, including lack of physical activity, irregular eating and sleeping habits, depression and emotional eating, certain medical conditions and medications, aging, genetics, and hormonal imbalances. Thyroid disorders, certain medications, and menopause can slow metabolism and contribute to weight gain. Musculoskeletal problems may limit physical activity. Genetics and leptin resistance can also play a role. Quitting smoking can result in weight gain due to changes in eating behaviors. The document provides information on weight gain and was reviewed by a medical advisor.