A lack of exercise contributes to weight gain by reducing the number of calories burned during daily activities. When physical activity levels decline, energy balance shifts, causing excess calories to be stored as fat. Additionally, sedentary behavior, such as prolonged sitting, slows metabolism, further exacerbating weight increase. Hormonal changes also play a role, as lower activity levels may increase the secretion of hormones that promote fat storage. Thus, consistent exercise is crucial for maintaining a healthy weight and preventing obesity.