Weight gain in humans is a biological process that occurs when the body stores more energy than it uses over time. This stored energy is mainly kept in the form of fat, though weight gain can also come from increases in muscle mass, water retention, or bone density depending on lifestyle and health conditions.
At the center of weight gain is a principle known as energy balance. When a person consumes more calories (energy from food and drinks) than their body burns through daily activities, metabolism, and bodily functions, the excess energy is stored. Over time, this leads to an increase in body weight.