Weight gain is an increase in body weight. This can be either an increase in muscle mass, fat deposits, or excess fluids such as water.

Weight gain has a latency period. The effect that eating has on weight gain can vary greatly depending on the following factors: energy (calorie) density of foods, exercise regimen, amount of water intake, amount of salt contained in the food, time of day eaten, age of individual, individual's country of origin, individual's overall stress level, and amount of water retention in ankles/feet. Typical latency periods vary from three days to two weeks after ingestion.