A gainer, also known as a weight gainer, is a type of nutritional supplement specifically designed to help individuals increase their calorie intake and gain weight, particularly in the form of lean muscle mass. It is commonly used by athletes, bodybuilders, and individuals who struggle to consume enough calories through their regular diet alone. It also helps to increase size and strength.

Gainers typically contain a blend of carbohydrates, proteins, fats, and sometimes added vitamins and minerals. The carbohydrate component provides a quick source of energy, replenishes glycogen stores, and supports muscle recovery. The protein component helps stimulate muscle protein synthesis, aiding in muscle growth and repair. The fats in gainers provide additional calories and contribute to overall energy intake.