Weight cutting is the practice of fast weight loss prior to a sporting competition. It most frequently happens in order to qualify for a lower weight class (usually in combat sports, where weight is a significant advantage) or in sports where it is advantageous to weigh as little as possible (most notably equestrian sports). There are two types of weight cutting: One method is to lose weight in the form of fat and muscle in the weeks prior to an event; the other is to lose weight in the form of water in the final days before competition.
Nutritional experts will rarely give advice on how to cut weight safely or effectively, and will simply recommend against cutting weight at all. However, many athletes choose to do it because they wish to gain an advantage in their sport.
In addition to improving performance through healthy eating, some athletes will seek to lose weight through dieting and aerobic exercise. By losing fat they hope to achieve a higher "strength to mass ratio" or "lean weight." This means more muscle and less fat, and should theoretically give them an advantage against other athletes of the same weight.