The three-day tuna diet is an informal eating plan often used by body builders to lose weight before competitions. It consists of eating equal portions of canned tuna six times a day for three days and drinking water.
The three-day tuna diet is supposed to promote weight loss while retaining muscle, since the tuna provides protein. This plan uses a weight-based formula to determine how much tuna to eat over the course of three days. During this time, the dieter should consume large quantities of water and take vitamins. This particular plan is not endorsed by medical professionals or supported by research, as of 2015.