Getting lean boils down to one thing: creating an energy deficit. Besides doing aerobic exercise, many precontest bodybuilders follow fairly strict low-calorie diets, most of which stress lean protein like skinless chicken breasts, egg whites, tuna and protein powders.
In addition, many dieting bodybuilders limit their carbohydrate intake to 200-350 grams a day, depending on their size, metabolism and level of activity. Keeping carbs low increases the energy deficit and helps control insulin, a hormone that stimulates hunger and plays a role in fat storage. It’s believed that lower insulin levels facilitate the burning of bodyfat.
But the downside to creating an energy deficit is that the body often adapts to the shortfall in calories by burning fewer of them. A break from a low-calorie diet (i.e., a cheat day) can interrupt this slowdown. There are other benefits to cheating. Fat-busting diets can cause a decline in levels of thyroid hormones and leptin (which directly affect fat-burning) and IGF (insulinlike growth factor, which supports muscle growth). Splurging a bit can bring them back to normal.