A negative calorie diet is a weight-loss plan built on the claim that certain foods cost more energy to digest than they provide, so eating them as often as possible should create a deficit without hunger. Celery, cucumber, lettuce, grapefruit, and similar watery, fibrous, low-energy foods form the core of the menu. The underlying physiology does not support the claim: the thermic effect of food runs at roughly 5 to 15 percent of the energy in a meal, so no food is genuinely negative in net energy. Dietitians view the approach as a gimmick, though the practical effect can still be weight loss, simply because the permitted foods are very low in energy density and displace richer foods. Extended versions risk nutrient gaps if protein, essential fats, and micronutrients are neglected.