A crash diet is a weight-loss regimen that slashes calorie intake drastically for a short period, promising rapid results. Very low calorie intakes cause quick initial loss, largely water and glycogen, but also muscle, and trigger metabolic adaptation, intense hunger, and often rebound bingeing. Side effects include fatigue, hair loss, gallstones, and nutrient deficiencies, and the weight almost always returns, sometimes with interest. Nutrition professionals condemn crash dieting as ineffective and harmful, advocating instead moderate deficits with balanced nutrition, activity, and behavioural support. Fast weight loss is seductive; lasting loss is gradual.