Calorie-restricted diets cause compensatory increases in hunger signaling that result in the failure to maintain weight loss in most individuals.

Study: Energetic adaptations in response to moderate calorie restriction-induced weight loss in normal-weight adults.

When people lose weight by eating fewer calories, their body responds by making less of a hormone called leptin, which tells the brain you're full. Less leptin means you feel hungrier, making it harder to keep the weight off.

Study: The effects of graded levels of calorie restriction: VI. Impact of short-term graded calorie restriction on transcriptomic responses of the hypothalamic hunger and circadian signaling pathways

When mice ate much less food, their brains sent stronger hunger signals, making them more eager to eat — which is exactly what the claim says happens in people on diets.

Study: Calorie restriction and calorie dilution have different impacts on body fat, metabolism, behavior, and hypothalamic gene expression.

When mice ate less food, they got hungrier and their brains showed signs of starvation—even if they ate the same number of calories from different food. This suggests cutting calories makes you hungrier, which might explain why people often regain weight after dieting.

Study: Metabolic and appetitive regulation of adipocyte mass during treatment of obesity

When people lose weight by eating less, their bodies fight back by making them hungrier and burning fewer calories, which is why most people regain the weight. The study shows this biological response is real and strong.