When healthy, normal-weight people eat 25% fewer calories for two years, they lose weight—but their bodies also burn even fewer calories at rest than you’d expect just from losing weight, as if their metabolism slows down on its own.

We analyzed the available evidence and found that when healthy, normal-weight people reduce their calorie intake by 25% for two years, they lose weight—and their bodies also burn fewer calories at rest than would be expected just from the weight loss alone. This suggests that metabolic changes occur beyond what’s explained by having less body mass to maintain [1]. What we’ve found so far is based on one assertion that supports this pattern, with no studies contradicting it. The data indicates that after long-term calorie restriction, the body may adjust its energy use in ways that go beyond simple physics—like a slowdown in resting metabolism that isn’t fully accounted for by changes in weight, muscle, or activity levels. This kind of adjustment is sometimes called metabolic adaptation, meaning the body appears to become more efficient at using energy, possibly as a survival response to lower food intake. We don’t know exactly why this happens or how universal it is, since only one assertion was available for review. There’s no information yet on whether this effect is the same in men and women, older or younger people, or those with different activity levels. We also can’t say if this slowdown continues beyond two years or reverses after returning to normal eating. The evidence we’ve reviewed leans toward the idea that long-term, significant calorie restriction can trigger metabolic changes that go beyond weight loss—but we need more studies to understand how common, lasting, or meaningful this effect is in real life. If you’re considering a long-term calorie cut, this suggests your body might respond by burning fewer calories than expected—even if you’re still losing weight. That doesn’t mean it won’t work, but it may mean progress slows in ways that aren’t just about eating less.