When healthy people eat 25% fewer calories for two years, their liver, heart, and muscles shrink a bit, which helps their body adjust to less food—but their brain and kidneys stay about the same size as people who eat normally.

We analyzed one assertion and found that when healthy adults reduce their calorie intake by 25% for 24 months, their liver, heart, and muscle mass tend to decrease slightly, while brain and kidney mass remain unchanged. This suggests the body may adapt to lower energy intake by reducing the size of organs and tissues that use more energy, while protecting vital functions like thinking and filtering blood. The evidence we’ve reviewed leans toward this pattern, with no studies contradicting it [1]. We don’t know exactly why this happens, but it may be related to how the body prioritizes energy use. The brain and kidneys are essential for survival, so they may be preserved even when calories are limited. The liver, heart, and muscles, while important, might shrink a bit to lower overall energy demands. This doesn’t mean these organs become unhealthy—just that their size changes in response to less food. It’s important to note this is based on a single assertion, and we haven’t reviewed detailed measurements or long-term health outcomes beyond size changes. We also don’t know if these changes reverse after returning to normal eating. If you’re considering long-term calorie reduction, this suggests your body may naturally adjust by slimming down some tissues while protecting your brain and kidneys. But whether this is beneficial, neutral, or risky for your overall health isn’t clear from what we’ve seen so far.