Do you actually live longer if you structurally eat fewer calories?

In mice and other laboratory animals the outcome is remarkably clear: eating 10 to 40% fewer calories extends average lifespan by 10 to 40%. This effect has been confirmed repeatedly, from yeast cells and worms to fruit flies and rodents. Few findings in ageing research are this robust.

In humans the picture is more nuanced. The strongest indication of a beneficial effect comes from a study in people with obesity and diabetes, in which caloric restriction was associated with 15% lower all-cause mortality. That is meaningful, but these are people with a serious health problem: it makes logical sense that eating less works in their favour. Whether the same holds for someone who is already at a healthy weight, you simply cannot know on the basis of current studies.

What caloric restriction does inside the body is somewhat clearer. A controlled study in healthy people showed that eating 14% less for two years improved the functioning of the thymus (a component of the immune system) and slowed inflammatory processes. Both are biological signals associated with healthier ageing. In addition, eating less activates a kind of cellular clean-up process in which damaged components are broken down and recycled. This is probably one of the pathways through which the effect occurs. One caveat: with prolonged, extreme caloric restriction, that same clean-up process can overshoot and damage cells.

Eating fewer calories is not the only dial to turn. When you eat also appears to matter: interrupted eating patterns (in which you eat nothing for a large part of the day) improve health markers even when total calorie intake stays the same. The same applies to eating less protein: lower protein intake is associated in humans with lower levels of a growth hormone (IGF-1) that has been linked to accelerated ageing. But here too the evidence in humans is associative, not cause-and-effect.

There are also real downsides. In animal experiments with 40% fewer calories, researchers observed reduced growth, poorer fertility, and a weakened immune defence. These side effects make it unrealistic to simply recommend strict caloric restriction as a therapy. Long-term mortality studies in healthy people are still entirely absent.