Caloric Restriction

at a glance: eating less, in a variety of ways, can make you live longer - but is your body just using number of calories as a signal?

In the 1930s, investigators wanted to do an experiment to see if stunted growth rates during the Great Depression might impact lifespan. They tested this in rats by feeding them less food than they would normally eat. To their surprise, this actually made the rats live longer! This was a seminal discovery. For the first time, we changed the environment of an animal to make it live longer than it normally would.

Since then, investigators have tried to uncover how this works. The effect depends on what genes you have, what you are eating and how much less you eat. If you take many genetically distinct mouse strains and put them on the same diet (cutting calories by ~40%), sometimes fewer than 1/5 of the mouse strains live longer. Diet composition also plays a role. Just decreasing protein or a specific amino acid, while keeping total calorie intake the same, can result in a lifespan extension in mice. Feeding mice a ketogenic diet also seems to help. Decreasing food intake by too much will result in starvation, so finding a diet that works can depend on the situation.

While long-term human studies are sparse, investigators have run two caloric restriction experiments in monkeys, one of which showed promising results for an increase in survival. To avoid the difficulty of continuous dieting, fasting ~8+ hours a day, or 5 days/month, or on a variety of different cycles might also be helpful. This is called intermittent fasting. Medically, intermittent fasting may aid recovery during chemotherapy. Some longevity-related pathways involve sensing amino acid levels, so it is possible that a specific biological process, not total calorie intake, controls the increase in lifespan.