It may soon be time to update the old adage about eating when you're hungry and stopping when you're full.
A growing body of research suggests that when we cut the amount of the food we would eat on a typical day by as much as a third or half, we enjoy benefits including more energy, less illness, and potentially longer lives.
Most of the comprehensive research on this strategy, called calorie restriction, has been done in animals, so its conclusions must be taken with a grain of salt. But some humans have gone full-steam ahead with their own research — on themselves.
In Silicon Valley, trendy calorie-restricting clans meet regularly for meals to break 14-36 hour periods of fasting. Followers of the fad say it helps them focus, increases their productivity, and speeds weight loss.
Many intermittent fasters simply give up one meal each day, an easy way of cutting roughly 30% of their daily calories.
The latest study on calorie restriction, published this month in the journal Nature, was conducted on mice and primates. Its results suggest that eating less could help slow the spinning gears of our biological clocks by interfering with an important genetic process called methylation, which is likely linked with aging. Researchers compared the cells of mice and primates who were either fed normal diets or calorie-restricted ones, and found that the restricted eaters had significantly fewer markers of the methylation process. In other words, their cells appeared "younger."
The measuring tool the researchers used for their study — methylation — is far from a perfect means of determining age. There's still a lot we don't know about the process, such as whether it effects all types of cells equally and if it can be used as a metric across different kinds of animals. Nevertheless, the study authors came away with some hopeful findings.
Rhesus monkeys in the study whose diets had been restricted by 30% for two-thirds of their lives (beginning in middle age) had cells that appeared, on average, seven years younger than their actual age. Mice in the study whose diets had been restricted by 40% for nearly their entire lives had cells that appeared two years younger. Taking into account the differences in the animals' average lifespan (rhesus monkeys live for about 25 years while mice live anywhere from 2-3 years), the results for the mice were more pronounced than those for the monkeys.
"Calorie restriction, which prolongs lifespan in mice and monkeys ... resulted in a significantly younger 'methylation age'," the researchers wrote in their paper.
That finding builds on several years of research in other animals on the potential benefits of calorie restriction. Researchers studying fruit flies, mice, rats, and worms have found that slashing calories (usually by about 30%) can double or even triple lifespan.
Still, a fly is not a human.
With that in mind, some scientists have moved onto tests in animals that are more like us, such as primates. In these animals, the research on calorie restriction and life extension has still been promising, although researchers haven't succeeded in going as far as tripling any monkey's lifespan. Several recent studies, including one important paper published in January in the journal Nature, have shown that dieting rhesus monkeys not only live longer than their non-dieting counterparts, but are also healthier and less prone to disease.
So how might these results translate to people? We don't know yet. In the meantime, biohackers, techies, and other trendy eaters will continue their intermittent fasting. Whether the benefits of the habit can be chalked up to real biological changes — or are simply the product of the placebo effect — remains to be seen.
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