Science split on longevity effects of eating less

If you put a lab mouse on a diet, cutting the animal’s caloric intake 30% to 40%, it will live, on average, about 30% longer. The calorie restriction, as the intervention is technically called, can’t be so extreme the animal is malnourished, but it should be aggressive enough to trigger some key biological changes.

Scientists first discovered this phenomenon in the 1930s, and over the past 90 years it has been replicated in species ranging from worms to monkeys. The subsequent studies also found many of the calorie-restricted animals were less likely to develop cancer and other chronic diseases related to aging.

But despite all the research on animals, there remain a lot of unknowns. Experts are still debating how it works, and whether it’s the number of calories consumed or the window of time in which they are eaten (also known as intermittent fasting) that matters more.

And it’s still frustratingly uncertain whether eating less can help people live longer, as well. Aging experts are notorious for experimenting on themselves with different diet regimens, but actual longevity studies are scant and difficult to pull off because they take, well, a long time.

Scientists don’t exactly know why eating less would cause an animal or person to live longer, but many hypotheses have an evolutionary bent. In the wild, animals experience periods of feast and famine, as did our human ancestors. Therefore, their (and conceivably our) biology evolved to survive and thrive not only during seasons of abundance, but also seasons of deprivation.

One theory is that, on a cellular level, calorie restriction makes animals more resilient to physical stressors. Another explanation involves the fact that, in both humans and animals, eating fewer calories slows down metabolism. Calorie restriction also forces the body to rely on fuel sources other than glucose, which aging experts think is beneficial for metabolic health and, ultimately, longevity. Several researchers pointed to a process known as autophagy, where the body eats up malfunctioning parts of cells and uses them for energy. This helps cells function better and lowers the risk of several age-related diseases.

There are a few notable exceptions to the findings around longevity and calorie restriction. Most striking was a study James Nelson, a professor of cellular and integrative physiology at the University of Texas Health Science Center at San Antonio, published in 2010 on mice that were genetically diverse. He found some of the mice lived longer when they ate less, but a larger percentage actually had a shorter life span.

Other researchers have disputed the significance of Nelson’s findings. “People cite this study as though it were general evidence that caloric restriction only works a tiny portion, or some portion of the time,” Miller said. “But you can reach that conclusion only if you ignore 50 years of strong published evidence saying that it works almost all the time.”

A more recent study conducted in mice explicitly tested the effects of calorie restriction with and without intermittent fasting. Scientists gave the animals the same low-calorie diet, but some had access to the food for just two hours, others for 12 hours and another group for 24. Compared to a control group of mice that could graze on a full-calorie diet at any time, the low-calorie mice with 24-hour access lived 10% longer, while the low-calorie mice that ate within specific time windows had up to a 35% increase in life span.