If we put a laboratory mouse on a diet, reducing the animal’s caloric intake by 30% to 40%, it will live, on average, about 30% longer. Caloric restriction, as the intervention is technically called, cannot be so extreme that the animal becomes malnourished, but must be aggressive enough to trigger some fundamental 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. Subsequent studies also found that 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 are still many unknowns. Experts are still debating how it works and whether it’s the number of calories consumed or the length of time they are eaten (also known as intermittent fasting) that is more important.
And it’s still frustratingly uncertain whether eating less can also help people live longer. Aging experts are famous for experimenting with different diets, but studies on longevity are scarce and difficult to conduct because they take a long time.
Here’s a look at what scientists have learned so far, mostly through seminal animal studies, and what they think it could mean for humans.
Why does reducing calories increase longevity?
Scientists don’t know exactly why eating less makes an animal or person live longer, but many hypotheses have an evolutionary bias.
In nature, animals go through periods of abundance and famine, just like our human ancestors. Therefore, their (and possibly our) biology has evolved to survive and thrive not only during times of abundance, but also during times of deprivation.
One theory is that, at a cellular level, calorie restriction makes animals more resistant to physical stressors. For example, calorie-restricted mice have greater resistance to toxins and recover more quickly from injuries, said James Nelson, professor of Cellular and Integrative Physiology at the University of Texas Health Science Center at San Antonio.
Another explanation is that, in both humans and animals, eating fewer calories slows down metabolism. It’s possible that “the less the body has to metabolize, the longer it can live,” said Kim Huffman, an associate professor of Medicine at Duke University School of Medicine who has studied calorie restriction in people. “You know, just slow down the wheels and the tires will last longer.”
Calorie restriction also forces the body to turn to fuel sources other than glucose, which aging experts consider to be beneficial for metabolic health and, ultimately, longevity. Several researchers have pointed to a process known as autophagy, in which the body consumes the defective parts of cells and uses them for energy. This process helps cells function better and reduces the risk of several age-related diseases.
In fact, scientists think that one of the main reasons calorie-restricted diets make mice live longer is because the animals don’t get sick as soon, if at all, said Richard Miller, professor of pathology at University of Michigan.
There are some notable exceptions to the conclusions about longevity and calorie restriction. The most impressive was a study that James Nelson published in 2010 on genetically diverse mice. He found that some of the mice lived longer when they ate less, but a larger percentage actually had a shorter lifespan.
“This was truly unheard of,” said James Nelson, noting that most articles on calorie restriction begin by saying, “Dietary restriction is the most robust and nearly universal means of extending lifespan across species in the animal kingdom, and blah.” , blah, blah.”
Other researchers have disputed the importance of these conclusions. “People cite this study as if it’s general proof that calorie restriction only works a small part or some of the time,” said Richard Miller. “But you can only reach that conclusion if you ignore 50 years of strong published evidence that says it almost always works.”
However, James Nelson’s study wasn’t the only one that didn’t find a universal longevity benefit from calorie restriction. For example, two studies carried out on monkeys over more than 20 years, published in 2009 and 2012, presented contradictory results. Animals in both experiments showed some health benefits associated with calorie restriction, but only one group lived longer and had lower rates of age-related diseases such as cardiovascular disease and diabetes.
