If you force a laboratory rat to go on a diet, reducing the animal’s caloric intake by 30% to 40%, it will live about 30% longer, on average. A calorie restrictionthe technical name of this intervention, cannot be extreme to the point of malnutrition, but must be aggressive enough to trigger some biological changes important.
Scientists discovered this phenomenon in the 1930s and, over the last ninety years, have replicated the experiment in species ranging from worms to monkeys. Subsequent studies also revealed that many of the calorie-restricted animals were less likely to develop cancer and other chronic diseases related to aging.
But despite all the animal research, there are still many unknowns. Experts are still debating how the phenomenon works and whether what matters more is the amount of calories consumed or the period of time over which they are ingested (also known as intermittent fasting).
And it’s still unclear whether eating less can also help people live longer. Aging experts often experiment with different diets on themselves, but more serious longevity studies are few and far between because, well, they take a long time.
Here’s an overview of what scientists have discovered so far, mostly through animal studies, and what they think it all might mean for humans.
Why would cutting calories increase longevity?
Scientists don’t know exactly why eating less would make an animal or person live longer, but many hypotheses have a evolutionary bias. In nature, animals go through periods of plenty and famine, just like our human ancestors. Therefore, their biology (and possibly ours) has evolved to survive and thrive not only in times of abundance but also in times of deprivation.
One theory says that, at the cellular level, calorie restriction makes animals more resistant to physical stress factors. For example, calorie-restricted mice have greater resistance to toxins and recover more quickly from injuries, says James Nelson, professor of cellular and integrative physiology at the University of Texas Health Science Center at San Antonio.
Another explanation involves the fact that, in both humans and animals, eating fewer calories slows down metabolism. It’s possible that “the less you make your body metabolize, the longer it will live,” notes Kim Huffman, an associate professor of medicine at Duke University School of Medicine who has studied calorie restriction in people. “If you drive your car less, your tires will last longer.”
Calorie restriction also forces the body to seek fuel sources other than glucose, which aging experts believe is beneficial for metabolic health and, ultimately, longevity. Several researchers have mentioned a process known as autophagy, in which the body consumes defective parts of cells and uses them for energy. This helps cells function better and reduces the risk of several age-related diseases.
In fact, scientists believe that one of the main reasons calorie-restricted diets make mice live longer is because the animals don’t get sick as soon, describes Richard Miller, a professor of pathology at the University of Michigan.
there are some exceptions notable for discoveries on longevity and caloric restriction. The most impressive was a study that Nelson published in 2010 on genetically different mice. He found that some of the mice lived longer when they ate less, but a larger percentage lived less.
Other researchers have disputed the importance of Nelson’s findings. “People cite this study as if it were general evidence that calorie restriction only works for a few people, or for a short time,” says Miller. “But you can only reach that conclusion if you ignore fifty years of strong published evidence saying it works almost all the time.”
However, Nelson’s study was not alone in failing to find universal benefits of calorie restriction for longevity. For example, two studies conducted on monkeys over twenty years, published in 2009 and 2012, reported conflicting results. Animals in both experiments showed some benefits associated with calorie restriction, but only one group lived longer and had lower rates of age-related problems such as cardiovascular disease and diabetes.
What does intermittent fasting have to do with it?
Given these conflicting results, Some researchers wonder if there isn’t another variable that is as important or more important than the amount of calories ingested by the animal: the period of time in which it ingests them.
An important difference between the two monkey studies was that in the 2009 experiment at the University of Wisconsin, the calorie-restricted animals received only one meal per day, and the researchers removed leftover food in the late afternoon, so that the monkeys were forced to fast for about 16 hours. In the 2012 study, carried out by the National Institute on Aging, animals were fed twice a day, and food remained available throughout the night. The Wisconsin monkeys lived the longest.
A more recent mouse study explicitly tested the effects of calorie restriction with and without intermittent fasting. The scientists gave the animals the same low-calorie diet, but some had access to the food for just 2 hours, others for 12 hours, and another group for 24 hours. Compared to a control group of mice that could eat a full diet at any time, mice on calorie restriction and with 24-hour access lived 10% longer, while mice on the low-calorie diet that ate in time windows specific species had an increase of up to 35% in lifespan.
Based on this set of findings, Rafael de Cabo, a senior researcher at the National Institute of Aging who helped lead the monkey study, now believes that while calorie restriction is important for longevity, the amount of time eating – and not eating – every day is equally essential. And this can happen not only to animals, but also to humans.
What does this mean for me?
It’s difficult to definitively answer whether intermittent fasting, calorie restriction, or a combination of the two can make people live longer.
“I don’t think we have any evidence that these methods extend the life of humans,” Nelson reports. That doesn’t mean they don’t work, he added, just that “it’s very difficult to gather evidence, because it takes a lifetime for us to collect that data.”
A clinical trial – called the Calerie study – attempted to answer this question by examining how a 25% reduction in calories over two years would affect a range of measures related to aging. More than 100 healthy adults were guided through meal planning and received periodic counseling sessions to help them achieve their diet goals. But because it’s so difficult to reduce calories, participants were only able to cut their intake by about 11 percent.
Compared to control group participants, dieters improved several aspects of cardiometabolic health, such as blood pressure and insulin sensitivity, and had lower levels of some markers of inflammation.
The study also included three measures of “biological age,” comparing blood tests taken at the beginning and end of the two years. Two of the tests showed no improvement in either group, but the third, which tries to measure how quickly people age, showed a difference between those who dieted. Calorie restriction “didn’t make people younger, but it slowed down how quickly they aged,” explains Huffman, who worked on the study.
For Miller, the most significant conclusion from this study is that calorie restriction of 25% to 40%, which has already been shown to be beneficial for animals, simply it’s not realistic for humans. “They did everything they could to help people cut calories,” he said, but participants still fell short of the 25% goal.
De Cabo presents a different opinion: “Even though they only achieved 11% calorie restriction, participants showed benefits”.
Other research has focused on the short-term effects of intermittent fasting on people with different body mass indexes (BMI). Some studies that tested different fasting schedules showed improved metabolic health and reduced inflammation. But a study of 116 people whose BMI classified them as obese or overweight found no benefit among those who ate within an 8-hour window but didn’t reduce calories, compared with a control group.
And we have one final twist: there is a remarkable body of evidence that seem to contradict directly the idea that calorie restriction or fasting, which normally leads to weight loss, prolongs human life.
Research has consistently found that people classified as overweight have a lower risk of death than people who are normal weight or underweight. One hypothesis is that people with a lower BMI may be thin because they are older or have a chronic illness. Another is that people with higher BMIs have more muscle, which weighs more than fat. But it’s also possible that, especially later in life, having more body mass is actually protective, Huffman reports.
Despite almost a century of research, there is still a long way to be covered until experts can say with certainty whether the longevity benefits seen in animals translate to humans. Some studies give reason to believe that calorie restriction and intermittent fasting will help you live longer, and there are likely to be short-term benefits, particularly when it comes to heart and metabolic health. But it’s also possible that eating less doesn’t do much more than make you hungry.
This article was originally published on New York Times. / TRANSLATION BY RENATO PRELORENTZOU
