Can Reducing Calories Really Help You Live Longer?

Most of us eat more than we need. Americans had an average supply of more than 3,800 calories daily in 2021, according to the Food and Agriculture Organization of the United Nations — well above the average adult’s recommended dietary allowance of between 1,600 and 3,000 calories. And our feasting surged in the early days of the COVID-19 pandemic: In a nationwide survey of more than 1,500 American adults, 39.7 percent reported over­eating more frequently after the pandemic began.

Many elements of our food system promote overindulgence. Some estimates suggest that more than one-third of U.S. adults eat fast food on any given day, in part because it’s convenient and hyperpalatable. And research suggests that our brains are primed to crave such ultraprocessed, high-glycemic fare — so if we drive through once, we’re likely to again.

In these circumstances, it’s easy to eat more than our bodies need, perhaps without even realizing it.

Given that context, calorie restriction (CR) — a consistent pattern of dieting in which one reduces overall calories by 25 to 50 percent while maintaining an intake of essential nutrients — might seem like a worthy alternative. Scientists have long been interested in the potential for it to afford us longer, healthier lives. CR seems to hold promise — but it’s not that straightforward.

What the Research Says

The idea of CR increasing longevity is based on two theories. The first is the rate-of-living theory: Some scientists believe large mammals live longer than smaller ones because their metabolisms are slower, allowing them to expend energy less quickly.

There’s also the free-radical theory. Free radicals are molecules created by our metabolic processes that damage our cells and can cause age-related diseases. The theory suggests that a slower metabolism produces fewer free radicals, less disease, and longer life. (For more, see “How Antioxidants Can Slow Free-Radical Damage.”)

These theories also fit with some of what we know about human meta­bolism — namely, that we require fewer calories as we age. As we get older, our bodies shift from “building” mode to “protecting and preserving” mode, explains functional-medicine trailblazer Frank Lipman, MD, in The New Rules of Aging Well. He recommends eating until you’re only 80 percent full after you reach age 45; at that age your body needs fewer calories than it previously did.

Many studies on CR in animals have produced promising results, showing that rodents, worms, and mouse lemurs lived longer on calorie-restricted diets. Beyond increased life­span, ­researchers found the ­approach resulted in fewer age-related diseases, including cancer, kidney inflammation, and others. Plus, it improved immunity.

[We] require fewer calories as we age. As we get older, our bodies shift from “building” mode to “protecting and preserving” mode.

Taken together, this research offers hope that similar results could be replicated in humans. Yet few human studies have been done, in part because long-term data is needed, and keeping participants in a lab for a long period of time to limit their diet is both unrealistic and unethical.

Outside of the lab, it’s hard to convince people to stick to a restrictive diet. “Adherence to diets is challenging for people, especially when they’re trying to embed the regimen into their normal lifestyle,” says Leanne Redman, MS, PhD, an investigator for the CALERIE trial at Pennington Biomedical Research Center in Baton Rouge, La.

Additionally, asking subjects to document their eating habits has drawbacks, even among those for whom CR is a long-term practice: People often forget what they’ve eaten, and they tend to underreport it. (For more on the trouble with nutrition ­research, see “The Trouble With Food Studies.”)

The CALERIE trial (Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy) is the first CR study conducted on humans.

Beginning in 2007, the research tracked 218 healthy people for two years. One group of participants ate a diet restricted in calories by 25 percent; another group ate their regular diets (an average of about 2,400 calories per day) as a control. The participants lived at home, and researchers supported those in the CR group with counseling sessions and nutrition education, as well as with supplements to ensure they met their nutritional needs.

The CR participants experienced improvements in body weight, cholesterol, inflammation, insulin-sensitivity index, and metabolic-syndrome score — all markers that indicate one’s risk of heart disease and stroke, two of the leading causes of death in the United States.

From Lab to Real Life?

The benefits of the CALERIE trial should be taken with caution, says University of Southern California gerontology and biological sciences professor Valter Longo, PhD, author of The Longevity Diet. He says the trial offers great information about cardiometabolic health, but it shouldn’t be interpreted as guidance for one’s life.

“The cardiometabolic effects are extraordinary, but because of compliance issues, safety, and also efficacy, I don’t recommend calorie restriction,” Longo says. “The research tells us that a lot of these problems — whether it’s diabetes, or cancer, or cardiovascular events — are preventable. But it also tells us that chronic calorie restriction is not the way to do it.”

That’s in part because it’s nearly impossible to effectively restrict caloric intake on your own. In real life, getting all the nutrients you need through a calorie-restricted diet (without the support of a team of researchers) is hard work, ­requir­ing careful planning and strict attention to everything you eat. (For more on the drawbacks of a calorie-focused mindset, see “Beyond Calorie Counting.”)

“You could do calorie restriction for a while, but eventually you’ll regain any weight you’ve lost,” Longo explains. “You’ve slowed your metabolism, so your energy expenditure goes down because your body is trying to save your life: It wants you back to your normal weight.”

And if your nutritional needs aren’t met, calorie restriction could lead to fatigue, anemia, weakened bones, and muscle loss. Women may experience menstrual disturbances and decreased fertility, while men may have lower testosterone levels. (For more on how to support sexual health with nutrition, see “How to Use Nutrition to Improve Your Libido.”)

Nutrient deficiencies can also come with a host of psychological effects, some of which were documented in physiologist Ancel Keys’s landmark Minnesota Starvation Experiment, which began in 1944. The study participants — men who ate about 1,500 calories per day for six months — grew weak and exhausted, became obsessed with food and body image, and suffered severe emotional distress.

Paradoxically, CR often leads to weight gain over the long term. “You could do calorie restriction for a while, but eventually you’ll regain any weight you’ve lost,” Longo explains. “You’ve slowed your metabolism, so your energy expenditure goes down because your body is trying to save your life: It wants you back to your normal weight.”

Your body is likely to respond to calorie deprivation by boosting production of the hunger hormone ghrelin, which may cause you to overeat. It’s a scenario familiar to any serial dieter, and it’s the principal reason diets don’t work. (For more on why going hungry is bad for your health, see “Why Dieting Doesn’t Work — and Never Has.”.)

The Risks of Restriction

For the CALERIE trial, CR was a specific intervention. Participants followed calorie-reduced diets, and they and the researchers took great pains to ensure they received proper nutrition. But if you simply see a breathless news headline about how “calorie restriction can increase lifespan,” you might assume any low-calorie diet will do.

“I worry about eating advice that could be taken out of context,” says Leah Graves, RDN, LDN, vice president of nutrition and culinary services at Accanto Health. Graves has treated people with eating disorders for more than 30 years. “I could see something like this being a reinforcer for someone who has developed an eating problem.”

Eating disorders are biopsycho­social disorders, she explains, meaning they have no single cause: There are many factors that make people vulnerable to them. “But the gateway to eating disorders for most of our clients is calorie restriction,” Graves adds.

Ultimately, the question of how much you should eat is for you and you alone to determine: The answer depends on your unique biology and your relationship with food.

“What I’ve seen over the years is people who make a shift in their eating that they believed was in the interest of health, yet that shift leads to more than they bargained for,” she continues. “And so, the pursuit of health actually leads to a potentially life-threatening eating disorder.”

Ultimately, the question of how much you should eat is for you and you alone to determine: The answer depends on your unique biology and your relationship with food.

It also depends on what you’re eating. Most of us would likely benefit from a focus on quality rather than quantity. If you’re getting plenty of satiating protein and fiber from nutritious whole foods, you won’t have room in your diet for the processed stuff that encourages overeating — and you’ll be improving your cardiometabolic health without the need to do calorie math.

This article originally appeared as “Eat Less, Live Longer?” in the June 2023 issue of Experience Life.

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