Calorie restriction can help animals live longer. What about humans?

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Living comes with a side effect: aging. Despite what you might hear on social media or in advertisements, there are no drugs that are known to slow or reverse human aging. But there’s some evidence to support another approach: cutting back on calories.

Caloric restriction (reducing your intake of calories) and intermittent fasting (switching between fasting and eating normally on a fixed schedule) can help with weight loss. But they may also offer protection against some health conditions. And some believe such diets might even help you live longer—a finding supported by new research out this week.

But the full picture is not so simple. Weight loss isn’t always healthy and neither is restricting your calorie intake, especially if your BMI is low to begin with. Some scientists warn that, based on evidence in animals, it could negatively impact wound healing, metabolism and bone density. Let’s take a closer look at the benefits—and risks—of caloric restriction.

Eating less can make animals live longer. This remarkable finding has been published in scientific journals for the last 100 years. It seems to work in almost every animal studied—everything from tiny nematode worms and fruit flies to mice, rats, and even monkeys. It can extend the lifespan of rodents by between 15% and 60%, depending on which study you look at.

At least it’s good news for lab animals. What about people? Also on Wednesday, another team of scientists published a separate review of research investigating the effects of caloric restriction and fasting on humans. That review assessed 99 clinical trials, involving over 6,500 adults.  Those researchers found that, across all those trials, fasting and caloric restriction did seem to aid weight loss. There were other benefits, too—but they depended on the specific approach to dieting. Fasting every other day seemed to help lower cholesterol, for example. Time-restricted eating, where you only eat within a specific period each day, by comparison, seemed to increase cholesterol, the researchers write in the BMJ. Given that elevated cholesterol in the blood can lead to heart disease, it’s not great news for the time-restricted eaters.

Cutting calories could also carry broader risks. Dietary restriction seems to impair wound healing in mice and rats, for example. Caloric restriction also seems to affect bone density. In some studies, the biggest effects on lifespan extension are seen when rats are put on calorie-restricted diets early in life. But this approach can affect bone development and reduce bone density by 9% to 30%.

It’s also really hard for most people to cut their caloric intake. When researchers ran a two-year trial to measure the impact of a 25% reduction in caloric intake, they found that the most their volunteers could cut was 12%. (That study found that caloric restriction reduces markers of inflammation, which can be harmful when it’s chronic, and had only a small impact on bone density.)

Unfortunately, there’s a lot we still don’t really understand about caloric restriction. It doesn’t seem to help all animals live longer—it seems to shorten the lifespan of animals with certain genetic backgrounds. And we don’t know whether it extends the lifespan of people. It isn’t possible to conduct a randomized clinical trial in which you deprive people of food from childhood and then wait their entire lives to see when they die.

It is notoriously difficult to track or change your diet. And given the unknowns surrounding caloric restriction, it’s too soon to make sweeping recommendations, particularly given that your own personal biology will play a role in any benefits or risks you’ll experience.

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