In the quest for a longer life, the secret might not be in how much we eat, but how resilient our bodies are when we eat less.
For nearly 100 years, scientists have known that animals fed less live longer, but the reasons behind this phenomenon have remained elusive. The Jackson Laboratory (JAX), however, might be pulling back the curtain.
In a new study out this week in Nature, researchers tracked the lives of almost 1,000 mice, putting them on various calorie-restricted diets and monitoring how long they lived. The study builds on a long history of research. But its findings are turning conventional wisdom about metabolism, weight, and aging on its head.
While the researchers confirmed that eating less extends lifespan, the study found that the results are far less straightforward than previously believed. Some of the mice on the restricted low-calorie diets lived much shorter, for instance. And now we are left to question whether losing weight is truly the key to living longer.
A More Complex Picture of Diet and Aging
Gary Churchill, a professor at JAX and the study’s lead scientist, studied a very large number of mice with very high genetic diversity. This was to make the results more relevant to humans, which have a lot of individual variability.
Over several years, the mice were placed on five distinct diets. Some could eat freely, while others received 60% or 80% of their usual calories. Two additional groups experienced intermittent fasting, eating nothing for either one or two days a week but feasting on the other days.
As the mice lived out their days, researchers carefully tracked their health, taking blood samples, measuring metabolic markers, and, ultimately, counting the months they survived. The results were, in some cases, expected — but also perplexing.
Mice on unrestricted diets lived for an average of 25 months. The intermittent fasters lived a little longer, clocking in at around 28 months. But those who consumed only 80% of their baseline calories lived about 30 months. The group eating just 60% of their regular food intake — extreme calorie restriction — had the longest average lifespan of all, reaching 34 months.
One may conclude from these figures that eating less will help you live longer (at least if you’re a mouse). However, within each group, lifespans varied wildly. Some mice on the low-calorie diet only lived a few months. Others’ lives extended to an astonishing four and a half years. These extreme variations pointed to something the researchers hadn’t expected: genetics mattered far more than the diet itself.
“Our study really points to the importance of resilience,” Churchill said. “The most robust animals keep their weight on even in the face of stress and caloric restriction, and they are the ones that live the longest.”
When Weight Loss Isn’t Always a Good Thing
While the average lifespans were illuminating, the real revelations came when Churchill and his team dug deeper. The mice that lived the longest were not necessarily the ones who lost the most weight. In fact, the opposite was true. The animals that shed the most pounds — those that dropped body fat as their calorie intake plummeted — tended to have shorter lives. These mice, though slimmer, showed signs of lower energy, compromised immune systems, and impaired reproductive health.
“The most surprising part of our findings,” Churchill explained, “was that losing weight on caloric restriction is actually bad for lifespan.”
It’s a stark contrast to what many of us might assume: that leaner equals healthier, and healthier means a longer life. But in the world of these mice, those that managed to maintain their weight, despite consuming fewer calories, thrived. They didn’t suffer from the negative effects of extreme weight loss and seemed better equipped to withstand the stresses of life — whether from food scarcity or other environmental factors. This suggests that resilience — both in terms of metabolism and immune function — plays a bigger role in longevity than was previously understood.
In other words, it’s not just about eating less; it’s about how well your body can handle the challenge.
“If you want to live a long time, what you want is a very old grandmother”
One of the most profound takeaways from the study is the role of genetics. While caloric restriction had a clear overall effect on lifespan, the genetic differences between the mice were what really determined how long they lived. Some mice on the same diet lived months longer than others, and Churchill’s team found that certain genetic traits — particularly those related to immune system health and red blood cells — were better predictors of longevity than traditional metabolic markers like blood glucose levels or body fat percentage.
This finding shakes the foundations of how we think about diet and aging. For years, doctors and scientists have looked to markers like body fat, blood sugar, and metabolism to gauge how healthy someone is and to estimate their future longevity. But the JAX study reveals that these markers may not tell the full story.
Instead, resilience — the body’s ability to function properly in the face of stress — could be the true key to a long life. The mice that lived longest were those that didn’t lose body fat late in life, didn’t suffer drops in immune cell health, and maintained their body weight, even on restricted diets.
So, where does this leave us? The study’s results suggest that when it comes to human aging, there may be limits to what we can control. Yes, caloric restriction can help, but it’s not a one-size-fits-all solution. Genetic factors — like the ones that gave some mice an advantage in this study — also play an important role in how we age.
Or, as Churchill put it more simply: “If you want to live a long time, there are things you can control within your lifetime such as diet, but really what you want is a very old grandmother.”
Rethinking Diets and Longevity
The implications of these findings are broad, particularly when it comes to how we think about human health and longevity. Many popular diets, from intermittent fasting to extreme caloric restriction, are based on the idea that eating less leads to a longer, healthier life. But the new study suggests that losing weight—especially too much weight—on these diets could actually be harmful in the long run for some people.
If the key to longevity isn’t just about cutting calories, but about maintaining resilience under stress, it may mean that human trials need to rethink their focus.
“So when we look at human trials of longevity drugs and see that people are losing weight and have better metabolic profiles, it turns out that might not be a good marker of their future lifespan at all,” Churchill said.
For now, though, the exact mechanisms behind longevity remain elusive. Diet and genetics are clearly important pieces of the puzzle, but they aren’t the whole picture — and it’s still not clear how they interact.
As this study shows, sometimes living longer isn’t just about how much — or how little — you eat, but about how well your body weathers the storms.
The findings were published in the journal Nature.
