Only one thing really helps you live longer, but diet helps, study finds

Eating less long-term is more likely to help people live longer than periodic fasting, according to new research. Consuming fewer calories had a greater impact on lifespan than fasting, say scientists.

They concluded that "more moderate" level of calorie restriction might be the way to balance long-term health and living longer following a study of mice. But losing weight may not be an indicator of a longer lifespan.

One surprising finding of the American study, published in the journal Nature, was that mice that lived the longest on restrictive diets were those that lost the least weight despite eating less. The mice that lost the most weight on restrictive diets tended to have low energy, compromised immune and reproductive systems, and shorter lives.

For nearly a century, lab studies have shown consistent results: eat less food, or eat less often, and an animal will live longer. But researchers have struggled to understand why restrictive diets work to extend lifespan, and how to best implement them in humans.

For the new study, scientists at The Jackson Laboratory (JAX) in Bar Harbor, Maine, tracked the health of more than 900 mice on a range of diets. The research was designed to ensure that each mouse was genetically distinct, which allowed the team to better represent the genetic diversity of humans.

The study concluded that eating fewer calories had a greater impact on lifespan than periodic fasting, revealing that very-low-calorie diets generally extended the mice’s lifespan regardless of their body fat or glucose levels - both typically seen as markers of metabolic health and aging. Research leader Professor Gary Churchill, of JAX, said: "Our study really points to the importance of resilience. 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.

"It also suggests that a more moderate level of calorie restriction might be the way to balance long-term health and lifespan.”

Prof Churchill and his colleagues assigned female mice to any of five different diets: one in which they could freely eat any amount of food at any time, two in which the animals were provided only 60% or 80% of their baseline calories each day, and two in which the animals were not given any food for either one or two consecutive days each week but could eat as much as they wanted on the other days.

The mice were then studied for the rest of their lives with periodic blood tests and extensive evaluation of their general health. Overall, mice on unrestricted diets lived for an average of 25 months, those on the intermittent fasting diets lived for an average of 28 months, those eating 80% of their normal diet lived for an average of 30 months, and those eating 60% of their previous diet lived for 34 months.

But within each group, the range of lifespans was wide with mice eating the fewest calories, for example, having lifespans ranging from a few months to 4.5 years. When the research team analysed the rest of their data to try to explain the wide range, they found that genetic factors had a "far greater" impact on lifespan than diets.

They say their findings highlight how underlying genetic features, yet to be identified, play a major role in how these diets would affect an individual person’s health trajectory. The team pinpointed genetically-encoded resilience as a "critical" factor in lifespan; mice that naturally maintained their body weight, body fat percentage and immune cell health during periods of stress or low food intake, as well as those that did not lose body fat late in life, survived the longest.

Prof Churchill said: “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."

The study also cast doubt on traditional ideas about why some diets can extend life. For example, factors such as weight, body fat percentages, blood glucose levels and body temperature did not explain the link between cutting calories and living a longer life.

Instead, the study found that immune system health and traits related to red blood cells were more clearly connected to lifespan. The researcher say their findings mean that human studies of longevity - which often use metabolic measurements as markers for ageing or youthfulness – may be overlooking more important aspects of healthy ageing.

Prof Churchill added: “While caloric restriction is generally good for lifespan, our data show that losing weight on caloric restriction is actually bad for lifespan. 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.”