Is this really the final word on whether calorie-restricted diets make you live longer?

The monkey on the left was given a diet with fewer calories while the monkey on the right was allowed to eat as much as it liked. Jeff Miller/University of Wisconsin-Madison

A new study claims to have settled the debate on calorie restriction and longevity, but it is a complex read and far from definitive.

We have known for many years that obesity is associated with many diseases, including type 2 diabetes, heart disease, liver disease and arthritis. So it should come as no surprise that scientists have wondered whether the opposite is also true – that is, whether calorie restriction confers health and survival benefits. It is, of course, impossible to conduct a randomised controlled trial in humans to test this calorie restriction-survival hypothesis. Who would agree to go on such a diet for years on end?

So, to find out if restricting calories does lead to better health and longer lives compared with a normal diet, we need to look to animal studies for evidence. Here there is plenty of evidence from rodent studies that show a survival benefit for calorie restriction. Unfortunately, rodents are far from human and many findings in rodents do not replicate when tested in people. So calorie restriction studies in other animals, specifically monkeys, would be more powerful. The problem with these types of studies is they are not easy to do, given the need for very long periods of intervention and follow-up.

Fortunately, groups in the US have performed two such studies that have been the subject of debate as one showed convincing evidence for survival benefits of calorie restriction and the other did not. To resolve these apparently different findings, the authors of the two studies went back to their original data and re-analysed the findings taking into account differences in design of the studies. In doing so, they worked out that the study that showed no initial overall benefit of calorie restriction on survival included a proportion of much younger monkeys, a group in whom calorie restriction seemed to show harm.

The authors have concluded that their combined analysis has resolved the debate once and for all and that there is clear evidence of benefit of calorie restriction when applied to adult monkeys.

Interesting but not definitive

Although this new paper is interesting, it has a couple of potential flaws. First, the number of monkeys in the studies was small so it’s difficult to draw firm conclusions from the analyses. Second, re-analysis of data is often biased, especially when scientists have firm pre-held beliefs about the true direction of the findings, as they appear to have done in this case.

Overall, though, it’s an interesting paper that provides a new hypothesis for why the two studies differed – calorie restriction being harmful in the young and beneficial in adults – but it is far from definitive. Sadly, further trials are required, but, as mentioned above, they are very hard to do.

So many tempting calories. hin255/

So what does this study mean for human health? Are there many people trying to restrict calories to live longer? Not many people who are a healthy weight would be persuaded by the evidence to restrict their calories to gain further survival or health benefits. By contrast, most people are fighting a daily battle against tempting calories thrust in their direction and are trying to cut calories from a baseline of being either overweight or obese.

In most wealthy countries, this represents more than 50% of the population. These people know that losing weight will help lessen their risks of getting type 2 diabetes, lower their blood pressure, improve their mobility and, perhaps most importantly, make them feel happier and improve their self-esteem.

To lose weight, calorie restriction is indeed needed for nearly all people, but there are many ways to achieve this, including the 5:2 diet which essentially calorie restricts on two days per week. So, for many, whether or not the primate data supports survival benefits of calorie restriction is a moot point. Most simply want to find ways to make sustainable changes to their lifestyles to stop or slow weight gain or, if possible, lose a few pounds.