Cut calories to live longer – Sensational headline or robust science?

Posted on July 16th, 2009 in Diet, Food, Health, Science

Monkey in Captivity You probably have read the headlines this week ‘Eat less and live longer’. Most of these sensationalising headlines written in the press, appear to have been copied from the scientist’s own press release without the journalist even reading the actual research paper. Researchers from Wisconsin National Primate Research Centre have published research in the journal Science suggesting that consuming fewer calories leads to a longer, healthier life.

Even the study’s title is misleading ‘Calorific restriction delays disease onset and mortality in Rhesus monkeys’ as the authors conclude that overall when reducing calories in the diet mortality is in the predicted direction but not statistically significant.

The theory behind the study is valid. Since the first studies of caloric restriction in rodents in the 1930′s, scientists have been intrigued by evidence that reducing calories can effectively extend lifespan.

This study involved a colony of 30 female and 46 male monkeys over a 20 year period, who were introduced to the study when they were adults (7-14yrs old). Rhesus macaques have an average life span in captivity of 27 years which can reach up to 40 years old.

Initially the animals were allowed to eat freely from a low fat diet containing 15% protein and 10% fat. For 3-6months the baseline daily food intake was collected for each animal, with half the monkeys allowed to continue eating what they liked as a control for the calorie restricted (CR) subjects who had their diet intake reduced by 30%.

Of the original 76 animals, 37% (14 out of 38) of control animals died of age related deaths compared to 13% (5/38) of the calorie restricted animals, suggesting that free eating animals had 3 times the rate of death from age related causes compared to animals who were calorie restricted.

Seven control and nine calorie restricted animals died from what the scientists called non age related causes, with the effect of CR on overall mortality in the predicted direction but not statistically significant (p=0.16). These non-age related causes included complications of anaesthesia, gastric bloat and endometriosis. The problem with scientific studies is that you can’t just choose which results you want to include. If you are testing a drug to treat cancer but your subjects die from a heart attack you can’t just ignore those results from the study, you have to include all subjects you intended to treat no matter how or why they left the study. The deaths in this trial could be due to adverse reactions to the reduced diet, so shouldn’t be dismissed.

In addition, the study never mentions how many calories the monkeys ate and how it equates to a human diet. The control group didn’t eat a recommended daily amount for monkeys but were instead allowed to eat freely. How many calories would humans eat if they were allowed to eat freely and had no knowledge of the risks of over eating? Without knowing the recommended calorie intake for women, I’m not sure that I would restrict myself to 2000 calories a day, and looking around the street there are quite a few people who don’t restrict their diet. So it wouldn’t take much stretching of the imagination to think that a given a free choice a monkey could happily eat as much as the equivalent of 2600 calories. Maybe the study is actually comparing over eaters with those eating a healthy amount.

There are benefits that can be concluded from the study with the incidence of cancerous tumours and cardiovascular disease in animals on a restricted diet less than half that seen in animals allowed to eat freely. Also while diabetes was common in monkeys that can eat all they want, it wasn’t observed in any animal on a restricted diet. There are clearly benefits from the restricted calorie diet but whether it can be claimed it delays mortality may be stretching the study’s findings to date. What can be concluded is that before you believe all you read in the press, no matter how prestigious the publication, you may wish to read the scientific paper yourself.