Recent study results suggest that calorie restriction may not be as effective at extending human life as it is at extending the lifespan of rodents.

Numerous animal studies have suggested that calorie restriction can significantly lengthen lifespan. Exactly how calorie restriction promotes longevity remains unclear, although pathways involving insulin-like growth factor-1 (IGF-1) have been implicated in many animal models, with levels of IGF-1 dropping by as much as 40% in some animals on a calorie restricted diet.

With this is mind, Luigi Fontana, assistant professor of medicine at Washington University at St Loius, and colleagues studied whether calorie restriction in humans had a similar effect on IGF-1 levels to that seen in animals. Their results showed that calorie restriction had no effect at all upon IGF-1 levels in humans.

This troubled the researchers as they could not understand why such different results had been obtained in rodents. However, the researchers also knew that protein intake can influence IGF-1 levels, so they began to investigate what effect eating less protein would have. This led the researchers to discover that protein does indeed have a major impact on IGF-1 levels. They found that IGF-1 levels plummeted dramatically if people following a calorie-restricted diet obtained just 10% of their calories from protein.

These findings suggest that humans wanting to lengthen their lifespan should possibly be thinking about lowering protein consumption instead of cutting calories.  "It's much easier to restrict protein than to restrict calories," Fontana says in a news release issued by Washington University. "If our research is on the right track, maybe humans don't need to be so calorie restricted. Limiting protein intake to 0.7 or 0.8 grams per kilogram per day might be more effective. That's just a hypothesis. We have to confirm it in future studies."

Fontana L, Weiss EP, Villarreal DT, Klein S, Holloszy JO. Long-term effects of calorie or protein restriction on serum IGF-1 and IGFBP-3 concentration in humans. Aging Cell. 2008;7. Doi: 10.1111/j.1474-9726.2008.00417.x

News release: Researchers note differences between people and animals on calorie restriction. Washington University in St. Louis. September 24th 2008.