NewScientist: Eat less, live longer?

... Fontana has found a notable difference in the way people and animals respond to calorie restriction, and it is not great news. It involves a hormone made by the liver called insulin-like growth factor 1.

IGF-1 has emerged as an important promoter of ageing. IGF-1 levels are lower than normal in worms, flies and mice on a restricted diet, and this is thought to be at least partly responsible for their longer lifespan. When it comes to people, however, CRONies have the same IGF-1 levels as the rest of us.

The explanation for this anomaly may lie in a new theory about how diet affects ageing. This says that it may not only be the drop in calories that is responsible for lifespan extension - in some species at least, perhaps it is also the accompanying drop in dietary protein.

One piece of evidence for this idea comes from studies in fruit flies and rodents. If these animals are fed special diets with less amino acids - the building blocks of proteins - they can eat as many calories as they want and still live longer. "These results clearly show that you don't need to restrict calories as a whole to get lifespan extension," says Piper, an author of the study on flies (Nature, vol 462, p 1061).

Further support for this idea comes from studying the molecular pathways inside cells that affect lifespan. A molecule called TOR has been found to set off a chain of reactions that boost cell growth at the expense of longevity. Blocking TOR increases lifespan in all organisms studied to date, including yeast and mice (Aging Cell, vol 9, p 105). Crucially, the most potent activators of TOR are amino acids.

Where does the protein theory leave the CRONies? Fontana noticed that the people in his study group were eating high levels of protein, about 1.7 grams per kilogram of body weight per day. This is more than the US government-recommended intake of 0.8 g/kg/day, and higher than that in a typical American's diet, about 1.2 g/kg/day.

So Fontana asked six CRONies to cut their protein intake to 0.95 g/kg/day while maintaining their usual calorie intake. After only three weeks on the low-protein diet, the CRONies showed a 25 per cent drop in their levels of IGF-1 (Aging Cell, vol 7, p 681). "Even if the CRONies are restricting their calories severely, if they're eating a high-protein diet, they're probably negating some of the most important beneficial effects," says Fontana.

If the new theory is right, then the whole concept of calorie restriction needs to be rethought. The very term would be misleading; Fontana and others have started referring to dietary restriction instead. As news of the study has spread, some CRONies have already reduced their protein intake.
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