Members of the Calorie Restriction Society call themselves CRONies. They believe they can add years to their lives by eating 1800 calories per day. But is cutting calories the key to a long life, or just another fad?

Studies on mice and rats give CRONies some scientific evidence to back their claims. Rodents given thirty to fifty percent less food live up to fifty-percent longer. But no one knew if the diet had the same effect on humans until recently.

Researchers believe that animals on restricted diets live longer because of an insulin-like growth factor called IGF-1, which is made in the liver. As food intake goes down in rodents, so does the blood level of IGF-1. So, one way to figure out if humans are benefiting from the diet is by measuring their IGF-1.

Scientists did just that. They took blood samples from CR Society members and compared them to people on different diets. They found that CRONies didn't have lower IGF-1 levels. As a matter of fact, vegans who were heavier and had more body fat than CRONies, had the lowest levels of IGF-1.

Puzzled, scientists looked at the nutrient that was low in the vegan diet-protein. Six CRONies agreed to decrease the protein in their diet and after three weeks their IGF-1 levels dropped dramatically. This suggested that it might be lower protein intake and not fewer calories that extends life spans.

The final word is still out on calorie restricted diets. Longevity may come not from fewer calories, but from eating less protein. More research is needed.