Recent research show that protein restriction may be far more effective than calorie restriction in prolonging the lives of humans. Many studies show that restricting calories prolongs the lives of yeast, worms, spiders, flies, insects, rats and probably monkeys. Humans who severely restrict calories have long-life characteristics, such as low cholesterol and blood pressure and hearts that are more than 15 years younger than those of other North Americans their age (Experimental Gerontology, August 2007).
However, most of the test group of humans who restrict calories do not have a drop in a hormone called Insulin-Like Growth Factor-1 (IGF-1) that appears necessary for living a long time. High blood levels of IGF-1 are associated with premature aging and diseases of aging such as diabetes and cancer. IGF-1 levels are lower than normal in worms, flies and mice on restricted-calorie diets, but not in humans. This week a report shows that IGF-1 shortens life by increasing cell DNA genetic damage, and causes cancer by blocking apoptosis that causes cancer cells to kill themselves before they destroy their host (Science Translational Medicine, February 16, 2011).
Luigi Fontana, a professor of medicine at Washington University in St Louis, noticed that most calorie-restricting humans eat high levels of protein, about 1.7 grams per kilogram of body weight/day. This is more than the US government-recommended intake of 0.8 g/kg/day, and even higher than the 1.2 g/kg/day that the average American eats. Dr. Fontana asked humans on calorie restricted diets to reduce their intake of protein to 0.95 g/kg/day. After just three weeks of reduced protein intake, their IGF-1 levels dropped markedly (Aging Cell, September, 2008).
Among the calorie-restricting humans, vegans have lower levels of IGF-1 than meat-eaters (Rejuvenation Research, February 2007). Strict vegans also have significantly lower IGF-1 levels than people who restrict just calories, even if they are heavier and have more body fat. Strict vegans take in about 10 percent of their calories from protein, whereas those on calorie restriction tended to get 24 percent of calories from protein. Other data show that diets lower in protein might protect against some cancers. So restricting protein may be more important than restricting calories.
If fruit flies and rodents are fed special diets that restrict protein, they can eat as many calories as they want and still live longer (Nature, December 2009). This suggests that as long as you are not overweight, you may not need to restrict calories. Instead, restrict only protein which is far easier to do.
Furthermore, you can probably eat all the fruits and vegetables you want and not restrict calories as long as you restrict protein. That's very good news because it is far easier to restrict protein than it is to restrict all foods. The only way that you can restrict calories and still remain healthy is to eat a diet based on vegetables. It now appears that you extend your life far more by reducing protein that you would by restricting just calories.