Diet is the key to a great many evolution-driven adaptations in the machinery of our bodies; changes in dietary intake cause the controlling mechanisms of metabolism to sit up and take notice. In particular, lowering the intake of calories by 30-40% or so, and while maintaining an optimal level of micronutrients, causes metabolic processes to operate in a mode that extends life and provides numerous other health benefits. Practiced as a lifestyle, this is known as a calorie restriction diet, and the laboratory version for animals is often called dietary restriction. Calorie (or dietary) restriction provides similar health and longevity benefits in almost every species tested to date.

Diet, of course, is more than a simple count of calories. Composition of food varies enormously, here thinking in terms of the proportions of different proteins, carbohydrates, fats, and so forth. This matters. Take methionine for example: this is one of the essential amino acids that we humans, and other mammals, cannot create and so must obtain through our diet. Through experimentation, researchers are starting to see that restricting the content of methionine in the diet of mammals - while leaving the calorie count unchanged - has many of the same results as restricting overall calories. Is the biological switch for the better state of human metabolism, that brought on by calorie restriction, essentially no more than a methionine sensor?

http://www.fightaging.org/archives/2010/01/methionine-restriction-as-the-cause-of-calorie-restriction-benefits.php