It's not surprising that people who are overweight and obese benefit from reducing their Calorie intake to shed the excess fat tissue. By contrast, it's more than a little counterintuitive that Calorie restriction is also beneficial to animals (and possibly humans) who are already lean to begin with. In animal studies, CR is one of only a very small number of interventions that actually slow down the degenerative aging process, and its "anti-aging" benefits kick in exactly starting from the point at which the animals' Calorie intake is reduced below the level required to sustain their "normal" weight, and continue to become more powerful as Calories are cut further and further — right up to the limits of starvation.
Even more surprisingly, there's also evidence that animals whose metabolism does a better job of "defending" their fat tissue as their Calories are cut actually benefit more from CR than animals who shed the weight easily. In a recent thread in the CR Society Forum, we discuss some of the latest evidence on this phenomenon.