A subject near and dear to my heart. It's not so much how much (within reason), but what you eat. And you've gotta expend!

Severe Diet Doesn’t Prolong Life, Study of Monkeys Concludes

Lab test results showed lower levels of cholesterol and blood sugar in the male monkeys that started eating 30 percent fewer calories in old age, but not in the females. Males and females that started dieting when they were old had lower levels of triglycerides, which are linked to heart disease risk. Monkeys put on the diet when they were young or middle-aged did not get the same benefits, though they had less cancer. But the bottom line was that the monkeys that ate less did not live any longer than those that ate normally. 

I'd really like to see studies around the stress levels in the "semi-starved" monkeys vs. the well-fed monkeys. I'm surprised this factor isn't addressed.