Not everyone who follows a calorie-restricted diet is hoping to lose weight. Some consciously choose fewer calories in hopes of living longer, as previous research has shown calorie restriction is an effective longevity-booster in rats and mice.

But new research suggests calorie-restriction alone may not crown you a centenarian, unless you simultaneously pay attention to your protein intake. Hmm, how much protein does 113-year-old Tomoji Tanabe eat? In animal longevity models, longer life is connected to lower levels of IGF-1 (insulin-like growth factor-1). When scientists compared IGF-1 measurements among short and long-term calorie restricters, regular exercisers and even a sedentary, Western diet group who changed nothing, IGF-1 levels were similar. Then the white coats took at look at long-term, strict vegans.

Vegans had significantly lower levels of IGF-1, even those heavier and fatter than calorie restricters. The vegans consumed about 10 percent of their calories from protein, while calorie-restricters were downing protein in the 23-24 percent zone. Here's the kicker, when six of the calorie-restricters agreed to try lowering their protein intake, their IGF-1 levels lowered dramatically in only three weeks. So to get in the running for Oldest Living Soul on the Planet, type of calories, not total calories may be key.