By Alexandra Sifferlin
February 12, 2015

We’re accustomed to thinking about diets as a short-term fix for unwanted weight gain, but eating for a long, healthy life requires a different approach. The priority should be a diet that prevents illness–and especially heart disease, the No. 1 killer in the U.S. Many experts believe that means a diet high in vegetables, whole grains and some fat. But a meal plan for longevity might also mean cutting back on protein–and, some experts say, reducing calories overall.

Many experts look to Europe–to the Mediterranean, specifically–for dietary secrets to a long life. While some debate remains about what people in the region actually ate, there’s near consensus about the benefits of fish, fruits and vegetables and extra-virgin olive oil.

In 2009 researchers randomly assigned 7,447 people at high risk for heart disease to one of three diets: the Mediterranean diet, with lots of olive oil; the Mediterranean diet, with extra nuts; and a low-fat control diet. Those who followed one of the versions of the Mediterranean diet, which was high in fat, had about a 30% lower risk of having a heart attack or stroke and a similar reduction in risk of dying of heart disease after five years. The findings were so impressive, the study ended early. (With results that strong, it’s considered unethical to withhold the advantageous approach from the other groups.) The findings were published in 2013 in the New England Journal of Medicine.

It’s impossible to parse which nutrients, exactly, produced the benefits. Many experts think it’s the result of the foods in combination. And at odds with some nutrition trends, the healthy diet was also relatively low in protein, which provided on average just 17% of daily calories, compared with up to 35% in the standard American diet. Also raising questions about protein is a 2014 study in Cell Metabolism. It showed that middle-aged Americans who ate a lot of animal protein were more likely to die of cancer and other causes, compared with people who opted for more plant-based protein. Study author Valter Longo, director of the University of Southern California’s Longevity Institute, recommends that people cut down on protein overall to live longer.

That advice may raise eyebrows, since many diets for weight loss, including the popular paleo diet, advocate high protein. “There’s a misconception that it’s O.K. to eat a lot of it,” Longo says. “People don’t understand it could lead to some major aging factors.” One such factor is the impact of the growth hormone IGF-1 (insulin-like growth factor 1). While it’s important for early development, getting too much from high-protein foods later may accelerate aging.

Longo and others were tipped off to the possibilities when studying a rural population in Ecuador with a genetic mutation that keeps their IGF-1 very low. They found that IGF-1-deficient people are typically short in stature but also rarely get diseases that tend to hit people as they age, like cancer or Type 2 diabetes. Limiting animal protein is a way to lower IGF-1 and keep aging effects at bay, says Longo.

A sizable camp of nutrition scientists also say we should cut back on how much we eat overall, with some recommending intermittent fasting–alternating between regular food consumption and short periods of eating almost nothing. Others say a diet with about 25% fewer calories than normal may extend life, as has been shown in many animal studies. In humans, studies have found that significantly reducing calorie consumption may reduce cardiovascular-disease risk–which could, in turn, impact longevity.

One thing experts can agree on is that we’d all benefit from less sugar, particularly added sugar in the form of fructose. A 2015 study in Mayo Clinic Proceedings pinpointed added fructose as the primary driver of Type 2 diabetes, which has reached epidemic proportions in the U.S.

For now, unsatisfying though it may be, the bottom line is that more research is needed before any one diet can be heralded as the key to a long, healthy life. But a diet low in sugar and high in plants, nuts, fruit, fat and some protein is a good bet. Just be sure to add the other secret ingredient too: exercise.

Contact us at editors@time.com.

This appears in the February 23, 2015 issue of TIME.

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