But here's the catch: The researchers also found that for older people, ages 65 and up, there may be a benefit to eating more protein. In this age group, higher levels of protein seemed to be protective against cancer and premature death.

So how might these age-dependent effects be explained? Well, Valter Longo, the director of the Longevity Institute at USC, who led the research, points to changes in the growth hormone known as IGF-I.

When we're young, IGF-1 help promotes growth, which is good. But as we age, too much protein in our diets may lead to overly high levels of IGF-1, which may contribute to aging and DNA damage, Longo explains.

Then, after 65, when IGF-I levels trail off, our bodies may benefit from more protein in the diet to help fend of frailty and decline.

"We've suspected that this is the case," Longo tells The Salt. In his view, most middle-aged Americans are eating too much protein.

The findings, he says, build on previous research he's done on IGF-I, including a study that looked at a group of Ecuadorians who had low levels of cancer and diabetes due to a genetic mutation that lowered levels of IGF-I.

In the new study, Longo and his colleagues found that high-protein foods derived from plants, such as beans and nuts, did not have the same effect on mortality as did high-protein foods from animals.

Singling out the effects of protein in the diet is hard to do. For starters, our diets are complex, and sussing out the independent effect of any one component is tough. What's more, surveying people on what they have eaten, as NHANES does, and then trying to figure out how that influences their health years later is a tricky business. So there are still lots of questions about how to interpret these findings.

In an age when advocates of the Paleo Diet and other low-carb eating plans such as Atkins talk up the virtues of protein because of its satiating effects, expect plenty of people to be skeptical of the new findings.

That said, as we've previously reported, several other studies have found a link between a high intake of red meat — especially processed meats like bacon and salami — and other animal proteins and an increased risk of mortality.

But could eating meat and cheese really be as bad for you as smoking, as the university news release describing the new Cell Metabolism paper suggested?

Well, that may be an exaggeration, according to Dr. Frank Hu, a researcher at the Harvard School of Public Health who studies the links between health, diet and lifestyle.

"The harmful effects of smoking on cancer and mortality are well-established to be substantial, while the harmful effects of red meat consumption are modest in comparison," Hu wrote to us in an email.

For instance, in a study Hu authored, people who ate a serving of red meat every day had a 13 percent increased risk of mortality, compared with those who ate little meat. By comparison, people who swapped out red meat for alternative sources of protein cut their risk of premature death. Choosing chicken and other poultry decreased the risk by 14 percent, fish decreased the risk by 7 percent and legumes decreased the risk by 10 percent.

So the debate about how much protein is ideal — and from which sources — will go on.

In the meantime, if you're feeling confused, consider the one strategy that almost all experts agree on: moderation.

The simplest way to maintain a healthy body weight and cut the risk of so many weight-related diseases is to limit calories.

So eat what you enjoy. Upsize servings of greens and other vegetables. And downsize servings of meat, cheese and other high-calorie foods.

Copyright 2014 NPR.

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Eat Plants and Prosper: For Longevity, Go Easy on the Meat, Study Says