Beau96080 /Wikimedia Commons

Meat and cheese are featured -- in the extreme -- in this Quad Stacker from Burger King.Beau96080 /Wikimedia Commons

Middle-aged adults who eat a lot of protein are four times more likely to die of cancer compared to people who eat much less protein, found a new study.

Deaths from diabetes and other causes were also higher in avid protein-eaters, particularly those who ate meat and dairy products as opposed to plant proteins.

Despite a strong belief for many people that a meal isn't a meal without a hunk of protein, the new findings join a growing body of research suggesting that most of us would be better off easing up on the chicken and cheese and opting for more grains and produce instead.

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"When people eat less than 10 percent of their calories from proteins, they are very well protected," said Valter Longo, director of the University of Southern California Longevity Institute in Los Angeles. "If the numbers we are showing are correct, this impact is as big as quitting smoking."

In the midst of the Paleo movement and the Atkins craze, evidence has been slowly accumulating for decades that dietary protein might have a detrimental effect on health through its influence on a growth hormone called IGF-1.

Studies show that eating protein can raise levels of IGF-1 in the body. But when levels of the hormone are reduced through diet or mutations, roundworms live several times longer than normal. Mice become much less likely to succumb to aging-related diseases.

And in a group of people in Ecuador who are deficient in the hormone, Longo's group has found, cancer and diabetes are virtually non-existent.

To see how protein consumption might affect long-term human health, Longo and colleagues turned to a database of health information called The National Health and Nutrition Examination Survey (NHANES). The researchers were particularly interested in middle-aged adults because they had noticed a shift in their lab rodents: at some point later in life, protein restriction went from helpful to harmful.

The team collected self-reported data on the diets of about 6,000 people. Half were between the ages of 50 to 65 when the study started. The rest were over 65. Then, the researchers looked at how many people from each group had died 18 years later and what the causes of death were.

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When people consumed less than 10 percent of their calories from protein, the researchers report today in the journal Cell Metabolism, they were about 25 percent less likely to die from cancer and about half as likely to die from any cause compared to people who ate more than 20 percent of their calories from protein.

Effects were worse when the bulk of protein came from animals than from plants. (Fish, Longo added, seems to offer more benefits than harm).

After age 65, though, restricting protein turned out to be detrimental to health. That's probably because IGF-1 levels tend to naturally decrease as we age, Longo said, causing a "pushing over the edge effect" that leads to weight loss and frailty. Past the age of 65, he said, it's probably best to eat more protein.

It might seem to the general public that science demonizes each food group, one at a time, leaving us with nothing to eat, Longo said. And taken alone, the new findings might not be strong enough to condemn protein as the next evil food group.

If you're middle-aged, you may not need that steak as much as you think. Peter Bagi/Corbis

But along with other findings, both in the new study and elsewhere, he said, there is a convincing story that leads from high protein consumption to high IGF-1 levels, which leads to rapid aging and DNA damage in cells.

Participants in the new study ate an average of 16 percent of their calories as protein, with 75 percent of those proteins coming from animal sources. According to the new findings, most people would do well to reduce their protein consumption by half.

"What I'm recommending is to switch as much as you can to plant-based protein and going to 0.36 grams of protein per pound of body weight," Longo said, which translates to 54 g for a 150-pound person.

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An egg has about six grams of protein. A cup of milk has 8 grams. There are 36 g in a five-ounce filet of cooked salmon. And there are 42 g in a cup of beans.

"One easy way to do it is to look at labels for a couple of months, which is what I do, and you just get used to it," he said. "The great majority of people can do it."

When people eat very little protein, some essential amino acids become so limiting that the process of cell division slows down, said Gerald Krystal, senior scientist at the BC Cancer Agency Research Centre. For children, the result would be restricted growth, which is one reason why kids still need plenty of protein. But in adults, slowed cell division is a good thing if it prevents or slows the growth of tumors and progression of cancer.

As for the difference between plant and animal proteins, Krystal suspects that it's not the nature of the proteins that matters but the package they come in. He thinks it's too soon to universally recommend protein restriction.

"Animal proteins typically come with a lot of saturated fat, while plant proteins come with a lot of healthy fiber," he said. "I don't think there is convincing evidence to limit protein in midlife, especially if plant-based. As a rule, limit red meats and cured meats, but I think it is very safe to eat high levels of fish and tofu and nuts like almonds."