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.
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."