There is so much to be bummed out about in middle age - your lost youth, lost hopes, lost hair, lost waistline, lost keys - so here's just what we need, one more thing: Middle-aged people who eat a diet high in meat and cheese are nearly twice as likely to die and four times as likely to die of cancer, according to a new study by USC. Not sugar. Not fat. Not carbs. In fact, the mortality risk of eating all that meat and cheese is similar to that of smoking.
That sound you just heard was France wailing.
Pooh-poohing faddish high-protein diets, "The question is not whether a certain diet allows you to do well for three days, but can it help you survive to be 100?" said study co-author Valter Longo, the Edna M. Jones Professor of Biogerontology at the USC Davis School of Gerontology and director of the USC Longevity Institute.
The study tracked the dietary habits of 6,318 adults over the age of 50 for almost two decades. It found that not only is excessive protein consumption linked to a dramatic rise in cancer mortality, but middle-aged people who eat lots of proteins from animal sources - including meat, milk and cheese - are also more susceptible to early death in general. In fact, protein-lovers were 74 percent more likely to die of any cause within the study period than their counterparts who ate a low-protein, plant-based diet. They were also several times more likely to die of diabetes.
The study was published March 4 in Cell Metabolism.
"How much protein we should eat has long been a controversial topic - muddled by the popularity of protein-heavy diets such as Paleo and Atkins. Before this study, researchers had never shown a definitive correlation between high protein consumption and mortality risk," the scientists say in a press release.
Paradoxically, the study also found that moderate protein intake after age 65 is good for you. Older adults who ate a moderate- or high-protein diet were less susceptible to disease.
"Rather than look at adulthood as one monolithic phase of life ... the latest study considers how biology changes as we age, and how decisions in middle life may play out across the human lifespan," the researchers write. "In other words, what's good for you at one age may be damaging at another."
They point out that protein controls the growth hormone IGF-I, which helps bodies grow but has been linked to cancer susceptibility. Levels of IGF-I drop off dramatically after age 65, leading to potential frailty and muscle loss, which protein can help.
"The research shows that a low-protein diet in middle age is useful for preventing cancer and overall mortality, through a process that involves regulating IGF-I and possibly insulin levels," said co-author Eileen Crimmins, the AARP Chair in Gerontology at USC. "However, we also propose that at older ages, it may be important to avoid a low-protein diet to allow the maintenance of healthy weight and protection from frailty." That's good news for bacon-and-egg-loving old folks.
Importantly, the researchers found that plant-based proteins, such as those from beans, did not have the same mortality effects as animal proteins in the middle-aged. Furthermore, rates of cancer and death also did not seem to be affected by controlling for carbohydrate or fat consumption, "suggesting that animal protein is the main culprit."
"The majority of Americans are eating about twice as much protein as they should," Longo said. "But don't get extreme in cutting out protein; you can go from protected to malnourished very quickly."
The researchers define a "high-protein" diet as deriving at least 20 percent of calories from protein, including both plant-based and animal-based protein. A "moderate" protein diet includes 10-19 percent of calories from protein, and a "low-protein" diet includes less than 10 percent protein.
Even moderate amounts of protein had detrimental effects during middle age, the researchers found. People who ate a moderate amount of protein were still three times more likely to die of cancer than those who ate a low-protein diet in middle age. Even decreasing protein intake from moderate levels to low levels reduced the likelihood of early death by 21 percent.
The researchers also extended their study to tumor rates and progression among mice and found lower cancer incidence and 45 percent smaller average tumor size among mice on a low-protein diet than those on a high-protein diet by the end of their two-month experiment.
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"Almost everyone is going to have a cancer cell or pre-cancer cell in them at some point. The question is: Does it progress?" Longo said. "Turns out one of the major factors in determining if it does is is protein intake."
So eat your legumes, stand back and watch all those smug hipsters on the Paleo diet ("I only eat bison and macadamia nuts!") drop dead before they hit 60.