People who eat a high-protein diet during middle age are more likely to die of cancer than those who eat less protein, a new study finds. 
 
 However, for people older than 65, a moderate protein intake may actually be beneficial, and protect against frailty, the researchers said. 
 
 The researchers looked at more than 6,000 people ages 50 and older, and followed them for 18 years. They found that people ages 50 to 65 who ate a diet rich in animal proteins during middle age were more than four times as likely to die of cancer during the study period than those who ate a low-protein diet, according to the study published today (March 4) in the journal Cell Metabolism. 
 
 "Popular diets in many cases have high proteins and low sugars. They may make you lose some weight, but that's not a good diet to increase life span," said study researcher Valter Longo, professor of biology at the University of Southern California (USC) and director of the USC Longevity Institute. 
 
 The risks of a high-protein diet are even comparable to smoking, the researchers said. People who smoke are four times more likely to die of cancer compared with nonsmokers, Longo said. 
 
 "Of course we cannot be sure of the reasons, but we have a pretty good idea, based on this study and also previous studies, as to why this is happening," Longo said. 
 
 The study also found that middle-age people who ate foods rich in animal proteins — including meat, milk and cheese — were 75 percent more likely to die of any cause than those who ate a low-protein diet within the study period. [5 Diets That Fight Diseases] 
 
 The researchers defined a "high-protein" diet as deriving at least 20 percent of daily calories from protein, a "moderate" protein diet as deriving 10 to 19 percent of calories from protein, and a "low-protein" diet as less than 10 percent of calories from protein. 
 
 However, what's bad for people at one age may be healthy at another. In the study, people older than 65 were less likely to die of cancer or other causes if they consumed more protein. 
 
 "So, not all people benefit from low protein. Older people actually seem to benefit from moderate intake of protein," Longo said. Having enough protein in the diet may be important for older people to maintain a healthy weight and protect against frailty, the researchers said. 
 
 The link between high-protein intake and risk of cancer almost vanished when the researchers considered participants whose protein mainly came from plants, such as beans. 
 
 This may be because proteins in plants have a different composition, and don't stimulate growth hormones as efficiently as meat proteins, Longo said. 
 
 The study suggests that the Mediterranean diet, which is low in animal protein and high in carbohydrates, may be best for extending life span, Longo said. 
 
 The researchers said they recommend eating about 0.8 grams of protein per kilogram of body weight every day in middle age. For example, a 130-pound person should eat about 45 to 50 grams of protein a day, with a preference for plant proteins, the researchers said. 
 
 To investigate why there may be a link between protein consumption and cancer, the researchers looked at mice that were injected with mouse skin cancer cells. They showed that a high amount of protein in the mouse diet promoted the tumor's survival and growth by increasing the levels of a growth hormone, called IGF-1. 
 
 The researchers also measured the levels of IGF-1 for 2,000 people, randomly selected among those in their study, and found that for every 10 nanograms per milliliter increase in IGF-1, those on a high-protein diet were 9 percent more likely to die of cancer than those on a low-protein diet. 
 
 "When you have a lot of protein, these growth factors go up, and we've shown that they help normal cells become cancer-like cells, and then they help the cells grow," Longo said. 
 
 Email Bahar Gholipour. Follow us @LiveScience, Facebook & Google+. Original article on Live Science. 

 People who eat a high-protein diet during middle age are more likely to die of cancer than those who eat less protein, a new study finds. 
 
 However, for people older than 65, a moderate protein intake may actually be beneficial, and protect against frailty, the researchers said. 
 
 The researchers looked at more than 6,000 people ages 50 and older, and followed them for 18 years. They found that people ages 50 to 65 who ate a diet rich in animal proteins during middle age were more than four times as likely to die of cancer during the study period than those who ate a low-protein diet, according to the study published today (March 4) in the journal Cell Metabolism. 
 
 "Popular diets in many cases have high proteins and low sugars. They may make you lose some weight, but that's not a good diet to increase life span," said study researcher Valter Longo, professor of biology at the University of Southern California (USC) and director of the USC Longevity Institute. 
 
 The risks of a high-protein diet are even comparable to smoking, the researchers said. People who smoke are four times more likely to die of cancer compared with nonsmokers, Longo said. 
 
 "Of course we cannot be sure of the reasons, but we have a pretty good idea, based on this study and also previous studies, as to why this is happening," Longo said. 
 
 The study also found that middle-age people who ate foods rich in animal proteins — including meat, milk and cheese — were 75 percent more likely to die of any cause than those who ate a low-protein diet within the study period. [5 Diets That Fight Diseases] 
 
 The researchers defined a "high-protein" diet as deriving at least 20 percent of daily calories from protein, a "moderate" protein diet as deriving 10 to 19 percent of calories from protein, and a "low-protein" diet as less than 10 percent of calories from protein. 
 
 However, what's bad for people at one age may be healthy at another. In the study, people older than 65 were less likely to die of cancer or other causes if they consumed more protein. 
 
 "So, not all people benefit from low protein. Older people actually seem to benefit from moderate intake of protein," Longo said. Having enough protein in the diet may be important for older people to maintain a healthy weight and protect against frailty, the researchers said. 
 
 The link between high-protein intake and risk of cancer almost vanished when the researchers considered participants whose protein mainly came from plants, such as beans. 
 
 This may be because proteins in plants have a different composition, and don't stimulate growth hormones as efficiently as meat proteins, Longo said. 
 
 The study suggests that the Mediterranean diet, which is low in animal protein and high in carbohydrates, may be best for extending life span, Longo said. 
 
 The researchers said they recommend eating about 0.8 grams of protein per kilogram of body weight every day in middle age. For example, a 130-pound person should eat about 45 to 50 grams of protein a day, with a preference for plant proteins, the researchers said. 
 
 To investigate why there may be a link between protein consumption and cancer, the researchers looked at mice that were injected with mouse skin cancer cells. They showed that a high amount of protein in the mouse diet promoted the tumor's survival and growth by increasing the levels of a growth hormone, called IGF-1. 
 
 The researchers also measured the levels of IGF-1 for 2,000 people, randomly selected among those in their study, and found that for every 10 nanograms per milliliter increase in IGF-1, those on a high-protein diet were 9 percent more likely to die of cancer than those on a low-protein diet. 
 
 "When you have a lot of protein, these growth factors go up, and we've shown that they help normal cells become cancer-like cells, and then they help the cells grow," Longo said. 
 
 Copyright 2014 LiveScience , a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. 

 Consuming high levels of protein — particularly animal protein — is a bad strategy if you’re at midlife and aiming to live into old age, new research finds. But a study out Tuesday reveals that in older age, fortifying one’s diet with more protein-rich foods appears to be a formula for extending life. 
 
 An article published in the journal Cell Metabolism says that, over an 18-year study period, middle-aged Americans who had the highest consumption of protein were more than four times as likely to die of cancer or diabetes, and twice as likely to die of any cause, than those whose diets were lowest in protein. 
 
 But a high-protein diet had the opposite effect on Americans 66 and older, a group of Americans and Italian researchers found. Those whose diets were highest in protein were 60% less likely to die of cancer and 28% less likely to die of any cause than were those whose protein intake was lowest. 
 
 “Your stage in life matters,” said biogerontologist Valter Longo, director of USC’s Longevity Institute and lead author of the paper, which explores the role of dietary protein from the cellular level to the population level. “Some have said for years that proteins are bad. That’s half right and half wrong.” 
 
 Tapping a national database of 6,381 Americans’ health and nutrition behaviors, Longo’s team found that in people between the ages of 50 and 65, following a diet in which protein accounted for 20% or more of daily calories consumed increased the risk of death during the 18-year study period to levels comparable to the effect of smoking cigarettes. 
 
 Whether the remainder of those younger individuals’ diet was dominated by fat or carbohydrates made no difference to the outcome. But the source of the protein mattered a great deal: for those whose sources of protein were heavily plant-based — nuts and legumes — the increased risk of dying of cancer declined and the increased risk of all-cause mortality disappeared altogether. 
 
 Among the study’s older subjects, by contrast, the source of proteins was less important. What was important, said Longo, appeared to be that those entering a period of growing frailty reduced their loss of weight and muscle mass with a higher intake of a nutrient that helps sustain and build both. 
 
 The article by the U.S.-Italian team is the culmination of two decades of study that has coaxed the researchers to look for clues well outside the lab: Longo and his team have studied the dietary habits of a little town in Italy with a high concentration of centenarians, and have traveled to Ecuador to gather information on an exceptionally long-lived family congenitally deficient in a growth hormone linked to cancer. 
 
 The findings of Longo’s team are in line with mounting research on the hazards of heavy consumption of red meats and the protective effects of plant-based nutrients. But unlike many large-population studies that have found links between poor health outcomes and animal protein consumption, the current study identified the potentially pivotal role that a hormone called insulin-like growth factor-1, or IGF-1, plays in driving age-related diseases such as cancer. 
 
 In a subset of the study’s human subjects who submitted their blood for analysis, as well as in laboratory mice, Longo’s team found that heavy protein consumption in middle age drove up levels of IGF-1. In the group’s rodent experiments, higher IGF-1 levels — whether induced by high protein consumption or genetic engineering — promoted rapid cancerous growth when the researchers implanted 20,000 cancer cells under the animals’ skin: 100% of these mice developed tumors. 
 
 Among mice that were fed a low-protein diet through middle age — and which therefore had lower IGF-1 levels — tumor formation was 10% to 30% lower. 
 
 At the same time, the researchers uncovered evidence that older mice were less able to absorb or process proteins. When they were fed high-protein diets, the older mice tended to maintain or increase their weight — a factor that appeared to keep them from becoming frail. By contrast, older mice fed a low-protein diet lost weight; that, says Longo, appeared to make them more vulnerable to diseases of aging. 
 
 IGF-1, produced largely in the liver, is essential to normal human growth and to the process of cell replacement after injury . But it is also implicated in a wide range of disorders associated with overgrowth and cell proliferation, including heart enlargement, the runaway cell growth common to all cancers, and obesity. 
 
 But while IGF-1’s role in promoting tumor growth has long been recognized, its link to a high-protein diet appears to be a new. 
 
 “People will say, ‘Here we go again: First you attack the fats, then you attack the carbohydrates, now it’s the protein,’ ” Longo said. But over 20 years of research linking heavy protein consumption to diseases of aging, and eventually to higher IGF-1 levels, he said, “we never changed our mind”: Americans’ protein-packed diets “are hurting them in a major way.” 
 
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– Good news and bad news for meat lovers: People who eat a diet high in protein in middle age are a staggering four times more likely to die of cancer than people on a low-protein diet, according to new research, but people over 65 who consumed more protein were less likely to die of cancer. The study, which tracked 6,000 people over 18 years, found that people between 50 and 65 with a diet rich in animal protein were 75% more likely to die of any cause over the study period as well as much more likely to die of cancer, Scientific American reports. But for the elderly, higher protein intake helped fight off frailty. "Your stage in life matters," the lead researcher tells the Los Angeles Times. "Some have said for years that proteins are bad. That’s half right and half wrong." The researchers believe the link between protein consumption and cancer is related to a growth hormone stimulated by high protein consumption, reports LiveScience. The source of protein mattered for the middle-aged people studied, with the risk of cancer going down steeply in those who consumed mainly plant-based proteins instead of animal proteins, though the beneficial effects of protein for the elderly weren't affected by the source.