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"There's a
misconception that because we all eat, understanding nutrition is
simple. But 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
corresponding author Valter Longo, the Edna M. Jones Professor of
Biogerontology at the USC Davis School of Gerontology and director of
the USC Longevity Institute.
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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, reveals the study to be published March 4 in
Cell Metabolism. Protein-lovers were 74 percent more likely to die of
any cause within the study period than their more low-protein
counterparts. They were also several times more likely to die of
diabetes.
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But 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.
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Rather
than look at adulthood as one monolithic phase of life, as other
researchers have done, the latest study considers how biology changes as
we age, and how decisions in middle life may play out across the human
lifespan.
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In other
words, what's good for you at one age may be damaging at another.
Protein controls the growth hormone IGF-I, which helps our 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.
The study shows that while high protein intake during middle age is
very harmful, it is protective for older adults: those over 65 who ate a
moderate- or high-protein diet were less susceptible to disease.
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The
latest paper draws from Longo's past research on IGF-I, including on an
Ecuadorian cohort that seemed to have little cancer or diabetes
susceptibility because of a genetic mutation that lowered levels of
IGF-I; the members of the cohort were all less than five-feet tall.
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"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."
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Crucially,
the researchers found that plant-based proteins, such as those from
beans, did not seem to have the same mortality effects as animal
proteins. 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.
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"The
majority of Americans are eating about twice as much proteins as they
should, and it seems that the best change would be to lower the daily
intake of all proteins but especially animal-derived proteins," Longo
said. "But don't get extreme in cutting out protein; you can go from
protected to malnourished very quickly."
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Longo's
findings support recommendations from several leading health agencies to
consume 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
45-50 grams of protein a day, with preference for those derived from
plants such as legumes, Longo explains.
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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.
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Even
moderate amounts of protein had detrimental effects during middle age,
the researchers found. Across all 6,318 adults over the age of 50 in the
study, average protein intake was about 16 percent of total daily
calories with about two-thirds from animal protein—corresponding to data
about national protein consumption. The study sample was representative
across ethnicity, education and health background.
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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,
the study shows. Overall, even the small change of decreasing protein
intake from moderate levels to low levels reduced likelihood of early
death by 21 percent.
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For a
randomly selected smaller portion of the sample – 2,253 people – levels
of the growth hormone IGF-I were recorded directly. The results show
that for every 10 ng/ml increase in IGF-I, those on a high-protein diet
were 9 percent more likely to die from cancer than those on a
low-protein diet, in line with past research associating IGF-I levels to
cancer risk.
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The
researchers also extended their findings about high-protein diets and
mortality risk, looking at causality in mice and cellular models. In a
study of tumor rates and progression among mice, the researchers show
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 the 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."
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