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, according to new research.
That chicken wing you're eating could be as deadly as
a cigarette. In a new study that tracked a large sample of adults for nearly
two decades, researchers have found that eating a diet rich in animal proteins
during middle age makes you four times more likely to die of cancer than
someone with a low-protein diet -- a mortality risk factor comparable to
smoking.
"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.
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.
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.
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.
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.
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.
"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."
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.
"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."
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.
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. 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.
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.
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.
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.
"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."
Story Source:
The above story is based on materials provided by University
of Southern California. The original article was written by
Suzanne Wu. Note: Materials may be edited for content and
length.
