http://news.usc.edu/#!/article/59199/meat-and-cheese-may-be-as-bad-for-you-as-smoking/
By Suzanne Wu
March 4
, 2014That
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, 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, revealed the study published
today 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 one 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 life span.
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 5-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, holder of 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 to 50 grams of protein a day, with preference for those
derived from plants such as legumes, Longo explained.
The researchers defined 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 to 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
backgrounds.
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 showed. 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
showed 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
showed 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 protein
intake.
Morgan Levine, Jorge Suarez and Pinchas Cohen of USC Davis were
co-authors of the study. The research was funded by the National
Institute of Aging of the National Institutes of Health (grants numbers
AG20642, AG025135, AG034906, P30AG017265 and T32AG0037) and a USC Norris
Cancer Center pilot grant given to Longo.
