Are High Protein Diets Bad For You?
Is there increased cancer risk with increased protein?
A great deal of research connects nutrition with cancer
risk. Overweight people are at higher risk of developing
post-menopausal breast cancer, endometrial cancer, colon
cancer, kidney cancer and a certain type of oesophageal
cancer. Now preliminary findings from researchers at
Washington University School of Medicine in St. Louis
suggest that eating less protein may help protect against
certain cancers that are not directly associated with
The research, published in the December issue of the
American Journal of Clinical Nutrition, shows that lean
people on a long-term, low-protein, low-calorie diet or
participating in regular endurance exercise training have
lower levels of plasma growth factors and certain hormones
linked to cancer risk.
"However, people on a low-protein, low-calorie diet had
considerably lower levels of a particular plasma growth
factor called IGF-1 than equally lean endurance runners,"
says the study's first author Luigi Fontana, M.D., Ph.D.,
assistant professor of medicine at Washington University and
an investigator at the Istituto Superiore di Sanità in Rome,
Italy. "That suggests to us that a diet lower in protein may
have a greater protective effect against cancer than
endurance exercise, independently of body fat mass."
The study involved three groups of people. The first ate a
low-protein, low-calorie, raw food vegetarian diet and was
made up of 21 lean men and women. Another group consisted of
21 lean subjects who did regular endurance running,
averaging about 48 miles per week. The runners ate a
standard Western diet, consuming more calories and protein
than group one. The third group included 21 sedentary people
who also consumed a standard Western diet, higher in sugars,
processed refined grains and animal products. The subjects
were matched for age, sex and other demographic factors, and
no one smoked or had diabetes, cardiovascular disease,
cancer, lung disease or other chronic illness.
Protein intake was, not surprisingly, lowest in the
low-protein group. They averaged a daily intake of 0.73
grams of protein per kilogram of body weight. Endurance
runners ate 1.6 grams and sedentary people on the Western
diet, 1.23 grams. The recommended daily allowance for
protein intake is 0.8 grams. That's about three ounces of
protein per day for a 220-pound man.
"It's interesting to us that both the runners and especially
the sedentary people consumed about 50 percent more protein
than recommended," says Fontana. "We know that if we consume
50 percent more calories than recommended, we will become
obese. But there is not a lot of research on whether chronic
over-consumption of protein also has harmful effects."
Fontana and colleagues found significantly lower blood
levels of plasma insulin-like growth factor 1 (IGF-1) in the
low-protein diet group than in either the equally lean
runners or the sedentary people eating a standard Western
diet. Past research has linked pre-menopausal breast cancer,
prostate cancer and certain types of colon cancer to high
levels of IGF-1, a powerful growth factor that promotes cell
proliferation. Data from animal studies also suggest that
lower IGF-1 levels are associated with maximal lifespan.
"Our findings show that in normal weight people IGF-1 levels
are related to protein intake, independent of body weight
and fat mass," Fontana says. "I believe our findings suggest
that protein intake may be very important in regulating
He calls the study a hypothesis-generating paper that
suggests connections between dietary protein and
epidemiological studies that show associations between IGF-1
levels and the risk of cancer. But he says more research is
needed to clarify what that connection is.
The researchers also found that the group of endurance
runners in the study consumed the highest number of
calories, averaging more than 2,600 per day. Those on a
standard Western diet consumed just over 2,300 calories
daily, while those in the low-calorie, low-protein group ate
just under 2,000 calories a day. Members of the latter group
also tended to weigh less than sedentary people but slightly
more than the endurance runners. The average body mass index
(BMI) in the low-protein, low-calorie group was 21.3. BMI
averaged 21.1 among the runners and 26.5 among those who
were sedentary. BMI is a measurement of weight divided by
height squared. People with a BMI greater than 25 are
Fontana says most of us don't eat nearly enough fruits and
vegetables or enough whole-grains, cereals or beans. "Many
people are eating too many animal products — such as meat,
cheese, eggs and butter — as well as refined grains and free
sugars," he says. "Our intake of vegetables and fruits is
low, and beans are vastly underconsumed in the U.S. and
Europe these days. "
He believes diets would be healthier if we ate more whole
grains, beans, fruits and vegetables and far fewer animal
products. He recommends mostly fish, low-fat dairy products
and, occasionally, some red meat. Such a diet would both cut
total calories and reduce the amount of protein we consume
to a level closer to the range recommended by the nutrition
experts of the Food Nutrition Board of the National Academy
of Sciences. It also might result in lower levels of IGF-1.
"Eating too many calories increases our risk of developing
obesity, diabetes, cardiovascular disease and of certain
types of cancer related to obesity," Fontana says. "We hope
to further clarify what happens to cancer risk when we are
chronically eating more protein than we need."
Fontana L, Klein S, Holloszy JO. Long-term low-protein,
low-calorie diet and endurance exercise modulat metabolic
factors associated with cancer risk. American Journal of
Clinical Nutrition, vol. 84; pp. 1456-1462, December 2006.
Funding from the National Institute of Diabetes and
Digestive and Kidney Diseases and the National Center for
Research Resources of the National Institutes of Health
supported this research.
Note: This story has been adapted from a news release issued
by Washington University School of Medicine.