Does too much protein in the diet increase cancer risk?

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"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 cancer risk."

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 considered overweight.

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."

Washington University School of Medicine in St. Louis