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 esophageal cancer.
Now preliminary findings from School of Medicine researchers suggest that eating less protein may help protect against certain cancers that are not directly associated with obesity.
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 the plasma growth factor called IGF-1 [insulin-like growth factor 1] than equally lean endurance runners,” said the study’s first author Luigi Fontana, M.D., Ph.D., assistant professor of medicine and an investigator at the Istituto Superiore di Sanità in Rome. “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.
Subjects in the low-protein group 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 ate 1.23 grams. The recommended daily allowance for protein intake is 0.8 grams, or about three ounces of protein daily 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,” Fontana said. “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 IGF-1 (a powerful growth factor that promotes cell proliferation) 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. 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 said. “I believe our findings suggest that protein intake may be very important in regulating cancer risk.”
Fontana said 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 slightly more than 2,300 calories daily, while those in the low-calorie, low-protein group ate slightly fewer than 2,000 calories a day.
Members of the low-calorie, low-protein 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. Among the runners, BMI averaged 21.1, and among those who were sedentary, 26.5. BMI is a measurement of weight divided by height squared. People with a BMI greater than 25 are considered overweight.
Fontana said 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 said. “Our intake of vegetables and fruits is low, and beans are vastly underconsumed in the U.S. and Europe these days. ”
He said 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.