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High Protein Diets May Boost Cancer Risk

Excess protein may also shorten life span, study suggests

THURSDAY, Dec. 7, 2006 (HealthDay News) -- Eating a low-protein diet may protect against certain cancers, while a diet high in protein may increase the risk for malignancies, a new study suggests.

The results of this preliminary study show that lean people on a long-term, low-protein, low-calorie diet or who participate in regular endurance exercise training have lower levels of plasma growth factors and certain hormones such as insulin-like growth factor 1 (IGF-1). These substances have been linked to an increased cancer risk, especially premenopausal breast cancer, prostate cancer and certain types of colon cancer.

"We know there is a link between nutrition and cancer," said lead author Dr. Luigi Fontana, an assistant professor of medicine at Washington University in St. Louis. "There are certain cancers that are linked with levels of IGF-1, which is an important growth factor that stimulates the proliferation of cells."

If there are high levels of IGF-1, there's a greater chance that mutated cells will become cancer cells, said Fontana, who's also an investigator at Istituto Superiore di Sanita, in Rome, Italy. "We found that people following a low-calorie, low-protein diet have lower IGF-1 than lean athletes who eat a Western diet. This suggests that low protein intake may reduce IGF-1, independent of body weight," he said.

The study is published in the December issue of the American Journal of Clinical Nutrition.

For the study, Fontana's team looked at three groups of people. The first, made up of 21 lean men and women, ate a low-protein, low-calorie, raw food, vegetarian diet. The second group of 21 people did regular endurance running, averaging about 48 miles a week. These runners ate a standard Western diet that included more calories and protein than the first group. The third group included 21 sedentary people who also consumed a standard Western diet, higher in sugars, processed refined grains and animal products.

People in the first group averaged a daily intake of 0.73 grams of protein per kilogram of body weight. The runners ate 1.6 grams, and sedentary people ate 1.23 grams of protein a day. The recommended daily allowance for protein intake is 0.8 grams, Fontana said. That's about three ounces of protein per day for a 220-pound man.

The researchers found that people in the first group had significantly lower blood levels of IGF-1 compared with the runners or the sedentary people. High levels of IGF-1 have been linked to premenopausal breast cancer, prostate cancer and certain types of colon cancer.

In addition, lower IGF-1 levels are associated with increased life span in animals, Fontana noted.

Fontana thinks that if people ate more whole grains, beans, fruits and vegetables and far fewer animal products, they would be healthier. He recommends fish, low-fat dairy products and, occasionally, some red meat. This type of diet reduces total calories and the amount of protein consumed, and it also might result in lower levels of IGF-1.

"Many people in the United States and Italy are eating 50 percent more protein than what is recommended," Fontana said. "If we eat 50 percent more calories than recommended, we become overweight and obese. What happens if you eat 50 percent more protein than required -- we don't know."

He speculated that eating too much protein increases the risk for cancer and also accelerates aging, "but we need more studies to see if my hypothesis is true or false."

One expert also thinks that a high-protein diet increases the risk for certain cancers.

"We recently published a paper that also shows that a high-protein diet is bad for you. It reduces survival; it increases the risk of cancer," said Dr. Dimitrios Trichopoulos, the Vincent L. Gregory Professor of Cancer Prevention at the Harvard School of Public Health's Department of Epidemiology.

In that study, published online in the Nov. 29 issue of the European Journal of Clinical Nutrition, the researchers collected data on the diets of 22,944 healthy adults and found that eating diets low in carbohydrates and high in protein was associated with increased mortality.

Trichopoulos thinks that levels of IGF-1 may be the reason for the increased cancer risk. However, other factors may be at work, he added.

Despite his and Fontana's findings, Trichopoulos isn't ready to recommend a low-protein diet to reduce the risk of cancer or to live longer. Another recent study contradicted this finding, Trichopoulos said. "At this stage, we should wait for clarification," he said.

More information

The American Cancer Society can tell you more about cancer and diet.

SOURCES: Luigi Fontana, M.D., Ph.D., assistant professor of medicine, Washington University, St. Louis, and investigator, Istituto Superiore di Sanita, Rome, Italy; Dimitrios Trichopoulos, M.D., Vincent L. Gregory Professor of Cancer Prevention, Department of Epidemiology, Harvard School of Public Health, Boston; December 2006, American Journal of Clinical Nutrition; Nov. 29, 2006, online edition, European Journal of Clinical Nutrition
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