Six-year study of men in Oakland, Calif., implicates carcinogen in cooked meat as risk factor

Photo of charred-cooked chicken Since 2001, more than 600 African-American men have participated in the study by answering detailed dietary questions, including how frequently they eat chicken, beef, pork and fish.

African-American men consume about twice as much PhIP, a carcinogen in cooked meat, as white men — and the greater their PhIP consumption, the higher their bloodstream concentrations of prostate-specific antigen, a marker for early prostate cancer.

The findings — which may help explain why black men die from prostate cancer at more than twice the rate of white men — stem from an ongoing study of diet and prostate cancer risk in African-American men in Oakland, Calif. The six-year-old study is led by a team of scientists from Lawrence Livermore National Laboratory and UC Davis Cancer Center. The scientists report their preliminary findings in the journal Prostate Cancer and Prostatic Diseases.

"We're increasingly confident that we're onto something," said lead author Kenneth Bogen, an environmental scientist at Lawrence Livermore and member of the UC Davis Cancer Center research program. "The hope is that we will really be able to tell people how to lower their risk."

Since 2001, more than 600 African-American men have participated in the study by answering detailed dietary questions, including how frequently they eat chicken, beef, pork and fish, in what quantities, using which cooking methods and to what degree of doneness. The study uses standardized food-model photographs to help participants report portion sizes and a set of meat-doneness descriptors and meat-doneness photographs to help standardize their reports of cooking preferences.

Interviews take place at the Markstein Cancer Education and Prevention Center at Alta Bates Summit Medical Center in Oakland and are conducted by trained dietary interviewers.

Study volunteers also have a blood test to measure PSA levels and a physical exam to look for signs of prostate cancer.

Study results

In the article, the scientists report results from the first 392 study participants. The researchers found that on average, the study volunteers consumed 17 nanograms of PhIP per kilogram per body weight per day — about double the consumption estimates reported for age-matched white men in the United States.

Moreover, the Oakland men who consumed PhIP at levels of 30 ng/kg per day or more were 30 times more likely to have a highly elevated level of the prostate-specific antigen known as PSA (defined as 20 or more nanograms of the antigen per milliliter of blood) when compared to those in the study who consumed PhIP in quantities of 10 ng/kg per day or less. The association remained significant even after researchers accounted for family history, saturated fat intake and total calories consumed, and it was strongest for men over age 50.

A PSA of less than four is generally considered normal.

Type of meat and cooking method explained about 89 percent of the differences in PhIP intake among study participants. Doneness preference explained the remaining 11 percent. Chicken accounted for 61 percent of the men's overall PhIP consumption, followed by hamburger, steak, pork, fish and bacon.

PhIP — short for 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine — is one of the heterocylcic amines, a family of potent mutagens formed in meats, chicken and fish during cooking.

James Felton, co-leader of UC Davis Cancer Center's Cancer Etiology, Prevention and Control Program and division leader of the Biosciences Directorate at Lawrence Livermore National Laboratory, was among the first scientists to discover PhIP and other heterocyclic amines in cooked meats in the 1970s. He and his team went on to show that the compounds begin to form when meat temperatures reach 350 degrees Fahrenheit, that at the same temperature, chicken has more than beef, pork or fish, and that cooking time greatly increases PhIP concentration — a gram of chicken cooked to medium has 5.9 nanograms of the carcinogen, for example, while a gram of extra well-done (blackened) chicken has 69.0.

In 2002, Felton was awarded a five-year, $1.5 million grant from the National Cancer Institute to explore the human cancer risk posed by PhIP and other heterocyclic amines. That grant, along with a three-year, $600,000 grant from the U.S. Department of Defense Cancer Research Program to Bogen, has funded the Oakland study.

"Our conclusions remain preliminary due to the small number of men in this prospective study to date," Felton said. "But they are supported by the consistency of the pattern of results observed and their level of statistical significance. It is possible that meat-cooking preferences alone may explain most or all of the disparity in prostate cancer incidence and mortality affecting African-American men."

UC Davis Cancer Center, a program of the University of California, Davis, is the nation's 61st National Cancer Institute center, serving a region of more than 6 million people in inland Northern California. Its joint research program with Lawrence Livermore National Laboratory, the first uniting a national laboratory and a major cancer center, includes more than 280 scientists on three campuses: the UC Davis Health System in Sacramento, Calif., UC Davis in Davis, Calif., and Lawrence Livermore National Laboratory in Livermore, Calif.