March 14, 2012
African Americans face higher cancer rates than whites for many types of cancer, but the reasons why are largely unknown. Epidemiologist Lisa Signorello hopes to help explain the disparities in her role as co-principal investigator on a long-term study of nearly 86,000 people living in the southeastern United States—two-thirds of whom are African American.
Signorello—associate professor in the department of medicine’s epidemiology division at Vanderbilt University and the Vanderbilt-Ingram Cancer Center, senior epidemiologist at the International Epidemiology Institute (IEI), and a 1998 HSPH graduate—spoke to a Harvard School of Public Health audience about the 10-year-old Southern Community Cohort Study (SCCS) on February 23, 2012.
Signorello and her colleagues, from Vanderbilt, Meharry Medical College, and IEI, have used the extensive data collected from the study participants to conduct numerous sub-studies on several factors that may play a role in racial disparities in both cancer and mortality risk, including Vitamin D levels, menthol cigarette smoking, and proteins called adipokines. Signorello’s co-principal investigator is cancer epidemiologist William Blot, professor of medicine at Vanderbilt University School of Medicine and IEI chief executive officer.
Life circumstances may contribute to the high cancer and cancer-related mortality rates faced by African Americans, Signorello said, including poor living conditions, social stressors, lack of access to health care, exposure to carcinogens, or poor health habits. Genetic or epigenetic factors may also be involved.
The role of socioeconomic status
Signorello and her colleagues have uncovered some intriguing preliminary findings, including the suggestion that household income may have more to do with mortality than race. Although national mortality rates among African Americans are higher than those for whites, no racial differences have been evident in the Southern cohort. “What we do see,” Signorello said, “are very alarming associations between total household income and mortality. The pattern holds for blacks, whites, men, and women.”
Lower Vitamin D levels
Vitamin D may play an important role in protecting against cancer and enhancing immune function. African Americans have lower levels than whites, partly due to darker skin pigmentation, which blocks the synthesis of Vitamin D that comes from sunlight, as previous studies have shown. Examining further, Signorello and her colleagues found that genes may also be involved; study participants with greater percentages of African ancestry had lower levels of Vitamin D.
In addition, African Americans who consumed foods and supplements containing Vitamin D—fish, eggs, fortified cereals and milk, or vitamin pills—did not appear to benefit as much as whites. There is a “blunting effect,” both of sunlight- and diet-generated Vitamin D, among those with high African ancestry, Signorello said.
Studying a sample of the cohort (about 3,600 people), SCCS researchers also found that mortality risk increased significantly as Vitamin D levels decreased. And the association was stronger among whites than African Americans.
“No matter how you slice it, the overall message from this study seems to be that Vitamin D may be an important predictor of mortality, both among African Americans and non-African Americans,” said Signorello.
Looking at menthol
Even though African Americans smoke fewer cigarettes per day than whites, they have significantly higher rates of both lung cancer and lung cancer-related mortality. Because African Americans smoke mostly menthol cigarettes, researchers have speculated that menthol may somehow increase exposure to harmful chemicals in tobacco smoke. But, so far, Signorello and her colleagues have found no link between African Americans’ menthol use and their blood levels of cotinine, a biomarker for tobacco smoke.
Some have also theorized that it’s harder to quit smoking menthol cigarettes than non-menthol, but Signorello’s group found no such evidence. They also found—contrary to existing literature—that African Americans are just as likely to quit smoking as whites.
In the future, Signorello and her colleagues plan to examine a host of other possible predictors of smoking cessation—such as attitudes about smoking and its risks, knowledge of smoking cessation strategies, neighborhood attributes, and genetic factors.
Proteins called adipokines are associated with some of the most common chronic diseases of adulthood, such as diabetes, metabolic syndrome, cardiovascular disease, and cancer, Signorello said. But a number of studies have shown that African Americans have a significantly worse adipokine profile than whites.
An unfavorable adipokine profile is related to obesity, which affects African Americans in greater numbers than whites. A healthy diet and exercise can effect positive changes in adipokine levels, but those lifestyle changes are often difficult to maintain. And obesity explains only a fraction of variability in adipokine levels among the population, Signorello said.
Some foods and supplements—magnesium, fiber, dairy, and nuts—can improve adipokine levels, but, according to preliminary data from the SCCS, only in whites. Signorello wants to know why. In the future, she and colleagues—including Frank Hu, Eric Rimm, and Donna Spiegelman of HSPH—plan to study whether racial differences in adipokine levels can be explained by dietary factors, genetic factors, or some interaction between the two.
Closing the gap
By examining factors such as Vitamin D, menthol cigarette smoking, adipokines, and more, Signorello and her colleagues hope to increase understanding of why African Americans face such markedly higher cancer and cancer-related mortality rates than other racial groups. “How all of these factors contribute to differential cancer rates by race—which are probably different for different types of cancer—needs to be understood if we hope to close the current gap,” said Signorello.