Mother's Travails May Appear in Offspring's DNA
Puzzled over health complications more persistent among African Americans than in other Americans, researchers invoke epigenetics.
For years, scientists have wondered why African Americans die of cardiovascular disease and diabetes complications at much higher rates than European Americans. Despite advances in health care and living conditions, African Americans succumb to heart disease at a rate 1.3 times higher than that of whites.
But even after accounting for income differences and known risk factors, researchers find there is still an unexplained "racial" disparity.
Some scientists have argued that the difference must be due to some underlying genetic difference between races, while others contend that the cause is fundamentally social: low incomes, psychosocial stress and persistent racial discrimination give rise to chronic health problems that take their toll in higher mortality rates.
Recently, a third explanation has emerged. Some anthropologists now think the adverse social and physical conditions a mother faces while pregnant can change the way her developing fetus's DNA is expressed, setting the stage for lifelong health problems.
Further, they argue, these deficits can actually be passed down through several generations, from grandmother to daughter to granddaughter in a phenomenon sometimes called "fetal programming." A European researcher has even proposed that contemporary African Americans are still suffering health consequences from the extreme privations of slavery and in the pervasive discrimination following emancipation.
"A lot of research has jumped to the conclusion that anything that can't be explained by the environment must relate back to the genome," said Christopher Kuzawa, a Northwestern University anthropologist, who with colleague Elizabeth Sweet published a paper called, "Epigenetics and Embodiment of Race: Developmental Origins of US Racial Disparities in Cardiovascular Health" in the American Journal of Human Biology.
"I just don't think the research supports that," Kuzawa said. In fact, Kuzawa and Sweet don't have much use for "race" itself as a biological concept. They write, "[W]e define race as a socially constructed category that has biological implications, rather than a genetically justified criteria for classifying human variation."
Data show African-American children are born with lower birth weight on average than their European-American counterparts —and often somewhat preterm. "What has been shown is if you're born with a low birth weight, you are at increased risk later in life for high blood pressure, diabetes and cardiovascular disease," Kuzawa said.
The question is, why are African-American children more likely to be born underweight in the first place?
Kuzawa thinks it is due to the cumulative, transgenerational impacts of chronic social stress. Developmental biologists specializing in a field called epigenetics have shown that the genetic legacy encoded in our DNA doesn't tell the whole story, he says. "What is modifiable is which of these genes is expressed," he says. Kuzawa doesn't deny the direct impact of racism and discrimination, which are bound to be stressful. "Stress is bad for you," he said, and by itself, stress is implicated in problems like high blood pressure and cardiovascular disease.
"What's new is the realization that when women experience this, especially when they're pregnant, that it affects their children," Kuzawa said. Stress and poor prenatal nutrition can lead to underweight babies by impairing the blood flow to the developing child and exposing them to the stress hormone cortisol. Scientists also know, for example, that individuals who were undernourished in the womb tend to have smaller kidneys and fewer nephrons, predisposing them to hypertension and renal failure.
Moreover, when an underweight child becomes a mother herself, she is more likely to suffer from hypertension and other problems that will hamper her unborn child's development. "These findings suggest that the intrauterine environment experienced by one generation (the mother) can influence the intrauterine environment that she creates for her offspring, in theory helping perpetuate certain biological or metabolic states, albeit in a fading fashion, across multiple matrilineal generations," Kuzawa and Sweet wrote.
Grazyna Jasienska, a reproductive ecologist specializing in women's fertility at Jagiellonian University in Krakow, Poland, has taken these ideas a step further. She believes that the persistence of underweight African-American babies may have its roots in the conditions of slavery their ancestors endured from the 17th through the 19th centuries. In effect, African Americans continue to carry a biological burden as a result of slavery.
Jasienska's paper, "Low Birth Weight of Contemporary African Americans: An Intergenerational Effect of Slavery?" also published in the American Journal of Human Biology, shares Kuzawa's emphasis on the importance of maternal health to the developing fetus. She noted, for example, that studies of egg donations show that the child's birth size is more closely correlated with the stature of the recipient than that of the donor.
She also acknowledges that contemporary living conditions play a powerful role in determining birth weight, but she points out that in comparison with low-income white women, low-income African-American women still have smaller babies. Poverty by itself does not fully explain the differences, she found. Jasienska decided to look further back in time.
"African Americans have a long, multigenerational history of nutritional deprivation, excessive workloads, and poor health due to years of slavery and the postslavery period of economic hardship," she wrote.
Combing through historical data, Jasienska found that 84 percent of slave children began working before the age of 11. Their heights, preserved in slave ship records, suggests that they suffered from terrible nutrition, she said. Meanwhile, slave women often did strenuous work in the fields while pregnant, placing further stresses on their unborn offspring. Nor did things improve in the decades after slavery was outlawed. "Some people say former slaves in fact became worse off with the share-cropping system," Jazienska said.
Like Kuzawa, Jasienska dismisses racial explanations for contemporary differences in birth weight. Because most African Americans trace their ancestry to West Africa, one would expect that if a racial difference was at work West African women would also have underweight children, but that isn't the case. "When they come to the United States, they have babies with much higher birth weight than African Americans," Jasienska said.
Jasienska acknowledges it's hard to verify this hypothesis. "Fortunately, slavery is gone, so it's hard to get data on calories that were expended."
Kuzawa, meanwhile, says the slavery theory is "very provocative and interesting," but still thinks the pervasiveness of racism in our society goes a long way to explain these health differences. "The overarching punch line of this work is that it's emphasizing the primacy of environmental impacts," he said. "It's good news, in the sense that you can change the environment — you can't change the genome."
But he added, "I don't think there is going to be a quick fix here . . . If we want to improve the health of future generations, we need to start working now."
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