If we can develop methods that can be applied in the same way for everybody, we could measure stress in much the same way we measure cholesterol levels,” says Brisa Sánchez, an assistant professor of biostatistics who’s working to develop improved methods for characterizing and analyzing salivary cortisol, a hormone whose levels reflect long-term exposure to stress.
Unlike adrenaline, a second human hormone whose levels fluctuate throughout the day and provoke the “fight or flight” response we commonly associate with the onset of stress, salivary cortisol levels typically rise in the morning and then decrease as the day progresses in what Sánchez describes as a circadian rhythm. But when someone is repeatedly exposed to stressful situations, the rise-and-ebb pattern of salivary cortisol levels changes. So by measuring those changes, scientists can effectively measure accumulated stress.
Together with fellow SPH faculty members Ana Diez-Roux and Trivellore Raghunathan and Ph.D. candidate Meihua Wu, Sánchez is developing tools to help scientists design study protocols to capture the most accurate cortisol measures possible—and more effectively analyze the data they collect. Sánchez notes that while some studies have shown an association between elevated cortisol levels in the evening and depressive symptoms, and other studies have shown an association between the rapidity of declining cortisol levels in the morning and subclinical measures of cardiovascular disease, still other studies have shown no links at all. “If we’re to better characterize these associations, we need to develop methods that can better characterize stress, give us unbiased inferences, and squeeze more information from the data,” she says. “Statistical methods are crucial to that process.
Thanks to decades of global lead pollution—particularly from leaded gasoline—anyone over 30 “is carrying around substantial deposits of lead in their skeleton,” says Howard Hu, NSF International Department Chair of the Department of Environmental Health Sciences. Recent research suggests that stress can deepen the ill effects of lead and other toxicants on human health, and Hu and his colleagues are conducting molecular epidemiology studies to test that hypothesis. By accurately measuring toxicant exposures and using a variety of tools—including both questionnaires and biological measures of salivary cortisol—to assess individual stress levels, they hope to shed new light on the interrelationships between stress, socioeconomic status, and race and ethnicity “so we can better understand the ultimate implications for health disparities,” Hu says.
As part of an ongoing study in Mexico, Hu and his associates have found that children who are exposed to lead and who have mothers with low self-esteem—a condition associated with higher stress—fare more poorly on neurodevelopmental tests at age two than children who are exposed to lead but have mothers with high self-esteem. “This is independent of any impact of socioeconomic status,” Hu explains, “and it seems consistent with animal experimental studies indicating that a stressful stimulus will magnify the impact of lead on neurodevelopment.”
In a separate study, Hu and his colleagues have found that aging men who are exposed to both lead and stress have a 2.7-times greater risk of developing hypertension, neurodevelopment, neurodegeneration, type 2 diabetes, and obesity.