Twin study evaluates role of environmental stress in cardiovascular disease

August 27, 2001

Whether environmental stress is a risk factor for cardiovascular disease is the focus of a study at the Medical College of Georgia following more than 500 pairs of twins. "We are looking at the role of environmental stress as a risk factor," said Dr. Frank Treiber, director of MCG's Georgia Prevention Institute and principal investigator on the four-year, $1.4 million continuation grant from the National Institutes of Health.

"Stress, at the moment, is considered a candidate risk factor, not an established one, for cardiovascular disease," he said. "We want to try and clear up its role, to see if indeed those who are under chronic stress show greater elevations in their blood pressure in response to stress and then whether they, over time, show increases in their resting blood pressure as well as deleterious changes in their heart structure and function," Dr. Treiber said

MCG researchers have been following the African-American and European-American identical and fraternal twins for four years to determine how genetics and environment affect the twins' blood pressure at rest and in response to stress. When the twins started the study, their average age was 14; they will be 18, on average, as they start the new study.

Researchers have monitored their blood pressure, heart rate and cardiac function, during stressful situations such as playing virtual reality games and while discussing personal problems and concerns. They already found that among both the African-American and European-American twins, genetics account for about 50 to 60 percent of the differences in blood pressure and reactivity to stress.

"We found very similar heritabilities, very similar influences from the environment," said Dr. Harold Snieder, genetic epidemiologist at the Georgia Prevention Institute and co-investigator on the study. "Now we have to find out what that means."

It may mean that the difficulties lie further along in life for some of these young people who may have chronic exposure to stress without good coping mechanisms. "This study will allow us to follow the twins across a total of eight to 10 years of important psychosocial changes," Dr. Treiber said. "The impact of stressful environments may start showing up as they get older and become young adults."

They also want to determine if the African-American twins - their race tends to have problems with hypertension and other cardiovascular disease much earlier in life and more often, than their European-American counterparts - are the most impacted by environmental stress.

The reasons behind increased cardiovascular disease and related risk factors among African-Americans are still being determined, but one theory is that they encounter more frequent and more intense psychosocial stressors - such as discrimination, violence and poor socioeconomic status - than European-Americans, Dr. Treiber said.

The researchers want to explore that issue as well, looking not only at how the twins react to stress, but if unrelenting environmental stress has a cumulative toll on their blood pressure reactivity and resting blood pressure.

Twins were picked for study because researchers could more easily determine the genetic influence on blood pressure by comparing identical twins who share 100 percent of their genes and fraternal twins who share 50 percent, about the same as any two siblings of the same age. "You would expect, if something is influenced by genetics, that the similarity in identical twins is much higher," Dr. Snieder said.

This phase of the study will follow the twins for four more years, continuing to look at resting blood pressure and reactivity, not only in the controlled setting of the Georgia Prevention Institute, but with 24-hour monitoring as well to measure reactivity to life's routine stressors.

"The other thing we are looking at now is why their blood pressure is going up more during stress," Dr. Treiber said. "Is it because they are retaining more sodium or producing more stress hormones which are contributing to the constriction of blood vessels?" They also are looking at whether blood vessels are becoming less elastic and so less responsive over time in the face of increasing pressures.

In a separate longitudinal study of young African-Americans and European-Americans with a family history of hypertension, the researchers already are finding that the African-Americans are showing telltale signs of blood pressure problems as they move into their late teens and early 20s.

Changes include pressures that are more reactive to stress and increases in the left side of the heart, which pumps blood into the body and, in the face of hypertension, must work against increased resistance.

"We want to see who is going to have the greatest increase in blood pressure over time," Dr. Treiber said. "We already know that if you are overweight, you are more likely to show increases. If you have a family history, you are more likely. If you already have a higher resting blood pressure, you are likely to have it later, too. Factor all that in and then say, 'Does reactivity to stress still help us tell who is going to show the greatest increases over time?''

The researchers suspect that the answer is, 'Yes, it does.' "We think part of it is how much stress they encounter, the other is their coping skills and lifestyle behaviors like physical activity, diet and smoking," Dr. Treiber said. "If we can see what are the environmental contributions that will help us understand what we can do to develop effective primary prevention strategies for people at risk."
-end-


Medical College of Georgia at Augusta University

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