People with more muscle than fat have increased ability to regulate their blood pressure in response to stress, according to a Medical College of Georgia study.
"Fitness facilitates the ability to regulate blood pressure; fatness impedes your ability to regulate blood pressure through your ability to regulate sodium," says Dr. Gregory Harshfield, hypertension researcher and second author on the study in the November issue of the American Journal of Hypertension.
"When you are under stress, your blood pressure should go up and when the stress is over, it should come back down," says Martha Wilson, research manager and the study's lead author. "Look at the Olympic athletes. Certainly their pressures are up when they are swimming or running or doing gymnastics, but I'm sure their pressures come back down relatively quickly afterward."
Previous studies have made the seemingly odd link between lean body mass and higher blood pressure in adults and children. And when the MCG researchers started analyzing their data on how 127 young adults with normal blood pressure responded to stress, they found the same thing. "I thought the data was wrong," says Dr. Harshfield.
What might be wrong is the notion that an increase in blood pressure is bad. "If you think about it, that concept doesn't make sense," Dr. Harshfield says. "If you are in a stressful situation, your blood pressure should go up. If it doesn't, then you do have a problem," he says, referencing the natural fight-or-flight mechanism that enables more blood and oxygen to get to the body during stress.
When the researchers looked at the percentages of fat and lean tissue on their study participants and looked at their ability to excrete sodium - the primary mechanism for dropping blood pressure back to normal - they found those with more fat had a decreased ability to excrete sodium.
MCG researchers say lean body mass may have gotten a bad rap because previous studies looked at casual blood pressure and did not factor in fat's contribution. Not unlike building muscle in response to lifting heavier weights, the body also builds more lean tissue to support more fat. "The argument was that you have to have greater lean mass to carry the greater fat mass, so it was really the greater fat mass that was the culprit," Dr. Harshfield says. "I am sure this is true, particularly in adults."
Fat, especially abdominal fat, secretes angiotensin which makes angiotensin II, a powerful vasoconstrictor that also directs the kidneys to absorb more sodium so blood vessels retain more fluid volume, says Dr. Harshfield, who is principal investigator on a separate National Institutes of Health grant looking at how fat contributes to high blood pressure. Research he published in 2003 in Hypertension also shows that overweight boys have greater blood pressure increase in response to stress than their female peers and decreased ability to return to normal.
For the newly published study, researchers measured the blood pressure of study participants every 15 minutes throughout a two-hour baseline period, an hour of competitive video games and a two-hour recovery period.
They found that while blood pressure increased an appropriate average of 5 percent in response to the stress of video games, participants who had more lean muscle mass than fat were better able to return to normal levels through this sodium excretion process called natriuresis.
Previous studies by Dr. Harshfield have shown that race also contributes to elevated pressures following stress because of blacks' reduced ability to excrete sodium. The new study shows that high body fat is another independent predictor of the abnormal response.
"The major finding of this study is that body composition is related to the pressure natriuresis response to mental stress," the researchers write. "Specifically, (lean body mass) was associated with higher (blood pressure) during stress. In contrast, greater body fat was associated with a slower natriuretic response to stress as well as slower natriuresis during stress, which is in part related to (angiotensis II)."
Those findings point back to the adage that fitness is good and should start early in life, Mrs. Wilson says.
The research was funded by grants from the NIH.