"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.