The hormone, aldosterone, influences the kidney's regulation of blood pressure, but too much of it can contribute to the development of hypertension because it causes the kidney to retain salt. Aldosterone, released by the adrenal glands on top of the kidneys, plays a role in the complex system used by the body to regulate blood pressure.
"Although the results are discouraging for African Americans and hypertension, it'll point us in other directions that may have more potential and could be the key to reducing hypertension," said Brown, who has a background in exercise physiology.
Many African Americans develop the salt-sensitive form of hypertension. Approximately 40 percent of African Americans have hypertension the highest rate of any racial or ethnic group in the United States but there is little data about what makes them more susceptible to this condition, Brown said.
This study is based on the premise that the prevalence of blood pressure sensitivity to salt is extremely high is African Americans. Alterations in aldosterone regulation may play a role because aldosterone causes the kidney to retain salt. Brown said he wanted to find out if exercise could lower the levels.
In the study, he found that the level of aldosterone was related to how the two racial groups distributed body fat. Caucasians generally stored fat in the abdomen area, whereas African Americans had fat distributed throughout the body in a layer under the skin.
The six-month study involving 35 Caucasians and African Americans with hypertension found that aerobic exercise training program reduced aldosterone levels in Caucasians by 32 percent, but levels for African Americans were reduced by only 8 percent. Total body fat was reduced only in Caucasians, which might be a clue to the drop in aldosterone.
"The kidneys help to regulate blood pressure by changing the levels of salt and water in our body. Sometimes the kidneys reset at a higher blood pressure level if it has retained too much salt," Brown said.
While the study showed exercise did not lower aldosterone in African Americans, exercise still has many other benefits for this population, Brown said."Exercise has the capacity to affect so many things. It's a way for the body to correct itself," he added.
Brown will continue his research in this area with a $3.5 million National Institutes of Health grant awarded earlier this year. In September, Brown will recruit African Americans with hypertension for a study on how exercise can improve the blood vessel condition. The study will also take an in-depth look at how genes can contribute to hypertension.
"Solving the cause of hypertension is similar to solving a big puzzle. Each piece of the puzzle represents a contributing factor to hypertension. Each of these pieces, or possible causes of hypertension, needs to be studied in a systematic way," Brown said.