Founded in 1981, the Georgia Prevention Institute is administratively housed in the Medical College of Georgia’s (MCG) Department of Pediatrics. It has a multi-disciplinary team focusing on the following areas of research: high blood pressure (hypertension), diabetes (types I and II), congestive heart failure, cancer (smoking prevention), obesity, and genes associated with the disease process.
The Institute has had an average of 10,000 outpatient visits a year in the past 3 years (2005-2007). Patients are evenly divided by sex and are about 50% black.
Although some adults participate in research projects at the Institute, most subjects are in the pediatric age group, from 4 to 19 years
Why does the Georgia Prevention Institute focus on hypertension?
- 1 in 3 Americans have hypertension
- High blood pressure levels are uncontrolled in 2 of every 3 cases
- The direct and indirect costs of hypertension in 2008 are estimated at $69.4 billion
Hypertension is more common in black people than in whites, and African American women have the highest prevalence of any group in the whole world: 46.6%.
African Americans develop high blood pressure earlier in life than white people and their average blood pressures are much higher. Compared with whites, black people have worse health consequences:
- 1.3 greater rate of nonfatal stroke
- 1.8 greater rate of fatal stroke
- 1.5 greater rate of heart disease death
- 4.2 greater rate if end-stage kidney disease
What accounts for the race differences in high blood pressure? Some say genetics and others say environmental factors, and this is a matter of debate. Environmental factors that might account for it include environmental stress and salt intake or how the body handles dietary salt. There is good evidence that an interaction between genetic and environmental factors – particularly stress – is involved; this is the focus of our research.
Hypertension is not a disease: it’s a symptom of underlying condition, genetic problems, flaws in the renin-angiotensin system, flaws in sodium handling.
Ideally, you identify the etiology of the problem, then treat it [based on what the underlying cause is]. But hypertension is treated with a “stepped-care” approach that starts with a front-line drug, followed by a string of other drugs in order. This is done with no regard for the underlying reasons for elevated blood pressure in the individual patient.
When you understand the underlying abnormality, you can treat it more effectively. But that’s not how it’s done now.
Black adults and, to some extent, black children, have a smaller drop in blood pressure at night following the stress of a normal day. So black people’s hearts work harder over days, weeks and years. This puts an increased load on the kidneys, heart and vascular system.
Why the difference? Our hypothesis is that stress impairs the body’s ability to excrete excess sodium in the urine, an essential function that helps keep circulating levels of sodium and blood pressure levels in the normal, healthy range. We believe this damages blood vessels at an early age, causing African American children to develop higher blood pressures at younger ages.
This could explain the increased risk among blacks for developing clinical hypertension. [Clinical hypertension means having a blood pressure level higher than 120/80 mmHg.]
All animals struggle to manage the sodium content of their bodies, maintaining the healthy balance known as homeostasis. Salt comes in through diet, and is regulated by natriuresis (salt excretion via urine). Blood pressure – determined by the volume of blood in circulation and the resistance of the vessel walls to that volume – helps maintain sodium homeostasis.
The hallmark of essential hypertension is that higher blood pressure is required to maintain homeostasis. In people with so-called “salt sensitive” hypertension, blood pressure rises and falls with dietary salt intake.
We conducted a large study to determine whether stress could have the same impact on blood pressure as consuming a large amount of salt, looking at whether stress had the same or different effects on African American and white teenagers.
We recruited kids [whose incentive] was a day off from school and we also paid them to participate. For three days before testing we put them on a low- sodium diet by providing them with low-salt versions of foods they like, such as hamburgers. Some 900 subjects have been put through this protocol, and we have achieved about 85% compliance with the diet.
On the fourth day, participants come to the lab for a 5-hour test protocol. For the first two hours they relax by watching a movie. This is followed by a 1-hour stress period during which they play a competitive video came, competing in pairs for cash prizes. Then there is a 2-hour recovery period.
Blood pressure measurements were made every 15 minutes and urine samples were collected hourly.
We saw two response patterns:
- Blood pressure and sodium excretion increased together, peaking during the high-stress period
- Blood pressure increased, but sodium excretion did not
The latter pattern was more common in African American teens: compared with white kids, these kids were holding onto salt. Overweight black teens retained even more salt than others. In general, whites were protected in terms of cardiovascular health, and blacks were not.
In a small pilot study focusing on teenaged girls, we saw racial differences in estrogen levels during the stress-inducing hour of the protocol. Estrogen is thought to protect women against cardiovascular disease, and in black females it fell during the video game competition. It did not fall in white volunteers.
We hypothesize that angiotensin – a hormone involved in blood pressure regulation – may be especially important in black girls.
So far, we have done one small clinical trial with an angiotensin receptor blocker that lowered blood pressure and increased sodium excretion in black males. This class of drugs is not typically used in black patients, but it could be.
New drugs are important because people whose blood pressure levels are continuously elevated are at risk for stroke, heart and kidney disease.
African Americans are also not the only population being affected by high blood pressure at a young age. When children in Houston’s largely Latino elementary schools had their blood pressure screened, about 9% had levels high enough to be diagnosed with hypertension.
Ideally, we need a simple way to identify people whose bodies react to stress the way a salt-sensitive person responds to a huge dietary intake of sodium. If we had this simple test, we would know who is at risk for hypertension and we would be able to predict their response to treatment.
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