Kidney Disease in African Americans

This article provides an overview of chronic kidney failure (aka-chronic renal failure) as it relates to African Americans. We will not discuss acute kidney failure (acute renal failure), because, despite its serious and often catastrophic course, it is most often reversible and has a much smaller impact on the overall health condition of African Americans. Chronic renal failure (CRF) is the general classification that describes kidney disease of all causes that is characterized by progressive deterioration in function over several years to eventually lead to end stage renal disease (ESRD). ESRD results in either death, dialysis, or transplantation. Chronic renal failure and ESRD disproportionately affects African Americans in such a dramatic way that being African American is considered a risk factor for developing chronic kidney disease.

Based on past studies and those currently in progress, compelling theories have been developed to explain this increased susceptibility to renal injury. They include epidemiological studies that analyze historical and cultural factors and population data; basic science studies of cellular physiology in human and animal models; and, clinical studies that compare physiologic (functional) mechanisms as well as responses to a variety of pharmacologic and non-pharmacologic treatments. A combination of genetic and environmental factors gives the best overall explanation. Delineating the multiple components characterizing each of these factors, and clarifying how they interrelate is a complex task.

Background

Most people are born with two kidneys. Each one is about the size of your fist. The kidneys occupy about 1% of the total body weight (one pound in a 100 pound person). However, under usual circumstances, the kidney receives about 20% of the amount of blood pumped from the heart at any given time. This means the kidneys get twenty times their weight in blood volume. This fits with the role the kidneys have in "purifying the blood of metabolic chemical waste. These chemicals are filtered from the blood to eventually end up in the urine. Of course, the kidney has other functions which include, maintaining the content and balance of the body fluids,

the maintenance of normal calcium balance and the prevention of anemia. In general, the measurement of kidney function involves determining how much blood the kidneys filter over a given period of time, usually per minute. On average, four ounces of blood is filtered of chemicals each minute. In a 24 hour period, this comes to 180 quarts of blood! Since the average size person has a total of five quarts of blood circulating in their body, the kidney filters our total blood volume roughly 36 times a day! This has to be accomplished every day to keep blood levels of toxic chemicals at the acceptable low levels. The two chemicals routinely measured are blood urea nitrogen (BUN) and serum creatinine. A doubling of the blood level correlates with a 50% reduction in kidney function. When kidney function is reduced to a level in which the accumulation of chemicals in the blood is life-threatening, the individual has reached end stage renal disease.

The Impact of Chronic Renal Failure

The United States Renal Data Systems documented 257,266 patients with ESRD at the end of 1995. This is up from 190,000 patients documented in 1990. The cost of treating these patients has increased each year and reached $8.83 billion in 1995. African Americans make up 32% of the ESRD population, compared to whites who make up 62.4% Since African Americans make up only 12.4% of the US population, their representation within the ESRD population is much higher than what one would expect. In fact, African Americans develop renal failure approximately 4.2 times more frequently that whites, and develop hypertensive renal failure 6 to 17 times more frequently.

Hypertension and Diabetes accounts for 66% of the new cases of ESRD. Although, the leading cause of ESRD in the United States is diabetes mellitus, among African Americans, the leading cause is hypertension and diabetes is second. Given the higher prevalence of hypertension and diabetes among African Americans, a greater incidence of ESRD is not unexpected. However, even when adjustments are made to offset the higher prevalence of diabetes and hypertension among African Americans, and also adjusting for differences in age and sex, still the higher risk for ESRD remains.

Hypertension and Chronic Renal Failure

Hypertension is a major problem in the United States. It affects nearly 60 million Americans and is associated with significant morbidity and mortality from both cardiovascular and renal diseases. African Americans have a 50% greater prevalence of hypertension. They also develop hypertension earlier in life, have more severe blood pressure elevations, and have higher rates of nearly all forms of renal disease (of which hypertension is often a major complication). Racial differences among hypertensives in kidney structure and function have been documented, especially in the vasculature and in the handling of salt (sodium).

There has also been evidence to suggest that African Americans have a greater degree of damage to the renal blood vessels than Whites for the same degree of blood pressure elevation. Finally, it is generally known that regardless of the type of renal disease, poor blood pressure control will promote a more rapid progressive decline in renal function. Furthermore, the converse is true; the better the blood pressure control, the slower the decline in renal function, although, African Americans benefit less from this effect.

Diabetes and Chronic Renal Failure

There are two major forms of diabetes: Type I (Insulin dependent diabetes mellitus-IDDM) and Type II (Non-insulin-dependent diabetes mellitus-NIDDM). These two types were previously called juvenile onset and adult onset, respectively. In both instances the body does not have enough insulin on board to allow it to utilize its principal energy source, glucose (sugar). As a result, the "sugar" accumulates in the blood in high concentrations. This damages blood vessels which results in damage to organs.

The most vulnerable organs include the brain and nervous system, eyes, heart and kidneys. In Type I there is no insulin made by the pancreas, which is the gland that manufactures insulin. Consequently, Type I patients must inject insulin to avoid the rapid onset of life threatening consequences. In Type II, the pancreas makes insulin often in excess quantities, but not sufficient to keep the blood glucose levels normal. Type II patients need their insulin production boosted by pills or supplemented by insulin injections. Twentyfive to 35% of patients with Type I go on to develop end stage renal disease. This occurs in 3% to 40% of Type II diabetic patients However, since there are many many more Type II patients, they constitute the majority of the diabetic patients with ESRD (59%).'

African Americans with diabetes have a 4.3 times higher rate of developing ESRD than in white diabetes patients. When one compares normotensive African Americans with Type II DM and those who are hypertensive with the same durations of diabetes, the hypertensive group has poorer renal function.

Other Causes of Chronic Renal Failure

African Americans have excess rates of ESRD from other causes of renal failure, although these have much less of a quantitative impact. In systemic lupus erythematosis, nearly twice as many cases reach ESRD than seen in whites. With HIV associated nephropathy, 26 times more cases reach ESRD in African Americans than whites. African Americans represent 75% of the cases of ESRD caused by Sickle Cell Anemia. Interestingly, the remaining 25% comes from Native Americans. In fact, if we take all causes of ESRD other than diabetes and hypertension, African Americans constitute 32% of those patients.

Issues in Treatment and Prevention

The mean survival after developing ESRD is less than 10 years. Thus the greatest impact can be made through preventing ESRD, or delaying the course leading to ESRD. In African Americans this can most effectively done by aggressively managing the course of Type II DM and Hypertension. Essential hypertension is often already present long before the onset of Type II DM. When hypertension follows the onset of diabetes, it generally heralds the development of significant renal disease.


There is strong evidence to support an important role of the kidney in the excess incidence of hypertension in African Americans. A unifying concept for the impact of hypertension and diabetes on the development of ESRD appears to be the increased hemodynamic stress on the functional vascular units within the kidney. To this end, there are blood pressure lowering medications that selective relieves some of this stress generated within the kidney (glomerular capillary pressure). The class of drugs called Angiotensin Converting Enzyme Inhibitors (ACE Inhibitors), have been best studied.

They have been definitively demonstrated in Type I diabetes to provide renal protection from progressive injury. Many smaller scale studies, yet significant ones, have demonstrated their beneficial renal protective role in Type II DM. Another class of blood pressure lowering drugs has been studied, the Calcium Channel Blockers (CCB's). Although these studies are not as extensive or conclusive as those using ACE's, two subclasses of the CCB's have been demonstrated to have renal protective actions (phenylalkalamines and benzothiazepines). The renal protective actions of the most potent subclass of CCB's (dihydropyridines) is doubtful. A new class of blood pressure lowering drugs that have great promise as renal protective blood pressure medications are called Angiotensin II receptor blockers. This promise is based on their mechanisms of action, however, they are too new and untested in this area to have any strong conclusions drawn.

Certainly, there remains an important role for early detection through screening and through supporting the general goals of disease prevention and health promotion. This includes healthy diets and regular exercise.

This is an overview and not a detailed analysis of the complex issues that deal with clarifying the mechanisms that characterize the susceptibility of African Americans to developing ESRD. However, a great deal of work is in progress in the areas of clinical and basic science research that will further our understanding and move us closer to solving this serious problem.

By Dr. Herman Anderson, Chief of Nephrology
Harlem Hospital Center, New York.

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