By Robert S. Dinsmoor
Diabetic nephropathy (kidney disease) is the leading cause of kidney failure in the United States. That’s the bad news. The good news is that there are now a number of measures you can take that have been scientifically proven to protect your kidneys and lower the risk of developing diabetes-related kidney disease.
Your kidneys, which are each about the size of your fist, are located near the middle of your back, just below the rib cage. By no coincidence, they are shaped like kidney beans. One of their jobs is to filter waste products and extra water from the bloodstream. This waste and excess water, in the form of urine, flow through tubes called ureters and into the bladder. The bladder stores urine until it is full enough to create the urge to urinate.
How does this filtering process work? Each kidney is made up of about one million tiny filtering units called nephrons. Tiny blood vessels called arterioles deliver blood to the nephrons. Within each nephron, the blood vessels form a complex called a glomerulus. It is within these glomeruli that the filtering activity actually takes place. The filtered blood leaves through another arteriole and is eventually carried back to the heart. Meanwhile, the material filtered from the blood passes through a tubule, where it is converted to urine, and then carried to the bladder through the ureters.
Diabetes sets the stage for kidney damage. Chronic high blood glucose levels, often in combination with hypertension (high blood pressure), damage the glomeruli and progressively diminish kidney function. (High blood pressure alone is the second-leading cause of kidney failure behind diabetes.) This type of kidney dysfunction is known as diabetic nephropathy. In its earliest stages, it has no symptoms; however, the “silent” damage going on behind the scenes can still pave the way for kidney failure.
Diabetic kidney disease develops slowly. In its earliest stages, excessive quantities of blood begin to flow through the kidneys, a condition called hyperfiltration. Eventually, the kidneys begin filtering greater and greater amounts of protein, which leak into the urine. One of these proteins, albumin, can be measured by laboratory tests, which can detect diabetic kidney disease in its early stages, when it is most treatable. In the later stages of kidney disease, the kidneys can no longer remove most of the body’s waste products from the blood. As a result, toxins begin to build up in the blood stream and anemia (a low red blood cell count) may ensue. In its final stage, called end-stage renal disease, the kidneys fail, causing uremia. Uremia, the build-up of urea and other waste products in the blood, can cause such symptoms as nausea, vomiting, fatigue, and fluid weight gain. People with kidney failure require dialysis, a treatment that takes over the function of the kidneys by filtering waste product from the blood. In some cases, they can get a kidney transplant, but most of them must wait a long time for a suitable kidney donor.
The American Diabetes Association (ADA) recommends measuring albumin levels and estimating the glomerular filtration rate (GFR) in patients who have had Type 1 diabetes for 5 years or more, all patients with Type 2 diabetes, and all people with diabetes who have high blood pressure. GFR cannot be measured directly. Currently, it is closely approximated, in part, by using the blood concentration of creatinine. Healthy kidneys excrete creatinine in the urine, so a buildup of creatinine in the blood shows that the kidneys aren’t working well. By measuring the concentration of creatinine in the bloodstream and using it in an equation that takes into account the person’s weight, age, sex, and race, doctors can estimate the GFR and get a sense of kidney function. The higher the blood creatinine level, the lower the GFR. Some physicians also use the blood creatinine level and GFR calculation to track the decline of kidney function and to gauge any benefits of treatment.
At least two large studies have shown beyond a shadow of a doubt that tight blood glucose control can significantly lower the risk of developing diabetic kidney disease. The glycosylated hemoglobin (HbA1c) test gives a measure of one’s long-term control of blood glucose and hence one’s risk for diabetic complications. The ADA currently recommends an HbA1c of less than 7 percent in many patients. It recommends stringent blood glucose goals (such as less than 6.5 percent) in people with a short duration of diabetes, a long life expectancy, and no cardiovascular disease — if they could be achieved without significant hypoglycemia. On the other hand, it suggests less stringent blood glucose goals (such as less than 8 percent) for those having difficulty maintaining target blood glucose levels, or with a history of severe hypoglycemia, long-standing diabetes, and a limited life expectancy.
Controlling blood pressure is important not only for reducing the risk of cardiovascular disease but also for protecting the kidneys. According to the ADA, high blood pressure can speed the progression of diabetic kidney disease, and aggressively treating it can greatly slow the decline in GFR. Treating high blood pressure has also been shown to raise life expectancy and reduce the need for dialysis and transplantation once diabetic kidney disease has developed. According to the ADA, people with diabetes should strive to maintain a systolic blood pressure under 140 mm Hg and a diastolic pressure under 80 mm Hg.
In some cases, initial therapy for high blood pressure should consist of lifestyle modifications, such as losing weight, cutting back on sodium and alcohol consumption, and getting more exercise. One of the most effective lifestyle changes would be to follow the DASH (Dietary Approaches to Stop Hypertension) eating plan, which is a diet low in saturated fat, total fat, and cholesterol that emphasizes fruits, vegetables, and low-fat dairy products. The DASH diet is most effective when combined with a reduction in sodium intake. You can download a copy of “Your Guide to Lowering your Blood Pressure with DASH,” which includes a week’s worth of menus or call (301) 592-8563 for a free copy.
For treating high blood pressure in people with diabetes — with or without diabetic nephropathy — specific blood-pressure-lowering drugs called angiotensin-converting enzyme (ACE) inhibitors and angiotensin-II receptor blockers (ARBs) are the drugs of choice. These drugs appear to have a protective effect on kidneys above and beyond blood pressure control.
ACE normally converts a hormone called angiotensin I to a related hormone called angiotensin II, which constricts blood vessels, increases sodium and water retention, activates the sympathetic nervous system, stimulates fibrosis (stiffening) of the heart and blood vessels, and promotes heart cell growth. The immediate net effect of these changes is to raise blood pressure, but over time this hormone can cause damage to the heart and kidneys. ACE inhibitors block the action of ACE, thus decreasing the amount of angiotensin II and in turn minimizing its effects.
ARBs also work to decrease the effects of angiotensin II, but at a different point in the process. For angiotensin II to exert its effects throughout the body, it must bind to certain receptors (much as a key fits into a lock) on cell surfaces. ARBs prevent angiotensin II from binding to its receptors and thus reduce its effects.
Many studies have shown that in people with diabetes, ACE inhibitors can have a host of beneficial effects, including preventing or delaying the progression of nephropathy in people with microalbuminuria or overt diabetic nephropathy, decreasing the risk of heart attack and stroke, and decreasing mortality, so people with diabetes and hypertension are routinely prescribed ACE inhibitors.
Like ACE inhibitors, ARBs decrease levels of albumin in the urine and have been shown to effectively prevent progression of nephropathy in people with microalbuminuria or overt diabetic nephropathy. If an ACE inhibitor or ARB used alone is not sufficient to lower blood pressure, other blood-pressure-lowering drugs such as diuretics may need to be added; ACE inhibitors and ARBs can even be used together. (Read “Analgesics and Kidney Health” for information on how various over-the-counter medicines affect the kidneys.)
Diabetic nephropathy is the leading cause of kidney failure in the United States. Yet with proper screening and diagnosis, some lifestyle changes, and good control of blood glucose and blood pressure with appropriate medicines, you can greatly reduce your chances of developing advanced kidney disease. The other good news: These same measures can also protect your heart, blood vessels, eyes, and nerves.
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