Kidney disease: an introduction
In the US, as well as throughout the developed world, diabetes is the leading cause of kidney failure, accounting for up to half of all new cases.1 While this is a sobering statistic, there is another way to look at the numbers. By making healthy changes as soon as you are diagnosed with type 2 diabetes, including controlling your blood glucose, losing weight and keeping it off, and keeping your blood pressure and lipids under control, you can prevent serious complications like kidney disease.
Even if you do develop diabetic kidney disease, important treatment advances have been made allow people with this complication to live longer and healthier lives.
How the kidney functions
To understand how kidney disease develops in people with diabetes, it is helpful to understand the basics about the kidney, including its role in the body, how it functions, and how it is affected in diabetes.
The kidneys (most people have two of them) are bean-shaped organs (this is how the kidney bean got its name), each about the size of a small- to medium-sized baking potato (about one and a half pounds), located in the back of the abdominal cavity, one kidney located on each side of the spine.
The kidneys have several important functions, including filtering waste out of the blood. In fact, about 1 quart of blood is filtered through the kidneys every minute. The waste products that the kidney removes from the blood are sent to the bladder, which gets rid of them by producing urine. Another important function of the kidneys is maintaining the balance of sodium, potassium, and other nutrients and minerals (sometimes these substances are called electrolytes) in the blood. By maintaining this balance, the kidney also plays an important role in regulating blood pressure in the body. The kidneys also produces several important hormones (chemicals transported in the bloodstream that serve as messengers carrying signals and instructions from one organ to another) that help them carry out their functions.1,2
Each kidney normally contains from 1 million to 1.5 million structures or units called nephrons (this word actually comes from the Greek word nephros for kidney). Nephrons take care of the actually work of the kidney, filtering blood, maintaining the balance between chemical elements. Each nephron is made up of a tiny blood vessel loop called a glomerulus, though which blood that comes from the heart is circulated and filtered. After waste products are removed from the blood by the glomerulus, they are sent to part of the nephron called the tubule, where they are excreted as urine and sent to the urinary bladder.2
What goes wrong in diabetes to cause kidney disease?
High blood sugar that is the hallmark of diabetes places a great deal of stress on the kidneys. The kidneys have to work overtime to carry out their function as a filter for blood and to maintain the correct balance of chemical elements in the blood. Over time, the increased demand placed on the kidney results in damage to the smallest blood vessels (called capillaries) that surround the glomerulus. Damage to these capillaries from high blood sugar, as well as high blood pressure, which is common in people with type 2 diabetes, decreases kidney function over time.3
What are the symptoms of kidney disease?
Unfortunately, damage to the kidneys can occur without any signs or symptoms. Only by the time extensive damage has already occurred will the symptoms associated with kidney failure appear. When kidney function is decreased to about 25% of normal, a person will develop uremia, in which waste products containing nitrogen normally filtered out by the kidneys accumulate in blood and tissues. The hallmark of uremia is edema (the accumulation of fluid in tissues), which results because you are no longer getting rid of as much excess fluid in the form of urine. Edema that occurs with uremia typically involves swelling in the legs, beneath the skin, and in the area of the eyes. Additionally, blood pressure will increase.1
Because kidney disease proceeds “silently” (without symptoms), it is very important if you have diabetes to have regular kidney function testing done to make sure that kidney disease is not developing and to take steps to address it if it is. Your doctor will give you kidney functions test that measures the ratio of albumin to creatinine in your urine. Typically all this test requires is a random sample of your urine, with no special requirements for eating or fasting. A normal result is anything under 30 micrograms (μg) of albumin per milligram (mg) creatinine. Abnormal results can be anything over 30 μg/mg creatinine.4
Stages of kidney disease
Five stages of kidney disease are recognized, although each person who experiences kidney disease may not go through all the stages. The stages include1:
- Microalbuminuria (30-299 μg/mg creatinine)
- Nephrotic syndrome
- Renal insufficiency (also called chronic kidney disease)
- End-stage renal disease (ESRD)
Hyperfiltration. During the initial stage of kidney disease, high than normal amounts of blood are filtered by the kidneys as they struggle to correct the imbalances that occur due to high levels of blood sugar. The kidneys may even enlarge as they try to accommodate a greater volume of blood. Your doctor may use a test that measures kidney clearance to determine whether you are experiencing hyperfiltration. This test measures the creatinine clearance or the glomerular filtration rate (GFR), which is defined as the volume of blood that is cleared of creatinine per unit of time. This test gives a good picture of how well your kidney is working to filter your blood. About half of people with diabetes who have hyper filtration will progress to more advanced stages of kidney disease. 1
Microalbuminuria.During this stage of kidney disease, small amounts of albumin, a type of protein, leak from the blood into the urine because glomeruli (the plural of glomerulus) are not functioning normally. A healthy person typically will get rid of less than 25 mg of albumin per day in urine. With microabluminuria, 30 to 299 mg of albumin per day are passed in urine (albumin levels above 299 mg per day are considered macroalbuminuria). Microalbuminuria occurs mostly in people who have had diabetes for at least 5 years and affects 4% to 15% of people with diabetes. Because the amount of albumin we pass can be increased by exercise, high blood pressure, infection, and other health conditions, it is necessary to measure levels of albumin in urine on 2 to 3 occasions to determine microalbuminuria. Microalbuminuria is sometimes treated with drugs called angiotensin-converting enzyme (ACE) inhibitors or angiotensin 2 receptor blockers (ARBs) that lower blood pressure and restore normal glomerulus function.1
Nephrotic syndrome. As kidney disease progresses and the ability of the kidney to filter blood is further compromised, larger amounts of albumin (up to 3,500 mg or more per day) leak into the urine. Increased amounts of albumin in urine, decreased levels in blood, increased lipids (cholesterol and triglycerides) in blood, and edema (retention of water) throughout the body are the hallmarks of nephrotic syndrome.1
Renal insufficiency (chronic kidney disease).Renal insufficiency (the word renal refers to the kidneys) occurs when the damaged kidneys are no longer able to keep all waste products out of the blood and albumin from leaking into the urine. Your doctor will use a creatinine blood test to monitor for development of renal insufficiency. This test measures levels of creatinine in your blood. Since the kidney filters creatinine from the blood, if your kidney function is not normal, the level of serum creatinine (creatinine in your blood) will be elevated. A normal result is 0.7 to 1.3 mg/dL for men and 0.6 to 1.1 mg/dL for women. Additionally, measurement of glomerular filtration rate (GFR), which shows how much fluid (in mL per minute) is filtered through your kidneys is also used to monitor kidney function and determine renal insufficiency. A GFR of 60 mL per minute or lower is a sign of renal insufficiency. However, symptoms of kidney failure may not be evident until GFR reaches 30 mL or less. Symptoms that occur when GFR reaches this level may include decreased appetite, difficulty concentrating, itching, feeling of cold, and nausea. When GFR reaches 15 mL or less, symptoms of kidney failure may include vomiting, weight loss, fatigue, insomnia.1
End-stage renal disease (ESRD). Renal insufficiency that persists for months and even years will eventually progress to ESRD, where kidney function decreases to the point where either dialysis or kidney transplant is needed to sustain life. If ESRD is not treated, it can lead to major abdominal swelling and accumulation of fluid around the heart, compromising cardiac function.1
Can I prevent or slow diabetic kidney disease?
If you have diabetes, there are some steps that you can take to decrease your risk for development of kidney disease. In fact studies have shown that damage to the kidney can be prevented or slowed by keeping your blood glucose levels at or close to normal and controlling your blood pressure. Additionally, there is evidence that smoking may contribute to kidney disease risk. So, from the standpoint of kidney disease, it is important to quit smoking.33