How to Keep Your Kidneys Healthy as You Age

Although it's been close to eighteen years since I dissected my first cadaver in anatomy class, I still remember being surprised when I got my first glance at a pair of kidneys - they were much smaller than I had expected. Up until that point, I had imagined the kidneys to be quite large, given the amount of work that they are responsible for.

Each of your kidneys is about 4 to 5 inches long and about 1 inch thick, weighing in at about 4.5 to 5 ounces. To put it into easy-to-visualize terms, each of your kidneys is a bit larger than a deck of cards.

Although your kidneys make up less than 0.5 percent of your total body weight, they receive close to 25 percent of the total amount of blood that your heart pumps while you're resting. Also, your kidneys use up about 20 to 25 percent of your body’s supply of oxygen.

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Why do your kidneys – such small organs – receive so much of your blood and oxygen? Because they are responsible for five critical functions:

1. Your kidneys keep your blood clean by filtering it of waste products and eliminating these waste products from your body as urine.

2. Your kidneys help maintain a proper balance of fluids throughout your body.

3. Your kidneys secrete a hormone called erythropoietin, which is responsible for stimulating the production of red blood cells in your bone marrow.

4. Your kidneys produce an enzyme called renin, which is needed to help maintain your blood pressure.

5. Your kidneys convert vitamin D to its most active form.

Your kidneys lie above your waistline, just underneath the surface of your mid/low back region. Although the position of your liver causes your right kidney to be slightly lower in your abdominal cavity than your left kidney, both kidneys are partially protected by the lower part of your ribcage.

With every beat of your heart, large amounts of blood are delivered to your kidneys via your renal arteries. Inside your kidneys, your renal arteries split up into a number of smaller branches that distribute blood to your nephrons, which are the microscopic processing units of your kidneys; you have about a million nephrons per kidney.

Within each nephron, there are specialized beds of capillaries (even smaller blood vessels) called glomeruli. The glomeruli filter your blood, and pass the filtrate on to a series of specialized tubules that are collectively known as the renal tubule – it’s in the renal tubule where urine is created.

The process of creating urine is complex, but in essence, what happens is this: about a fifth of the blood that passes through each of your kidneys gets filtered by your glomeruli to enter your the renal tubules; the stuff that passes through is referred to as filtrate, which includes waste materials, water, chloride ions, sodium ions, bicarbonate ions, glucose, potassium ions, urea, uric acid, and protein. As the filtrate travels through the renal tubule, about 99 percent of it is reabsorbed into your blood circulation - this number alone gives you a good idea of how hard your kidneys work to produce urine; of the approximately 40 gallons (150 litres) of filtrate that enters your kidneys on a daily basis, only about 1 to 2 quarts (1 to 2 litres) turns into urine. The 99 percent that is reabsorbed into your circulation is how your kidneys help to maintain your body’s fluid composition and pH level.

If you want to understand exactly how your nephrons create urine, I recommend that you read the chapter on Kidneys and Body Fluids in Guyton's classic textbook on human physiology – this is the go-to book when you want a detailed look at how your body works on a microscopic level.

Once urine is created in your renal tubules, it is shuttled through a series of collecting ducts until it reaches the inner middle section of your kidney, where urine is collected by your ureter, the tube that allows urine to travel from your kidney to your bladder. From your bladder, urine exits your body through your urethra.

In understanding the work that your kidneys are forced to undertake to filter your blood and produce urine, I hope it’s clear that drinking large amounts of water when you’re not thirsty is a good recipe for prematurely wearing down your kidneys as you age. Your body is not like a plumbing tube that gets cleaner by flushing large amounts of water through it. A number of your organs, including your kidneys, are designed to keep your body clean by continuously eliminating waste materials. If you want to prevent illness as you age, a top priority should be to prevent unnecessary burden to your kidneys and other waste-eliminating organs.

Beyond using your sense of thirst to dictate how much water and water-rich foods you ingest, here are two important ways to protect your kidneys from prematurely breaking down:

1. Don’t eat too much protein.

   Eating more protein than you need leads to greater workload on your kidneys, which must filter a by-product of protein metabolism called blood urea nitrogen (BUN) out of your blood. This increased workload can contribute to premature breakdown of the glomeruli in your kidneys.

   If you have healthy kidneys, you can safely eat up to half of your body weight (in pounds) in grams per day. For example, if you weigh 150 pounds and are in good health, you can safely eat up to 75 grams of protein from minimally processed foods per day. If you have problems with your kidneys, you should decrease this amount to a level that results in a healthy blood urea nitrogen level.

   If your current health status is such that you need an objective way to monitor how well your body is responding to the amount of protein that you are eating, ask your doctor about monitoring your BUN level. Whenever you eat protein, your body breaks it down into amino acids that contain nitrogen. Nitrogen separates from amino acids and combines with other molecules to form urea. Urea is eliminated from your body when your kidneys filter it out of your blood and into your urine.

   A healthy range for BUN is between 4 to 17 mg/dL. Anywhere between 18 to 21 mg/dL is a sign that you may be eating too much protein, and possibly that your kidneys are under excessive strain. More than 21 mg/dL is a strong sign that you need to significantly reduce your protein intake.

   The link between eating too much protein and developing kidney disease is one that is not often talked about by supporters of a high-protein diet. While it is important to keep your blood sugar and insulin at healthy levels by avoiding sugar and other simple carbohydrates, please know that a high-protein diet poses many dangers to your health, especially if most of your protein is cooked using high temperature techniques. Your health is best served by replacing simple carbohydrates with lots of high quality fat, and moderate amounts of healthy protein and non-starchy vegetables.

2. Don’t take over-the-counter pain pills on a regular basis.

   Non-steroidal anti-inflammatory drugs like ibuprofen (Advil, Motrin), naproxen (Aleve), and aspirin are known to cause kidney damage if taken regularly. Acetaminophen (Tylenol and Excedrin) can also cause kidney damage and failure if used regularly. All of these over-the-counter pain medications probably don’t pose significant danger if your kidneys are relatively healthy and you use them for emergencies only.

   As many professional athletes have discovered during the past several years, regular use of prescription anti-inflammatory pain medication like Vioxx, Indocin, and Naprosyn poses even greater danger to kidney health than over-the-counter pain killers.

I hope that this article has helped you understand the complex design of your kidneys and the key steps that you can take to prevent premature breakdown of your kidneys as you age. Please remember that the best "medicine" for all of your organs, your kidneys included, is eating a plant-based, minimally processed diet, getting enough physical and emotional rest, getting regular exposure to fresh air and sunlight (without getting burned), and being physically active.