The following is an excerpt from “Living Low Carb: Controlled Carbohydrate Eating for Long Term Weight Loss” (Revised and expanded edition, 2010) by Jonny Bowden, PhD, CNS
References and footnotes can be found in the original.
MYTH #5: High-Protein Diets Cause Damage to the Kidneys
You will often hear from ill-informed sources that a high-protein diet damages the kidneys. Not so. Consider the following: everyone knows about step classes and aerobics. They are great calorie burners, get the blood and oxygen flowing, are good conditioners of the cardiovascular system, and, with certain variations, can even be good for muscle toning. So they’re a good thing, right?
Except if you have a broken leg.
If you have a broken leg, or a sprained ankle, or shin splints, I’m going to suggest that you not take a step class until the injury heals. Under these special circumstances, the very weight-bearing that does so much good for the normal person is going to be more stress than you need during the healing phase. I’m going to tell you to stay off the leg, let it heal, and avoid putting additional stress on it at this time.
Does the fact that step class is not good for a person with a broken leg mean that the step class led to the broken leg?
No. And ketogenic diets do not—I repeat, do not—cause kidney disease. If your doctor says they do, politely ask him or her to show you the studies. (They don’t exist.) Ketogenic diets are, however, not a good thing if you have an existing kidney disease, much the way a step class is not a good thing if your leg is already broken.
High Protein Causes Kidney Disease? Not.
The oft-repeated medical legend that high-protein diets cause kidney disease came from reversing a medical fact. The medical fact is that reducing protein (up to a point) lessens the decline of renal (kidney) function in people who already have kidney disease. Because restricting protein seems to be a good strategy for those with existing kidney failure (or even some kidney weakness), some people drew the illogical conclusion that the obverse must also be true—that large amounts of protein lead to kidney failure.
In any case, it is not proteins per se that cause problems, even for those who already have renal disease: it is the glycolated proteins (see chapter 2). These sugar-sticky proteins, you may remember, are the result of excess sugar in the blood bumping into protein molecules. These sugar-coated proteins are called AGES, advanced glycolated end-products. The AGES themselves then stick together, forming even bigger collections of molecules, which are too large to pass through the filtering mechanisms of the glomerulus, the network of blood capillaries in the kidneys that acts as a filter for waste products from the blood. This reduces GFR (glomerular filtration rate), a measure of kidney function.
High protein intake does not cause this to happen in normally functioning kidneys. A recent study of 1,624 women enrolled in the Nurses’ Health Study concluded that “high protein intake was not associated with renal function decline in women with normal renal function.”17 Another study in the American Journal of Kidney Diseases showed that protein intake had no effect on GFR in healthy male subjects.18 And a third study in the International Journal of Obesity compared a high-protein with a low-protein weight-loss diet and concluded that healthy kidneys adapted to protein intake and that the high-protein diet caused no adverse effects.19
If you don’t currently have kidney disease, a low-carbohydrate diet is actually an ideal way to help control the blood-sugar levels that can eventually lead to kidney disease. Of course, just to be safe, you should check with your doctor to make sure you don’t have any undiagnosed kidney impairment; but if you don’t, you’re sure not going to develop it from being on a low-carb diet.
Higher protein intakes do not cause any damage whatsoever to healthy kidneys.
Jonny Bowden, PhD, CNS is the author of the Poliquin Manual for Nutrition. He's a board certified nutritionist, a nationally known expert on weight loss, health and nutrition, and the best-selling author of 8 books including “The 150 Healthiest Foods on Earth”. Visit him at www.jonnybowden.com