One of the most enduring critiques of high-protein diets is that they are hard on the kidneys. On a superficial level that appears to make sense. After all, it’s the kidneys that work with the liver in excreting the nitrogenous waste products that result from protein consumption, primarily urea.
When you eat large amounts of protein, the waste products do increase the stress on the kidneys, specifically the filtering units of the kidney known as the glomeruli. The effect is known as “hyperfiltration.” The theory of how a chronic high-protein diet harms the kidneys suggests that the excessive hyperfiltration required to get rid of protein waste products eventually leads to the formation of scar tissue in the glomeruli. That, in turn, can result in chronic kidney disease.
The problem with the theory is that thus far it has been shown to occur only in selected animal studies and in those with preexisting kidney disease. In fact, renal, or kidney, hyperfiltration is now considered to be a normal adaptation of the kidneys in order to handle the increased load provided by protein metabolic waste products efficiently. In that sense, it’s similar to the way the heart enlarges as a result of exercise to handle the extra blood circulation it requires.
Recent animal-based studies, however, have suggested that a protein intake of 35 percent of total calories may indeed impose stress on the kidneys. The 35 percent figure is often suggested as the upper limit for safe protein intake and is about the amount taken in by most competitive bodybuilders.
Typical of the recent animal studies was one in which rats got either a 15-percent-protein diet or 35 percent protein, made up of both animal and plant proteins. The rats were all normal and not suffering from any preexisting kidney impairment They were divided into groups that stayed on the respective diets for four, eight,12 and 17 months.
The rats’ renal function was accessed by measuring creatinine and urinary protein, which are common techniques for testing human renal function. The researchers also examined the rats’ kidney tissues for signs of damage.
The rats getting 35 percent protein showed 17 percent greater kidney weight, three times more protein excretion in their urine and 27 percent higher creatinine clearance, all signs of kidney stress. Indeed, examination of the rodents’ kidneys showed larger glomeruli and increased scar tissue in the higher-protein rats. Those findings are consistent with past findings in some human studies that pointed to a degree of moderate stress imposed on renal function by consumption of a higher-protein diet.
While it’s clear that eating a high-protein diet increases the work of the kidneys, the question remains whether that will cause long-term damage to those with normal kidneys. No one argues with the fact that eating less protein will reduce the load on those with existing kidney problems, and so slow the progression to outright kidney failure. The problem is—and it’s a major one— that kidney disease in its early stages is silent. You only become aware of subtle kidney damage by undergoing specific tests.
People with high bodyfat often excrete greater amounts of a blood protein called albumin in their urine than is normal. That is also common in those with diabetes and insulin resistance. It’s considered an early sign of kidney disease. Unfortunately, most of the popular diets suggested for those who want to lose bodyfat feature a high protein content. Will such a diet cause more harm than help in those with early-stage kidney disease?
Whether it does depends on several factors, including genetic predisposition. The major risk factor for chronic kidney disease is high blood pressure, which accounts for 30 percent of all cases of CKD in the United States. Those who are obese and follow poor diets without exercise often have high blood pressure, a.k.a. hypertension. For them the first order of business would be to lose weight and exercise as a means of lowering blood pressure. That, in turn, would greatly relieve kidney stress.
Interestingly, several recent studies have clearly shown that high-protein diets lower elevated blood pressure. So consuming a higher-protein diet would be renal protective in that sense. Getting nutrients known to modulate blood pressure, such as potassium, magnesium and calcium, would add to the blood-pressure-lowering effect of a high-protein diet.
What about the hyperfiltration in the kidneys induced by eating a high-protein diet? As noted, it may be just a normal adaptation of the kidneys. One theory is that eating more protein leads to a greater release of glucagon, a pancreatic hormone that works with another substance, cyclic AMP, to increase the filtration through the glomeruli of the kidneys.
The notion that hyperfiltration may be just a normal adaptation of the kidneys is bolstered by what happens during pregnancy, when the glomerular filtration rate increases by as much as 65 percent in healthy women but returns to normal about three months after they give birth. The increased filtration is needed to deal with the waste products produced by both mother and fetus. More important, it does not cause any degree of harm to normally functioning kidneys.
Hyperfiltration also occurs when one kidney is removed. The remaining kidney increases its filtration rate to make up for the loss. Again, no harm whatsoever occurs due to this upgrading of renal function, and those with one remaining kidney live normal lives.
Most of the critiques leveled at high-protein diets in relation to kidney function stem from the publication of a 1982 review in the New England Journal of Medicine, in which the author suggested that anything that increases glomerular filtration and pressure can cause sufficient renal injury to set you up for future chronic kidney disease. Singled out in the study as prime suspects for CKD were high-protein diets; however, the author had based his assumptions entirely on studies with animals and with humans who had preexisting kidney disease. It never occurred to him—or to those who often quote his study—that changes in kidney function and structure may be just normal adaptations to higher protein, just as muscles adapt to weight resistance by increasing in size.
As discussed above, the notion that high-protein diets stress the kidneys is based on the increased urea they bring. There is a bit of truth there, however. One of the most protective things you can do to preserve kidney function is to make sure you get adequate fluids. That is particularly important for those on high-protein diets.
The excretion of urea does require a lot of water to aid the kidneys in processing it. Bodybuilders who dehydrate themselves either by severely restricting fluids before a contest or using potent diuretic drugs are without question stressing their kidneys. I’ve seen some competitors actually cause kidney damage with their extreme water-restriction techniques.
Even so, a normal intake of water by drinking it and eating foods rich in it, such as fruits and vegetables, should be enough to protect the kidneys. Indeed, a study of bodybuilders published a few years ago found no renal impairment in 37 who regularly got 1.4 to 1.9 grams of protein per kilogram of bodyweight each day.
There is some evidence that a higher-protein diet may increase the risk of kidney stones but only in those who are predisposed to developing them. In short, preexisting metabolic abnormalities must exist before stone formation occurs.
There is one other instance where a high-protein diet may adversely affect kidney function. I reported in this publication about a study involving bodybuilders, some of whom showed evidence of a disease that involves scarring the glomeruli. One subject even had outright kidney failure. The commonality of the group was that all used large doses of anabolic steroids.
When I interviewed the author of the study, she suggested that a combination of a very high-protein diet—more than 300 grams a day—and large doses of steroids could lead to kidney damage. She also noted that a normal protein intake would have no adverse effects on kidney function.
She speculated that anabolic steroids may be toxic to the kidneys, although she also admitted that she had no definitive proof of that. She felt that the increased pressure in the kidneys caused by higher protein, combined with the toxic effects induced by steroids, might have explained the kidney disease in her subjects. Indeed, when one of them went off steroids, all signs of his kidney disease abated. When he went back on the drugs, however, the symptoms returned.
So it may be that bodybuilders who eat extremely high-protein diets for years and also use anabolic steroids, do have a higher risk for chronic kidney disease; however, at present, that remains speculative.
1 Wakefield, AP, et al. (2011). A diet with 35% of energy from protein leads to kidney damage in female Sprague-Dawley rats. Br J Nutr. 106(5):656-63.