Protein has always been my safe space in the diet wars. As the pendulum swings back and forth between “carbohydrates are going to kill you” and “fat is going to kill you,” everyone agrees that protein is good for your muscles, satiating, and non-lethal. Everyone except nephrologists, that is.
Back in 1982, a nephrologist—that is, a kidney doctor—named Barry Brenner, co-author of one of the definitive textbooks on nephrology, published a paper in the New England Journal of Medicine suggesting that eating too much protein over a long period of time might overwork the kidneys and eventually lead to chronic kidney disease and even kidney failure. This idea has been hotly debated ever since, with opinions loosely settling into two distinct camps: nephrologists think too much protein is a problem, and sports nutrition researchers think it isn’t.
As someone who pays a lot of attention to the sports nutrition literature and very little attention to the latest nephrology news, I haven’t worried much about Brenner’s hypothesis. But a recent review paper by three nephrologists in Sports Medicine—in my world, in other words—caught my attention.
Alberto de Lorenzo of HM Sanchinarro University Hospital in Madrid, along with two colleagues, argues that chronic kidney disease might be “the real finish line” for athletes and bodybuilders on a high-protein diet. Given all the articles I’ve written about the joys of protein, I figured it was worth digging into.
Why Kidney Specialists Worry About Too Much Protein
Most of the protein you eat is broken down into amino acids which are then used to build or repair muscle. If there’s any extra protein, it will be burned to produce energy. One of the byproducts of burning protein is a waste product called urea. The job of filtering out waste products like urea from your bloodstream is handled by your kidneys, which send the unwanted waste to be excreted in your urine. The more protein you eat, the more urea you’ll produce—and, consequently, the harder your kidneys will have to work.
It’s reasonably well established that eating more protein will lead to an increase in kidney filtration rate, presumably to handle the higher waste load. This is accompanied by dilation of the blood vessels leading to the kidney and an increase in the pressure on the filtering units. Similarly, eating less protein reduces the filtration rate. For this reason, people who already have chronic kidney disease but aren’t on dialysis are often advised to keep their protein intake relatively low.
What some nephrologists fear is that, even in people with healthy kidneys, this increased blood flow and pressure will eventually damage the delicate filtering structures in the kidney—which in turn will force the remaining kidney structures to work even harder, triggering a kidney death spiral.
The evidence for this sequence of events is clear for people (and animals) who already have kidney problems. But it’s basically non-existent for people with normal kidney function. That means it becomes a question of weighing different kinds of indirect evidence.
Why Sports Nutritionists Don’t Worry About Protein
The International Society of Sports Nutrition, in its official position stand on protein and exercise, is unequivocal: “no controlled scientific evidence exists indicating that increased intakes of protein pose any health risks in healthy, exercising individuals.” Moreover, the position stand cites a series of studies led by Jose Antonio, the cofounder of the ISSN, in which athletes consumed as much as 4.4 grams of protein per kilogram of body mass each day, more than five times the recommended daily allowance of 0.8 g/kg/day, without any ill effects.
De Lorenzo and his colleagues aren’t impressed by these studies, or by the larger body of studies in healthy non-athletes. The sample sizes were small, and their durations ranged from six weeks to a year, which might not be long enough to observe a decline in kidney function. The tests used to estimate kidney filtration rate are designed for use with kidney disease patients, and are less accurate in people with healthy kidneys, particularly if they have high muscle mass. There were no true low-protein-diet control groups. The authors have ties with sports nutrition companies that sell protein supplements, and none of them were nephrologists.
These critiques are all worth considering, but you have to weigh them against the strength of whatever other evidence is available—which, in this case, is not much. I dug up a few of the individual studies and meta–analyses to look at more closely, and my general impression was that it was basically a Rorschach test. Depending on which outcomes you look at and what threshold you use to define a clinically significant change, you can convince yourself that high protein intake has a strong effect on kidney function or has no effect on kidney function.
How Much Protein is “Too Much”?
Half a century after Brenner’s original warning, I still don’t see any compelling evidence that healthy people need to worry about eating too much protein. I can’t rule it out, though. So it’s worth considering some numbers. The protein RDA is 0.8 g/kg/day, which for a 150-pound person works out to about 55 grams of protein per day. That’s a couple of 5-ounce cans of tuna.
De Lorenzo’s paper defines a “high-protein diet” as more than 1.5 g/kg/day. The ISSN suggests that “the majority of exercising individuals” should aim for 1.4 to 2.0 g/kg/day, though it notes some “preliminary evidence” that higher doses even above 3 g/kg/day might help build muscle. A major meta-analysis in 2018 concluded that muscle- and strength-building gains max out at around 1.6 g/kd/day on average.
The average daily protein intake among Americans, as of 2015-2016, was 97 grams for men and 69 grams for women. Depending on weight, that’s in the range of 1.0 to 1.5 g/kg/day. In broad strokes, then, I suspect that the average person could probably still benefit from upping their protein intake.
In particular, focusing on getting more protein throughout the day rather than cramming in a huge load at dinner might (though not necessarily) help build more muscle. But I’d pause before pushing too far into the extremes, like 3 or 4 g/kg/day—not because there’s strong evidence that it’s dangerous, but because it’s conceivable that there might be downsides and not clear that there are significant upsides.
As it happens, that balancing act is evident in another new study that was published last month looking at protein intake and mortality in older adults who already have chronic kidney disease—in other words, the group that we know will be hurt by too much protein. The results were the opposite of what you’d expect: the more protein subjects ate, the less likely they were to die during the study. Higher protein intake is associated with all sorts of interconnected benefits in older people: more muscle, stronger bones, less frailty, lower rates of heart disease, and so on. In this cohort, these benefits outweighed whatever theoretical harm protein might inflict on the kidneys. For muscle-challenged endurance athletes like me, that’s a message to keep in mind.
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