The Protein Scaries: What the Research Says About Your Kidneys, Cancer, Bones, and Body Fat

Jordan Feigenbaum (0:00)

Somewhere in the next month, somebody who cares about you is going to text you a screenshot, a headline. And it is going to tell you that the protein you eat is quietly hurting you. This year, the target is your kidneys. A couple years ago, it was cancer. Before that, your bones. And you wait a while and it's going to be your liver or your heart. It's the same story every few years, just pointed at a different organ. So I'll give you the answer now and then spend the rest of the episode defending it. If you're a healthy adult, you can't eat enough protein to really damage your kidneys, feed a cancer, weaken your bones, or turn a big meal into body fat. The amount it would take to do real harm is so far past what any human actually eats, that you couldn't really get there if you tried. Now, every one of those fears does start from something real. There's a genuine physiological mechanism within the body behind each one. The problem is that somebody took that mechanism and ran it straight to a terrifying conclusion, with no clinical outcomes underneath it. You'll see that move again and again, a real mechanism extrapolated way past what the evidence really supports. I call it mechanistic masturbation, and once you see it, you can't unsee it. And while the whole internet argues about whether you're eating too much protein or too little, almost nobody stops to ask the question that actually matters.

Why are we arguing about one nutrient in isolation at all? I'm Dr. Jordan Feigenbaum, and this is the Barbell Medicine Podcast.

There are four big fears about protein, and we're gonna take all four of them apart, one at a time. One is that it wrecks your kidneys. Two, that it feeds cancer. Three, that it weakens your bones. And four, the one that sounds the most scientific, that your body can only use 20 or 30 grams at one sitting, so the rest is wasted or stored as body fat. Not one of them holds up once you actually read the studies behind it. And I want to leave one fact with you right now, because it tells you almost everything about how these studies work. The single most famous study behind the idea that protein feeds cancer also found that, after age 65, the people eating the most protein had the least cancer. Same study, same table.

We'll get there. But first, let's start with the kidneys, because it is the oldest version of the scare and the one with the most real medicine behind it. There are really two questions in here and they have different answers. So let's take them one at a time. What protein does to a healthy kidney and what it does to a kidney that already has some disease?

Here's what actually happens when you eat protein. Your body breaks it down into its constituent amino acids, just the amino acids that it's made out of, and it uses what it needs to build and repair, and is left with some nitrogen to get rid of. It packages that nitrogen into a waste product called urea. Your kidneys filter the urea out of your blood and you pass it in your urine. Eat more protein and your kidneys filter a little faster to keep up. That filtration speed has a name and it's called glomerular filtration rate or GFR. It is the main number we use to judge how well a kidney is working, unless we do a more extensive workup.

Now, there's a wrinkle here almost nobody hears about. We rarely measure GFR directly. We estimate it usually from a blood marker called creatinine. Creatinine is a breakdown product of muscle. It goes up when you carry more muscle in general, and it goes up for a day or two after you eat a big steak or start taking creatine. So a lean muscular person or anyone who ate meat before their blood drop or someone who just started taking creatine monohydrate, well, they can show a lower estimated GFR and it kind of looks like their kidneys are failing when nothing has actually changed. Part of the protein-rextra kidneys signal is that measurement quirk rather than any real damage.

So where did the scare come from? Decades ago, researchers took animals, removed most of one kidney and watched the small piece that was left work harder and eventually wear out. This gave us the hyperfiltration hypothesis, the idea that making a kidney filter harder ages it faster. More protein, more filtering, more wear. On paper, seems like a reasonable worry. And in 1982, that animal idea became a human recommendation in one famous paper in the New England Journal of Medicine by Brenner and colleagues. They argued the same hyperfiltration was slowly damaging human kidneys and that we should cut protein to protect them. That single paper launched decades of low protein diets for kidney patients. It's a perfect example of the thing I told you to watch out for. A mechanism became medical advice years before anyone had the outcomes to justify it. So let's test that in healthy people. In 2018, a meta-analysis in the Journal of Nutrition pooled 28 controlled trials. More than 1300 adults with normal kidneys. Higher protein against lower. The higher arms were up around 1.5 to 2 grams per kilo of body weight or more. The lower arms sat near the old RDA. Around 0.8 grams of protein per kilogram body weight. Now, the fair knock on these trials is that they're short, mostly weeks to a couple of years and not decades. So, hold that in mind while we go through them. But, across that time period, the higher protein groups did filter faster and their actual kidney function didn't decline at all. The kidney did more work the way any healthy organ does when you ask more of it and it held its own. Now, you see the same thing in people you would recognize. In one study, resistance trained men ate more than 3 grams of protein per kilogram body weight, over 300 grams of protein a day for a 200 pound guy. Then they tracked them for a full year and their kidney markers, their liver enzymes and blood lipids didn't move at all. It's a small study, just 14 dudes, and they reported their own food, so it's not proof on its own, but it does agree with the larger review.