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

0:00Dr. Jordan Feigenbaum: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.

0:31The 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.

1:08Why are we arguing about one nutrient in isolation at all? I'm Dr. Jordan Feigenbaum, and this is the Barbell Medicine Podcast.

1:23There are four big fears about protein, and we're going to 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.

1:59Same 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'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.

2:42That filtration speed has a name and it's called glomerular filtration rate or GFR and 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 and 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 draw 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.

3:22Part of the protein wrecks your 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.

3:57They 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 pulled 28 controlled trials, more than 1 ,300 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.

4:31The 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.

5:08They tracked him 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. Now the part that actually matters clinically. The largest long-term look at this followed about 1 ,600 women for 11 years. In the women who started with normal kidneys, higher protein had no link to any decline in kidney function. But in a smaller group who already had mild impairment at the start, meaning that their filtration was already somewhat reduced, higher protein, and animal protein in particular, tracked with a faster drop in their GFR.

5:46Now there's a reason that makes mechanistic sense. A healthy kidney carries huge reserves, far more filtering capacity than it needs on a given day. So a little extra work is nothing. A kidney that has already lost units is running the survivors closer to their limit just to keep up, so asking for more may lean on tissue that is already maxed out. Still, this is an observational study, so it can't really prove that protein did anything, and animal protein is not a clean variable on its own. In these datasets, a lot of animal protein typically rides along with a low-quality diet, more processed food, more of everything that comes with it, added sugars, added sodium, so on and so forth.

6:22So even here, the truth is that a lousy dietary pattern is doing a lot of the work that we keep pinning on protein. Which brings us to real kidney disease. Chronic kidney disease means the kidneys have been losing function for months or years, usually staged by how far the filtration rate has fallen. The logic here is reasonable. If more protein makes a damaged kidney work harder, then cutting protein should protect it. That is why low-protein diets have been prescribed to kidney patients for decades. So does it actually work? The biggest test we have is a 1994 trial called MDRD, the Modification of Diet and Renal Disease Study.

6:58840 patients with real chronic kidney disease, protein was cut hard, the main comparison taking people down to about 0.6 grams per kilogram body weight against a usual intake of near 1.3 grams of protein. And again, this was tracked for years. The main result, the one the study was built to find, came back with no significant benefit. it. The low protein group didn't decline meaningfully slower, and there was no reduction in how many people reached dialysis or died. The study everyone remembers as proof that cutting protein saves kidneys didn't actually show that. Now to be fair, there was a hint in the details.

7:33The most aggressive arm slowed decline a little, but it fell short of statistical significance, a p-value of 0.07, which means we can't rule out that it happened by chance. It's a maybe, and it's unlikely to be clinically meaningful, and you don't build a medical recommendation on a maybe. The most recent word is a 2020 Cochran review, 17 trials of about 3 ,000 patients. A moderate low-protein diet, around 0.6 to 0.8 grams of protein per kilogram body weight, the kind of thing a person can actually live on, made little or no difference in whether patients reached dialysis, and no difference in whether they died.

8:09Only when you pushed all the way down to a very low protein diet, about 0.3 to 0.4 grams of protein per kilogram body weight, did you see a real reduction in progression to dialysis. Now for a normal size adult, that's roughly 30 grams of protein for the entire day, which is almost nothing. And to hit it, you would basically be living on sugar and fat. And even that only delayed dialysis. It didn't help anyone live longer. It delayed the machine. Nothing more. Now, a good nephrologist would stop me right there, because MDRD and that Cochrane review were mostly in people without diabetes. And diabetes is the number one cause of kidney disease in this country.

8:44In people with diabetic kidney disease, cutting protein under 0.8 grams of protein per kilogram body weight does track with a slower decline in GFR and less protein spilling into the urine. And the clear signal is in people with type 1 diabetes. Type 2, which is the far more common cause, the case is much weaker. But here's what I can't unsee. And I'll say up front, this is not my lane. I'm not a nephrologist, and there's certainly some nuance here. Still, every time I look at this, it kind of smells funny. The people who actually managed to cut their protein also changed their entire diet. They lost weight, they dropped sodium, they cleaned up the whole dietary pattern.

9:20I find it very hard to believe the protein number itself is the thing that helped, rather than everything else that came with lowering it. The condition is mostly what it is, and a better diet probably helps, and that better diet happens to be lower in protein for these folks. but it doesn't make protein the lever. And I'm not really out in left field here. In 2022, three nephrologists published a piece in the Journal of the American Society of Nephrology and titled it, bluntly, Protein Restriction for Kidney Disease. It's time to move on. They go through these same trials and conclude that the guidelines overstate the evidence.

9:52And they land on the point that runs through this entire episode. When you look at what actually tracks with the kidneys failing, it comes down to red meat, specifically. In a study of 63 ,000 people, red meat raised the risk of kidney failure in a dose-dependent way, while total protein did not, and neither did poultry, fish, eggs, or dairy. It's the same story as the cancer segment we're going to get to. The red meat and the dietary pattern it comes along with is doing most of the work. And one more point from those authors. Almost all of the low-protein diet research was done before the drugs we now use to actually protect the kidneys, the ACE inhibitors and ARBs, and the newer SGLT2 inhibitors.

10:30So whatever small benefit protein restriction might once have had is even smaller today, layered on top of treatments that actually work. And even where cutting protein moves something, we gotta look at what it moves. A slower change in GFR and less protein in the urine are surrogate markers, numbers on a lab report. What a patient actually cares about, living longer, staying off dialysis, feeling well enough to have a life, mostly doesn't change. and quality of life usually is not even measured. On top of that, almost nobody stays on a low-protein diet once they leave the office. A benefit that only exists on paper from a diet almost no one can follow doesn't really seem like a big benefit.

11:08And step back for a second because this is the part that really gets me. The whole framing is that you can eat all the protein you want unless your kidneys are bad. But why are the kidneys bad? Usually because of the same metabolic mess, the diabetes, the blood pressure, the dietary pattern that we should have been talking about the entire time. Zeroing in on the protein number in someone whose kidneys are already failing is kind of like arguing about the deck chairs on the Titanic. And most people flagged with early kidney disease aren't headed for dialysis at all. For a middle-aged person with stage 3 chronic kidney disease, the lifetime risk of kidney failure is in the single digits.

11:41She isn't zero, but we're asking them to give up the food they love for years to maybe delay something that the large majority were never going to reach. Now there's an old line in nephrology quoted at the very top of that same piece we were talking about. All a low-protein diet does is shrink the patient down to the size of his kidneys. And cutting protein isn't free. These are often older patients who are already losing muscle, and strength and physical function predict who stays independent, who avoids a fall and a fracture, and who lives longer better than almost any number on any lab report.

12:12When you push their protein down to protect a lab value, buffing the chart, you can speed up the muscle loss that shapes the rest of their life. And if someone truly has to restrict their protein, that is the exact person who should be lifting. A 2001 randomized trial out of Tufts put older patients with kidney disease on a low-protein diet and had half of them do resistance training. The ones who trained built muscle and got stronger on that same protein-restricted diet. The ones who only did the diet, well, they lost strength. It's the same low protein for both groups. The training is what protected the muscle.

12:45That fits what we actually know about aging muscle. Older muscle does respond a little less to dietary protein on its own, what we call anabolic resistance. But that is mostly not about aging itself. It's driven by the medical conditions and the inactivity that pile up as people get older, and it's worse when those two are combined. Fix the inactivity, and a lot of it's going to improve. Eating more protein helps a little. Loading the muscle helps a lot. So an older patient with kidney disease is really managing two problems at once. The kidney, with their nephrologist, and the muscle, in the gym.

13:17And most older adults have the opposite problem entirely. The expert consensus is now that they need more protein than the old guidelines say, up around 1.2 grams per kilogram body weight and beyond, just to hold on to the muscle as they age. They don't need less protein. Advanced kidney disease is the real exception, and even there, the muscle still needs training. So the kidneys. In a healthy adult, protein doesn't damage them. And the large studies we have all agree on that. On someone whose kidneys are already diseased, it becomes a real careful conversation with a nephrologist, weighed against muscle and quality of life.

13:49But even there, I suspect the protein number matters far less than the dietary pattern it's attached to.

13:54Dr. Jordan Feigenbaum:Either way, it's a universe away from a healthy adult being told that their protein shake is dangerous. Every few episodes, someone asks if eating more protein is going to wreck their kidneys. Now, for most people, the data on that is very reassuring, but lead in your drinking water is a different conversation. The EPA and CDC agree there is no known safe level of lead exposure. The EPA's health-based goal for lead in drinking water is zero, and we're nowhere close to that. The problem is that there are millions of lead service lines still in the ground across the country. So the pipes, the solder, and the plumbing between the treatment plant and your faucet break down over time, and that's how lead gets into the tap water.

14:32Dr. Jordan Feigenbaum:There's a federal mandate to replace them, but it's on a 10-year timeline, and most of that work hasn't started yet. In adults, chronic low-level lead exposure has been linked to all sorts of things like kidney damage, high blood pressure, and cardiovascular disease. And these are claims from the WHO and CDC's position. The thing is, you'd never know from looking at your water or tasting it. So that's why I've started filtering my water with Cove Pure. It's a countertop reverse osmosis system. There's no plumber, no installation needed. You plug it in, it sits on your counter. And reverse osmosis is one of the most effective filtration methods for removing lead, PFAS, arsenic, and nitrates.

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17:19Dr. Jordan Feigenbaum:So you're paying for the quality and not somebody's logo or marketing campaign. And it's not just clothing anymore. Quince has turned into my go-to since I've moved into a new home and they've got travel stuff as well. I use this and you should too. Make your summer wardrobe easier. Go to quince.com slash bbm for free shipping on your order and 365 day returns. Now available in Canada too. That's quince.com slash bbm for free shipping and 365 day returns. Now, the scariest one this whole episode and the one with the most real science tangled up in it. Protein feeds cancer. I'm going to give you the strongest version of that case first because that's the version worth answering.

17:58When you eat protein, your body releases a hormone called insulin-like growth factor 1, IGF-1 for short. Now, this is a growth signal. It tells the cells to grow, to divide, and to hold off on dying. And a cancer, at its core, is a group of cells growing, dividing, and refusing to die on schedule. So, a diet that keeps that growth signal turned up, the thinking goes, gives a tumor a friendlier place to live.

18:21Dr. Jordan Feigenbaum:Now, that's a real concern. IGF-1 is a real hormone doing a real thing, and serious scientists have spent careers on the link between it and cancer. If you're going to worry about protein, this is the most defensible place to do it. So let's go to the study this whole idea comes from, even though most people have never heard its name. It was published in 2014 in the journal Cell Metabolism, and it's the reason that protein causes cancer is out there in the first place. Headline's not made up. The finding is real. It just got badly misread. So here's what they did. They took a national U.S. health survey, about 6 ,300 adults over the age of 50, and asked each person to recall everything they ate one time for a single day.

18:59Then they followed those who died over the next 18 years, and they sorted those people into high-protein groups, meaning that they ate 20 % or more of their daily calories from protein, and they also classified them into moderate and low groups. And put that into real food.

19:13Dr. Jordan Feigenbaum:20 % of calories is about 100 grams of protein on a normal day for an average adult. That's a chicken breast, a cup of Greek yogurt, and a couple of eggs. The high-protein group wasn't eating some carnivore mountain of meat. So let's think about that design for a second. A single day of eating, standing in for 18 years of a life. Whatever you happen to have for lunch on a Tuesday in 1994, treat it as your diet forever. Food recalls are kind of janky anyway, everybody knows that. But the deeper issue is what a one-day snapshot is even capturing. At best, it's a rough proxy for a person's overall dietary pattern, and that pattern is what really tracks with health.

19:49Dr. Jordan Feigenbaum:And here's the part nobody screenshots. When they looked at everyone over 50 together, higher protein had no link to dying of cancer at all. None. The scary number only shows up when they split the group by age. In the 50 to 65 band, the high protein eaters were about 4 times as likely to die of cancer. That's the number that started the panic. But 4 times as likely is a comparison, not your odds. If your baseline risk of cancer is small, 4 times a small number is still a small number. And that 4 is shaky on its own. Slice the study that thin, and the real range runs anywhere from about double up to 10 times.

20:25Dr. Jordan Feigenbaum:A range that wide means the study can't actually pin the number down. Then look at the other half of that same table in the same study. The people over 65. There, the effect doesn't just fade, it reverses. Over 65, the people eating the most protein had about 60 % less death from cancer. The same diet that supposedly feeds tumors at 60 protects against them at 70. Now, a new tree doesn't change its nature on your birthday. When one thing points in the opposite direction in two halves of the same data set, what you're looking at is noise or something else traveling along with the protein. And there's plenty that's also been traveling with protein.

21:03Dr. Jordan Feigenbaum:The risk only showed up for animal protein, never for plant protein. And realistically, the person eating a lot of animal protein in a survey like this is eating a lot of processed food and red meat, specifically processed red meat, as part of an overall low-quality diet. It's the whole dietary pattern. The fast food, the processed meat, everything clusters together. It's kind of like a Trojan horse. That person differs from somebody who's eating a lot of legumes in about 40 different ways besides just protein. Now, the authors also ran a mouse experiment to try to prove causation. And in the mice, low protein did slow tumor growth.

21:37Dr. Jordan Feigenbaum:But the low protein mouse diet was 4-7 % protein, a level of near starvation that no human being can really eat. and they had injected the tumor cells directly into the animals. And the growth signal they pinned the whole thing on, IGF-1, in their own human data, it didn't carry the effect. When they tested whether IGF-1 was the thing actually linking protein to death, the numbers didn't support it. The mechanism they built the entire story on didn't show up in their own people. So what does this study actually show? That in one survey from one day of eating, animal protein tracked with more cancer death in one age group and less than another.