It seems like every other week there is a media uproar about the dangers of any number of foods. Lately the target has been meat specifically high-protein diets and its supposed connection to a range of chronic diseases and early death.

I've discussed high-protein diets a couple times on my podcast (here and here) but with the recent press surrounding a new study on high-protein diets and cancer risk there's been yet another upsurge in concern over their safety.

In this article we'll take a look at the research behind three of the most common concerns about high-protein diets: kidney health cancer and longevity. By the end of this article you'll know what the real danger underlying high meat consumption may be and how you can easily avoid it by eating a Paleo diet.

High-protein diets don't cause kidney disease in healthy people

Before getting into this I want to make something clear. Research does show that high-protein diets can be harmful for people who already have chronic kidney disease and low-to-moderate protein diets are generally advisable for these patients.

However just because a low-protein diet can be therapeutic for those with kidney disease doesn;t mean a high-protein diet causes kidney disease in the first place. (This is the same distinction I made when critiquing Dr. Perlmutter's broad recommendation for a low-carb diet to prevent neurological disorders.) What I'm addressing here is the notion that high-protein diets cause kidney disease in healthy people which is not as you'll find out supported by research.

Since one of the main biological roles of the kidney is to metabolize and excrete nitrogen byproducts from protein digestion many people believe that eating more protein will strain the kidneys. This is similar to the argument made against acid-forming diets. There is an upper limit to the body's ability to metabolize protein (studies suggest that this limit is around 35 percent of total calories) but the brain has specific mechanisms that regulate desire for protein and these mechanisms are difficult to override through willpower alone. (1)

It's clear from controlled trials that high-protein diets do induce measurable changes in kidney function. (2 3 4) These changes include increases in glomerular filtration rate (GFR) (often referred to as hyperfiltration) and an increase in the size and volume of glomeruli which are the functional filtration units of the kidney. (5) The sticking point seems to be in how these changes are interpreted because while some researchers view hyperfiltration as a sign of kidney stress and even damage others view it as the kidneys simply getting better at doing their job.

The best paper I've found to explain this issue shows that hyperfiltration is a normal adaptive response to additional protein in the diet as opposed to a pathological condition that will eventually lead to kidney disease. (6) Pregnancy is discussed as one case where GFR increases significantly but does not increase the risk for kidney disease.

A more compelling example is that of someone who has donated one of their kidneys because in these cases GFR in the remaining kidney increases as an adaptive response and remains elevated. One would expect that if hyperfiltration leads to or indicates kidney disease increased kidney disease would be found in these patients down the road. However studies have not found a higher risk for kidney disease in patients with one kidney even 20 years after donation.

After reviewing all of the published research on high-protein diets and kidney disease the authors of this paper concluded that while high-protein diets can be harmful for those with kidney disease they do not harm the kidneys in healthy individuals. Since that paper was published new studies have tested the effects of high-protein diets on renal function in healthy individuals and generally their conclusions are the same. (7 8 9)

The newest meat will give you cancer study

Now to the primary motivation for this article: the study behind headlines such as Diets high in meat eggs and dairy could be as harmful to health as smoking. In this study researchers reported a positive association between high protein intake and cancer incidence based on observational evidence from people between the ages of 50 and 65. (10)

However they found that high-protein diets were actually associated with lowermortality from cancer in people over 65. (In this study high protein was defined as at least 20% of calories from protein.) Importantly this relationship only existed for animal protein; researchers found no relationship between a high consumption of plant protein and cancer or mortality.

For a thorough critique of this study I recommend reading Denise Minger's analysis. As she points out the oft cited healthy user bias is less relevant to this study because protein hasn't been demonized such that people who eat more protein would be expected to have unhealthy lifestyle habits. In fact the high-protein participants in the study actually had slightly lower rates of smoking.

The major downside of most observational studies is that they don't shed light on the mechanisms behind the associations they uncover. On this point this study is designed better than most. Researchers hypothesized that increased IGF-1 activity due to high protein consumption may lead to a higher instance of cancer and an earlier death so after observing a positive relationship between these variables they designed a mouse study to test the mechanism of their hypothesis.

They found that mice eating a higher protein diet (18% of calories) grew larger tumors at a higher rate than mice on a low protein diet (4-7% of calories) and that the high-protein mice had higher levels of circulating IGF-1. (IGF-1 is a hormone that prompts cell growth in almost every tissue of the body.)

This is unsurprising as earlier animal studies have shown that increasing protein intake (especially from isolated casein which tends to promote cancer growth more than other sources of protein anyways) increases IGF-1 levels and it's well-known that IGF-1 encourages the growth of cancer cells as well as healthy cells. However as Denise brings up in her analysis total protein restriction is only one way that researchers have been able to decrease circulating levels of IGF-1 in rodent studies.

Is protein to blame or is methionine?

This is also where the issue of longevity comes into play because increased levels of IGF-1 are thought to contribute to accelerated aging and shorter lifespans. Many early studies found that calorie restriction reduced IGF-1 and increased lifespan in many animal models in addition to protecting against cancer. (11) Researchers then discovered that restricting total protein but not total calories accomplished the same goal often more effectively than calorie restriction. (12 13)

Recently the amino acid methionine was targeted as the primary operator in the protein/IGF relationship and new animal studies demonstrated that methionine restriction alone was able to reduce IGF levels and extend lifespan. (14 15 16) Methionine is an amino acid found primarily in muscle meats and eggs and I frequently emphasize the importance of maintaining a healthy methionine-to-glycine ratio by consuming glycine-containing foods like bone broth and gelatin.

This evolution of research from calorie restriction to methionine restriction is already fascinating but here's the kicker a study done in 2011 found that supplementing with glycine had the same life-extending IGF-reducing health-promoting effects as restricting methionine intake (and restricting protein intake or overall calories)! (17)

Eat the odd bits!

Granted this is one study and it was done in mice. But from an evolutionary perspective this connection makes perfect sense! Until recently humans were not getting a majority of their protein from high-methionine foods such as muscle meat and eggs. We just didn't have the luxury of heading to the nearest grocery store and picking out steak chicken breasts and pork tenderloin.

It can be easy to forget when these cuts are all packaged up nicely in the store but those “prime cuts used to be attached to bones cartilage skin organs and all the other odd bits that now usually end up in pet food (so at least Fido is getting his glycine!).

These odd bits (especially liver) also have other nutrients in addition to glycine that help the body metabolize methionine including vitamins B6 B12 folate betaine and choline. As is often the case traditional foods have a range of nutrients that work together synergistically and whole foods tend to be much healthier when they're left whole.

So will a high-protein diet give you kidney disease and cancer? As far as kidney disease goes the research suggests that the answer is no. But when it comes to cancer and longevity it depends on the overall context of your diet. If you're getting a high percentage of calories from protein and you eat muscle meats and eggs without glycine-rich foods and organ meats there is reason to believe you may be at higher risk for cancer. Fortunately if you're following the advice I outlined in Your Personal Paleo Code and eating nose-to-tail you need not be concerned about eating a high-protein diet.

by Chris Kresser M.S. L.Ac

Chris Kresser M.S. L.Ac is a practitioner of integrative and functional medicine and the creator of ChrisKresser.com one of the most respected natural health sites in the world. He is widely known for his in-depth research uncovering myths and misconceptions in modern medicine and providing natural health solutions with proven results. He developed the Personal Paleo Code based on over ten years of research his own recovery from a debilitating decade-long illness and his clinical work with patients. Chris maintains a private practice in Berkeley California where he lives with his wife and daughter. 

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How do you feel about the research Chris presented? Are you planning on making any changes to your diet or has Chris simply boosted your confidence that your current diet is appropriate? Let us know in the comments below!