Dear Mark: Is a High Protein Diet Really As Bad for You As Smoking?

It’s about that time, boys and girls. A new “protein kills” study has arrived to throw you into the pit of existential angst and self-doubt you recently managed to crawl out of from the last one. As you may know, I’ve just spent a week in Tulum, Mexico for PrimalCon (which was amazing, by the way, absolutely fantastic) where I managed to avoid most contact with my inbox. Oh, I took a couple glimpses here and there, enough to notice an endless stream of frenzied email subject lines, but I didn’t read the contents until the flight home. I still knew what was coming.

I finally have a little time to dig into this paper (which actually covers two studies) to see if there’s anything we can learn. Let’s jump right in…


A co-worker informed me of a news clip she’d heard regarding an increased cancer risk for those who stick to a high animal protein-based diet. She said that smoking cigarettes is healthier than eating high fat/high protein diet. My first reaction was to chuckle (just another CW media attack against Paleo/Primal) and I did once I read the actual piece in Forbes. I wanted to get your thoughts on it as a lot of the article’s evidence once again seems inconclusive. Seems like there are a lot of caveats.


Right off the bat, we can read the paper and see an important part that the journalists conveniently left out:

Conversely, high protein intake was associated with reduced cancer and overall mortality in respondents over 65, but a 5-fold increase in diabetes mortality across all ages.


Using Cox Proportional Hazard models, we found that high and moderate protein consumption were positively associated with diabetes-related mortality, but not associated with all-cause, CVD, or cancer mortality when subjects at all the ages above 50 were considered.

Huh. From age 50-65, protein modestly increases mortality, especially cancer- and diabetes-related mortality. In ages 50+, though, a high protein intake has no association with all-cause mortality (the most important endpoint), while in ages 65+, high protein is downright protective. The only consistent positive association across all ages is with diabetes mortality. How can this be?

It’s well known that seniors are less efficient at utilizing protein than younger people. Muscle protein synthesis is slightly impaired in the elderly, for example. As such, they need to eat more of it to get the same benefits as the rest of the population. It may be that they also have a higher upper threshold for protein until harm ensues – a higher “protein ceiling.”

Other populations also have higher ceilings. Runners, cyclists, dancers, swimmers, weight lifters (especially weight lifters) all need more protein than the sedentary, just like the seniors. Unfortunately, the authors did not control for exercise. Too bad, because it probably would have modified the relationship between protein intake and mortality in the other age groups.

I think if anything can be taken from this study, it’s that you should only eat as much protein as you require. We likely all have “safe protein ceilings” depending on various factors, like age and activity level. The ceiling is higher the older and more active you are.

Since the connection between diabetes-related mortality and protein persisted even among the older crowd, let’s look at that a little further. Does other research agree that protein is bad for diabetics?

Well, in the most recent meta-analysis dealing with diet and diabetes, researchers concluded that “low-carbohydrate, low-GI, Mediterranean, and high-protein diets are effective in improving various markers of cardiovascular risk in people with diabetes and should be considered in the overall strategy of diabetes management.” They looked at both epidemiology and clinical trials to reach that conclusion, so it’s not clear how protein is increasing diabetes mortality in the current study if the relationship is causal.

I’m not suggesting that you eat all the protein you want as long as you’re exercising or elderly. I’ve even softened my “high-protein” leanings over the years, advocating instead for a more moderate approach. One gram protein per pound of lean tissue is plenty for most active people, and many can get by on half that.

The Mouse Study

Okay, that’s one part of the paper. There was also a study that found both breast cancer and melanoma tumors grew more slowly in mice on a 4-7% protein diet than mice on an 18% protein diet. High protein mice also had higher levels of IGF-1, a growth promoter, and the rate of tumor growth was associated with the amount of IGF-1 secreted. The protein was entirely casein, with a touch of added methionine.

When I saw that they used casein, it reminded me of something from Chris Masterjohn’s analysis of the T. Colin Campbell casein/cancer studies. While high intakes of casein promoted the growth of existing tumors in those rodents, it was also protective against the mutagens that cause the initial appearance of tumors. Protein was protective against cancer until they had it, at which point it accelerated the cancer’s progression. Rodents on the low protein diet were more susceptible to getting cancer after aflatoxin exposure. Once the rodents already had cancer, low protein was protective against further growth.

Does this phenomenon apply to the most recent study? Impossible to tell. The mice were directly injected with melanoma and breast cancer cells, not exposed to a cancer-causing toxin or exposed to high levels of UV radiation. This usually isn’t how cancer develops in the real world, unless we’re talking about mistakenly receiving a cancerous organ transplant. We tend to develop cancer in response to mutagens in the environment.

That said, we can’t discount the results entirely. Getting your protein only from casein is unwise (and unfeasible when eating whole foods). Methionine (an amino acid common in muscle meat which does increase IGF-1) probably should be balanced with glycine (an amino acid common in connective tissue and gelatinous cuts of meat). I think glycine is especially important for a few reasons:

Realize that the Primal eating plan doesn’t prescribe “eating lots of meat.” Instead, we “eat animals,” and animals have bones and gristle and organs and tendons and loins and ribs and shoulders and all kinds of edible bits and pieces with extremely varying amino acid profiles. Over the years, I’ve been emphasizing these “alternative” cuts more and more. The folks profiled in the study’s data sets probably weren’t eating whole animals. The mice definitely weren’t. We largely just eat meat, and I think that should probably change if we want to maintain optimal health and avoid some of the diseases the epidemiology keeps pointing at.

There are also untoward effects of ultra-low protein intakes to consider. While this paper was only concerned with the cancer endpoint, free-living, free-thinking, autonomous humans with hopes, desires, aspirations, and goals aren’t only worried about cancer. We’d certainly prefer not to have cancer, but what if the narrow pursuit of the ultimate anti-carcinogenic diet has unpleasant side effects that affect the enjoyment of our lives?

Consider that low protein diets:

In other words, starving yourself of protein (the mice in the study achieved the protective effects by eating just 4-7% of calories as protein, which qualifies as protein starvation in most books) may reduce tumor growth, but it might make you fatter and weaker, give you brittle bones, prevent you from taking care of yourself in the later years, and make you more susceptible to illness. Oh, and it may even make you more vulnerable to initially getting cancer (if the Campbell casein studies are applicable to people). But that’s all worth it if once you have cancer the tumors grow slightly more slowly, right?

Even IGF-1 isn’t evil. It promotes growth across the board – in muscles and in tumors. Tumors suck, but not everyone has tumors. Everyone does have muscles. IGF-1 secretion also promotes neural growth and resistance to neurological degeneration. In a study mentioned in yesterday’s Weekend Link Love, elderly women improved muscular and cognitive function by eating a diet high in protein derived from red meat and lifting heavy things. Another study found that increased IGF-1 was linked to improved memory and vigor in both middle-aged and elderly subjects. I don’t know about you, but “living long and dropping dead” is all about vigor for me.

Think about it. Vigor. Sure, it’s not exactly a traditional health biomarker, but who’s got it? What kind of older people display vigor?

The sassy centenarian who eats bacon for breakfast every single day.

The old guy with the vise-grip for a handshake.

That old dude surfing with all the guys a third his age.

The 85 year-old great grandmother who shoulders past you at the farmer’s market to get the best leeks.

The old lady who beats you at raquetball.

I will take vigor every single time even if it modestly increases the speed of tumor growth on the off chance I get cancer. That’s just me, though.

Summing Up

As I see it, there are some potential takeaways, keeping in mind that half the paper was based on epidemiology:

Don’t eat more protein than you actually need. Get extra calories from fat and, if activity warrants it, carbs.

Eat the whole animal. Don’t just eat muscle meat, but also bones, ligaments, shanks, cartilage, tails, and organs. Supplementary gelatin is a good idea if you don’t eat the aforementioned foods.

Don’t eat a diet of casein, methionine, soybean oil, corn starch, and sucrose, especially if you have cancer.

And some questions:

High IGF-1 levels are linked to bad things like cancer, but also good things like strong muscles and a sharp mind. How do we balance the two?

Were processed meat and fresh meat lumped together? Is pepperoni pizza “meat”?

What about people younger than 50? They weren’t addressed at all.

How valid is the dietary data? It was gathered by calling people on the phone and asking them what they’d eaten the previous day and whether that was typical of their normal diet. Seems awfully unreliable and prone to human error.

That’s my take. There are many others out there, but this is mine. What do you think?

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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