Dear Mark: Fat Causes Brain Damage?

It feels good to get back to our regularly scheduled Monday morning programming. “Dear Mark” is a way for me to keep my finger on the pulse of the community, to respond quickly and directly to any issues that may arise. I try to keep abreast of all this stuff, but there’s a lot, and some will inevitably slip through the cracks. When that happens, you guys pick up the slack and keep me honest and informed. These Mondays give me a chance to respond to the things I would have otherwise missed or put off until another time. Thanks for that.

This past week, I received a massive influx of emails from readers worried about the results of a new study. I figured it was a good idea to address it. Here are a few of the messages:

I’m very interested in what your commentary would be in relation to this article. The brain inflammation part is very interesting, but it worries me that this is seemingly being contributed to “high fat” meals. I can’t seem to find more info detailing exactly what the test subjects (in this case rats) were fed. What kind of fat?! In what ratios!? I know from reading your blog to be suspicious of data like this and that the devil is often in the details. In any case this research is intriguing and I wanted to draw your attention to it.




My point of contact is to share a study that just came out from the University of Washington a few days ago.  I know you rarely lack blog topics but this is just ridiculous since we are not told what the high fat diet consisted of.

Thanks again for all of your time and research,




Can you please debunk this article:

Study: Brains show evidence of injury after you eat fat. I’m hoping the rats ate bad food, not just fat, or they ate trans fat.


The study in question comes out of the University of Washington and is entitled “Obesity is associated with hypothalamic injury in mice.” Huh. That doesn’t sound so bad. But then you go and read what the journalists wrote about it and you understand, because they are real fear mongers. “High fat diets cause brain scarring,” out of the mega-obscure Cable News Network, was my personal favorite headline. Not only do you get claims of unequivocal causality with that one, but also promises of permanent brain damage – “brain scarring.”  Scary stuff, for sure. And when a loved one or a coworker or a concerned friend shoves it at you, making smug “I told you so” eyes, it can be frustrating, and it can even make you feel like maybe you’ve got it all wrong regardless of your own success, and maybe you should just give it all up. Media is a powerful force indeed, difficult to completely ignore and laugh off. It works, really really well. That’s why it’s there.

Anyway, I actually found this study interesting and useful. One of the friends of MDA, Dr. Stephan Guyenet of Whole Health Source, was fourth author on it. He collaborated on the research and wrote about it on his blog a few days ago, having foreseen the uproar it might provoke from the Primal/ancestral health community. Stephan sums up the study, essentially telling those crazies among us who aren’t scared of butter and lard to rest easy and avoid jumping to conclusions about the effect of a high-fat diet on the brain, because he and his colleagues certainly were not. They were looking at something very different and far more basic: what happens to the brain as an animal begins to grow obese.

The high-fat diet wasn’t the target; obese mice brains were. This study wasn’t really about the diet. The diet was a means to an end – a way to make the rodents obese so that the effect of obesity on the brain could be studied. It’s well-known that high-fat diets make mice pretty fat, even when they’re relatively low in sugar. (Remember, different species with a different ancestral environment.) So, in order to study obesity, the researchers used the most foolproof obesogenic rodent diet around. As Stephan says, “We choose rodent strains that are susceptible to obesity on purified high-fat diets simply because we’re studying obesity, and we know that feeding this diet to the right strains of rats and mice produces it readily.” The scientists simply wanted to make some mice fat and see what happens to their brains. That’s it. They chose the best diet for doing that to mice, and they even took away their running wheels (mice love Chronic Cardio, actually respond well to it, get chubby without it, and have no real substitutes for it; studies indicate that although mice are enthusiastic about the idea of burpees, rodent anthropometry makes actually performing them impossible) to really make sure it happened. 

This doesn’t tell us a whole lot about humans following and losing fat on a high-fat Primal eating plan. I haven’t heard from or met many, if any, people who got fat eating healthy plants and animals. I’m sure it’s happened, and I’m sure it continues to happen, but the vast majority of obesity stems from refined, high-carb, high-industrial-fat, high-sugar, processed diets paired with chronic stress, bad sleep, and sedentary living. It probably is applicable to lots of “regular” people today, many of whom eat the aforementioned diet that makes them fat and most of whom lead sedentary lives. If that’s the case, they should take note of the study’s (not the journalists’ interpretations of the study’s) findings. Bottom line, if you’re getting fat from the food you eat, this study may be relevant to your situation.

There are a couple more things that deserve mention:

The quality of science reporting, especially with diet studies, is just awful. What the journalists derived from the study was the problem; the study itself was not. In a way, I understand. What’s more likely to catch a lay reader’s eye – “High fat diet injures the brain” or “Obesity associated with hypothalamic inflammation”? Exactly. And technically, the science writers didn’t even really lie. They just seized on the fact that a high-fat diet, typically used to make fat rats, was used to make rats fat. And everyone knows fat makes you fat, and that fat people eat “fatty food” (like chips, pizza, burgers, and sweets, low-carb choices all of ’em). This was just more fuel for the fire.

The quality of the diet was not the greatest, and it doesn’t really resemble what people actually eat beyond the macronutrient ratios. It was highly purified. With these studies, consistency is crucial. You can’t hit up the local farmer’s market for pastured lard, organic corn, evaporated cane sugar, pastured egg yolks and expect it’ll be the same as the lab rat diet of industrial lard, maltodextrin, sucrose, soybean oil, and choline. The former are not always the same, not like the latter food-like substances are alike and identical every single time. This pastured egg yolk will have 1.4 times the choline, 2 times the vitamin A, and half the vitamin E as that one. This slab of lard might be 8% PUFA, while that slab might be 12% and the next might be 17%, depending on what the animal ate. You’d probably end up with healthier rats, but controlled diet studies aren’t interested in that. They need to minimize variability to maximize accuracy, so they opt for purified diets with zero nutrient variation between batches. Two 50 gram chunks of Research Diets D12492 (the popular high-fat rodent diet used in the study) will be identical in every way – but neither will be very identical to your real life diet made of real whole foods.

There’s also the fact that the lard the RD12492 diet (PDF) uses is actually far higher in PUFA than previously reported. Until they had the diet directly analyzed, Research Diets (the company that makes the high-fat rat chow) had been using the USDA database listing for lard to determine the fatty acid composition of RD12492. The new analysis revealed that instead of its fat being 17% PUFA, RD12492’s fat was 32% PUFA. That’s a huge difference. Of course, if you’re eating commercial lard from conventionally-raised pigs, you’re most likely getting more PUFA than Fitday or Cronometer is going to indicate, just like the obese rodents. So keep that in mind when thumbing your nose at this study with Oscar Mayer bacon-slicked thumbs (besides, better bacon is worth it)!

Okay, that’s it for today. I just wanted to clear a few things up for people and prevent wide-spread panic. I hope I succeeded. For a discussion of the actual study, I suggest reading the guy who actually did the thing.

Take care, and thanks for reading!

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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