Dear Mark: Saturated Fat More Harmful to Liver Than Sugar?

For today’s Dear Mark, I’m answering just one question from a reader. What are we to make of the new study purporting to show that saturated fat is the most harmful substance a liver can encounter? Should we remove all traces of it from our diets? Should we eat pure sugar? Quaff soybean oil? How relevant is an overfeeding study to a community of people dedicated to eating a sustainable, weight-reducing or -maintaining diet that includes saturated fat?

Let’s find out:

Hi Mark,

Did you see this study?

What’s your take on it? Just got one of those classic emails from my vegetarian friend with only a link to the study and a smug emoji.

Oh yes, I knew this would come up after I linked to in on Weekend Link Love.

The title is “Saturated Fat is More Metabolically Harmful for the Human Liver Than Unsaturated Fat or Simple Sugars.”

I have a human liver. Do you? Probably. This is highly relevant to all of us, right?

Well, the first thing to understand is that this was an overfeeding study. Participants didn’t just eat a eucaloric diet with different energy sources. They each got a daily 1000 calorie snack on top of their normal diet for three weeks.

The SFA group ate 1000 extra calories in the form of butter, coconut oil, and blue cheese. The SFA:MUFA:PUFA ratio was 76%:21%:3%. Their liver fat increased by 55%.

The UFA group ate 1000 extra calories in the form of pecans, olive oil, pesto, and butter. Their SFA:MUFA:PUFA ratio was 21%:57%:22%. Their liver fat increased by 15%.

The Sugar group ate 1000 extra calories in the form of orange juice, soda, and candy. Their snack had no fat, all sugar. Their liver fat increased by 35%.

Saturated fat also increased lipolysis—the breakdown of fatty acids for energy—while unsaturated fat decreased it. Lipolysis normally increases during fasting or exercise. In that context, it’s a good thing and you end up losing body fat. If you have a steady stream of extra fat calories coming in, you won’t lose body fat. The researchers could have seized on this point and screamed “unsaturated fat inhibits lipolysis!” but they didn’t. Wonder why.

Also notable is the observation that overeating sugar increased de novo lipogenesis (creation of fat from sugar) in the liver by 98%. I was told that didn’t happen in humans, that de novo lipogenesis was a myth. Guess it can happen.

But, again, the most important thing to realize is that this was an overfeeding study. It wasn’t a weight loss study. It wasn’t designed to see which kind of diet spontaneously results in the most weight loss. It was designed to get people to eat 1000 extra calories from different nutrient sources to see how they affect liver fat. And they didn’t construct the entire diets around the hypothesis. From what I can tell, the participants ate their normal diets. Only the 1000 calorie overfeeding snacks were provided by the researchers.

You also have to consider choline. The more fat you eat, the more choline your liver needs to process that fat. In rats with fatty liver, supplementing with extra choline directly reduces liver fat. In Chinese women, a high choline intake protects against fatty liver. If the subjects didn’t increase choline as they increased fat overfeeding, they were bound to gain more liver fat. Remember: this was the whole point of the study.

Now with that out of the way, who is this study relevant for?

Those people who think keto gives them free license to consume as many calories as they can cram into their mouths. Don’t do that. Especially don’t do that with isolated sources of fat, including saturated fat. Overfeeding anything is bad news, unless you’re trying to gain lean mass, lifting hard and heavy on a regular basis, and the thing you’re overfeeding is whole food rather than isolated fat.

That’s it for today, folks. What are your thoughts? Does this study tell you anything new, or is it old news?

Let me know what you think down below. Take care, be well, and have a great day.

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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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25 thoughts on “Dear Mark: Saturated Fat More Harmful to Liver Than Sugar?”

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  1. A thousand extra calories from anything is a LOT. For context, a large piece of cheesecake is about 450 calories. Since it contains both sugar and fat, eating cheesecake (or chocolate cake, take your pick) on a daily basis is going to be a bad idea for both your weight and your liver. Likewise, a stick of butter contains about 800 calories, but realistically, who eats that much butter every day? Maybe fans of the “Fat won’t make you fat” meme, but the rest of us would rather use a little common sense. Moderation is a wonderful thing when it comes to food.

    Definition of “moderation”: Somewhere between total avoidance and the opposite extreme of going completely overboard (such as 1,000 extra calories a day). It can vary, but most of us know what moderation is as pertains to us personally. Unfortunately, knowing isn’t the same as doing.

  2. Can’t comment on that study, as the full study appears to be pay only. Without seeing their exact methods, it’s difficult to interpret.

  3. I guess I learned one thing: I’m like Pavlov’s dogs after reading about all that delicious fat. Had to clean the drool off the keyboard and go eat some lunch.

  4. I wouldn’t be as quick to dismiss the study because of overfeeding fat by 1000 calories per day. It sounds (and is) very extreme, but keep in mind that many Americans, especially many of those living in areas of the country where food choices are poor to start with, may be doing something very much like this with their diets. Sad but there may be some truth in it.

    1. Not with saturated fat, they aren’t. Cheap carbs and polyunsaturated oils are the cheap foods.

      Potato chips and potatoes fried in polyunsaturated oils, processed meats and cheese “foods”, I would suggest. Oh, and carbs with a pancreas load of sugar.

  5. Hmmmm, it would be interesting to learn about the “normal” diets of the participants. Without knowing that, I think the most important take-away is that eating too much of anything (except maybe leafy greens…?) is not healthy. This is why it is vital to pay attention to satiety signals: to stop eating BEFORE I feel full. I love my primal/keto foods so much that I have to really focus on this at meal time.

  6. They only looked at impacts on liver as well. Where did all the unsaturated fat go? Oxidised and started to deposit around the arteries???

  7. Such a problem is like a double-edged sword. On the one side, the results of the study don’t cover all types of diets. And on the other side, the consumption of keto/primal meal that reduces saturated fat and maybe sugar. But, as it was written in the article, overfeeding is much worse than consequences of fat’s side effects. So choose natural supplements and healthy food.

  8. Thank you again for another great response to a study.

    What made me curious was this: I love coconut oil, and I love butter, and I really love cheese, but I find these foods – and protein – self-limiting at any given meal. Did the people in the two ‘fat’ groups report back that it was very difficult for them to eat that much (of either type of) fat every day?

  9. Like Katie, I’m curious about what else these people were eating. And yes, there are many people that are probably consuming 1000 calories more per day than they should be. It’s probably a mix of sugar and crappy processed fats. I’m thinking most of the people in this group are not doing this. A study like this certainly does not scare me away from saturated fat, but it’s a good reminder that overeating anything is not a good idea. And I’d argue that you can even overdo it on the veggies. I’ve had digestive issues sometimes from too many of the cruciferous ones.
    Love how Mark explains these studies in a way we can understand.

  10. In response to Joanne K. (the “reply” button isn’t working)…

    I agree that certain foods are self-limiting. For me it’s meat protein, in particular. I love steak and prime rib, for example, but can’t manage more than about 6 oz. at one sitting. It simply becomes unappetizing, regardless of how well-cooked and delicious it might be, and then I don’t want any more. This is one reason why I wouldn’t do well on a carnivore diet. Evidently some people naturally have a stronger satiety response than others. I think a Paleo or keto diet further develops that instinct.

  11. As far as I’m concerned, this study is meaningless. Fed 1,000 calories of fat in addition to a standard American diet.

    Are you kidding?

    It’s like the rats being fed “fat heavy diets.” When in reality they are fed sugar mixed with fatbso they will eat it.

    Ridiculous study.

  12. I just eat, dont care about fat, sodium, carbs, etc…has no affect on me…

  13. Was it voluntary? What makes people want to participate in something like this? I would guess a financial incentive? It makes me question how much they bother to take care of themselves in the first place. Maybe it depends on how much money I’d make but I don’t think I’d want to be a participant in any potentially dangerous health studies, especially drug trials.

  14. Very depressing that “studies” like this get funded since there is no actionable outcome (once one considers the details that Mark points out) that could possibly have come from it, no matter what te outcome. Money wasted that could have been spent on something useful or just returned to the tax payers.

  15. I am not sure this study is sufficiently powered to really make a difference in the fat vs. sugar war. Just going on the abstract, as no way am I that interested to pay $35 to look at their data.

    38 (presumably Finnish subjects, as most of the authors from finland) divided up into three groups and given controlled snacks. That is 12-13 subjects per group. NO way to tell what their normal diet was like. No control group who just ate 1000 extra calories made up of varied fats/sugar/protein. Did they control for smoking, exercise, fasting blood sugars pre-study, etc. What was the quality of the fats (from CAFO animals fed conventonal grain), etc. etc.


  16. This review article summarizes all the overfeeding studies. If you paste the title into google scholar you can find a free pdf download.

    Authors Conclusions: The challenge with the current obesity epidemic is to understand how to facilitate healthy AT remodeling expansion with hyperplasia, involving adipocyte differentiation, rather than dysfunctional AT remodeling with hypertrophy, induction of insulin resistance and inflammation. In doing so we can reduce ectopic fat and potentially ectopic fat-related complications, T2DM, NAFLD and CVD.

    Prediction of personal fat thresholds would help individuals maintain their metabolic health as long as possible. Despite the numerous overfeeding studies performed, conclusions are hampered by significant heterogeneity in study design and the limited number of studies involving a controlled environment. However, such studies are technically and ethically difficult, with optimal study duration and design unclear, and the issue of controlling for confounding factors challenging. Considering such limitations, the fundamental question of adipose tissue, metabolic and cardiovascular responses to excess calories from fat vs. carbohydrate intake remains a major public health concern and is a knowledge void that needs filling with carefully designed interventions.

    So long story short, science still along way away form understanding “personal fat thresholds”. Likely that the macronutrient ratios required to maintain healthy metabolisms will vary by the individual. Surprise.

  17. The study makes no sense. They want to study how different macronutrient composition affects something, so they do it by overfeeding specific macronutrients.

    But all that shows is what happens when you overfeed.

  18. I think it’s important to recognize this study had all of 38 subjects. Regardless of what the calculated p-factors are, it is impossible to draw valuable hard conclusions from this small a sample size.

  19. Hi Mark, thanks for clearing the doubts of readers. I appreciate this work. Its really very important to know about the harms of saturated fat.

  20. Off topic, but why has the “reply” button been disabled? I, for one, have always enjoyed the animated although usually respectful exchange of ideas that it provided. Sometimes people get a bit feisty (and I include myself in that category), but I’ve never seen this comment board degenerate to the level that I’ve seen with other websites.

  21. The only thing I understand well that excess of everything is bad. Eating one thing excessively will definitely going to harm your health. For example, food and vegetables are good for health. But due to my experience, I can say, if I eat them too much, they upset my stomach. Same is with the soup and everything.

  22. Ok, but what about the state of the participant’s livers and blood when they entered this study? MOST people are on their way to a fatty liver, evidenced by weight gain due to fat, not muscle; their livers are becoming compromised by all the viral, hvy metal, environmental toxins and now, excess fat of which only so much can be stored. The rest of course, gets recirculated in the blood stream, to be diagnosed as dirty or thick, un-oxygenated blood- a prime precursor to a myriad of diseases. What’s more, heavy metals like mercury and aluminum are oxidized/ leached in the presence of blood fat and there is evidence now, strongly linking dementia and Alzheimer’s with fat-soluble heavy metal interference of signals passing through the neural transmitters across the synapses!.
    I would happily provide the references backing up my statements, upon request.
    A stagnant liver can’t contribute to strong digestive acid mix which leaves the body open to pathogens.
    Vegans aren’t off the hook either- fats from avocados, nuts, oils, etc are just as detrimental to a healthy liver, in excess.

  23. I am one of those who overate butter while thinking it healthy to do so because it is saturated fat.

    I was eating 12 to 16 oz of butter every day for 5 months. I had steatorrhea during this time but merely dismissed it.

    I had to stop when I came across Konstantin Monastyrsky’s article in which he said that excessive fat causes bile to leak into the colon where bacteria convert them to carcinogens.

    Steatorrhea also hurts the colon lining and increases the risk of appendicitis.

    I am worried now about the carcinogenic damage I accumulated, but what’s done is done. All I can do now is to stop before doing even more damage.

    Butter should be treated like a condiment for keto, and not a meal.