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

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Updated on September 9, 2020

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? https://care.diabetesjournals.org/content/early/2018/05/24/dc18-0071

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.

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.