844. A recent study claimed to identify a mechanism by which soybean oil drives obesity (and coconut oil does not). But the interactions between diet and biology are rarely as black-and-white as the headlines suggest.
844. A recent study claimed to identify a mechanism by which soybean oil drives obesity (and coconut oil does not). But the interactions between diet and biology are rarely as black-and-white as the headlines suggest.
References:
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Find a transcript here.
You may have seen some headlines about a new study on soybean oil as a unique contributor to obesity. But this is really a pretty misleading interpretation. Today, we’re going to look at what this study actually found (it was definitely interesting!). But the takeaways are a lot more nuanced than most of the news coverage I have seen.
Hello! I’m Monica Reinagel and this is the Nutrition Diva podcast, a show where we take a closer look at nutrition news, research, and trends so that you can make more informed decisions about what you eat.
If you follow nutrition news (and, honestly, even if you don’t!), you have probably seen some coverage of a new study that claimed to reveal a biological mechanism that makes soybean oil more fattening than other types of fat.
Now, the study in question does suggest that the way our bodies process different types of fats might play a role in whether they are burned for energy or stored as fat. But there’s quite a bit more to this story. For a lot of people, the takeaway seemed to be that we need to avoid seed oils or outlaw processed foods. My takeaways were a bit different.
Let’s start with the study itself. It was published recently by researchers at UC Riverside. The study involved juvenile mice (the equivalent of toddlers) who grew to adulthood over the course of the study. But the mice grew up eating different diets. One group ate a diet enriched with soybean oil. Specifically, about 10% of their calories came from linoleic acid (LA) which is the primary fat found in soybean oil. And the other group ate a diet with the same amount of fat and calories but enriched with coconut oil instead of soybean oil.
As the mice grew to full size, the group that was eating the coconut oil diet maintained a lean adult weight but the group eating the soybean oil diet kept gaining weight and they became obese, developed fatty liver, and their metabolic health took a nosedive. What could possibly explain this?
Well, the researchers had one more twist up their sleeve. They also fed these two diets to a group of mice that had been genetically modified to lack a specific set of liver enzymes that are involved in the metabolism of dietary fats. In this genetically modified cohort, the group eating the coconut oil diet grew to adult size and then stopped gaining weight–just like the unmodified group. But–and here’s the payoff–the genetically modified mice who were fed the soybean oil enriched diet ? They also remained lean.
So: the only mice in this study who became obese were the ones without the genetic modification and who were eating the soybean oil enriched diet. And this is the group of mice that is thought to most closely resemble the American public. Most of us have not been genetically modified and most of us are eating twice as much linoleic acid (as a percentage of total calories) as these mice were.
At this point, the headline writers had their story: Linoleic acid (or soybean oil or maybe seed oil) drives obesity and other fats don’t. But that is not actually the story here. Nor is the advice to avoid soybean (or seed) oil the relevant takeaway.
The real story here is that these scientists designed an elegant experiment that shed some light on how our bodies (or the bodies of mice, anyway) deal with different sources of fat, and how that might contribute to obesity.
When a normal mouse’s liver encounters polyunsaturated fats, such as the linoleic acid in soybean and other plant-based oils, enzymes convert that fat into metabolites called oxylipins. These oxylipins turn out to be signaling molecules. They send a signal to the body, “Okay, store this energy as fat.” Not necessarily a problem, unless this particular type of fat is being consumed in very high quantities.
Incidentally, omega-3s are also polyunsaturated fats and are also processed into oxylipins through this same metabolic pathway. Theoretically, if we ate sufficient amounts of omega-3s, this could also promote obesity. However, virtually none of us consume enough omega-3 fats to be a concern on this count.
Saturated fats (like those found in coconut oil) and monounsaturated fats (like those found in olive oil) have a different molecular structure that essentially makes them invisible to those specific enzymes. Because they cannot be converted into oxylipins, they follow a different metabolic pathway. Instead of preferentially being stored as fat they are more readily burned for energy. That doesn't mean that they cannot be stored as fat. Excess calories will always be stored as fat, one way or another. It just means that it is not their default metabolic pathway.
Now because the genetically modified mice couldn’t produce that particular liver enzyme, they couldn’t convert polyunsaturated oil into oxylipins. So, instead of storing it fat, their mitochondria burned it for heat and energy–similar to the way the body handles saturated fats like those found in coconut oil.
One other thing I find interesting is that the liver appears to get more and more efficient at converting polyunsaturated fats (PUFAs) into oxylipins as we get older. Only in this case, efficiency is not a good thing. It means that the PUFAs we eat as adults are more likely to be stored as fat than when we are children. This could be one reason why we seem more prone to weight gain as we get older, even if our diet hasn’t changed all that much. Essentially, as we age, we may get too good at processing these fats into the signals that promote storage.
Now, if your brain works the way mine does, you might be wondering: OK, then why can’t we just suppress this enzyme in people, in order to treat or prevent obesity?
Well, the researchers actually tried that. They gave some of the nonmodified mice a drug designed to block the specific enzyme responsible for converting PUFAs into oxylipins but it failed completely—the mice still became obese. As for attempting actual gene editing in humans, that remains a dream for now, because the gene in question is a sort of 'master switch.' It doesn't just control the liver; it also regulates the kidneys and pancreas.
In fact, the genetically altered mice developed smaller kidneys as a side effect. So, flipping this genetic switch in humans could theoretically fix your waistline while accidentally damaging your other organs. But it remains an interesting area for future research.
OK, that’s the deal with this study. Now, I want to talk about what–if anything–this actually means to adult humans navigating the real world foodscape. Because here is where the media coverage really went off the rails.
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First off, when we talk about the effects of a diet high in soybean oil, we are not talking about a diet high in soybeans.
Whole soy foods—like tofu, tempeh, edamame, soy milk, and plant-based meats—are excellent sources of high-quality plant protein and fiber. They have a completely different nutritional profile than oil extracted from the bean.
So, if you enjoy tofu stir-frys or plant-based burgers or you like to put soy milk on your cereal, this study is not a reason to stop doing that. The concentration of linoleic acid in whole soy foods is much lower, and you’re getting all that beneficial fiber and protein alongside it.
Secondly, soybean oil was the substance used in this particular study but the real finding here is not about soybean oil in particular but linoleic acid in general. They could have used sunflower, safflower, corn oil or any number of other vegetable oils to provide the PUFAs that were converted into oxylipins. For that matter, large quantities of flax oil, which is rich in omega-3 PUFAs, could have had the same results.
Thirdly, this study is also not a blanket indictment of seed oils because seed oils are not all high in PUFAs. Canola oil, for example, is 60-65% lower in linoleic acid than corn or soybean oil.
Another false conclusion I’ve seen in response to this study is that avoiding (or maybe even banning) processed foods is the answer – based on the assumption that most of the linoleic acid in our diets is coming from ultra-processed foods. I have two issues with this conclusion.
It’s true that processed foods that are high in fat may be high in linoleic acid, because the oils that are used in these foods generally tend to be polyunsaturated. But not all ultra-processed foods are high in fat. There are half a dozen ultra-processed foods that are a regular part of my diet, primarily because they are convenient ways to get more protein or fiber, or they’re foods that allow me to eat healthy even when I’m on the road.
And while ultra-processed foods are drawing a lot of fire these days, there’s another category of foods that may contribute even more PUFAs (and calories) to the standard American diet–and yet this category seems to be flying below the radar: I’m talking about fast food, fried foods, carryout, and restaurant meals. These are frequently high in fat and also quite high in PUFAs, because those oils are relatively inexpensive and they have a high smoke point.
In other words, you might be stressing out about the trace amounts of sunflower oil in your oat milk, all the while the massive dose of vegetable oil in your Friday night takeout is completely invisible to you.
So, let’s pull all of this together. What does this mouse study actually mean for how we should eat? If it works in people the way it works in mice (a very big “if”), then diets that are very high in PUFAs may contribute to weight gain.
But even if that is true, that doesn't mean that we need to completely avoid or eliminate this category of fats from the diet. Indeed, linoleic acid is an essential fatty acid. Our bodies need it in order to function properly. We just want to avoid diets that are very high in this specific type of fat.To that end:
I hope that helps clarify some of the confusion around this recent study and puts it into better perspective for you.
If you have a question or topic you’d like me to tackle on the podcast, you can email me at nutrition@quickanddirtytips.com. And if you’d like to find out about having me speak at your next in person or virtual event, you can learn more at wellnessworkshere.com
Nutrition Diva is a Quick and Dirty Tips podcast. Holly Hutchings is our Director of Podcasts. Steve Riekeberg is our audio engineer, Morgan Christianson heads up Podcast Operations & Advertising, Rebekah Sebastian is our Manager of Marketing and Publicity and Nat Hoopes is our Marketing and Operations Assistant. Thanks to all of them and thanks to you for listening!