A serious but overlooked health risk in the American diet is hiding in plain sight.

That risk is soybean oil — the most widely consumed vegetable oil in America.

Outside the home, soybean oil is commonly used as a frying oil in restaurants and fast-food chains, as an ingredient in packaged and takeout foods, and as a livestock-feed ingredient.

The startling results of a University of California, Riverside (UCR) study in mice show that soybean oil produced changes in the animals that would promote brain/mood problems like autism, Alzheimer’s disease, anxiety, and depression — on top of the obesity- and diabetes-promoting changes seen in the UCR team's prior research.

Even before the disturbing new findings appeared, we had ample, persuasive evidence that soybean oil presents risks — ones we should quickly review before turning to the just-released evidence of a new, brain-related health risk from soybean oil.

Soybean oil’s unhealthful fat profile
Researchers in the field of fatty acids and how they affect human health have been sounding alarm bells about the average American’s extremely high, historically unprecedented, intakes of omega-6 fats from vegetable oils.

Soybean oil features very high, almost unsurpassed, proportions of an omega-6 fat called LA, in which the average American’s diet is awash — to proven-unhealthful effect. For more on the ill effects of that dietary flood of omega-6 LA fat, see our Omega-3/6 Balance page, and the Omega-3/Omega-6 Balance section of our newsletter archive.)

There’s good, widely accepted evidence that the diets our hominid ancestors ate as they evolved — and that people ate until very recently — were much higher in omega-3 fats and lower in omega-6 fats, with about three times more omega-6s than omega-3s.

Consumption of edible oils in the U.S.

But the average American now consumes 10 to 20 molecules of omega-6 fat for every molecule of omega-3 fat — an “omega imbalance” that creates a pro-inflammatory state in the body and influences our genes in undesirable ways.

The pie chart at right reflects USDA data showing that soybean oil is by far the most commonly consumed vegetable oil in America — accounting for a whopping 20% of calories in the average American’s diet — followed by corn oil and sunflower oil, which are also very high in omega-6 LA. Copious consumption of these vegetable oils explains why the average American gets an unhealthful 9% of their calories from omega-6 LA. 

The University of California research team’s new study shows that soybean oil isn’t good for mice. And as they wrote, they suspect that “in all likelihood” it’s not healthy for humans either.

Study sees mysterious gene and hormone harms from soybean oil
Five years ago, the same University of California, Riverside (UCR) research team behind the new study reported that soybean oil caused obesity, diabetes, insulin resistance, and fatty liver in mice (Deol P et al. 2015).

Then, two years later, the California group confirmed the unhealthful role played by soybean oil’s high levels of omega-6 fat, when they found that a special low-omega-6 soybean oil caused much less obesity and insulin resistance in mice (Deol P et al. 2017).

For their new study, the UCR team divided mice into three groups and fed each a different diet:

  • High in coconut oil
  • High in special, low-omega-6 soybean oil
  • High in standard, high-omega-6 soybean oil

While their prior studies looked only at soybean oil’s effects on markers of cardiovascular and metabolic health, this one also examined its effects on the animals’ brains (Deol P et al. 2020).

Overall, the two different kinds of soybean oil didn’t produce significantly different impacts on the animals’ brains — except that both types of soybean oil exerted disturbing effects on the hypothalamus, which is responsible for several essential processes.

“The hypothalamus regulates body weight via your metabolism, maintains body temperature, is critical for reproduction and physical growth as well as your response to stress,” said the study’s lead author, associate professor Margarita Curras-Collazo, Ph.D.

In all, the UCR team discovered that both types of soybean oil affected more than 100 genes — alterations likely to exert negative effects on the animals’ metabolisms and brain functions, and raise the risks for rodent versions of autism, Alzheimer’s disease, anxiety, and depression.

Critically, those changes included dysfunctions in genes responsible for producing the so-called “love” hormone, oxytocin, causing oxytocin levels to drop in the animals’ hypothalamus.

Oxytocin is critical to establishing people-to-people connections, especially love — including mother-child, romantic, and familial relationships. (Female mice bred to lack oxytocin-production capacity don’t bond with their offspring, while people diagnosed as sociopaths or psychopaths typically suffer from impaired oxytocin functions.)

Research by NIH clinical psychiatrist Joe Hibbeln, M.D., and other scientists shows that rising homicide rates in the U.S. and other Western countries over the past 20 years closely parallel the rise in consumption of high-omega-6-laden soybean oil (Hibbeln JR et al. 2004).

Accordingly, Dr. Hibblen and others propose that Americans' over-consumption of omega-6-laden oils may help explain evidence of upticks in emotional isolation, anger, depression, and anxiety, and hostility.

And if the UCR team’s findings in mice apply to humans, soybean oil's mystery oxytocin-impairing factor(s) may also play a role.

Importantly, the team's hypothalamus-harm findings only applied to soybean oil — not to other soy products or to other vegetable oils. And as UCR professor Frances Sladek, Ph.D., said, “Many soy products only contain small amounts of the oil … [so there’s no need to] … throw out your tofu, soymilk, edamame, or soy sauce.” (Dr. Sladek is pictured above, standing, with study co-author Poonamjot Deol, Ph.D.)

This study only involved male mice, and the researchers noted that because oxytocin is critical for mother-child bonding, it should be repeated using female mice.

Source of hypothalamus harm is unknown, but it isn’t omega-6 fat
The UCR team hasn’t identified the soybean oil chemicals responsible for the gene changes they found in the hypothalamus.

Critically — because both types of soybean oil produced the same genetic disruptions — they know the villain isn’t omega-6 LA fat. Nor is the villain a cholesterol-like chemical called stigmasterol, because the same levels occurred in both types of soybean oil.

Accordingly, the UCR team intends to try identifying the chemical or chemicals in soybean oil that produce the potentially risky genetic effects seen in mice.

“This could help design healthier dietary oils in the future,” said Assistant Project scientist Poonamjot Deol, Ph.D. “The dogma is that saturated fat is bad and unsaturated fat is good. Soybean oil is a polyunsaturated fat, but the idea that it’s good for you is just not proven.”

For example, coconut oil — which, according to the USDA database, is about 82% saturated fat — produced very few changes in the animals’ hypothalamic genes.

We have to agree with Dr. Deols’ cautionary comment: “If there’s one message I want people to take away, it’s this: reduce consumption of soybean oil.”


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