Soybean oil, the second most important oil in the world – and by far the most widely produced and consumed edible oil in the US – is probably not healthy for human consumption.
A 2015 study found a diet high in soybean oil causes more obesity and diabetes than a diet high in fructose, a sugar commonly found in soft drinks and processed foods.
New research shows soybean oil causes genetic changes in the brain, leading scientists to suspect it may be associated with neurological conditions like autism, Alzheimer’s disease, anxiety and depression.
Both studies came from the University of California, Riverside (UCR) – and involved experiments on mice, and the results can’t be meaningfully asserted as evidence of human vulnerability.
The authors acknowledge this limitation, but they’re urging consumers to limit their amount of soy oil intake anyway.
The first study
Male mice were fed a series of four diets that contained 40 per cent fat, similar to what Americans consume.
Diet one: Coconut oil, which consists primarily of saturated fat.
Diet two: About half of the coconut oil was replaced with soybean oil, which contains primarily polyunsaturated fats and is a main ingredient in vegetable oil.
The researchers note that the second diet “corresponded with roughly the amount of soybean oil Americans currently consume”.
Diet three: Coconut oil with added fructose.
Diet four: Coconut and soybean oils, with added fructose.
All four diets contained the same number of calories and there was no significant difference in the amount of food eaten by the mice on the diets.
This allowed the researchers to study the effects of the different oils and fructose in the context of a constant caloric intake.
According to a prepared statement from the university, compared to mice on the high coconut oil diet, mice on the high soybean oil diet showed increased weight gain, larger fat deposits, a fatty liver with signs of injury, diabetes and insulin resistance, all of which are part of the Metabolic Syndrome.
Fructose in the diet had “less severe metabolic effects than soybean oil” although it did cause more negative effects in the kidney and a marked increase in prolapsed rectums, a symptom of inflammatory bowel disease that – like obesity – is on the rise.
The mice on the soybean oil-enriched diet gained almost 25 per cent more weight than the mice on the coconut oil diet and 9 per cent more weight than those on the fructose-enriched diet.
The second study
Male mice were fed three diets: Coconut oil, soybean oil and genetically modified soybean oil, engineered to be low in linoleic acid (a 2017 study found it induced less obesity and insulin resistance, but was harmful to liver function).
The scientists found that the soybeans, modified and naturally grown, had “pronounced effects” on the hypothalamus, which regulates body weight via your metabolism, maintains body temperature, is critical for reproduction and physical growth as well as a body’s response to stress.
The researchers determined a number of genes in mice fed soybean oil were not functioning correctly.
One such gene produces the love hormone, oxytocin. In soybean oil-fed mice, levels of oxytocin in the hypothalamus went down.
(Coconut oil, which contains saturated fats, produced very few changes in the hypothalamic genes.)
The research team discovered roughly 100 other genes also affected by the soybean oil diet.
They believe this discovery could have ramifications not just for energy metabolism, but also for proper brain function and diseases such as autism or Parkinson’s disease.
They were careful to make the point that there is no proof the oil causes these diseases.
A soy latte is still OK
They also advised that their findings only apply to soybean oil – and not to other soy products or to other vegetable oils.
“Do not throw out your tofu, soy milk, edamame or soy sauce,” said Dr Frances Sladek, a UCR toxicologist, professor of cell biology and corresponding author.
“Many soy products only contain small amounts of the oil, and large amounts of healthful compounds such as essential fatty acids and proteins.”
The research team has not yet isolated which chemicals in the oil are responsible for the changes they found in the hypothalamus – a problem they intend to address in future research.
“This could help design healthier dietary oils in the future,” said Dr Poonam Jot Deol, an assistant project scientist in Sladek’s laboratory and first author on the study.
The take away: “If there’s one message I want people to take away, it’s this: Reduce consumption of soybean oil … In all likelihood it’s not healthy for humans.”
But according to a market report, “the low cost, easy availability and eco-friendly nature of soybean oil has further facilitated its use in various sectors such as food, industrial and feed.”
Demand is expected to continue to grow.