Soybean oil, the most consumed cooking oil in the U.S., induces genetic changes in the hypothalamus — the brain region controlling appetite and metabolism — a study in mice suggests.
The findings indicate that, besides its effects on obesity, this oil affects the brain control of insulin signaling and inflammation, while also having an impact on neurological pathways important in depression and Alzheimer’s disease.
The study, “Dysregulation of Hypothalamic Gene Expression and the Oxytocinergic System by Soybean Oil Diets in Male Mice,” was published in the journal Endocrinology.
The hypothalamus harbors brain cells that play pivotal roles in the control of body fat — adipose tissue — by regulating the balance between caloric intake and energy expenditure. Hypothalamic cells release small molecules, such as the hormone oxytocin, whose role in regulating food intake and energy spending has increasingly been recognized.
“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,” Margarita Curras-Collazo, a professor of neuroscience at University of California –Riverside (UCR) and the study’s lead author, said in a press release.
Diets rich in saturated fats, like those found in fatty meats, are known to induce obesity. In contrast, unsaturated fats are perceived as good.
“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,” said Frances Sladek, a UCR toxicologist and professor of cell biology, and a study co-author.
In fact, increasing evidence suggests that polyunsaturated fatty acids (PUFA), such as those found in soybean oil, also contribute to obesity.
Soybean oil is used in fast food frying and packaged foods, and fed to livestock. Its use in the U.S. increased 1,000 times throughout the 20th century, the researchers said.
Studies in mice, however, have shown that soybean oil induces obesity, diabetes, insulin resistance, and fatty liver. The team at UCR in 2017 found that reducing the amount of of PUFAs — namely linoleic acid — in soybean oil led to less obesity and insulin resistance.
Yet, this oil’s effect on the hypothalamus remains unknown.
To bridge this information gap, the researchers fed mice with three different diets high in fat — soybean oil, soybean oil modified to be low in linoleic acid, and coconut oil (control). The mice were fed twice weekly for up to 24 weeks.
The results showed that both soybean oil diets induced significant changes in over 100 hypothalamic genes compared with the coconut oil diet. The coconut oil diet used that oil as a regular oil without linoleic acid or stigmasterol — a cholesterol-like molecule that also is a main component of soybean oil — as well as a coconut oil supplemented with stigmasterol.
The genes with altered activity were involved not only in processes such as inflammation and insulin signaling, but also in signaling pathways important in anxiety, depression, and Alzheimer’s.
One such genes was the Oxt gene, which generates oxytocin. Levels of this hormone, which also has been associated with obesity, were reduced in the hypothalamus and appeared to lead to an increase in glucose sensitivity — an indicator of diabetes. Still, the animals’ body weights were unchanged.
In fact, Oxt was the only gene associated with neurological, metabolic, and inflammatory diseases that showed increased activity in both soybean oil diets.
The researchers noted that their findings are not to be extended to other soy-containing products or other vegetable oils.
“Do not throw out your tofu, soymilk, edamame, or soy sauce,” Sladek said. “Many soy products only contain small amounts of the oil, and large amounts of healthful compounds such as essential fatty acids and proteins.”
In additional experiments, the scientists confirmed that the impact of soybean oil on the hypothalamus was not linked with linoleic acid nor stigmasterol.
Identifying which components have effects on the brain is a future aim of the researchers, as “this could help design healthier dietary oils in the future,” said Poonamjot Deol, an assistant project scientist in Sladek’s laboratory and the study’s first author.
These results “underscore the need for a careful evaluation of the extensive use of soybean oil-based food products, including infant formula, animal feed and other processed foods,” the researchers said.
“If there’s one message I want people to take away, it’s this: reduce consumption of soybean oil,” Deol said.