A newly introduced soybean oil genetically modified to be low in trans fats was only marginally healthier than conventional soybean oil — and not as healthy as coconut oil — in mouse studies assessing the metabolic impact of various high-fat diets.

Compared with control mice fed low-fat diets or high-fat coconut oil diets for 6 months, mice fed soybean oil-rich diets gained significantly more weight, had fattier livers, and had more glucose intolerance, researcher Frances M. Sladek, PhD, of the University of California, Riverside, noted in a March 6 presentation at the Endocrine Society’s 97th annual meeting (ENDO 2015) in San Diego.

Coconut Oil-Fed Mice Gained Less Weight

Mice whose diets consisted of 40% soybean oil genetically modified to be low in linoleic acid did show slightly better metabolic profiles than mice fed diets containing 40% of their calories from conventional soybean oil, and the low-linoleic acid soybean oil-fed mice had little evidence of insulin resistance, Sladek said.

But their weight gain over the 6-month study was similar to that of mice fed the conventional soybean oil diets, and it was significantly greater than that seen in the coconut-oil fed mice.

Compared with control mice fed a low-fat diet (5% of calories from fat), the coconut oil group weighed about 13% more at follow-up, the genetically modified soybean oil group weighed 30% more, and the conventional soybean oil group weighed 38% more.

“Compared with conventional soybean oil, the genetically modified soybean oil did appear to be a bit healthier,” Sladek told MedPage Today in a telephone interview. “But both soybean oils appeared to be worse than coconut oil.”

Genetically Modified Oil Common in U.S. Diet

Soybean oil is ubiquitous in the American diet — a common ingredient in vegetable oil blends and processed foods — and about 90% of the soybeans used in the oil processed in the U.S. are genetically modified to be pesticide resistant, Sladek said.

“If you ever eat processed foods or eat at just about any restaurant in the U.S., you are probably consuming genetically modified soybean oil,” she said.

But the newly introduced soybean oil, marketed by the company Dupont Pioneer as Plenish, has also been genetically modified to have high levels of oleic acid and low levels of the linoleic acid.

Earlier studies in mice conducted by Sladek along with Poonamjot Deol, PhD, and colleagues suggested that linoleic acid may be a major contributor to soybean oil’s negative metabolic effects.

Their new study was designed to further explore this hypothesis.

The researchers fed 12 mice each either a low-fat chow diet (5% of calories from fat), or high-fat diets with the same total calories in which 40% of calories came from either conventional soybean oil, genetically modified oleic-acid rich soybean oil, or coconut oil.

Coconut oil is very high in saturated fat but it is naturally low in linoleic acid. Animal fat tends to have more linoleic acid than coconut oil because vegetable oils high in the fatty acid are commonly used in their feed, Sladek said.

“To determine whether linoleic acid was responsible for the metabolic effects of soybean oil, we designed a parallel diet in which the regular soybean oil was replaced, on a per gram basis, with genetically modified high oleic acid soybean oil,” the researchers wrote in an abstract presented at ENDO 2015. “Surprisingly, this diet induced weight gain and fatty liver essentially identical to that of a linoleic acid (rich) high-fat diet.”

Sladek said linoleic does appear to promote insulin resistance, but she said the findings suggest that other as yet unidentified components of soybean oil also have a negative impact on metabolic profiles.

She said she also hopes to compare the metabolic impact of the genetically modified, oleic acid-rich soybean oil with olive oil, which has a similar fatty acid make up.

“Based on our studies, I would say that coconut oil is definitely healthier than soybean oil,” she said. “Olive oil also appears to be very healthy, and it is a staple of the Mediterranean diet. I would be surprised if it wasn’t as healthy or healthier than coconut oil.”

• Reviewed by F. Perry Wilson, MD, MSCE Assistant Professor, Section of Nephrology, Yale School of Medicine