While scientists are moving ahead with the genetic modification of plants, there is still much debate among consumers about whether they offer any benefit, or if they are harmful. New work from scientists at the University of California, Riverside has found that one type of genetically modified (GM) soybean oil has a similar effect on the liver and induces the same amount of diabetes that conventional soybean oil does, but the GM oil causes less obesity and insulin resistance. The results of this study have been published in Nature Scientific Reports.
The use of soybean oil is increasing around the world and is the most popular vegetable cooking oil in the United States. It is high in unsaturated fats like linoleic acid, and soybean oil can cause obesity and associated health problems like diabetes, insulin resistance, and fatty liver in mice. For this study, DuPont supplied a GM soybean oil called Plenish® (but did not fund the research). Plenish was made to have lower levels of linoleic acid, making it more similar to olive oil, considered a more healthful oil.
The investigators compared Plenish to conventional soybean oil as well as coconut oil. "We found all three oils raised the cholesterol levels in the liver and blood, dispelling the popular myth that soybean oil reduces cholesterol levels," revealed the leader of the project, Frances Sladek, a Professor of Cell Biology.
Next, the researchers compared Plenish to olive oil. Both oils have high levels of oleic acid; a fatty acid believed to reduce blood pressure and help with weight loss. "In our mouse experiments, olive oil produced essentially identical effects as Plenish - more obesity than coconut oil, although less than conventional soybean oil - and very fatty livers, which was surprising as olive oil is typically considered to be the healthiest of all the vegetable oils," noted Poonamjot Deol.
"Plenish, which has a fatty acid composition similar to olive oil, induced hepatomegaly, or enlarged liver, and liver dysfunction, just like olive oil," continued Deol, the co-first author of the report and an assistant project scientist working in the Sladek lab.
Sladek suggested that some of the negative health consequences of animal fat seen in rodents may be caused by high levels of linoleic acid since many farm animals in the U.S. are fed soybean meal. "This could be why our experiments are showing that a high-fat diet enriched in conventional soybean oil has nearly identical effects to a diet based on lard," she explained.
The researchers have speculated that the increasing amounts of soybean oil that Americans have been consuming since the 1970s may be contributing to the obesity epidemic. The Centers for Disease Control and Prevention estimates that 35 percent of adults are obese. Obesity was designated a disease by the American Medical Association in 2013 and is associated with cancer, diabetes, and heart disease.
"Our findings do not necessarily relate to other soybean products like soy sauce, tofu, or soy milk - products that are largely from the water-soluble compartment of the soybean; oil, on the other hand, is from the fat-soluble compartment," Sladek cautioned. "More research into the amounts of linoleic acid in these products and others is needed.”
As an essential fatty acid, humans and animals must get linoleic acid from the diet. "But just because it is essential does not necessarily mean it is good to have more of it in your diet," Deol cautioned. "Our bodies need just 1-to-2 percent linoleic acid from our diet, but Americans, on average, have 8-to-10 percent linoleic acid in their diets."
Deol and Sladek have concluded that it’s best to avoid using conventional soybean oil. "This might be difficult as conventional soybean oil is used in most restaurant cooking and found in most processed foods," Deol admitted. "One advantage of Plenish is that it generates fewer trans fats than conventional soybean oil."
"But with its effects on the liver, Plenish would still not be my first choice of an oil," Sladek said. "Indeed, I used to use exclusively olive oil in my home, but now I substitute some of it for coconut oil. Of all the oil we have tested thus far, coconut oil produces the fewest negative metabolic effects, even though it consists nearly entirely of saturated fats. Coconut oil does increase cholesterol levels, but no more than conventional soybean oil or Plenish."
One important caveat of this study is that the cardiovascular effects of coconut oil were not assessed. "As a result, we do not know if the elevated cholesterol coconut oil induces is detrimental," Sladek said. "The take-home message is that it is best not to depend on just one oil source. Different dietary oils have far-reaching and complex effects on metabolism that require additional investigation."
The work adds to previous findings by this lab that compared soybean oil and a high-fructose diet, and that soybean oil causes more obesity and diabetes than coconut oil. The scientists plan to continue this investigation, next examining whether intestinal health is impacted by conventional and GM soybean oil. The previous work is described in the following video.
Sources: AAAS/Eurekalert! via UC Riverside, Scientific Reports