Soybean Oil: The Newest Nutritional Villain?

By Jane Farrell

A diet high in soybean oil causes more obesity and diabetes than a diet high in fructose, a sugar commonly found in soda and processed foods, according to a new study.

Research from scientists at the University of California, Riverside, fed male mice a series of four diets that contained 40 percent fat, similar to what Americans currently consume. In one diet the researchers used coconut oil, which consists primarily of saturated fat. In the second diet about half of the coconut oil was replaced with soybean oil, which contains primarily polyunsaturated fats and is a main ingredient in vegetable oil. That diet corresponded with roughly the amount of soybean oil Americans currently consume.

The other two diets had added fructose, comparable to the amount consumed by many Americans. 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. Thus, the researchers were able to study the effects of the different oils and fructose in the context of a constant caloric intake.

According to a news release 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. 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 (IBD), which like obesity is on the rise.

The mice on the soybean oil-enriched diet gained almost 25 percent more weight than the mice on the coconut oil diet and 9 percent more weight than those on the fructose-enriched diet. And the mice on the fructose-enriched diet gained 12 percent more weight than those on a coconut oil rich diet.

“This was a major surprise for us — that soybean oil is causing more obesity and diabetes than fructose — especially when you see headlines everyday about the potential role of sugar consumption in the current obesity epidemic,” said Poonamjot Deol, the assistant project scientist who directed the project in the lab of Frances M. Sladek, a professor of cell biology and neuroscience.

The paper, “Soybean oil is more obesogenic and diabetogenic than coconut oil and fructose in mouse: potential role for the liver,” was published July 22 in the journal PLOS ONE.

In the U.S. the consumption of soybean oil has increased greatly in the last four decades due to a number of factors, including results from studies in the 1960s that found a link between saturated fatty acids and the risk of cardiovascular disease. As a result of these studies, nutritional guidelines were created that encouraged people to reduce their intake of saturated fats, commonly found in meat and dairy products, and increase their intake of polyunsaturated fatty acids found in plant oils, such as soybean oil.

Implementation of those new guidelines, as well as an increase in the cultivation of soybeans in the United States, has led to a remarkable increase in the consumption of soybean oil, which is found in processed foods, margarines, salad dressings and snack foods. Soybean oil now accounts for 60 percent of edible oil consumed in the United States. That increase in soybean oil consumption mirrors the rise in obesity rates in the United States in recent decades.

During the same time, fructose consumption in the United States significantly increased, from about 37 grams per day in 1977 to about 49 grams per day in 2004.

The researchers also did a study with corn oil, which induced more obesity than coconut oil but not quite as much as soybean oil. They are currently doing tests with lard and olive oil. They have not tested canola oil or palm oil.

The findings were published in the journal PLOS One.