Soybean oil recently made headlines when researchers from the University of California, Riverside, shared the findings from a new study at a medical conference. They discovered that mice fed a diet rich in soybean oil — both the regular stuff and a variety genetically modified to have lower levels of omega-6 fatty acids (which most Americans over-consume) — gained more weight, had fattier livers and more glucose intolerance (a risk factor for diabetes) than mice fed a diet rich in coconut oil.

An earlier mouse study by the same research team suggested a link between soybean oil and an increased likelihood of developing metabolic syndrome — a cluster of  heart disease and diabetes risk factors that include belly fat, unhealthy cholesterol levels, high blood pressure and high blood sugar.

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So does that mean soybean oil belongs in the nutritional doghouse? Time — and specifically, more research — will tell.

For now, most experts and organizations, including the American Heart Association and the Academy of Nutrition and Dietetics, give soybean oil the thumbs up, along with olive, canola and other plant-based oils.

"While soybean oil has some saturated fat, the majority is polyunsaturated, which studies have found can help reduce  bad cholesterol levels in the blood and lower the risk of heart disease and stroke," says Holley Granger, MS, RDN, a dietitian based in Birmingham, Alabama.

Like other vegetable oils, soybean oil also is a good source of vitamin E, says Kristen F. Gradney, RDN, LDN, spokesperson for the Academy of Nutrition and Dietetics. That antioxidant supports heart, eye and brain health.

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America’s most-consumed oil

Americans consume far more soybean oil than any other oil, including canola and olive. It's the most common ingredient in generic vegetable oil, and it's used in all kinds of processed foods, turning up in everything from salad dressing to crackers,  baked goods, potato chips and nondairy creamer. It's cheap and has a neutral flavor and a high smoke point (the temperature at which a fat begins to smoke and smell acrid).

From a culinary perspective, "soybean oil's mild flavor makes it ideal for baking, roasting, sautéing and marinating," says Holley Granger, MS, RDN, a dietitian based in Birmingham, Alabama.

New worries

The UC Riverside research calls soybean oil's virtue into question. Researchers used the same kind of soybean oil available on supermarket shelves and in amounts comparable to those found in the American diet, say researchers Frances M. Sladek, PhD, and Poonamjot Deol, PhD. The results weren’t encouraging.

In the study, they compared conventional soybean oil with two other oils. One was Plenish, a type of soybean oil genetically modified to be low in linoleic acid (the main fat in soybean oil) and high in oleic acid (the type of fat found in olive oil). The other was coconut oil, which is rich in saturated fat.

The mice fed either type of soybean oil were more likely to gain weight and become diabetic and insulin resistant. "They also had large lipid droplets in their liver and ballooning, a sign of liver injury," note Sladek and Deol. But the genetically modified oil seemed to be slightly less harmful than the conventional. The mice who consumed it had less insulin resistance than the ones who consumed conventional soybean oil.

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"The GM Plenish seems to have fewer negative effects [than conventional soybean oil], indicating that genetic engineering can introduce beneficial properties to a crop. But both conventional and Plenish soybean oil appear to be less healthy in terms of obesity and diabetes than coconut oil."

More research is needed to understand what component of soybean oil may be contributing to weight gain and other health problems. The research team also is investigating the effects of different fats on gut bacteria, for example.

In the meantime, they say, "It's highly unlikely that small amounts of soybean oil are detrimental."

The soybean oil hazard everyone agrees on

"Soybean oil is often used in processed foods in an unhealthier version, as a trans-fat-containing partially hydrogenated oil," says Grainger.

Partially hydrogenating soybean (or any) oil helps it stay solid at room temperature — think margarine or shortening. This type of trans fat boosts “bad” LDL cholesterol while lowering beneficial HDL cholesterol, making it just about the worst fat you can eat for your heart.

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Grainger's advice: Read labels and skip products that list partially hydrogenated soybean (or other) oil among the ingredients.

Limiting processed foods, which most health experts recommend, is an easy way to avoid trans fats from hydrogenated soybean oil.

If generic vegetable oil is your go-to cooking fat, Grainger suggests branching out for both flavor and nutrition. "I always recommend eating a variety of foods from different sources to maximize your intake of vitamins and minerals," she adds. "So experiment with other oils in the kitchen, like olive, grape seed, peanut or canola oil. And check out nuts, fatty fish or avocado for more heart-healthy food sources."

Alison Ashton is a freelance food writer, recipe developer and Cordon Bleu-trained chef based on Los Angeles.