Heart-healthy olive oil may not be as beneficial as it once was because farming practices have changed in recent years. This was the conclusion researchers at Ohio State University came to after studying the effects of consuming certain oils. While many oils’ quality has dropped in recent years, they found that grapeseed oil remains one of the healthiest on grocery store shelves.

Researchers analyzed two previous studies that involved a total of 139 male and female participants. For both studies, participants had their fat and muscle mass measured. They also submitted blood samples after a 12-hour fast to measure for linoleic acid, oleic acid, and omega-3 fatty acid levels.

Low-cost cooking oils typically contain higher levels of linoleic acid, also known as polyunsaturated omega-6 fatty acids, which have been shown to play a positive role in the health and function of cells. Linoleic acid is found mostly in sunflower oil, flaxseed oil, and corn oil. Oleic acid, on the other hand, is a monounsaturated omega-9 fatty acid found mainly in avocados, olive oil, and canola oil. While it’s healthy in small doses and has been shown to improve blood flow, oleic acid is also higher in calories and less necessary to our diet because the body naturally produces it.

Researchers found participants who had higher levels of linoleic acid tended to have less heart-threatening fat cushioning their vital organs, less inflammation, and more lean body mass. Their higher levels also conferred a lower risk of insulin resistance — a precursor to Type 2 diabetes. These findings suggested those whose diets are high in linoleic acid might also be in better health.

Yet, despite the benefits, researchers warned it might not be as easy as it once was to consume more linoleic acid. Cooking oils now contain less than 20 percent of the healthy acid than they did five years ago. They said this resulted from a push in the agricultural industry to yield genetically modified plants that produce higher levels of oleic acid-rich oils at a lower cost.

As a result, safflower, sunflower, and soybean oils, formerly high in linoleic acid, are now a less healthy choice than they once were. The only exception is grapeseed oil, lead author Martha Belury said in a press release, because 80 percent of its fatty acids are still linoleic acid. "Vegetable oils have changed. They’re no longer high in linoleic acid. It really kind of popped out and surprised us.”

Although the participants were slightly heavier than the population average, Belury believes consuming linoleic acid could benefit everyone by reducing risk of heart disease and Type 2 diabetes, as well as lowering body mass index. As little as 1.5 teaspoons each day is all it takes, she said. Still, it will take further study to see how linoleic acid affects larger groups with different body types.

Source: Belury M, Cole RM, Bailey BE, Ke JY, Aldridge RR, and Kiecolt-Glaser JK. Enterocyte linoleum acid, but not oleic acid, is associated with improvements in body composition in men and women. Molecular Nutrition & Food Research. 2016.