Around the middle of the 20th century, the Western diet changed. Dietary guidelines encouraged people to eat less meat, eggs and dairy, and more fruits, veggies, whole grains and vegetable oils.

That last point signaled the most dramatic change.

Americans slowly ditched their butter for margarine, their lard for vegetable shortening and their bacon grease for vegetable products like corn and soybean oil (it’s vital to note that what’s called “vegetable oil” on supermarket shelves is nearly always soybean oil). And by 1990, even McDonald’s had made the switch from frying French fries in beef tallow to a blend of canola, soybean and hydrogenated soybean oil.

In these pages, we’ve explored the theory that these oils can lead to inflammation and disease. But it appears they also have a unique ability to drive weight gain.

If you chart out the rise in obesity in the U.S. — which topped 40 percent in 2017 (CDC, 2021), compared with only 10-15 percent in 1960 (NIH, 2021) — it maps suspiciously closely to the rise in vegetable oil consumption over that time (Eades, 2018).

Researchers estimate that per capita consumption of soybean oil increased 1,000-fold from 1909 to 1999 in the U.S. (Saini & Keum, 2018). On average, people today in the U.S. get a whopping six percent of their calories from the linoleic acid found in most vegetable oils — though they likely don’t know it (Jandacek, 2017), as the oils are typically used as tasteless, cheap fillers in snack and fried foods.

Of course, researchers have yet to find the smoking gun linking vegetable oils to an increase in BMI. And there are other dietary factors that could have changed over that time. But some experts are beginning to take a much closer look at the strong correlation between these unhealthy oils and the obesity epidemic (Eades, 2018).

Vegetable oils and the causes of obesity

Experts like physician and New York Times bestselling author Dr. Michael Eades and registered dietician Joy Kiddie think the connection between vegetable oils and obesity comes down to the ways different types of fats are metabolized in your body.

All whole foods — those that haven’t undergone artificial processing — contain a mix of fats, both saturated and unsaturated. But the amounts of different kinds of fat vary from food to food, and that can make a big difference for our health.

Saturated fats, like those found abundantly in meat and butter, and polyunsaturated fats, like the linoleic acid found abundantly in vegetable oils, have different chemical structures.

close up of butter sliced on a cutting board
Butter, long demonized as an unhealthful fat, appears to actually have many positive properties – possibly including a better ability to control appetite than vegetable oils.

When fatty acids are broken down inside our cells, two of the resulting molecules are called FADH2 and NADH.

These molecules supply our bodies with energy.

Here’s the important point. Saturated fats are broken down into a higher ratio of FADH2 to NADH than polyunsaturated fats are.

So the “bad” ratio we get from linoleic acid, Eades says, triggers a complex chain reaction that leads to energy being stored as fat instead of being used to fuel the body. That increases hunger cues, leading people to eat more even when they don’t need to.

The process is similar to the sequence of events that lets our bodies store blood glucose as energy. The higher blood glucose levels are, the more gets stored in cells for later. Linoleic acid is stored in cells much like glucose is, but with a fatty twist. The compound acts like a “supercharged carbohydrate,” containing over twice as many calories per gram as a regular carbohydrate.

Tellingly, studies show linoleic acid levels in fat cells have increased 136 percent over the past 50 years (Guyenet & Carlon, 2015). If Eades is right, those are calories that could have been used by our bodies for fuel, but weren’t.

The end result could be calories going to your belly fat.

And what about seafood? Like vegetable oil, it is high in polyunsaturated fats, but these fats take the form of highly beneficial omega-3s EPA and DHA, not linoleic acid.

EPA and DHA play a vital role in brain and cellular function, and research indicates they do not have the downsides of linoleic acid, the polyunsaturated fatty acid derived from plants.

That might be because we have a long evolutionary history with EPA and DHA, and essentially none with large quantities of free linoleic acid that’s been industrially extracted from seeds.

Putting soybean oil to the test

In 2015, researchers from the University of California, Riverside, tested a diet laden with soybean oil on mice to see how it compared to a diet rich in coconut oil (a mostly saturated fat) as well as a diet containing the sugar fructose. Despite eating the same number of calories of each, the mice eating soybean oil fared worse than mice eating either saturated fat or sugar-heavy diets. The researchers found the soybean-oil-fed mice gained significantly more weight, more fat, and had greater glucose intolerance and insulin resistance compared to the other mice (Doel et al., 2015).

Linoleic acid could be one of the culprits. It’s an omega-6 fatty acid, something our bodies need, but only in moderation. Research shows when omega-6 levels get out of hand, our health suffers, especially when we’re not getting enough of their omega-3 fatty acid cousins, which are found predominantly in seafood.

Two years later, the same research group followed up their study with a test of the genetically modified soybean oil Plenish, which is low in linoleic acid. They found that it induced less obesity and less insulin resistance in the mice. This time, they analyzed what was happening inside the mice’s cells, and found a healthier balance of omega-6 and omega-3 fatty acids between the Plenish-fed mice and those fed standard soybean oil (Doel et al., 2017).

(That said, we don’t necessarily recommend this highly processed oil. Read on for more on the oils and fats to favor.)

Vegetable oils high in omega-6 fatty acids

The prevalence of soybean oil in processed foods is one of the reasons why some studies suggest highly processed foods are a key driver of chronic diseases. Recent research has even implicated soybean oil in the brain dysfunction that leads to a number of conditions ranging from depression, and anxiety to Alzheimer’s and autism.

Omega-6 fatty acids, like those found in vegetable oils, cause inflammation, and they compete with inflammation-busting omega-3s in the body, so it’s important to maintain a healthy ratio in your diet. Researchers recommend a ratio of omega-6/omega-3 of 1:1 or 2:1 — but most people eating a Western diet consume more than a 15:1 ratio of omega-6s as omega-3s (Saini & Keum, 2018).

That key omega-6/omega-3 ratio could be influencing health everywhere in our bodies. Omega-3s possess important anti-inflammatory properties and are linked to brain development, as well as prevention of cancer, diabetes, obesity, infections, and cardiovascular disease (Saini & Keum, 2018).

Which oils are healthiest?

man working an ancient roman olive oil press
Olive oil is a fruit oil, not a seed oil. That means the oil fraction comes out easily and naturally with a simple pressing (such as with this vintage stone press). That’s quite different from the high heat, pressure and solvents needed to liberate oil from seeds. Fruit oils and animal-based oils are healthful alternatives to seed oils.

A good rule of thumb for picking a healthy oil, some experts say, is to think about the oiliness of its source. If you squeeze an olive, for example, oil will come out. You can see the oily grease coming off your sizzling bacon, hamburger or steak.

But squeeze a kernel of corn or a soybean until there’s nothing left, and you’ll never produce a drop of oil. These are oils that can only be produced by industrial processes that involve a significant amount of heat, pressure, and sometimes chemical solvent, to extract the oil from its “tight bond” within the seed’s structure.

And, oils with a high omega-3/omega-6 ratio can be some of the healthiest options out there.

Generally, it’s a good idea to take the advice of Cate Shanahan, M.D., author of the “Deep Nutrition” and a lifelong student of fatty acid metabolism. In her own cooking, she uses “just the good fats: mostly butter, and olive oil for cooking. Olive oil is a fruit oil, not a seed oil – it gives up its fat easily with simple pressing, and is mostly monounsaturated, not polyunsaturated. Fat from cattle, pigs and fish is also part of a healthy diet.”

 

Sources:

CDC. (2021, June 7). Adult Obesity Facts. Retrieved August 16, 2021, from Centers for Disease Control and Prevention website: https://www.cdc.gov/obesity/data/adult.html

National Institute of Diabetes and Digestive and Kidney Diseases. (2021, August 16). Overweight & Obesity Statistics | NIDDK. Retrieved August 16, 2021, from National Institute of Diabetes and Digestive and Kidney Diseases website: https://www.niddk.nih.gov/health-information/health-statistics/overweight-obesity

Eades, M. March 4, 2018. A New Theory of Obesity. Low Carb Breckenridge. Via Kiddie, J. 2018 PART 1 of 2: The Evolving Hypothesis of Obesity – the role of polyunsaturated fats. The Low Carb Healthy Fat Dietician. Accessed 11 Aug 2021.

Jandacek RJ. 2017. Linoleic Acid: A Nutritional Quandary. Healthcare. 5(2):25. DOI:10.3390/healthcare5020025

Guyenet SJ, Carlson SE. 2015. Increase in adipose tissue linoleic acid of US adults in the last half century. Advances in Nutrition. 6(6):660-4. DOI:10.3945/an.115.009944

Saini RK, Keum YS. 2018. Omega-3 and omega-6 polyunsaturated fatty acids: Dietary sources, metabolism, and significance — A review. Life Sciences. 203:255–267. DOI:10.1016/j.lfs.2018.04.049 

Deol P, Evans JR, Dhahbi J, et al. 2015. Soybean Oil Is More Obesogenic and Diabetogenic than Coconut Oil and Fructose in Mouse: Potential Role for the Liver. PLoS One. 10(7):e0132672. DOI:10.1371/journal.pone.0132672.

Deol P, Fahrmann J, Yang J, et al. 2017. Omega-6 and omega-3 oxylipins are implicated in soybean oil-induced obesity in mice. Scientific Reports. 7:12488. DOI:10.1038/s41598-017-12624-9