Healthy Inspiration

They’re everywhere. Like a crude oil spill in the Gulf, vegetable oils — those slippery, modern elixirs — have seeped their way into all the nooks and crannies of our food supply.

If you eat out, chances are your food is cooked in — or doused with — some type of vegetable oil. If you buy packaged goods like crackers, chips or cookies, take a look for it on the label; there’s a very good chance that vegetable oils have oozed their way in. If you buy spreads, dips, dressings, margarine, shortening or mayo, can you guess the star ingredient? Yup — vegetable oils.

Vegetable oils have quickly become a major source of calories in our food supply. Is that a good thing? To find out, let’s review what we know, and what we don’t know, about these pale and innocent-looking plant-based fats. As you will see, we have a pretty good handle on the basics, but, beyond that, there is a lack of evidence and scientific consensus about the healthfulness of these glistening food science darlings.

Ready for a deep dive into the slick, new man-made fats in our diets? Click any of the links to jump to that section, or simply keep reading!

Let’s start with the basics. We actually know a lot about the origins of vegetable oils and their chemical makeup.

All fats contain a blend of saturated (SFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids (learn more), and vegetable oils are no exception. Each type of seed has its own signature blend of the dozens of possible fatty acids that occur in nature, and each fatty acid is either a saturated, monounsaturated or polyunsaturated fatty acid.

Take a look at the percentage makeup of each of the three main vegetable oils in our food supply, compared to coconut oil, a traditional plant fat:

For more on the origin and structure of different types of fat, check out our guide below.

What about omega-6?

Most vegetable oils contain high levels of omega-6 fatty acids. There are exceptions (like high-oleic canola oil), but vegetable oil varieties like soy, corn, cottonseed, safflower and sunflower all contribute to the abundance of omega-6 fatty acids (over omega-3 fatty acids) in the standard American diet.

This is a decidedly modern pattern; traditional diets were never high in omega-6 fatty acids. A high ratio of omega-6 to omega-3 is often linked to inflammation and chronic disease.

What happens when we cook with vegetable oils?

Vegetable oils contain mostly polyunsaturated and monounsaturated fatty acids, which means they are liquid at room temperature. It also means that they are innately less stable than a predominately saturated fat. This is because unsaturated fatty acids have one or more unsaturated (double) chemical bonds that react more easily than the stable single bonds in a fully saturated fatty acid.

Even if vegetable oils can be stabilized during production to achieve a reasonable shelf life, adding heat can quickly oxidize them.

As Nina Teicholz notes in her talk, “The unknown story of vegetable oils,” and in her bestselling book, The Big Fat Surprise, putting vegetable oils into deep fryers creates an oxidized, gummy mess. It is hard to predict how our bodies manage these already oxidized, unstable fatty acids once ingested.

Fats that include higher levels of saturated fatty acids, like clarified butter and coconut oil, are the most heat-stable fats, and are much safer for cooking.

These mostly saturated fats are solid at room temperature, do not become rancid when stored, and resist oxidization when heated. Lard and extra virgin olive oil, both made up of predominately monounsaturated fatty acids, are also quite heat stable.

What happens to the vegetable oils that we eat?

Fatty acids can be burned for energy, so vegetable oils are a source of fuel. If we do not need that energy immediately, our bodies store it in our fat cells. But fatty acids are also used to build and repair body parts and create signaling molecules (like eicosanoids), so the vegetable oils you eat literally become a part of you. Your mother was right — you really are what you eat!

Your body needs to incorporate fatty acids into a variety of structures, particularly cell membranes. And the selection of fatty acids in the food you eat provides the array of building blocks available to the body. Your body can rearrange some of these fatty acids, and make a somewhat different mix to suit its needs. But the raw materials provided by vegetable oils are not identical to the building blocks that a diet containing more traditional fats provides. As a result, we end up with less stable fatty acids incorporated into our cell membranes, which affects membrane fluidity and cellular function.

This begs the question: Does a diet high in vegetable oils change our bodies in a healthy way? We don’t know. What a perfect segue into the next section of this guide…