In the middle of the 20th century, concerned by the growing heart-attack epidemic, Americans ditched butter and other saturated fats in favor of vegetable oils.

According to Dr. Catherine Shanahan, author of the new book “Deep Nutrition: Why Your Genes Need Traditional Food” (Flatiron Books, out now), that was a fatal mistake.

Shanahan — a family physician based in Denver with a degree in biochemistry and genetics — has built a career around bucking the nutritional norm. Unlike many doctors and dieticians who suggest diets packed with fruits and vegetables, Shanahan recommends an eating plan based on animal fats and proteins, along with traditional healthy foods such as vegetables and nuts.

She began her career practicing medicine and studying traditional diets in Hawaii, and gained a fan in Kobe Bryant. She overhauled his diet, taking out vegetable oils and sugar and adding in foods such as bone broth, and went on to became the director of the Lakers’ nutrition program, a position she still holds. With the release of “Deep Nutrition,” she’s made it her mission to get Americans to ditch vegetable oils — or, in her words, “your brain’s worst enemy.”

To understand the problem with the substances — which she says include canola, palm, corn, soy, sunflower, safflower, cottonseed, rice bran and grapeseed oils, but not olive, coconut, avocado or peanut oils — Shanahan says, you have to understand why we began eating vegetable oils in the first place. “In the 1950s, we were told we couldn’t have saturated fats,” she says. “We stopped eating butter and natural fats, but still craved fatty foods, so we turned to vegetable oil[s]. And then restaurants started using them because they’re dirt cheap.”

While unsaturated vegetable oils seemed healthy at first, recent research has shown that the processed oils may be more destructive than the fats they replaced. One re-evaluation of a heart study from the 1970s, published in April in the British Medical Journal, questioned the original finding that consuming vegetable oils instead of saturated fats can lower the risk of heart disease and death. In fact, it found that consuming too much vegetable oil may actually increase the risk of heart disease.

Researchers believe that this may be due to what happens when vegetable oils are processed or heated when cooking food.

“The way the oils have been treated strips away the anti-oxidants,” says Shanahan. As a result, the balance between disease-fighting anti-oxidants and free radicals is tipped in favor of free radicals, which have been shown to cause disease growth. (It’s important to note that Shanahan’s claims have a foundation in previous studies, but most of her specific ideas have not been scientifically tested themselves.)

While Shanahan claims that vegetable oils can wreak havoc on all areas of the body, she’s particularly concerned about Poker the brain. Regularly consuming vegetable oils can lead to lethargy and trouble concentrating in the short term, she believes, and, in her view, might trigger brain conditions, such as Alzheimer’s or dementia, down the line. “Oxidative stress disrupts the function of the brain, so the cells cannot do their normal housekeeping,” in her opinion, which allows plaque to build up in the brain.

“The No. 1 symptom someone would feel is migraines,” she believes, adding that most people would experience general brain fog and fatigue after consuming vegetable oils. “They don’t realize how poorly they were thinking until they’re off this stuff.”

Still, says Shanahan, growing up on a vegetable oil-rich diet doesn’t mean you’re stuck with a misfiring brain. “At any point in time, if you get those things out of your diet, you’re going to improve,” she says.

Maria Elena Rodriguez, a registered dietitian and diabetes program manager at Mount Sinai Hospital, has a more moderate view. “On a scale of oils, vegetable oils land somewhere in the middle” in terms of how bad they are, she says.

Still, Rodriguez says, vegetable oils shouldn’t be the basis of anyone’s diet. “When we’re talking about heart health, diets that are high in vegetable oils can lead to plaque buildup and heart disease.” They can also cause inflammation.

But she’s not a fan of replacing vegetable oils with saturated fats from butter or coconut oil, either. “Saturated fat is shown to have a higher risk of plaque, heart disease, diabetes and inflammation,” Rodriguez says — claims that Shanahan calls outdated and based on faulty science.

New research appears to back up Shanahan’s concern.

The European Food Safety Authority issued a report in May warning against high levels of contaminants in palm oil, an inexpensive vegetable oil used in Nutella, margarine and Girl Scout Cookies, among other treats. The report cautioned against potential gerotoxins and carcinogens that are created when the oil is heated over roughly 400 degrees Fahrenheit during the refining process. The EFSA announced last week that it is going to reexamine the warning.

A rep for Ferrero, the company that makes Nutella, tells The Post that the palm oil used in the chocolate spread is refined at a lower heat, and therefore contains fewer contaminants. The Girl Scouts say they are aware of potential risks and are working closely with their bakers to determine if there is any cause for concern.

According to an official statement from the Girl Scouts of the United States of America, “We are aware of recent reports related to the potentially adverse health effects of high-temperature heating of all refined vegetable oils, including palm oil. We are working closely with our bakers to determine if there is any cause for concern. The EFSA has not advised the public to stop consuming products with palm oil, noting further study is required before the level of risk can be properly calculated. Providing safe products is always our top priority.”

The FDA, typically less cautious than the EFSA with such matters, has yet to weigh in on palm oil.

But that’s hardly the only claim against vegetable oils. In 2015, researchers at England’s De Montfort University cautioned that heating sunflower and corn oils produced much higher levels of aldehydes — compounds that may cause heart disease and cancer — than fats such as olive oil and goose fat. Other studies have raised concerns over potential disease-causing properties of vegetable oils.

At the same time, there’s been a resurgence in appreciation for old-school fat sources. Health nuts are blending butter, once a pariah, into their morning coffee. Fatty avocados are topping everything from toast to burgers. And lard — unthinkable even a decade ago — is returning to farm-to-table restaurants nationwide.

With so many options on the table, says Shanahan, there’s no need for vegetable oils in a modern diet.

“Why not go with a healthier, tastier fat?”

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