Highly processed vegetable oils made from corn, soy, sunflower, canola, and other plants are easy to find in home kitchens and grocery stores these days, but they’ve been around in human diets for only a short time, relatively speaking. It wasn’t until the early 1900s that the technology required to process these oils was invented, resulting in a product still found on grocery shelves: Crisco. Introduced in 1911, it was a cooking fat derived from cottonseed oil that resembled lard, but was manufactured by Procter & Gamble through a chemical process. After bringing Crisco to market, the Ohio-based company went on an advertising blitz selling the notion that its oil was more healthy than the saturated animal fats typically used for cooking at the time. It worked: In a matter of five years, the company was selling tens of millions of pounds of Crisco. Other refined vegetable oils followed over the course of the 20th century—these days they’re often referred to as “seed oils” as a catchall term.
In recent years, however, seed oils have become something of a nutritional bogeyman. While most mainstream nutrition experts don’t consider them particularly unhealthy, many doctors and nutritionists claim that the specific fatty acids found in processed vegetable oils lead to inflammation and chronic health conditions. Last year, Joe Rogan talked to doctor and carnivore diet evangelist Paul Saladino about this for more than three hours. On another podcast in 2020, board-certified family physician Cate Shanahan referred to the most common seed oils on the market today as the “hateful eight,” to be avoided at all costs. So what gives? Should we forswear all seed oil?
All cooking oils are combinations of fatty acids, but it’s the generally higher levels of polyunsaturated fats found in seed oils that make them undesirable in the eyes of some dietitians and health professionals.
A healthy person has about 2 percent of polyunsaturated fat in their body fat; on average, however, people contain as much as 30 percent of polyunsaturated fat in their body fat. The cause, Shanahan notes, is due to seed oils. Higher-than-normal levels of polyunsaturated fats can lead to reduced energy and bodily inflammation, which in turn can contribute to obesity, diabetes, and heart disease.
Furthermore, the highly processed nature of seed oils—there are multiple steps and chemicals involved in their production—makes them ultra-rich in omega-6 fatty acids, a key contributor to inflammation. According to Chris Kresser, co-director of the California Center for Functional Medicine, the human body works best when its ratio of omega-6 fatty acids and omega-3 fatty acids (the ones you see on any bottle of grocery-store fish-oil supplement) is balanced.
Many Americans’ diets, however, have thrown this balance way out of whack: We generally get far too much omega-6 than we do omega-3 thanks to our food choices and how we’re cooking our meals. “[S]eed oils are perhaps the most significant contributor to the imbalanced omega-6-to-omega-3 ratio,” Kresser writes, “and thus play a significant role in chronic inflammatory diseases.”
In short, one main argument against consuming seed oils is that the fatty acids they contain promote inflammation, the follow-on effects of which are chronic diseases that we’d all like to avoid. There are studies out there that bear out this conclusion, several of which call out omega-6 fatty acids specifically for the roles they play in increased risk for obesity and long-term disease. The Sydney Diet Heart Study from the 1960s also shows that participants who replaced saturated fats with polyunsaturated fats high in linoleic acid—another fatty acid also found in many seed oils—“had a higher risk of death from coronary heart disease,” as STAT reported several years ago.