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Keith Ayoob recently wrote a piece for MedPage Today concerning the benefits of polyunsaturated fats (PUFAs). In it, he recounts his advice to patients to cook and fry with high-PUFA oils such as corn oil, rather than expensive olive oil.
Aside from cost issues, is corn oil actually healthy? Let's take a step back and look at why we're using the oil of the corn plant in the first place.
Corn Oil and Double Bonds
Certain plant foods quite obviously contain fat, such as olives and avocado. You can taste the fat and quite easily press it out of the plant. Other plants aren't so high in fat, like corn. To get the fat out of corn, you have to bathe it in a hexane bath and chemically extract the fat, then bleach and deodorize it, among other production steps.
It turns out that processing corn for fat can make it a bit rancid. This is because corn oil is high in PUFAs, which are easily oxidized due to their chemical structures containing multiple double bonds. After purchasing it, cooking with corn oil can make it even more rancid.
Oxidized PUFA can be dangerous when in our bodies, especially since oxidative damage to fat-containing LDL particles is a primary factor in the development of heart disease. And an omega-6 rich diet greatly increases oxidized LDL levels. The more PUFA we eat, the more that accumulates in our bodies' adipose tissue. Hence, we have more than double the linoleic acid in our body fat as do our parents' generation, and fatty acid turnover in adipose tissue can take years. So you may feel the health effects of your diet from the past year or two, even if you switch to a healthier diet.
Framing "Healthy" Versus "Unhealthy"
Taking a step back, neither saturated nor polyunsaturated fats are inherently "bad." Most dietary components have roles in promoting physiological function. For example, sugar (in fruits) and starch (in tubers) were a primary source of energy for humans for millennia, and only now that food is cheap, hyperpalatable, and always available, are sugars and starches demonized.
Similarly, fats weren't demonized until relatively recently. PUFA, specifically, is useful for providing fluidity to cell membranes when flexibility is needed, as well as for certain signaling molecules. Saturated fats, conversely, are helpful when stability is needed. We don't require tons of PUFA in our diet (even omega-3), and saturated fats can be made from other dietary components. Neither PUFA nor saturated fat is likely to be harmful at moderate doses, but eating large amounts can lead to health issues in some people.
The Omega-6 to Omega-3 Ratio
People in the U.S. generally eat loads of corn oil and other oils high in omega-6. In fact, soy and corn oils alone make up almost a third of our calories. In centuries past, the amount of omega-6 we ate was much lower, with a ratio of around 4:1 omega-6 to omega-3.
As cheap vegetable oils became prominent in the the middle of the 20th century, the ratio has increased, all the way up to the current ratio of around 15:1. While people occasionally make a tiny dent in their omega 6:3 ratio by eating fish or taking some fish supplements, the main driver of this skewed ratio is ubiquitous and cheap vegetable oils.
A high ratio would be fine if it's healthy. But it's not -- too much omega-6 compared to omega-3 is linked to a variety of chronic conditions. Some of that may be due to inflammation, but the exact mechanisms aren't fully understood. The ratio of omega-6 to omega-3 in our cells determines which eicosanoids (signaling molecules created from fats) are produced by our body. Omega-6 tends to produce more pro-inflammatory eicosanoids, while omega-3 tends to produce the opposite.
Insufficient Evidence for Disease Outcomes
Without many trials linking intake directly to disease and mortality, we have to rely on other types of evidence. Compared with diets comprised of more saturated fats, diets rich in linoleic acid (a common type of PUFA, especially in corn oil) positively impact heart disease. But that might be a bit misleading. Typically, high-saturated fat foods explored in trials aren't very healthy, as opposed to healthier high-saturated fat foods like grass-fed beef and pastured eggs, which contain substantial amounts of omega-3s as well as naturally-produced trans fats which may curb heart disease. High-PUFA diets also haven't been shown to be superior to diets rich in monounsaturated fats, such as from olive oil.
While some mechanistic studies suggest that linoleic acid is linked to increased inflammation and risk of disease, a more rigorous systematic review hasn't found a connection. The available evidence is unfortunately not sufficient to draw a conclusion, however, as longer-term trials that are more strictly controlled are needed.
Studies have shown that a reduction in omega-6 fats is a primary factor in reaching a healthy omega-3 intake that can curb chronic disease. Experts also suggest that recommendations to increase PUFA intake for the general population might be misguided, given the uncertainty as to whether it may increase chances for heart disease.
In animal studies, corn oil and linoleic acid hasten the progression of prostate and other cancers, when compared with saturated fat. Human trials have also found harm, such as corn oil more than tripling mortality and doubling the number of major cardiac events. While other more recent studies have shown mixed results, none have shown benefit to actual mortality or cardiac events from corn oil, and most have suggested some degree of harm.
Where To Go From Here?
So why do people still eat so much corn, soy, and other oils high in PUFA? First of all, these oils are extremely cheap, and hence highly prevalent in junk foods. Second, there haven't been many long-term randomized trials of high-PUFA oils. That's because it would be extremely expensive and difficult to conduct a study with a regimented cooking oil intake (since plant oils are ubiquitous in many different foods, every meal would have to be provided by the research team), and the trial would have to go on long enough for the diets to influence chronic conditions, which could take many years.
Another important point: merely citing evidence isn't enough to prove an argument. There are studies on both sides of most nutrition-related debates, and some papers examining the health effects of PUFAs happen to be authored by researchers who might benefit from advocating a particular viewpoint (for example, the author of this paper is the scientific advisor to the Vegetable Oils Industry of Canada).
While authors having affiliations don't at all negate study findings, it's important to put nutrition research in the context of funding: food and agriculture conglomerates have an incentive to show their products are healthy, and also have money to spend on trials. On the flip side, there isn't a conglomeration of well-funded anti-PUFA advocates willing to fund trials. Thus, it's important to look at not just randomized trials, but also mechanistic studies, animal evidence, and observational evidence.
In that vein, one of the strongest predictors of heart disease is tissue omega-6 to omega-3 ratio, which can be estimated by a blood test. Americans typically have three times more omega-6 than omega-3 in their tissues, which is the reverse of what is optimal. Japanese and Mediterranean populations have far lower ratios, and also happen to have lower heart disease rates and longer lifespans. While this cannot prove causation, it's clear that eating high-PUFA oils such as corn oil is not necessary for optimal health and may very well be detrimental.
Coupled with the intuitive mechanistic findings showing that PUFA is easily oxidized, along with the fact that omega-6 competes with omega-3s within our bodies, there does not seem to be much reason to purposely choose corn oil as a kitchen staple.
Kamal Patel, MPH, MBA, is a nutrition researcher and director of Examine.com, a nutrition and supplement resource.