Soybean oil is the most widely consumed cooking oil in the United States and its usage is rapidly growing globally. It’s in countless processed foods and a staple in restaurants due to its affordability. For decades, unsaturated fats, especially from plant sources like soybean oil, have been championed as healthier alternatives to saturated animal fats. However, emerging research is prompting a closer look at the potential downsides of high soybean oil consumption, particularly concerning its impact on our gut health and overall well-being. So, Why Is Soybean Oil Bad For You? Let’s delve into the science.
The Rise of Soybean Oil and its Hidden Problem: Linoleic Acid
Soybean oil’s dominance in the food industry began in the 1970s, initially driven by the demand for soybean meal as animal feed. Soybean oil emerged as a readily available and cost-effective byproduct. Soybeans themselves are a valuable source of protein and are easy to cultivate, further solidifying soybean oil’s position in the market.
However, the potential issue with soybean oil lies in its high content of linoleic acid, an omega-6 polyunsaturated fatty acid. While linoleic acid is essential in small amounts – our bodies can’t produce it and need it for various functions – excessive intake, primarily from soybean oil, is becoming a concern.
Soybean oil is currently the most highly consumed cooking oil in the U.S. (UCR/Stan Lim)
Researchers like Poonamjot Deol, an assistant professional researcher in the Department of Microbiology and Plant Pathology at the University of California, Riverside (UCR), are challenging the long-held belief that all plant-based unsaturated fats are inherently healthy. Deol and her colleagues published a study in Gut Microbes investigating the effects of high soybean oil diets, and their findings raise significant questions about its impact on gut health. According to Deol, the problem isn’t unsaturated fat itself, but the type and quantity of unsaturated fat, specifically linoleic acid in soybean oil.
“While our bodies need 1-2% of linoleic acid daily, based on the paleodiet, Americans today are getting 8-10% of their energy from linoleic acid daily, most of it from soybean oil,” Deol explains. “Excessive linoleic acid negatively affects the gut microbiome.”
How Soybean Oil Impacts Your Gut Microbiome
The UCR study, conducted on mice, revealed that a diet consistently high in soybean oil for up to 24 weeks led to significant alterations in the gut microbiome – the complex community of microorganisms living in our intestines. The researchers observed a decrease in beneficial bacteria and a surge in harmful bacteria, particularly adherent invasive Escherichia coli (AIEC).
This is concerning because AIEC is implicated in inflammatory bowel disease (IBD), including ulcerative colitis. The study demonstrated that linoleic acid in soybean oil acts as a food source for AIEC, promoting its growth in the gut. Conversely, many beneficial bacteria are unable to thrive in an environment rich in linoleic acid, leading to a microbial imbalance or dysbiosis.
A diet high in soybean oil is found to encourage the growth of harmful bacteria such as adherent invasive E. coli in the gut. (UCR/Sladek lab)
“It’s the combination of good bacteria dying off and harmful bacteria growing out that makes the gut more susceptible to inflammation and its downstream effects,” Deol states. This microbial shift is a key reason why soybean oil is bad for you, as it disrupts the delicate balance necessary for a healthy gut.
The Gut Barrier and Systemic Inflammation
Beyond altering the gut microbiome, the study also found that high linoleic acid consumption from soybean oil compromises the intestinal epithelial barrier. This barrier, crucial for gut health, acts as a gatekeeper, selectively allowing nutrients in while preventing harmful substances from leaking into the bloodstream.
Linoleic acid was shown to make this barrier more “porous” or “leaky.” When the gut barrier becomes compromised, toxins and bacteria can escape the gut and enter systemic circulation. This phenomenon, often referred to as “leaky gut,” can trigger chronic inflammation throughout the body, increasing the risk of not only colitis but potentially other inflammatory conditions and diseases.
The researchers point out a concerning correlation: the rise in IBD incidence in the U.S. mirrors the increasing consumption of soybean oil. While correlation doesn’t equal causation, it raises a significant red flag and warrants further investigation into the link between soybean oil and inflammatory bowel diseases in humans.
Challenging the “Healthy Unsaturated Fat” Myth
For decades, health advice has emphasized replacing saturated fats with unsaturated fats to protect heart health. While this advice has merit, toxicologist Frances M. Sladek, a professor of cell biology at UCR and a co-corresponding author of the study, emphasizes that not all unsaturated fats are created equal.
“Since studies showed that saturated fats can be unhealthy, it was assumed that all unsaturated fats are healthy,” Sladek notes. “But there are different types of unsaturated fats, some of which are healthful. For example, the unsaturated fat fish oil is well known to have many beneficial health effects. People therefore assumed that soybean oil is perfectly safe and healthier to consume than other types of oils, without actually doing a direct comparison as we have done.”
(Source for data: Dahlhamer et al, 2016; USDA)
While linoleic acid is an essential fatty acid and the American Heart Association recommends some intake (5-10% of daily calories from omega-6 polyunsaturated fatty acids), the crucial point is moderation. Soybean oil, with its high linoleic acid content (around 19% in the oil used in the study), contributes significantly to the overconsumption of this fatty acid in many Western diets. Other seed oils like safflower and sunflower oil are also high in linoleic acid, while animal fats can also contribute.
The issue is not that linoleic acid is inherently harmful, but that excessive amounts, largely driven by soybean oil consumption, may be detrimental. As Sladek points out, “Just because something is needed does not mean a lot of it is good for you.”
Soybean Oil in the Modern Diet and Processed Foods
The widespread use of soybean oil in processed foods and restaurants makes it challenging to avoid excessive consumption. From salad dressings and mayonnaise to snack foods and fried items, soybean oil is often a primary ingredient due to its low cost and versatility. This pervasive presence in the food supply contributes to the high levels of linoleic acid intake in many individuals.
James Borneman, a professor of microbiology and plant pathology at UCR and another co-author, highlights the human relevance of their mouse study. “Adherent invasive E. coli contributes to IBD in humans, and the fact that we find this E. coli in these mice is concerning,” he states. “Sometimes, it can be unclear how research done in mice translates to humans, but in this study it is fairly clear.”
Furthermore, the researchers observed that mice on a high soybean oil diet experienced a decrease in gut endocannabinoids, molecules that regulate various physiological processes, and an increase in inflammatory oxylipins. These changes further support the link between soybean oil and gut inflammation, potentially contributing to conditions like IBD and potentially other health issues previously linked to soybean oil, such as obesity, diabetes, and neurological conditions.
Chart depicts consumption of edible oils in the U.S. for 2017/18. (USDA)
Healthier Alternatives and Dietary Recommendations
Given the potential negative impacts of high soybean oil consumption, what are healthier alternatives? The researchers recommend opting for oils with lower linoleic acid content. Olive oil, a cornerstone of the Mediterranean diet, is highlighted as a healthy choice. “Olive oil, the basis of the Mediterranean diet, is considered to be very healthy; it produces less obesity and we have now found that, unlike soybean oil, it does not increase the susceptibility of mice to colitis,” Sladek explains.
Other beneficial options include coconut oil and avocado oil. Conversely, corn oil, another common cooking oil, contains a similar amount of linoleic acid as soybean oil and should also be used with caution.
To minimize your intake of soybean oil and excessive linoleic acid, the researchers offer practical advice:
- Reduce processed food consumption: Processed foods are often high in soybean oil. Focus on whole, unprocessed foods.
- Read nutrition labels: When purchasing oils or packaged foods, check the ingredient list and nutrition facts label to identify soybean oil and other high linoleic acid oils.
- Choose healthier cooking oils: Opt for olive oil, avocado oil, or coconut oil for cooking and salad dressings.
- Consider air frying: Air fryers require significantly less oil compared to traditional frying methods.
In conclusion, while soybean oil has been a dominant cooking oil for decades, emerging research suggests that its high linoleic acid content may pose health risks, particularly concerning gut health and inflammation. Excessive consumption, driven by its prevalence in processed foods and restaurants, can disrupt the gut microbiome, compromise the intestinal barrier, and potentially contribute to inflammatory conditions like ulcerative colitis. While further research, especially in humans, is needed, the current evidence suggests that being mindful of soybean oil intake and choosing healthier alternatives is a prudent step towards promoting better health. The take-home message from Deol and her colleagues is clear: “We recommend keeping track of the soybean oil in your diet to make sure you are not consuming excessive linoleic acid.”
