Soybean oil is a commercial cooking oil produced by extracting the oil from the seeds of the soybean plant (Glycine max).
This oil is incredibly common, and it is the second most-consumed vegetable oil in the world behind palm oil (1).
However, what does soybean oil offer nutritionally, and it is a good choice of cooking oil?
This article examines the nutritional values and scientific research on soybean oil.
How Is Soybean Oil Made?
Soybeans contain approximately 20% fat by weight, but extracting their oils is quite difficult without the appropriate equipment (2).
Therefore, the majority of commercially produced soybean oil relies on a mechanical and solvent extraction process (3).
This process extracts the edible oil from the soybeans, and the finished product is known as a ‘refined, bleached, and deodorized’ (RBD) oil.
The entire production process is quite long and complex, but here is a quick summary of the primary steps:
- First, the soybeans go through a mechanical press that uses heat and pressure to extract their oil.
- After mechanical pressing, there is still residual oil left in the soybean meal, which is then extracted by soaking the soybean meal in a solvent called hexane. The hexane then needs removing before the soybean oil can be sold, and this removal process uses steam and vacuums (4).
- Following this solvent extraction step, the oil also needs bleaching, This bleaching process removes impurities from the oil (5).
- Finally, the oil undergoes a deodorization process to remove volatile compounds in the oil. This deodorizing stage involves a high-temperature steam distillation process (180-220°C) (6).
Although this method of oil production may not sound as “clean” as cold-pressed oil, all the solvent residues are removed before the product can be sold.
Additionally, this process is not unique to soybean oil.
For example, there are many different vegetable oils (such as Canola oil and refined olive oil) that undergo refining, bleaching, and deodorization processes.
Based on data from the USDA FoodData Central Database, the following table shows the nutritional values for a tablespoon (13.6 gram) serving of soybean oil (7).
Daily values have been calculated using the FDA’s latest published daily values (8).
|Name||Amount||% Daily Value|
|Vitamin E||1.11 mg||7%|
|Vitamin K||25.0 mcg||21%|
Despite not being a major source of vitamins or minerals, soybean oil does provide moderate amounts of vitamin E and K.
The five primary fatty acids in soybean oil are as follows (9):
- Palmitic acid
- Stearic acid
- Oleic acid
- Linoleic acid (omega-6)
- Linolenic acid (omega-3)
Linoleic acid is the predominant fatty acid in soybean oil.
Soybean oil has a smoke point of 234°C, which is approximately 453°F (10).
The smoke point of a cooking oil represents the temperature at which it starts to smoke. For instance, if soybean oil reaches the ‘smoke point’ temperature during cooking, it will start to burn.
It is important to note that the smoke point of a cooking oil very much depends on the amount of free fatty acids (FFAs) present. A higher amount of FFAs results in a lower smoke point and in turn, a lower level of FFAs means a higher smoke point (13).
Since the refining process largely removes free fatty acids from oil, refined oils tend to have a higher smoke point than unrefined oils. For this reason, soybean oil has a higher smoke point than cold-pressed extra virgin olive oil (14, 15).
Another consideration to be aware of is that the given smoke point of cooking oil actually reduces during heat exposure. Also, this rate of decline depends on the oxidative stability of the specific type of cooking oil (16).
For this reason, the smoke point alone isn’t necessarily the best indicator of how heat-stable a particular oil is.
Various different factors affect the oxidative stability of a specific oil, and these include:
- Degree of saturation
- Polyphenol content
- Amount of antioxidants (such as vitamin E) present
Firstly, out of the three main types of fatty acids, saturated fats are the most stable when exposed to high heat temperatures (17).
This is because they are “fully saturated.”
In other words; the carbon atoms in the fat are tightly surrounded by hydrogen atoms, which helps to prevent oxygen from ‘oxidizing’ the carbon atoms.
On the other hand, polyunsaturated fatty acids have numerous double bonds (gaps) exposing the carbon atoms to oxygen.
This means that instead of linking to a hydrogen atom, the carbon atom has two links with another carbon atom.
However, this ‘double bond’ makes polyunsaturated fatty acids more prone to oxidation.
As shown in the diagram above, this particular polyunsaturated fatty acid has two double bonds. Others may have more.
For this reason, soybean oil has poor oxidative stability compared to fats with a higher degree of saturation, such as butter, extra virgin olive oil, and avocado oil.
Studies On Soybean Oil’s Oxidative Stability
One study investigated the rates of oxidative deterioration for soybean oil, corn oil, and olive oil. After using the oils for frying at 180°C, soybean oil oxidized more (and faster) than the other two oils (18).
Another study showed that heating soybean oil at 185°C (365°F) for two hours led to the formation of 4-hydroxy-2-trans-nonenal (HNE). HNE is an oxidation product that has mutagenic and cytotoxic properties (19).
Additionally, in a rat study, prolonged intake of re-heated soybean oil caused the rats to develop high blood pressure and vascular inflammation (20).
Of course, rats are not humans, and we should try not to re-heat any oil. That said, it is not unusual for fast food restaurants to use (or re-use) oil for extended periods.
Furthermore, a study examined the oxidative stability of soybean oil during storage. This study found that blending soybean oil with other oils, including sesame oil and peanut oil, made it more stable (21).
The researchers concluded that the greater oxidative stability of the blended oil was likely because polyunsaturated fatty acids decreased while monounsaturated fatty acids increased.
Excessive Soybean Oil Intake and Obesity
Look around, and it’s clear to see that Western society has an obesity problem.
For instance, 2 out of every 3 adults in the United States (68.8%) are either overweight or obese (22).
Some people lay the blame for this on red meat and animal fat, but the data does not support this.
As you can see, the consumption of red meat has fallen over the past several decades while leaner poultry increases in popularity.
Rates of sugar consumption, which is often a scapegoat for almost every health condition, have also been decreasing since the 1990s (23).
However, if we are looking for a possible dietary villain, then soybean oil consumption has grown by a significant magnitude.
It is in almost everything we eat; not only as a cooking oil but also in salad dressings and processed foods.
Specifically, domestic consumption of soybean oil has more than quadrupled from 1,652,000 tons in 1964 to 6,576,000 tons as of 2016 (24).
If we go even further back, soybean oil consumption increased 1000-fold between the years 1909 and 1999 (25).
Animal Studies Suggest Soybean Oil Could Be Obesogenic
If excessive soybean oil intake is helping to drive the obesity crisis, then isn’t that all just about calories in vs. calories out?
Well, calories certainly matter, and soybean oil contributes a substantial number of them to our daily diet.
However, some animal studies suggest that there could be more to it;
- A mouse study compared the impact of a diet high in soybean oil to high fructose and coconut oil diets. In this study, the mice fed large amounts of soybean oil had statistically significant greater weight gain, diabetes, and insulin resistance. Additionally, the soybean oil diet upregulated cancer, inflammation and obesity-related genes (26).
- Another animal study fed mice a high-fat diet of either soybean, olive oil, MCT oil, peanut oil, or tea oil. Following the study, body weight and body fat were significantly higher in the soybean oil group compared to all others. Also, the soybean oil group had visible fatty liver disease—the MCT, olive oil, and tea oil mice did not (27).
- In a study comparing butter and soybean oil, fat oxidation increased in the group fed butter. In the soybean fed group, subjects showed an inability to oxidize fat while resting (28).
Are these animal studies proof that soybean oil is obesogenic in humans?
Of course not, but with a lack of human trials on this issue, these animal studies certainly suggest it is worth looking into.
Refined, Bleached and Deodorized Oils May Contain Hexane Residues
Soybean oil manufacturers state that the finished oil doesn’t contain an appreciable amount of hexane because of the cleaning and refining process the oil undergoes.
However, studies show that residual amounts usually remain in the finished oil. Based on equivalent-dose animal studies, these amounts are safe for human consumption (29).
That being said, independent tests have investigated the issue and shown that hexane residues “10 times higher than what the FDA considers normal” can appear in soy products (30).
Additionally, the FDA has set no maximum values on hexane residues in soy products, and FDA regulation does not require testing vegetable oils for hexane contamination (31).
Soybean Oil Promotes the Excessive Omega-6 To Omega-3 Ratios In Our Diets
As previously mentioned, soybean oil is a significant source of omega-6 fatty acids.
Although both omega-3 and omega-6 are essential fatty acids, some researchers believe we should consume them in relative balance.
For example, research shows that ancestral diets generally had an omega-6 to omega-3 ratio approaching 1:1 (32).
Due to the excessive amount of vegetable oils in the modern diet, people are consuming more omega-6 (and less omega-3) than at any point in human history.
Notably, estimates place the modern diet as having an omega-6 to omega-3 ratio of 20:1 or higher (33).
Omega-6: Healthy Until It’s Not?
Omega-6 is an essential fatty acid that is pro-inflammatory, and this is a good thing.
For instance, some inflammation in the body is necessary for healing cuts and wounds.
However, in excessive amounts, some research suggests that it may cause our body to stay in an inflammatory state — and chronic inflammation is a known risk for the majority of illnesses (34).
On this note, studies suggest that this excessive intake of omega-6 could cause the following problems;
- As omega-6 competes with omega-3 for uptake into our cells, too much omega-6 could mean a deficiency of anti-inflammatory omega-3 (35).
- Studies show that a lower dietary omega-6 to omega-3 ratio reduces inflammation and mortality risk from heart disease (36).
- Chronic diseases such as diabetes, obesity, IBD and arthritis all have associations with increased omega-6. Imbalances in the omega 6 to 3 ratio have likely increased the prevalence of multiple inflammatory, disease-causing processes (37).
That said, this excessive omega-6 content is not a problem unique to soybean oil. Other oils rich in polyunsaturated fats such as grapeseed oil have the same problems.
Soybean Oil Has Changed the Fatty Acid Composition of our Cells
The above graph shows the explosion in the consumption of polyunsaturated fat over the past century.
As soybean oil is the most significant source of omega-6 in the US, it is a significant part of this trend.
Excessive Omega-6 Intake May Cause Problems
As discussed, polyunsaturated fats have several double bonds, which means their carbon atoms are susceptible to oxidization.
The first thing to remember here is that our cells and tissues predominantly contain saturated and monounsaturated fat.
For one thing, this makes the fat in our cells stable. Just like vegetable oils can oxidize due to air, light, or heat exposure, so can fats inside our body.
One theory speculated on by researchers is that as our cell membranes incorporate a higher amount of polyunsaturated omega-6, they become less stable.
Several studies on soybean oil/omega-6 seem to offer some support for this theory;
- Omega-6 is a “melanoma stimulator” because it makes our skin cells susceptible to damage from UV rays (38).
- An animal study separated mice into different groups and fed them varying amounts of dietary omega-6 PUFA. The higher their dietary intake, the more they developed cancerous tumors (39).
- One study fed female rats either soybean oil or ghee as 10% of their total diet. The soybean oil group had higher tumor incidence (65.4% versus 26.6%), larger tumor size, and more rapid onset of cancer (40).
Overall, there is a lack of direct evidence from human trials to claim that soybean oil is harmful with any certainty.
However, in my view, many studies justify being wary about the potentially detrimental effects soybean oil can have.
Especially when we consider just how much of this oil many people consume.
To summarize; soybean oil is prone to oxidation, offers predominantly omega-6 fatty acids, and has links to adverse health effects in animal studies.
With all this being said, it is probably a better idea to opt for cooking oil that does not have these concerns.
Fruit oils such as avocado, coconut, and olive oil appear to be a good choice since studies show these oils have much better oxidative stability.