SOYBEAN OIL CAN POSITIVELY CONTRIBUTE TO A HEALTHFUL DIET

Soybean oil is the most widely consumed oil globally and in the United States, as it accounts for approximately 30%1 and 57%2 of total edible oil consumption, respectively.  It also accounts for more than 7% of U.S. total caloric intake and more than 40% of the intake of both essential fatty acids, linoleic acid, an omega-6 fatty acid and alpha-linolenic acid, an omega-3 fatty acid.3

Soybean oil is comprised of approximately 58% polyunsaturated fat, 23% monounsaturated fat, and 16% saturated fat (table 1).  Replacement of saturated fat with unsaturated fat, like is found in soybean oil, improves circulating lipids and lipoprotein levels, the main targets for cardiovascular disease prevention and management. The first clinical trial demonstrating the cholesterol-lowering effect of soybean oil was published in 1993.4 Several other trials have subsequently been published.5-11

This cholesterol-lowering replacement effect of soybean oil was formally recognized by the U.S. Food and Drug Administration (FDA) in 2017, when it approved a qualified health claim for soybean oil and coronary heart disease.12  Language allowed by the FDA in support of the soy oil health claim is much stronger than allowed for other oils with approved health claims, including canola oil and olive oil. The claim states:

“Supportive but not conclusive scientific evidence suggests that eating about 1½ tablespoons (20.5 grams) daily of soybean oil, which contains unsaturated fat, may reduce the risk of coronary heart disease. To achieve this possible benefit, soybean oil is to replace saturated fat and not increase the total number of calories you eat in a day.” 

Despite the favorable fatty acid profile and the FDA health claim, the results of a small number of animal studies conducted by UC Riverside researchers have raised concerns about the healthfulness of soybean oil.13-15 While animal studies are part of the scientific literature, they carry much less weight within the scientific community than human studies do. Findings from animal studies are generally used for hypothesis generation, not as a basis for reaching conclusions about health effects in humans. In contrast to the aforementioned animal studies, observational studies and clinical studies support the safety and health benefits of soybean oil.  Concerns that have been raised about soybean oil are addressed individually below.

Dietary omega-6:omega-3 fatty acid ratio and inflammation

At one point, there was scientific consensus that a high dietary omega-6:omega-3 fatty acid ratio leads to a proinflammatory state.16  Some experts concluded that the ideal dietary ratio is as low as 1:1;17  which is much different than the current U.S. dietary ratio of about 10:13 and the ratio in soybean oil of approximately 7.7:1.

Inflammation was thought to result from the pro-inflammatory potential of arachidonic acid, a metabolic product of linoleic acid. However, there is now widespread agreement that linoleic acid is not adversely associated with cardiovascular disease18,19 and does not lead to a pro-inflammatory state.20,21  To the contrary, evidence indicates that linoleic intake reduces risk of coronary heart disease.18 Further, authoritative organizations worldwide no longer consider the omega-6:omega-3 dietary fatty acid ratio to be important, but instead emphasize the importance of consuming adequate amounts of each essential fatty acid.19,22-29  Although a high dietary omega-6:omega-3 fatty acid ratio may inhibit the conversion of alpha-linolenic acid to the long chain omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the American Heart Association concluded that limiting linoleic acid intake with the aim of improving this conversion would do more harm than good.18

Polyunsaturated fat and oxidative stress

Polyunsaturated fatty acids are particularly susceptible to oxidation because of their multiple double bonds.30 The increased susceptibility of polyunsaturated fat-enriched low-density-lipoprotein (LDL) to oxidation and the role of oxidative stress in atherogenesis has led to the assertion that polyunsaturated fat promotes atherosclerosis. However, as noted, substantial evidence indicates that when polyunsaturated fat replaces saturated fat, coronary heart disease risk is reduced.18

Several studies have directly examined the effects of soybean oil on markers of oxidation. One 4-week randomized, crossover, controlled feeding trial compared the effects of consuming a diet that included 20g/day of palm oil-based mayonnaise with a diet that included 20g/day of soybean oil-based mayonnaise. No difference between the diets in levels of thiobarbituric acid reactive substances, a measure of lipid peroxidation in response to oxidative stress, was noted.5 Similarly, a 4-week randomized parallel trial of adults with metabolic syndrome showed no effect of 10mL/day of soybean oil on lipid peroxidation, measured by malondialdehyde equivalents, compared to 10mL/day of Brazil nut oil.31 Finally, one study did find that in comparison to palm oil, the consumption of soybean oil led to a reduction in small-dense, LDL oxidation lag time.8  However, soybean oil significantly decreased levels of LDL-cholesterol and small-dense LDL-cholesterol by 14% and 10%, respectively. Thus, overall risk of coronary heart disease was likely reduced.

Diabetes

Observational studies show that the replacement of saturated fat with polyunsaturated fat is associated with better glycemic control and increased insulin sensitivity.32  Also, a meta-analysis of 20 prospective studies involving nearly 40,000 adult men and women found that higher proportions of linoleic acid biomarkers as percentages of total fatty acids were associated with an approximate 30% reduction in risk of developing type 2 diabetes.33  These findings suggest that soybean oil, which is high in polyunsaturated fat, will not increase risk of diabetes.  This suggestion is consistent with the results of a recently published clinical trial that found circulating glucose levels did not differ among study participants according to whether they consumed soybean oil, high oleic soy oil, or palm oil.6

Obesity

No scientific consensus exists on whether manipulating the macronutrient content of the diet affects weight gain or loss in humans,34,35 although some evidence suggests higher-protein diets may be advantageous in promoting weight loss.36  The notion that soybean oil contributes to the obesity epidemic conflicts with global and U.S. data.  U.S. soybean oil consumption began markedly increasing long before obesity rates began to skyrocket.3 Furthermore, the increase in obesity rates in the United States parallels those in the world, although the U.S. starts at a higher body mass index.37 Many countries consume relatively little soybean oil, but nearly all countries have experienced an increase in obesity. Finally, U.S. obesity rates for both children and adults have continued to climb over the past decade,38 even though U.S. per capita soybean oil consumption has declined during this time period.39

Alzheimer’s disease

The results of a recently published meta-analysis indicate that because of its high polyunsaturated fat content, soybean oil consumption would not likely be a risk factor for Alzheimer’s disease. This analysis included 8,630 participants and 633 cases from four prospective cohort studies.40 A diet higher in saturated fat was significantly associated with a 39% increased risk of Alzheimer’s disease and 105% increased risk of dementia. Dose-response analysis indicated a 4g/day increment of saturated fat intake was related to a 15% higher risk of Alzheimer’s disease. In contrast, no significant association between dietary intake of total, monounsaturated, or polyunsaturated fat and Alzheimer’s or dementia risk was found. These finding contradict rodent research suggesting soy oil could increase risk of developing neurological conditions such as Alzheimer’s disease.

Non-alcoholic fatty liver disease

Emerging data suggests that in comparison to polyunsaturated fat, saturated fat enhances visceral fat accumulation, especially liver fat, and subsequent risk of developing non-alcoholic fatty liver disease.41  Cross-sectional data suggest that dietary fat composition could play a key role in liver fat accumulation with polyunsaturated fatty acids inversely,42 and saturated fat, directly associated with liver fat and liver fat markers.43,44  There are also clinical data implicating saturated fat.  For example, in 2012, an isocaloric diet rich in polyunsaturated fat given for 10 weeks reduced liver fat content and tended to reduce insulin resistance, compared with a diet rich in saturated fat in individuals with abdominal obesity and type 2 diabetes.45

In 2014, Rosqvist et al.46 found that when 39 young and normal-weight individuals were overfed muffins high in either saturated fat or omega-6 polyunsaturated fat, although both groups gained weight, in those consuming the muffins with saturated fat, liver fat increased markedly compared with those consuming the polyunsaturated fat muffins. Individuals fed the muffins high in saturated fat experienced a twofold greater increase in visceral adipose tissue.  Conversely, polyunsaturated fat caused a nearly threefold larger increase in lean tissue than saturated fat. More recently, a 12-week overfeeding trial involving 61 overweight or obese men, showed that saturated fat markedly induced liver fat content (50% relative increase) along with liver enzymes (indicating hepatocellular injury) and atherogenic serum lipids.41 In contrast, despite similar body weight gain, polyunsaturated fat did not increase liver fat or liver enzymes or cause any adverse effects on blood lipids. Overall, the evidence suggests that given its high polyunsaturated fat content, relative to sources of saturated fat, soybean oil will not promote liver fat accumulation.

Autism

The role that prenatal dietary intake may have on risk of offspring developing autism spectrum disorder is unclear and has not been rigorously investigated.47 Of note is a cross-sectional study in which dietary information was collected on 317 mothers of children with autism spectrum disorder and 17,728 control mothers who were participants in the Nurses’ Health Study II.  Maternal intake of linoleic acid was significantly inversely associated with autism spectrum disorder risk in offspring, corresponding to a 34% reduction in risk in the highest versus lowest intake quartiles.48  In contrast, one cross-sectional study found that a lower plasma omega-3:omega-6 fatty acid ratio (due primarily to high plasma linoleic acid) was associated with an increased risk of autistic traits in children49 whereas a more recently published trial involving 258 mother-child pairs found linoleic acid intake was unrelated to risk of autism spectrum disorder.50  Overall, the evidence is too limited to draw meaningful conclusions about the impact of maternal fat intake on risk of autism and; therefore, no inferences about the impact of prenatal soybean oil intake on autism risk can be made.

Summary and Conclusion

Soybean oil plays an important role in the global and U.S. food supply. It is comprised predominantly of polyunsaturated fat. When replacing saturated fat in the diet, soybean oil lowers LDL-cholesterol. There is also intriguing evidence that relative to saturated fat, polyunsaturated fat improves glycemic control and decreases liver fat accumulation. Although some animal studies have raised concerns about the healthfulness of soybean oil, these concerns are contradicted by observational and clinical data.

Table 1. Fatty Acid Composition of Selected Plant Oils (g/100g)

OILUSDA
NDB #		TOTAL
SATURATED		OLEIC ACID (N-9)LINOLEIC ACID (N-6)ALPHA LINOLENIC ACID (N-3)N-6:N-3 RATIO
SAFFLOWER45106.214.174.60—-
SUNFLOWER450610.319.565.70—
WALNUT45289.122.252.910.45.1
CORN451813.427.251.91.0449.9
SOYBEAN404414.921.450.96.67.7
SESAME405814.239.341.30.3137.7
PEANUT404216.944.832.00—-
CANOLA45827.461.718.69.12.0
AVOCADO458111.667.912.50.9613.0
OLIVE405313.871.39.70.812.1
PALM OIL405549.336.69.10.245.5
COCONUT404782.56.31.70—

Source: U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, 2019. fdc.nal.usda.gov

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