FATTY ACIDS, HEALTH, AND SOYBEAN OIL

Heart disease continues to be the leading cause of death in the United States, and resulted in more than 655,000 deaths in 2018.¹ Data from observational and clinical trials have led to the identification of dietary approaches that reduce the risk of heart disease. Evidence indicates that soybean oil, when used as a replacement for saturated fat, improves blood cholesterol levels and may lower the risk of heart disease. In addition, clinical trials show soybean oil does not cause inflammation or oxidative stress.

Soybean Oil and Healthy Dietary Patterns

Dietary recommendations support incorporating soybean oil into healthy dietary patterns for general health and chronic disease prevention. Intake of soybean oil in amounts recommended by the 2015-2020 Dietary Guidelines for Americans (i.e., 27g/2,000 kcal),² will result in adequate intake recommendations³ for both essential fatty acids (linoleic acid and alpha-linolenic acid) being met. Soybean oil, when used as a replacement for saturated fat, improves blood cholesterol levels and may lower the risk of heart disease, making it a heart healthy dietary choice.

Recommendations for Healthy Dietary Patterns

Dietary guidelines for general health and chronic disease prevention recommend a dietary pattern that is abundant in fruits, vegetables, legumes, whole grains, and nuts and seeds, and includes low-fat/fat-free dairy, vegetable protein or lean animal protein, and oils that are composed predominantly of monounsaturated (MUFA) or polyunsaturated (PUFA) fatty acids. The diet should be low in saturated fat, sodium, processed meats, and added sugars.^2,4 Several dietary patterns meet these criteria, including the 2015-2020 Dietary Guidelines for Americans eating patterns (i.e., Healthy U.S.-Style, Healthy Mediterranean-Style, and Healthy Vegetarian diets). Non-tropical oils such as soybean and canola are emphasized as part of these healthy eating patterns because they are a major source of essential fatty acids, linoleic acid and alpha-linolenic acid (ALA).² At 2,000 kcal, all 3 of the eating patterns include 27g/day of oils, which are incorporated to replace sources of saturated fat.¹ Replacement of saturated fats with MUFAs and PUFAs is recommended for general health and heart disease prevention.^2,5

Soybean Oil and Heart Health

Figure 1 shows the fatty acid composition of commonly consumed fats and oils. Soybean oil predominantly contains PUFA (58g/100g) in the form of essential fatty acids, linoleic acid (51g/100g) and ALA (6.6g/100g), and is low in saturated fat. Importantly, soybean oil is one of the few commonly used and widely available oils that contains a significant amount of ALA -- a plant-derived omega-3 fatty acid. ALA intake is consistently associated with moderately lower risk of fatal coronary heart disease.^6,7

The fatty acid profile of soybean oil makes it a healthful choice, particularly for heart health. Consistent evidence shows that replacing saturated fat with soybean oil lowers total cholesterol and low-density lipoprotein cholesterol (LDL-C);^8-13 the main targets for heart disease prevention.¹⁴ Formal recognition of the cholesterol-lowering effects of conventional (commodity) soybean oil came in 2017 when the FDA authorized the following qualified health claim:

“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.”¹⁵

The effect of soybean oil on blood cholesterol levels aligns with the well-established effects of replacing saturated fat with PUFA, which results in greater blood cholesterol lowering than replacement of saturated fat with MUFA.¹⁶ The 2020 U.S. Dietary Guidelines Advisory Committee concluded that strong evidence supports replacing dietary sources of saturated fat with PUFA to reduce risk of coronary heart disease and cardiovascular disease mortality.¹⁷ In alignment, a meta-analysis of 4 trials satisfying rigorous criteria for causality that was included in a 2017 American Heart Association Presidential Advisory showed a 29% relative risk reduction for coronary heart disease when saturated fat was replaced with PUFA primarily from soybean oil.¹⁸

Formal recognition of the cholesterol-lowering effects of soybean oil came in 2017 when the FDA authorized the following qualified health claim:

“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.”

Soybean Oil and Inflammation

Despite the well-established beneficial effect of soybean oil on blood cholesterol levels and that it may lower the risk of heart disease, concern has been raised that the high linoleic acid content of soybean oil promotes inflammation.^19-21 Linoleic acid, an essential omega-6 fatty acid, is a metabolic precursor to arachidonic acid. Arachidonic acid is converted to eicosanoids, bioactive oxidized lipid molecules (oxylipins), which are implicated in inflammatory pathways.²² Initially, it was thought that omega-6 PUFA (i.e., linoleic acid) derived oxylipins were pro-inflammatory and, in contrast, omega-3 PUFA derived oxylipins were anti-inflammatory. This led to the assertation that a high dietary ratio of omega-6 PUFA to omega-3 PUFA promotes inflammation.¹⁹ However, the interaction between omega-3 and omega-6 PUFA and their lipid mediator derivatives in the development of inflammation is complex and still not completely understood.²² Authoritative organizations worldwide no longer include guidance on the dietary ratio of omega-6 PUFA to omega-3 PUFA, but rather emphasize the importance of consuming sufficient amounts of each essential fatty acid.³

Considerable evidence shows that intake of linoleic acid only modestly affects tissue arachidonic acid levels because the latter is tightly regulated.²³ Examination of the inflammatory effects of soybean oil in randomized controlled trials has demonstrated no change in markers of inflammation including C-reactive protein (CRP),^24-27 an established marker of inflammation and target for heart disease risk reduction.¹⁴ In alignment, data from 15 randomized controlled trials showed no effect of linoleic acid intake on a broad range of markers of chronic inflammation including CRP, fibrinogen, plasminogen activator inhibitor type 1, soluble vascular adhesion molecules, and tumor necrosis factor-α.²⁸ Similarly, a more recent meta-analysis of 30 randomized controlled trials, involving 1,377 participants, showed that higher linoleic acid intake does not affect the blood concentrations of 11 pro-inflammatory markers.²⁹ These findings are consistent with the concordant evidence base showing linoleic acid is associated with lower heart disease risk.^18,30 In summary, current evidence does not show that intake of soybean oil or linoleic acid, at amounts consistent with dietary recommendations, causes excess inflammation.

Soybean Oil and Oxidative Stress

The susceptibility of the double bonds in PUFA to oxidation has led to concerns about higher intake of PUFA promoting oxidative stress. There is particular concern about the oxidation of PUFA-rich low-density lipoprotein (LDL) particles, since oxidized LDL is implicated in atherosclerosis development. However, most of the evidence related to oxidative stress is from in vitro and ex vivo experiments. The clinical significance of these findings remains unclear. The few randomized controlled trials that have been conducted to directly assess the effect of soybean oil on circulating markers of oxidative stress have shown no adverse effects of soybean oil.^24,31 Therefore, concerns that intake of soybean oil or linoleic acid (at recommended amounts) may increase oxidative stress, including LDL oxidation, are not supported by evidence from clinical trials.

Figure 1: Fatty Acid Profiles (g/100g) of Commonly Consumed Oils

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

References

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