Edible plant oils are commonly referred to as vegetable oils and include oils derived from vegetables, nuts, seeds, fruits, and cereal grains. The term “seed oil” has recently gained popularity and refers to oils derived from plant seeds. Seed oils, which are high in unsaturated fatty acids, include canola, corn, cottonseed, soybean, sunflower, safflower, grapeseed, and rice bran oil. Social media misinformation about seed oils includes claims that they cause inflammation, type 2 diabetes, and other chronic diseases, and has led to the “anti-seed oil movement”. As a result, some consumers have decided to avoid seed oils.
This movement has risen in prominence despite the preponderance of evidence showing no adverse health effects of seed oils or their constituents, and strong evidence demonstrating cardiometabolic health benefits of consuming unsaturated fatty acids.1-6 This article will review the evidence on the association between fatty acid and seed oil intake on risk factors for major diet-related chronic diseases with a focus on cardiometabolic disease.7
Seed oils differ widely in their fatty acid composition (Table 1); however, all are higher in unsaturated fatty acids, including monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), than in saturated fatty acids (SFA). While humans can endogenously synthesize adequate amounts of SFA and MUFA, humans are unable to synthesize the essential omega-6 PUFA linoleic acid (LA) and the essential omega-3 PUFA alpha-linolenic acid (ALA). Therefore, LA and ALA must be consumed to ensure nutrient adequacy. The U.S. recommendations for LA and ALA intake are presented in Table 2. These adequate intake recommendations are based on the U.S. median intake where deficiency is non-existent among healthy individuals.8
Fatty acid composition of commonly consumed seed oils, animal fats, and tropical oils ordered by saturated fat content.
Data are from USDA FoodData Central (https://fdc.nal.usda.gov/).
Table 2: Adequate intake recommendations for linoleic and alpha-linolenic acid by life stage group.8
Epidemiological Evidence
Meta-analyses of epidemiological studies show higher intake of omega-6 PUFA, predominately LA, is associated with lower risk of cardiovascular disease (CVD) and type 2 diabetes.9,10 In addition, higher intake of LA is associated with lower total mortality risk as well as lower risk of mortality from CVD and cancer.11 In alignment, pooled analyses of observational studies examining biomarkers of omega-6 PUFA intake show higher proportions of LA biomarkers are associated with lower risk of CVD, cardiovascular mortality, ischemic stroke, and type 2 diabetes.12,13
Epidemiological evidence relevant to the association between MUFA intake and CVD is less consistent. Studies generally show no association between total MUFA intake and risk of CVD14 or mortality from CVD or cancer.15 However, higher MUFA intake from plant sources is associated with lower risk of CVD.14
Consistent epidemiological evidence also demonstrates that the replacement of SFA with PUFA, predominately LA, is associated with lower risk of CVD events and CVD mortality.9,16 In addition, replacement of SFA with PUFA is associated with lower risk of mortality from cancer, neurodegenerative diseases, and respiratory diseases.5
Clinical Trials
Evidence demonstrates that increased consumption of essential fatty acids is associated with lower risk of cardiovascular disease and type 2 diabetes.
Evidence from clinical trials demonstrates that intake of unsaturated fatty acids as a replacement for SFA reduces the risk of cardiovascular events and improves major risk factors for CVD. The most recent Cochrane Collaboration review, which included 15 randomized controlled trials, examining the effect of SFA replacement, concluded that replacing SFA with PUFA is associated with lower risk of CVD.17 A similar finding was reported in an American Heart Association Presidential Advisory on dietary fats.5 A meta-analysis of the core randomized controlled trials showed that replacing SFA with PUFA reduced CVD risk by 29%.
Strong and consistent evidence from clinical trials shows that consuming unsaturated fatty acids instead of SFA improves atherogenic lipoproteins. A systematic review and meta-regression that included 84 randomized controlled trials showed that isocalorically replacing SFA with PUFA or MUFA reduced total cholesterol, LDL-cholesterol, and apolipoprotein B to a clinically relevant extent.18 In alignment, clinical trial evidence shows that intake of canola,19,20 corn,21-23 cottonseed,24,25 soybean,26 sunflower,27,28 safflower,29,30 and rice bran oil31 improves atherogenic lipoproteins. Finally, a network meta-analysis of randomized controlled trials examining direct and indirect evidence on the effects of 13 oils and solid fats (safflower, sunflower, canola, hempseed, flaxseed, corn, olive, soybean, palm, and coconut oils as well as beef fat, lard, and butter), showed that replacement of 10% of energy from butter with an equivalent amount of safflower, sunflower, canola, olive, flaxseed, corn, or soybean oil lowered LDL-cholesterol by 10–16 mg/dl.32 In summary, replacement of dietary sources of SFA with oils rich in PUFA and MUFA consistently improves atherogenic lipoproteins.
Evidence indicates unsaturated fatty acids may help reduce risk of type 2 diabetes.
Evidence from clinical trials also demonstrates that replacing SFA with PUFA improves glucose-insulin homeostasis. A meta-analysis of 102 controlled-feeding trials showed that replacing SFA with PUFA improved fasting glucose, HbA1C, C-peptide, and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR).33
Evidence shows no effects or reduced inflammation.
Findings from clinical trials also show that intake of omega-6 PUFA, particularly LA, does not promote inflammation or oxidative stress.34-37 A systematic review and meta-analysis that included 30 randomized controlled trials showed that higher intake of LA did not increase markers of inflammation including tumor necrosis factor-α, interleukin-6, adiponectin, monocyte chemoattractant protein 1, or C-reactive protein.34Similarly, clinical evidence shows no effect of omega-6 PUFA on markers of oxidative stress including oxidized LDL cholesterol37 or markers of lipid peroxidation included F-2 isoprostanes.36
Recommended Intake
Guidelines for general health and chronic disease prevention focus on following a healthy dietary pattern throughout life.38,39 Healthy dietary patterns emphasize minimally processed fruits, vegetables, whole grains, healthful protein sources (seafood, legumes, soy foods, nuts, and seeds), and liquid non-tropical vegetable oils. These patterns are also low in sources of saturated fats, added sugars, and sodium. To limit sources of SFA, it is recommended to replace rich sources of SFA, such as butter, tallow, lard, palm and coconut oils, duck fat, and ghee with vegetable oils including seed oils.Summary
Strong evidence supports consuming unsaturated fatty acids, particularly PUFA, instead of SFA to reduce the risk of CVD and type 2 diabetes. It is recommended that non-tropical liquid vegetable oils, including seed oils, be consumed as part of healthy dietary patterns. Vegetable oils should be used as a replacement for rich sources of SFA including butter, tallow, lard, palm and coconut oils, duck fat, and ghee.
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