Soybean oil can make important contributions to the health of Americans
by Mark Messina, PhD, MS
Health Attributes of Soybean Oil and Its Contribution to US Diets
Soybean oil is the most widely produced vegetable oil in the United States. In 2014, over 20.6 billion pounds were produced with 1.9 billion pounds exported to more than 50 countries. It is also the most commonly consumed oil in American diets accounting for more than half of all US vegetable oil consumption and for a little over 7% of total caloric intake.1 Because soybean oil is nearly always marketed and labeled as vegetable oil, most consumers don’t recognize the extent to which it plays in a role in their diets.
The reason for this labeling approach isn’t entirely known. One theory is that it is linked to the introduction of “all vegetable” shortenings in the 1960s which successfully replaced lard and beef tallow-based shortenings and were largely soybean oil-based. “All vegetable” cooking oils soon followed in the marketplace.
Given the contribution that soybean oil makes to caloric intake, it is important to have an understanding of how it impacts the health and nutrition status of Americans. This is especially relevant given the impact of different dietary fats on coronary heart disease risk.
Fatty Acid Composition of Soybean Oil
Soybean oil is low in saturated fat which comprises about 12% of total fatty acids. About 29% of the fat in soybean oil is monounsaturated. Like sunflower, sesame, and corn oils, soybean oil is rich in polyunsaturated fats (PUFAs) which make up 59% of its total fat content.2 What sets soybean oil apart from these other oils is that the polyunsaturated fat is comprised of both linoleic acid and alpha-linolenic acid. Both of these fatty acids are essential nutrients. Linoleic acid is the essential omega-6 fatty acid while alpha-linolenic acid is an essential omega-3 fatty acid.
This is a unique aspect of soybean oil since most other oils that are rich in PUFAs contain only omega-6 fats and have only negligible amounts of omega-3 fats. Because of its widespread use, soybean oil accounts for over 40% of the US intake of both of the essential fatty acids.1
The omega-3 fat found in soybean oil is not the same as the long chain omega-3 fats that are found in fatty fish. Although these long chain fats have been linked to lower risk for heart disease, they are not considered essential. In part, this is because the body can synthesize them from alpha-linolenic acid, the omega-3 fat that is present in soybean oil.
Effects of Polyunsaturated Fat on LDL-cholesterol
Saturated fat has long been known to raise blood levels of LDL-cholesterol which is linked to increased heart disease risk. In contrast, polyunsaturated fat lowers LDL-cholesterol. Since the 1980s, the US Dietary Guidelines have recommended limiting saturated fat intake. The most recent recommendation is to consume a diet that provides no more than 10% of calories as saturated fat. The American Heart Association recommends limiting intake to only 5 to 6% of calories.3
Soyfoods and soybean oil fit well into diets that adhere to these guidelines. In 2006 the American Heart Association stated that “ … soy products … should be beneficial to cardiovascular and overall health because of their high content of polyunsaturated fats … and low content of saturated fat.”4
One of the first studies to directly demonstrate the cholesterol-lowering effect of soybean oil was published in 1993 by researchers from Pennsylvania State University.5 The study involved 18 healthy young men who consumed test diets for 26 days. The diets were 37% fat and 80% of total fat came from one of four different fats: cocoa butter, dairy butter, olive oil or soybean oil. Soybean oil was the most effective in lowering LDL-cholesterol levels. The ending LDL-cholesterol values for cocoa butter, dairy butter, olive oil and soybean oil were 113, 103, 92 and 83 (mg/dl), respectively.
Dietary fat and coronary heart disease
Headlines declaring that “bacon is back” have created misconceptions about the effects of saturated fat on heart disease risk. The confusion stems from population studies that have failed to find a connection between the intake of saturated fat and coronary heart disease. That is, in these studies, people eating diets high in saturated fat are no more likely to experience a heart attack than those whose diets are low in saturated fat.
However, recent research from Harvard University provides an explanation for these findings. The investigators analyzed two prospective cohort studies involving over 130,000 health professionals in the United States. One was the Nurses’ Health Study which includes 84,638 female nurses and the other is the Health Professionals Follow-up Study which involves 42,908 male health professionals. The participants were followed for approximately 30 years and their dietary intake was assessed every two years.
Results showed clear benefits of consuming foods rich in polyunsaturated fats. Replacing 5 percent of calories from saturated fat with polyunsaturated fat decreased risk for coronary heart disease by 25%. Replacing saturated fat with monounsaturated fat also reduced risk but to a lesser extent, about 15%. Replacing saturated fat with whole grains also had a beneficial effect, although it was much smaller reduction in risk of 9 percent. In contrast to the beneficial effects of polyunsaturated and monounsaturated fats and of whole grains, refined carbohydrates had no benefits. In fact, replacing saturated fat with refined carbohydrates and added sugars caused a slight increase in heart disease risk.
This explains the confusion about the effects of saturated fat on health. In an attempt to reduce saturated fat intake, many Americans turned to special reduced-fat products, which also happened to be high in refined carbohydrates like white flour. It placed them at greater risk for heart disease. The findings from the Harvard study show that both saturated fat and refined carbohydrates raise heart disease risk. But replacing saturated fat with polyunsaturated or monounsaturated fats or with whole grains lowers risk. It also lowers risk for overall mortality based on a follow up analysis of this group of 130,000 health professionals.6
Fat, blood sugar levels and inflammation
Not only does polyunsaturated fat have more robust effect on reducing LDL-cholesterol levels, it also helps regulate blood glucose levels. These findings come from an analysis of 102 clinical studies involving 4,220 adult men and women.7 The researchers measured the effects of different dietary patterns on levels of glycosylated hemoglobin, abbreviated as HbA1c. This is a long term measure of blood glucose levels, reflecting average blood glucose levels over a period of three months. Based on this comprehensive analysis, replacing 5 percent of calories from carbohydrate with saturated fat had no significant effect on HbA1c levels. In contract, replacing carbohydrate with polyunsaturated fat significantly lowered HbA1c. In fact, the authors of this analysis estimated that replacing 5 percent of energy from carbohydrate with 5 percent of calories from polyunsaturated fat could reduce risk of developing type 2 diabetes by 22.0% and cardiovascular disease by 6.8% based on the effects on HbA1c.
Finally, recent research has challenged the idea that vegetable oils promote inflammation, an underlying cause of a number of chronic diseases including coronary heart disease. Specifically, it has long been believed that diets with high amounts of omega-6 fats relative to the amount of omega-3 fats promote inflammation. Recommendations have been to aim for a ratio of about 4:1 for optimal health. In soybean oil, the ratio is much higher, about 8:1.
But evidence for any harmful effects of this higher ratio are lacking. In fact, in 2008, the Food and Agriculture Organization of the United Nations concluded that that there is no rationale for recommending a specific ratio of omega-6 fats to omega-3 fats.8 In agreement, In a review of this issue, internationally-recognized CHD expert William S. Harris concluded that the focus should not be on dietary ratios but rather on intake levels of each type of essential fat.9
In conclusion, as the most widely consumed vegetable oil in the United States soybean oil can make important contributions to the health of Americans. Its low content of saturated fat and high content of polyunsaturated fat makes it a perfect fit to heart-healthy diets. It is one of a very few good sources of both essential fatty acids, the omega-6 fatty acid, linoleic acid and the omega-3 fatty acid, alpha-linolenic acid. Soybean oil lowers LDL-cholesterol levels and population studies show that diets high in polyunsaturated fat lower risk of coronary heart disease and favorably affect blood sugar levels.
Mark Messina, Phd
- Blasbalg TL, Hibbeln JR, Ramsden CE, Majchrzak SF, Rawlings RR. Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century. The American journal of clinical nutrition. 2011;93:950-62.
- Slavin M, Kenworthy W, Yu LL. Antioxidant properties, phytochemical composition, and antiproliferative activity of Maryland-grown soybeans with colored seed coats. Journal of agricultural and food chemistry. 2009;57:11174-85.
- Eckel RH, Jakicic JM, Ard JD, et al. 2013 AHA/ACC guideline on lifestyle management to reduce cardiovascular risk: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2014;63:2960-84.
- Sacks FM, Lichtenstein A, Van Horn L, Harris W, Kris-Etherton P, Winston M. Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory for professionals from the Nutrition Committee. Circulation. 2006;113:1034-44.
- Kris-Etherton PM, Derr J, Mitchell DC, et al. The role of fatty acid saturation on plasma lipids, lipoproteins, and apolipoproteins: I. Effects of whole food diets high in cocoa butter, olive oil, soybean oil, dairy butter, and milk chocolate on the plasma lipids of young men. Metabolism: clinical and experimental. 1993;42:121-9.
- Wang DD, Li Y, Chiuve SE, et al. Association of specific dietary fats with total and cause-specific mortality. JAMA internal medicine. 2016;176:1134-45.
- Imamura F, Micha R, Wu JH, et al. Effects of saturated fat, polyunsaturated fat, monounsaturated fat, and carbohydrate on glucose-insulin homeostasis: A systematic review and meta-analysis of randomised controlled feeding trials. PLoS medicine. 2016;13:e1002087.
- Fats and fatty acids in human nutrition. Report of an expert consultation. Food and Nutrition Paper 91. Food and Agriculture Organization of the United Nations. Rome, 2010.
- Harris WS. The omega-6/omega-3 ratio and cardiovascular disease risk: uses and abuses. Current atherosclerosis reports. 2006;8:453-9.