By David Baer, PhD
Soybean oil is a critically important oil in the global food supply. Conventional soybean oil, also known as “commodity” soybean oil, is high in polyunsaturated fatty acids (PUFA) which are consistently associated with decreased risk of chronic diseases such as diabetes and cardiovascular disease.1 And while some individuals have suggested that the n-6 PUFA found in commodity soybean oil are “proinflammatory” and detrimental to health,2 those claims are not supported by the scientific evidence.3 Despite their positive impact on health, a limitation of oils high in PUFA is that they are susceptible to oxidation, which can reduce its shelf- and fry-life.
To overcome this limitation, the process of partial hydrogenation has been used. However, it has been recognized for some time that partial hydrogenation produces trans fatty acids which are extremely detrimental to health. As a result, the U.S. Food and Drug Administration has required the labeling of trans fatty acids since 2003, and in 2015 removed partially hydrogenated vegetable oils from the list of foods that are “Generally Recognized as Safe.” Since partially hydrogenated oils will no longer be widely available, alternatives are needed.
In 2011, high oleic soybean oils became commercially available. Unlike commodity soybean oil, high oleic soybean oils have a higher concentration of monounsaturated fatty acids and lower concentration of PUFA. More specifically, as the high oleic name implies, oleic acid is increased in these oils. Oleic acid is the predominant fatty acid in olive oil. While these oils are liquid at room temperature, they have a longer shelf- and fry-life than commodity soybean oil. Further, by blending high oleic soybean oil with other oils, it is possible to create oil blends that increase functionality for a greater variety of food applications.
In 2015, scientists at the USDA’s Agricultural Research Service from the Beltsville Human Nutrition Research Center conducted a clinical study to evaluate how high oleic soybean oil and blends of high oleic soybean oil affect cardiovascular disease risk factors.
For this study, four oil treatments were employed:
- Commodity soybean oil
- High oleic soybean oil (50:50) blend of the two commercially available high oleic soybean oils
- Blend of two high oleic soybean oils and fully hydrogenated soybean oil (40:40:20)
- Blend of palm oil and palm kernel oil (50:50)
Using these oils, it was possible to address three objectives. The first objective was to demonstrate that the replacement of high oleic soybean oil for saturated fats improves LDL cholesterol and other biomarkers of cardiovascular disease. Since high oleic soybean oil is less prone to oxidation than commodity soybean oil (because of its reduced PUFA content), it is a reasonable replacement for partially hydrogenated vegetable oils or oils high in saturated fatty acids, for some food system applications. Additionally, use of an oil high in saturated fats (the palm blend) serves as a positive control for this study.
The second objective was to determine the effect of replacing commodity soybean oil with high oleic soybean oil on cardiovascular disease biomarkers.
Finally, the third objective was to compare the effect of two oils with similar functional properties but different mono- and polyunsaturated fatty acid profiles on cardiovascular disease biomarkers. To accomplish this objective, high oleic soybean oil was blended with fully hydrogenated soybean oil. This oil blend was compared with the palm blend– as both of these blends have similar functional properties (semisolid at room temperature) yet very different fatty acid profiles. The high oleic soybean oil blended with fully hydrogenated soybean oil has a lower saturated fatty acid content (and more of the saturated fat from stearic acid) and a higher monounsaturated fatty acid composition than the palm blend.
Over 200 individuals were screened for this study to make sure they met the criteria to participate. The eligibility criteria were selected to represent a group of individuals who did not have heart disease but were at-risk for developing it. Eligibility criteria included that they have at-risk lipoprotein profiles (higher LDL cholesterol and triglycerides, and a lower HDL cholesterol) as well as three of five metabolic syndrome risk factors, they were between the ages of 30-70 years, and not taking medications to lower cholesterol. Also important was their willingness to eat all of the foods that were provided and nothing else for the 4-month duration of the study. After the recruitment process ended, 60 individuals initiated the feeding phase of the study.
Since the study utilized a cross-over design, each volunteer ate each of the four treatments for about one month in random order with a two-week washout between treatments. During the active period, volunteers came to the Research Center Monday through Friday and ate breakfast and dinner in our research dining room under our supervision. Lunches and meals for weekends were prepared by the staff but consumed off site.
Menus were developed for the feeding phase of the study using foods that most people enjoy. Many of the food items were identical across all four treatments (beef, chicken, vegetables, etc.). In order to incorporate the four treatment oils into the diet, recipes for fourteen foods were developed. These recipes included sauces, gravies, cakes, muffins, fried potatoes, and fried donut holes. Again, these recipes were identical except for the oil. Fried foods were included in the menus since some of the oils (especially palm blend and high oleic soybean oil) are suitable for frying applications. Baked products were developed since some of the oils (especially the palm blend and high oleic soybean oil and fully hydrogenated soybean oil) are suitable for baking applications. Using these “real life” food preparation methods was an important way to incorporate the oils into the menus, and not typically done in research clinical studies.
At the beginning and end of each of the four treatment periods, blood was collected to measure biomarkers and risk factors for cardiovascular disease. Concentration of markers associated with increased risk of coronary heart disease [LDL cholesterol, apolipoprotein B, Non HDL cholesterol, LDL particles] were higher after consumption of the palm blend compared to high oleic soybean oil and the other treatments. LDL cholesterol and apolipoprotein B were higher after consumption of the high oleic soybean oil compared to commodity soybean oil. There were no significant differences in the atherogenic markers between the blend of high oleic soybean oil and fully hydrogenated soybean oil and the high oleic soybean oil. Thus, replacement of up to 20% of high oleic soybean oil with fully hydrogenated soybean oil provides solid fat functionality with no change in these atherogenic markers.
Consumption of the palm blend increased HDL cholesterol (good cholesterol) and apolipoprotein AI compared to all other treatments, with no differences among the other treatments for HDL cholesterol and apolipoprotein AI. Ratios of total cholesterol-to-HDL cholesterol (see Figure 1) and LDL cholesterol-to-HDL cholesterol were highest after consumption of the palm blend compared to the other treatments (higher ratios are associated with increased risk of coronary heart disease). Triglycerides, fasting glucose, and blood pressure were similar across all four treatments.
High oleic soybean oil, an important option for replacement of solid fat and liquid oils in multiple food applications, maintains or improves lipid and lipoprotein profiles in humans compared to alternative functional fats. Additional analyses of data from this clinical study is ongoing. To date, it appears that high oleic soybean oil will provide new opportunities for food formulation and heart health.
References
1. Wang DD, Li Y, Chiuve SE, Stampfer MJ, Manson JE, Rimm EB, et al. Association of Specific Dietary Fats With Total and Cause-Specific Mortality. JAMA Intern Med. 2016;176(8):1134-45.
2. Okuyama H, Ichikawa Y, Sun Y, Hamazaki T, Lands WE. Omega3 fatty acids effectively prevent coronary heart disease and other late-onset diseases--the excessive linoleic acid syndrome. World Rev Nutr Diet. 2007;96:83-103.
3. Johnson GH, Fritsche K. Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic review of randomized controlled trials. J Acad Nutr Diet. 2012;112(7):1029-41, 41 e1-15.
Figure 1. The TC:HDL ratio at the end of each treatment period. There is an overall treatment effect, and bars with different letters have a significantly different ratio.