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Clinical Trials Highlight the Benefits of Soybean Oil and Contrast with Results in Mice

By October 11, 2017 No Comments

Conventional Soybean Oil and High Oleic Soybean Oil Favorably Affect Lipid Levels

Clinical studies showing the benefits of soybean oil by far outweigh a new mouse study on soy oil, but you would never know it by reading a recent announcement put out by its authors.

According to a press release from the University of California Riverside: “Genetically Modified Soybean Oil Causes Less Obesity and Insulin Resistance but Negatively Impacts Liver Function.” This attempt to negatively portray high oleic soybean oil (HOSoy) is little more than clickbait and ignores prior human clinical research. What is not clear from the headline, and to some extent from the entire press release, is that the research in question was conducted in mice, and that the findings from it contrast with the known effects of soybean oil in humans.

The press release, and for that matter the referenced published paper by Doel et al.,1 act as though the scientific community decided to turn the hierarchy of evidence, which is often illustrated as a pyramid, upside down with animal studies now carrying more weight than human studies.

In the press release, the senior author of this research, Frances M. Sladek, commented that the research she and her colleagues conducted “dispels the popular myth” that soybean oil reduces cholesterol levels. That is quite a comment to make considering that recently the U.S. Food and Drug Administration approved the use of a health claim for soybean oil based on its ability to lower circulating LDL-cholesterol levels.

The UC-Riverside mouse study focused primarily on comparing the effects of high-fat diets containing coconut oil, a 50-50 mix of conventional soybean oil, and coconut oil or a 50-50 mix of HOSoy and coconut oil. A chow diet was used as low-fat control. In addition, Doel and colleagues1 compared diets containing olive oil, conventional soybean oil, HOSoy or lard. As the name suggests, the predominant fatty acid in HOSoy is oleic acid. In comparison to conventional soybean oil, HOSoy is also low in linoleic acid. The oil was developed to help address the needs of the food industry stemming from the eventual banning of trans fats.

Note that Sladek isn’t arguing that soybean oil doesn’t lower risk of heart disease, an outcome which in nutrition intervention studies is only rarely examined. She is commenting about a routinely measured outcome. Nearly 25 years ago Penny Kris-Etherton and colleagues2 from Pennsylvania State University showed that in adults, after one month exposure, LDL-cholesterol levels were lowest in response to soybean oil when compared to three other diets containing coconut butter, dairy butter, or olive oil.  A few years later, researchers at Tufts University also showed soybean oil lowered LDL-cholesterol.3

Furthermore, more than a decade ago, Lichtenstein et al.4 found that both conventional soybean oil and HOSoy lowered LDL-cholesterol compared to partially hydrogenated oil.  In agreement, in soon to be published research, David Baer and colleagues from the USDA found that both conventional soybean oil and HOSoy lowered the total cholesterol:HDL-cholesterol ratio in comparison to adults fed a blend of palm oil and palm kernel oil.

The contrasting effects of soybean oil on cholesterol levels in mice and humans should call into question the value of the work by Doel et al.1 At the very least, these researchers should have cited the relevant human research in the discussion of the implications of their findings. Furthermore, the contrasting effects between mice and humans aren’t limited to effects on cholesterol. In mice, soybean oil increased insulin resistance, whereas a recently published meta-analysis of the clinical data concluded that polyunsaturated fat may decrease insulin resistance.5

Another contention made by Doel et al.1 is that soybean oil causes obesity. This notion is again based on the findings in mice, plus the correlation between the rise in the consumption of soybean oil beginning in the 1970s and the increase in obesity in the U.S. and the world. This correlation is an excellent example of correlation not equaling causation. It is not difficult to think of a host of societal factors that have changed over the past 50 years that could have contributed to the obesity epidemic.

In fact, it is far from clear that the macronutrient composition of the diet influences weight gain or weight loss.6 But it is interesting to note that there is clinical work indicating that monounsaturated fat and polyunsaturated fat are more readily oxidized7 and have a greater thermogenic effect than saturated fats.8-10 Diet induced thermogenesis refers to the rise in energy expenditure from the fasted state that is caused by energy spent digesting, transporting or absorbing, metabolizing and storing the absorbed, but not immediately oxidized, nutrients.11,12

It is also noteworthy that in the mouse study by Doel et al.,1 HOSoy resulted in hepatomegaly (enlarged liver) and liver dysfunction to the same extent as olive oil, which is not surprising given that they have a similar fatty acid composition. But since when has the nutrition community raised concerns about the harmful effects of olive oil, which has been extensively studied?

Finally, there is one other comment in the paper by Doel et al.1 that is particularly perplexing.  The authors suggest that the reason sources of saturated fat such as lard cause harmful effects in laboratory animal studies is because it is high in linoleic acid as a result of livestock being fed soybean meal.  Livestock is indeed the main outlet for soybean meal but the meal (protein) is fat free so it can’t possibly contribute to the linoleic acid content of lard.

All in all, it is difficult to see how the findings from this new mouse study make much of a contribution to the literature when placing them within the context of a large body of human evidence supporting the health benefits of conventional soybean oil and high oleic soybean oil.

Clinical trials show that both conventional soybean oil and high oleic soybean oil favorably affect lipid levels. Human evidence does not implicate soybean oil as a causative factor in the obesity epidemic.The contrasting effects of soybean oil noted in research involving mice in comparison to humans should call into question the value of this new animal study.

References

  1. Deol P, Fahrmann J, Yang J, et al. Omega-6 and omega-3 oxylipins are implicated in soybean oil-induced obesity in mice. Scientific reports. 2017;7:12488|.
  2. 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. 1993;42:121-9.
  3. Lichtenstein AH, Ausman LM, Jalbert SM, Schaefer EJ. Effects of different forms of dietary hydrogenated fats on serum lipoprotein cholesterol levels. N Engl J Med. 1999;340:1933-40.
  4. Lichtenstein AH, Matthan NR, Jalbert SM, Resteghini NA, Schaefer EJ, Ausman LM. Novel soybean oils with different fatty acid profiles alter cardiovascular disease risk factors in moderately hyperlipidemic subjects. Am J Clin Nutr. 2006;84:497-504.
  5. 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 Med. 2016;13:e1002087.
  6. Williamson DA. Fifty years of behavioral/lifestyle interventions for overweight and obesity: Where have we been and where are we going? Obesity (Silver Spring). 2017.
  7. Piers LS, Walker KZ, Stoney RM, Soares MJ, O’Dea K. The influence of the type of dietary fat on postprandial fat oxidation rates: monounsaturated (olive oil) vs saturated fat (cream). Int J Obes Relat Metab Disord. 2002;26:814-21.
  8. Casas-Agustench P, Lopez-Uriarte P, Bullo M, Ros E, Gomez-Flores A, Salas-Salvado J. Acute effects of three high-fat meals with different fat saturations on energy expenditure, substrate oxidation and satiety. Clin Nutr. 2009;28:39-45.
  9. Clevenger HC, Kozimor AL, Paton CM, Cooper JA. Acute effect of dietary fatty acid composition on postprandial metabolism in women. Exp Physiol. 2014;99:1182-90.
  10. van Marken Lichtenbelt WD, Mensink RP, Westerterp KR. The effect of fat composition of the diet on energy metabolism. Z Ernahrungswiss. 1997;36:303-5.
  11. Tappy L. Thermic effect of food and sympathetic nervous system activity in humans. Reprod Nutr Dev. 1996;36:391-7.
  12. Denzer CM, Young JC. The effect of resistance exercise on the thermic effect of food. International journal of sport nutrition and exercise metabolism. 2003;13:396-402.

 

Dr. Mark Messina

Author Dr. Mark Messina

PhD in Nutrition, Executive Director, Soy Nutrition Institute. Expert in soyfoods and isoflavones.

More posts by Dr. Mark Messina

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