Omega-6 and omega-3 oxylipins are implicated in soybean oil-induced obesity in mice
The following is an excerpt of a study published in the open access journal Nature Scientific Reports which showed that a genetically modified soybean induces less obesity and insulin resistance than conventional soybean oil.
While humans have been cultivating soybeans for ~5000 years1, soybean oil has become a substantial part of our diet only in the last few decades2. This increase in soybean oil consumption is due in part to a reaction to large-scale population studies in the 1950s and 60s, which showed that a typical American diet rich in saturated fats from animal products was linked to an increased risk of cardiovascular disease3,4. It was subsequently assumed that most if not all saturated fats are unhealthy and conversely that all unsaturated fats are healthy, this despite the ambiguity of evidence of cardio-protective effects of vegetable oils, which are rich in unsaturated fats5,6. Similarly, it was assumed that whatever is healthy for the heart is also healthy for the rest of the body although this assumption was never rigorously tested7,8. Nonetheless, vegetable oil, and, in particular, soybean oil, began to replace animal fat in the American diet starting in the 1970s, resulting in an exponential rise in soybean oil consumption that parallels the increase in obesity in the U.S. and worldwide2,9,10. Indeed, soybean oil is the component in the American diet that has increased the most in the last 100 years2. It constitutes >60% of all edible vegetable oil consumption in the U.S11. and is ubiquitous in the American diet, especially in cooking oil and processed foods.
Soybean oil is comprised of primarily polyunsaturated fatty acids (PUFAs), particularly linoleic acid (LA, C18:2), an omega-6 (ω6) fatty acid that makes up ~55% of soybean oil. Omega-3 (ω3) fatty acids, especially those found in fish oil, and their ratio to ω6 fatty acids have also received considerable attention. Numerous studies have shown that high ω3:ω6 (and hence low ω6:ω3) ratios are generally healthful12. However, like saturated and unsaturated fats, a distinction between different types of ω3 and ω6 fatty acids is often not made, even though this could be relevant to their metabolic effects.
While most experimental diet-induced obesity studies use high fat diets composed of lard or milk fat (rich in saturated fats), a few recent studies (including one from our group) have examined the effects of a diet rich in soybean oil and found that this vegetable oil does in fact increase adiposity, diabetes, insulin resistance and fatty liver9,13,14,15. Furthermore, soybean oil induces more metabolic effects than an isocaloric diet made from coconut oil13, which is nearly all saturated fats, albeit of shorter chain length than those in animal fat.
One study proposed, but did not formally prove, that linoleic acid (LA) drives the metabolic effects of soybean, and other vegetable oils16. To investigate the role of LA in soybean oil-induced metabolic disease, we compared conventional soybean oil to a new genetically modified (GM) soybean oil (Plenish) which was engineered to generate fewer trans-fats by blocking the desaturase gene FAD2-1 which converts oleic acid (C18:1) to LA17 (Supplementary Fig. S1a). The net result is an oil low in LA and high in oleic acid, similar to that of olive oil (Supplementary Fig. S1b), which, as a component of the Mediterranean diet, is considered to be healthful18,19. Our results show that the GM oil Plenish does indeed induce less obesity and insulin resistance than conventional soybean oil, although not less diabetes or fatty liver. Plenish also induced hepatomegaly and liver dysfunction, as does olive oil. Importantly, extensive metabolomic and proteomic analyses indicate that oxylipin metabolites of LA and α-linolenic acid (ALA, C18:3ω3) correlate positively with obesity.
To read the entire study, please visit Nature Scientific Reports.