Linoleic Acid in Soy Strongly Linked to Obesity Epidemic

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Photo by Fotolia/Sergey Bogdanov
Much of our linoleic acid consumption comes from meat and dairy products from industrially raised animals that are fed soy and corn.

Seldom does a story stand so starkly illuminated, boldly outlined by the lines of a graph. The focal point is a period in the mid-1960s, when forces aligned to launch the current obesity epidemic and a host of health problems in the United States. As you’ll see, it was a perfect storm.

The data for this graph comes from an ambitious 2011 study called Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century, which was published in the American Journal of Clinical Nutrition. Researchers posed a question fundamental to understanding modern health concerns, including diabetes, heart disease, obesity and mental health: How does our collective diet today differ from the common diet 100 years ago? The study examined consumption of 373 foods, but then went deeper by looking at how the composition of those foods varied over time, from 1909 to 1999. This latter detail — for example, how a modern chicken breast is different from its 1909 counterpart — turned out to be critical. Specifically, researchers examined fat consumption — not just fat in general, but the quantities of particular fats.

An Era of Cheap Vegetable Oil

Despite what you may have heard, per capita fat consumption hasn’t increased substantially in the United States throughout the past century. Per capita carbohydrate consumption has increased, however, causing low-carb advocates to cite this factor alone as the cause of the obesity epidemic. The data from the fat study doesn’t contradict this hypothesis, but certainly refines the picture to pin at least some blame on the dramatic increase in our consumption of soybean oil, thanks to industrial agriculture’s concentration on this single crop.

Myriad food sources provide dietary fats, from lard and butter — the mainstays of the Edwardian-era kitchen, when the study’s data stream began — to margarine, canola oil, flaxseed oil and olive oil. Starting in the mid-’60s, what stands out — indeed, leaps off the graph — is the thousand-fold increase in per capita consumption of soybean oil (see graph). No other food in the study comes even close to matching that explosion.

Partly this is because of the ubiquitous vegetable oils, made mostly from soy, that now lurk on grocery shelves and in processed foods. But it’s also related to the way animals are raised. Today’s industrially raised livestock, poultry and farmed fish are almost universally fed soybean meal and oil. Their feed’s components are then found in the meat, milk and eggs sold to consumers. This study and others like it are explicit about this: Consumers get much of the soy in their diets secondhand from eating industrial meat, eggs and dairy products, as well as farmed fish.

Linoleic Acid: Omega-6 Overload

Why is this a problem? Soybean oil is very high in linoleic acid, an essential omega-6 fatty acid that is linked to obesity. Because we now consume a thousand times more soybean oil than we did a few decades ago, this means we’re getting a much higher amount of linoleic acid. A diet that has a high omega-6 to omega-3 fatty acid ratio is linked to inflammation, but the evidence on obesity connects specifically to linoleic acid.

Nothing is inherently wrong with linoleic acid; it is, after all, an essential fatty acid. But it’s problematic in excess, as evidenced from a 2012 study published in the journal Obesity. Feeding lab animals a diet in which 8 percent of their calories come from linoleic acid, a mirror of the modern U.S. diet, made them fat. Reducing their linoleic acid intake to 1 percent (in line with our ancestral diet), and replacing those calories with calories from other fats, made these same animals skinny again, like throwing a switch. Same amount of calories, same high-fat diet — but with different kinds of fats — and obesity reversed itself.

Yet even these studies may understate the long-term damage. A 2010 French paper moved the focus to epigenetics, the study of problems created by the behavior of one generation that are then passed on to the next. That study confirmed the role of linoleic acid in causing obesity. Researchers then bred the fat mice for several generations and found that the same diet made every succeeding generation fatter still. The results revealed a cumulative effect across generations.

Industry and Government Influence

How did we arrive at this excess of certain kinds of fats in our diets? Any Midwesterner can tell you that today’s industrial agriculture really is, more specifically, the cultivation of corn and soybeans, most of which are genetically modified. The large-scale, mechanized, chemical-dependent system made popular by the so-called Green Revolution of the mid-20th century allowed farmers to grow monoculture commodity crops in enormous surplus. The trend began with cereal grains, but eventually U.S. corn and soy production skyrocketed. This created a massive increase in available carbohydrates and linoleic acid, which were cheaper than animal fats, such as lard and butter. Food processors switched from traditional animal fats to vegetable oils, and livestock farmers began including corn and soy in animals’ rations. Both we and the livestock we raise for food were left with little choice but to eat up all that cheap linoleic acid.

Also in the ’60s, the American Heart Association and the USDA recommended that we stop eating butter and eggs and switch to “heart-healthy” vegetable oils. Massive subsidies were put in place that still pay farmers billions of dollars annually to grow soybeans, corn and other commodity crops. Aggressive marketing campaigns persuaded us to eat products containing vegetable oils. All of this converged exactly when that line for soybean oil consumption took off in the graph of this key study, and the obesity epidemic began within a decade after that point. We have been victims in this complex interplay that’s led to ill health. Only in recent years has the cover been lifted on the bad science about fats. We now understand that butter, lard, meat and eggs from pastured animals provide the kinds of omega-3-rich fat profiles that actually contribute to a healthful diet.