How vegetable oils make us fat

Something has changed in the last hundred years, during which time obesity has transitioned from anomaly to epidemic. Why have we spontaneously started storing so many calories as fat? This is a fascinating, and credible, look at another hypothesis on one of the major NCDs blighting modern man.

Highlights

  • Over-consuming vegetable oils, specifically seed oils, which contain evolutionarily unprecedented amounts of the omega-6 fat linoleic acid, can cause obesity.
  • There is no tribe, population, or nation that has started consuming vegetable oils and not seen obesity rates climb.
  • Vegetable oil breaks down into the obesogenic toxin HNE (4-Hydroxynonenal), which appears to be a cause of obesity.
  • Linoleic acid sensitizes fat cells to the fat-storing hormone insulin. It can divert energy toward fat storage rather than fat-loss, up to a point.
  • Linoleic acid activates the body’s own (endo)cannabinoid system via the CB1 receptor in a way that may interfere with hunger signals.
  • For many animals, seasonal increases in linoleic acid consumption are a signal to hibernate and fatten up — might this be relevant to how human obesity develops?

Introduction

What causes obesity? It’s a seemingly simple question that has a thousand different complex answers, depending on who you ask. Carbs, sugar, fat, calories, chemicals, sedentary lifestyles, food processing, and genetics have all been blamed. 

The NHS in the U.K. states that “Obesity is generally caused by eating too much and moving too little,” and the CDC in the U.S. seems to agree: “If you take in more calories than you use, you will gain weight [*].” However, these explanations say nothing about the underlying cause of storing excess fat. And recommendations to simply eat less and move more clearly aren’t working.

Obesity is a systemic problem, not a result of widespread loss of personal willpower to cut calories or a new unwillingness to exercise. If weight loss were as easy as deciding to ignore hunger signals and taking the stairs instead of the elevator, we would not have an obesity rate of 42.4% in the U.S., with the rest of the world quickly catching up. And we would not have an increasing percentage of American children who are overweight or obese, having grown from 36% to 46% of American kids in the last two years alone [*,*].

Something has changed in the last hundred years, during which time obesity has transitioned from anomaly to epidemic. Why have we spontaneously started storing so many calories as fat?

It has been repeatedly demonstrated in animal models and, more recently, in human studies that so-called “high-fat diets” lead to increased body fat [*]. And in mice, even when these fats bypass the mouth and are injected into the gut, the mice spontaneously start eating more [*].

However, it may not be the amount of dietary fat that causes obesity but rather the type of fat. And if American dietary trends are any indication, it’s not saturated fat, beef fat, or heavy cream that’s causing obesity, it’s vegetable oils [*,*]:

Line graph showing how much particular sources of dietary fat contributed to our daily calorie total (left Y-axis) over the years (bottom X-axis) as the adult obesity rate increased (right Y-axis).

As we’ll see in the data to come, much of which has only been published in the last few years, a driving force behind increasing obesity rates may be the overconsumption of vegetable oils — soybean, sunflower, corn, and canola oil — all of which contain large amounts of linoleic acid, a type of omega-6 polyunsaturated fat.

Horizontal stacked bar chart of linoleic (omega-6) and alpha-linolenic acid (omega-3) as a percentage of total calories from different sources of added fat.

Omega-6 fats, while necessary in extremely small amounts, may contribute to general inflammation and other health issues when eaten in excess [*,*]. And, as we’ll see, high levels of dietary linoleic acid can get converted to certain metabolites which activate key biological pathways that may contribute to rising obesity rates. 

Vegetable oil and the rise in obesity rates

Different groups of people have eaten a myriad of diets throughout history, from predominantly carbohydrates like potatoes and honey to mostly protein and fat like meat and dairy, and obesity rates have never been as high as they are today. 

There is no tribe, population, or nation that has started consuming vegetable oils and not seen obesity rates climb.

So, is it possible that rising obesity rates aren’t primarily an issue of macronutrient ratios and that there’s something else at play? 

The rise in obesity is strongly associated with the rise in vegetable oil and linoleic acid consumption — arguably more so than any other dietary change — notably because there’s evidence that total calories, carbohydrates, sugar, saturated fat, and red meat consumption have stayed relatively steady and in some cases have decreased while obesity rates have increased.

For example, the Kitavans of Papua New Guinea in the Pacific Islands were estimated to eat 69% of their calories as carbohydrates, primarily from sweet potatoes, of which they have an abundant caloric supply, yet they remain thin and fit [*].

Adult Kitavan man displaying the naturally lean human physique (left panel) and a young Kitavan woman. Photo by now deceased paleoanthropologist Staffan Lindeberg.

For much of the 20th century, in many East Asian countries, white rice (a refined carbohydrate) accounted for the majority of calories consumed. But even when white rice contributed 84% of daily calories, as was the case in Japan in the 1950s, most people remained thin and morbid obesity was extremely rare.

On the other end of the spectrum, and opposite side of the planet, the Inuit of Greenland traditionally ate a diet that was nearly 100% seal meat and blubber, with more than half of their calories coming from fat. Yet, just like the Kitavans and Japanese, there was no obesity epidemic. Similarly, the Massai of East Africa eat a diet composed almost entirely of blood and milk and remain fit.

You might imagine that we’re eating more today than ever, contributing to the increase in obesity rates. But in 1939, high-income Americans living in cities may have eaten even more daily calories than the average American eats today; yet an early USDA report estimated that the average man living in North Atlantic and Pacific only weighed 154 pounds [*]. The average American man today weighs 198 pounds [*]. 

Before Europeans brought flour and sugar to the Americas, many Native Americans subsisted on a mixed diet of corn, beans, squash, fruits, greens, nuts, meats, and maple syrup [*]. Far from overweight, Native Americans were described by early Europeans as some of the fittest people they had ever seen.

While all these healthy populations have varying diets, a major commonality is a lack of or very minimal use of vegetable oil in the diet. The consumption of dietary linoleic acid (the primary fat in many vegetable oils) was very low in pre-industrial cultures, between only 0.3-2.0% of calories [*]. 

Today, however, many of these cultures have begun consuming vegetable oils in massive quantities.

After industrial vegetable oils were introduced to East Asia, the Middle East, the Americas, and Pacific Island nations near Papua New Guinea, these nations saw obesity rates soar. While not every individual gets obese as a result of vegetable oil consumption, it appears that every population does. 

By analogy, not everyone who smokes gets lung cancer, but every population that starts smoking sees increased lung cancer rates. Of course, different individuals have different genes. They, therefore, react to potential negative inputs — like vegetable oils and cigarettes — differently, but what we can say with confidence is that based on historical data, eating more vegetable oil is always associated with more obesity on a population level.

This population-wide association between vegetable oils and obesity may also extend to pregnant mothers and their offspring. For example, a large observational study in the U.K. found that the more linoleic acid the mother ate, the greater the child’s fat mass at both 4 and 6 years of age [*]. And a study conducted on over 500 American women in their 50s found that higher baseline levels of linoleic acid in the blood, which was representative of recent diet, were correlated with weight gain resulting in excess body weight or obesity after 10 years [*].

A study conducted in a Spanish population found a correlation between fat tissue levels of linoleic acid (which also correlated with dietary linoleic acid intake), degree of obesity, and “central adiposity,” better known as belly fat [*,*]. 

As you can see, there’s widespread recognition that a “dietary transition” is taking place, and one of the major elements in this transition is an increase in dietary vegetable oils [*]. In most populations, widespread weight gain (and other health issues) began occurring before the connection between vegetable oil consumption and obesity was recognized.

For example, the Tsimane are a primitive horticulturalist group living in the Bolivian Amazon jungle. They are renowned among cardiologists because they “have the lowest reported levels of coronary artery disease of any population recorded to date” [*]. Until recently, obesity was also rare among the Tsimane.

But even among the Tsimane, as they gain access to “market foods”, excess body fat is increasing. Researchers trying to discern what was driving the change in body fat found that it was only the consumption of vegetable oils [*,*,*]. The experience of the Tsimane suggests that weight gain starts quickly after the introduction of vegetable oils into a population, which is consistent with what happened in the US. [*]

Although observing these populations reveals clear correlations between linoleic acid and obesity, it’s essential to test the hypothesis that reducing linoleic acid reverses obesity. Well-controlled and preferably randomized clinical trials are the tool for doing that.

In one randomized trial, 41 adult women with excess body fat were given a daily high-fat breakfast for nine weeks that contained either high linoleic soybean oil or lower linoleic olive oil. Body fat loss was 80% greater in the group that consumed less linoleic acid [*]. 

In an even longer experiment, this time in Asian Indian males, replacing high linoleic soybean and safflower oil with lower linoleic oils for six months led to a significant decrease in body weight. The men consuming high linoleic soybean/safflower oil (60-75% linoleic acid) lost 1.7 lbs, the group consuming 21% linoleic canola oil lost 4.2 lbs, and the group consuming 11% linoleic olive oil lost 5 lbs. The less linoleic acid the men consumed, the more weight they lost [*].

The timing of widespread seed oil consumption and the link between high linoleic diets and obesity rates in the research make seed oils an excellent candidate for being a major “obesogen” — or a factor that promotes obesity by interfering with metabolism. But other than environmental changes like diet, could our genetics have turned on us and made us obese?….

ZeroAcre.com: Read the full scholarly article here