Vegetable oil is truly an "external factor", something in our environment that has changed dramatically in the last 100 years (as opposed to our genes, which are essentially the same). A century ago, nearly all fats eaten in the U.S. came from animals. That's how it's been for humans since the beginning of time, with the exception of seasonal nuts and oily seeds for some groups. Polyunsaturated fat (PUFA) consumption in the US has more than tripled in the last 100 years, with the increase coming almost solely from omega-6 linoleic acid-rich seed oils like soybean and corn oil (source): This amount of linoleic acid is totally foreign to the human body. It causes countless detrimental effects to many, if not all organ systems. And it's exactly what mainstream health authorities have been telling us to eat for the last 50 years. You may have noticed that the recommendations have changed recently to focus on monounsaturated oils like olive oil. They are backpedaling in the face of an avalanche of studies demonstrating the detrimental effects of linoleic acid on health.
I found a well-controlled study in which investigators put rats on three different isocaloric, high-fat diets. Each one contained an identical amount of total fat, protein, carbohydrate, omega-3 alpha-linolenic acid (from flax oil), and variable amounts of omega-6 linoleic acid:
- 59% of calories from beef tallow, for a total n-6 of 4.4% of calories.
- 59% of calories from olive oil, for a total n-6 of 7.7% of calories.
- 59% of calories from safflower oil, for a total n-6 of 36.6% of calories.
This effect seems to be specific to mammals, because linoleic acid does not cause weight gain in birds. I think it's worth noting that homemade tallow from beef suet is only 2.4% linoleic acid, far lower than the 7.4% (as a percentage of total fat) fed to the tallow group in this experiment.
Weight gain occurred even at 4.4% of calories from linoleic acid. We don't know what they would have looked like on less linoleic acid under the same conditions. However, rats fed a high-fat diet rich in coconut oil or butter (low n-6) do not gain weight, despite a greatly increased caloric intake (although they do gain some fat mass). It's also worth mentioning that rats in the tallow group had higher tissue concentrations of omega-3 fatty acids, though they had an identical intake to the other two groups. This suggests that linoleic acid somehow drives omega-3 out of the tissues, and is consistent with studies showing a precipitous drop in muscle concentration of omega-3 in cows fed grains and soybeans in a feedlot.
Time to back this up with human data. When I'm digging for studies, I typically look at the oldest ones first. They are often the most informative. In this case, I had another reason as well. In the 1960s, Americans ate about half the linoleic acid we do today, and our body fat linoleic acid stores were also about half as big (although still high). What happens when you feed a 1960s American man with a diet high in linoleic acid-rich vegetable oils? He gains weight as his body fat stores of linoleic acid increase, just like the rats above. It's like putting your body fat in a time machine and beaming it to 2008!
They don't design studies like this anymore. 782 elderly men were placed on isocaloric controlled diets for 5 years, one group eating a typical "high-saturated fat" diet and the other eating a diet in which total energy, fat, carbohydrate and protein intake were identical to the first group, but linoleic acid-rich vegetable oils were substituted for animal fats. This study is unique in its long duration, large number of participants, specific isolation of the effects of linoleic acid, and the fact that it was conducted when body fat stores of linoleic acid were lower than today.
In the beginning, the body fat of both groups was 11% linoleic acid. By one year on the diet, the unsaturated group had risen to just over 20%. This is where the weight gain began. Between year one and three, the two groups diverged by just under 5% body weight, or about 8 pounds (3.6 kg). This is an estimate based on the graph provided, because they didn't include the raw data. If you compare these numbers to the body fat/weight gain statistics for the rat study above, they are quite similar.
At five years, body fat in the unsaturated group was 32% linoleic acid, and the weight difference between groups remained the same as it was at 3 years. Hunter-gatherers almost certainly have values in the low single digits (possibly less than 1% for some groups), putting this completely outside the range of our natural fat composition. The body fat of Americans is currently hovering between 15 and 25% omega-6 linoleic acid, due mostly to a high intake of industrial vegetable oils.
If you believe the studies I presented above, Americans and other industrial cultures throughout the world are in the range where excess linoleic acid causes the accumulation of body fat. According to the human study, adding more linoleic acid to body fat after about 20% may not cause further weight gain. That means any similar study in the last 20 years could be confounded by excess stored linoleic acid. The half-life of fat tissue turnover is about 2 years, so restoring balance is a long-term process.
In both the rat and human experiments above, "low"-linoleic and high-linoleic groups ate the same number of calories, yet the high-linoleic groups gained more weight. How could that be? In the next post, I'll describe what I believe is the mechanism. Any guesses?