Industrial seed oils (corn, cottonseed, soybean, safflower, sunflower, etc.) have not been a part of the human diet up until relatively recently, when misguided groups like the AHA and the ADA started promoting them as “heart-healthy” alternatives to saturated fat.
Throughout 4-5 million years of hominid evolution, diets were abundant in seafood and other sources of omega-3 long chain fatty acids (EPA & DHA), but relatively low in omega-6 seed oils.
Anthropological research suggests that our hunter-gatherer ancestors consumed omega-6 and omega-3 fats in a ratio of roughly 1:1. It also indicates that both ancient and modern hunter-gatherers were free of the modern inflammatory diseases, like heart disease, cancer, and diabetes, that are the primary causes of death and morbidity today.
At the onset of the industrial revolution (about 140 years ago), there was a marked shift in the ratio of n-6 to n-3 fatty acids in the diet. Consumption of n-6 fats increased at the expense of n-3 fats. This change was due to both the advent of the modern vegetable oil industry and the increased use of cereal grains as feed for domestic livestock (which in turn altered the fatty acid profile of meat that humans consumed).
The following chart lists the omega-6 and omega-3 content of various vegetable oils and foods:
Vegetable oil consumption rose dramatically between the beginning and end of the 20th century, and this had an entirely predictable effect on the ratio of omega-6 to omega-3 fats in the American diet. Between 1935 and 1939, the ratio of n-6 to n-3 fatty acids was reported to be 8.4:1. From 1935 to 1985, this ratio increased to 10.3:1 (a 23% increase). Other calculations put the ratio as high as 12.4:1 in 1985. Today, estimates of the ratio range from an average of 10:1 to 20:1, with a ratio as high as 25:1 in some individuals.
In fact, Americans now get almost 20% of their calories from a single food source – soybean oil – with almost 9% of all calories from the omega-6 fat linoleic acid (LA) alone! (PDF)
This reveals that our average intake of n-6 fatty acids is between 10 and 25 times higher than evolutionary norms. The consequences of this dramatic shift cannot be underestimated.
So what are the consequences to human health of an n-6:n-3 ratio that is up to 25 times higher than it should be?
The short answer is that elevated n-6 intakes are associated with an increase in all inflammatory diseases – which is to say virtually all diseases. The list includes (but isn’t limited to):
- cardiovascular disease
- type 2 diabetes
- metabolic syndrome
- irritable bowel syndrome & inflammatory bowel disease
- macular degeneration
- rheumatoid arthritis
- psychiatric disorders
- autoimmune diseases
The relationship between intake n-6 fats and cardiovascular mortality is particularly striking. The following chart, from an article entitled Eicosanoids and Ischemic Heart Disease by Stephan Guyenet, clearly illustrates the correlation between a rising intake of n-6 and increased mortality from heart disease:
As you can see, the USA is right up there at the top with the highest intake of n-6 fat and the greatest risk of death from heart disease.
On the other hand, several clinical studies have shown that decreasing the n-6:n-3 ratio protects against chronic, degenerative diseases. One study showed that replacing corn oil with olive oil and canola oil to reach an n-6:n-3 ratio of 4:1 led to a 70% decrease in total mortality. That is no small difference.
Joseph Hibbeln, a researcher at the National Institute of Health (NIH) who has published several papers on n-3 and n-6 intakes, didn’t mince words when he commented on the rising intake of n-6 in a recent paper:
The increases in world LA consumption over the past century may be considered a very large uncontrolled experiment that may have contributed to increased societal burdens of aggression, depression and cardiovascular mortality.
And those are just the conditions we have the strongest evidence for. It’s likely that the increase in n-6 consumption has played an equally significant role in the rise of nearly every inflammatory disease. Since it is now known that inflammation is involved in nearly all diseases, including obesity and metabolic syndrome, it’s hard to overstate the negative effects of too much omega-6 fat.