Dr. William Lands, one of the pioneers of the eicosanoid field, compiled this graph. It may be the single most important clue we have about the relationship between diet and ischemic heart disease (heart attacks).To explain it fully, we have to take a few steps back. Dietary polyunsaturated fatty acids (PUFA) are primarily omega-6 and omega-3. This is a chemical designation that refers to the position of a double bond along the fatty acid's carbon chain. Omega-6 fats are found abundantly in industrial vegetable oils (corn, soybean, sunflower, cottonseed, etc.) and certain nuts, and in lesser amounts in meats, dairy and grains. Omega-3 fats are found abundantly in seafood and a few seeds such as flax and walnuts, and in smaller amounts in meats, green vegetables and dairy.The body uses a multi-step process to convert omega-3 and omega-6 fats into eicosanoids, which are a diverse and potent class of signaling molecules. The first step is to convert PUFA into highly unsaturated fatty acids, or HUFA. These include arachidonic acid (AA), an omega-6 HUFA, eicosapentaenoic acid (EPA), an omega-3 HUFA, and several others in the 20- to 22-carbon length range.HUFA are stored in cell membranes and they are the direct precursors of eicosanoids. When the cell needs eicosanoids, it liberates HUFA from the membrane and converts it. The proportion of omega-6 to omega-3 HUFA in the membrane is proportional to the long-term proportion of omega-6 and omega-3 in the diet. Enzymes do not discriminate between omega-6 and omega-3 HUFA when they create eicosanoids. Therefore, the proportion of omega-6- to omega-3-derived eicosanoids is proportional to dietary intake.Omega-6 eicosanoids are potently inflammatory and thrombotic (promote blood clotting, such as thromboxane A2), while omega-3 eicosanoids are less inflammatory, less thrombotic and participate in long-term repair processes.Many of the studies that have looked at the relationship between HUFA and heart attacks used blood plasma (serum lipids). Dr. Lands has pointed out that plasma HUFA do not accurately reflect dietary omega-6/3 balance, and they don't correlate well with heart attack risk. What does correlate strikingly well with both dietary intake and heart attack risk is the proportion of omega-6 HUFA in tissue, which reflects the amount contained in cell membranes. That's what we're looking at in the graph above: the proportion of omega-6 HUFA in the total tissue HUFA pool, vs. coronary heart disease death rate.You can see that the correlation is striking, both between populations and within them. Greenland Inuit have the lowest proportion of omega-6 HUFA, due to a low intake of omega-6 and an exceptionally high intake of seafood. They also have an extraordinarily low risk of heart attack death. The red dots are from the Multiple Risk Factor Intervention Trial (MRFIT), which I'll be covering in a bit more detail in a later post. They're important because they confirm that the trend holds true within a population, and not just between populations.In the next post, I'll be delving into this concept in more detail, and explaining why it's not just the ratio that matters, but also the total intake of omega-6. I'll also be providing more evidence to support the theory.
I have to take a brief intermission from the heart disease series to write about a very important paper I just read in the journal Obesity, "COX-2-mediated Inflammation in Fat is Crucial for Obesity-linked Insulin Resistance and Fatty Liver". It's actually related to cardiovascular disease, although indirectly.First, some background. Polyunsaturated fatty acids (PUFA) come mostly from omega-6 and omega-3 sources. Omega-6 and omega-3 are precursors to eicosanoids, a large and poorly understood class of signaling molecules that play a role in basically everything. Eicosanoids are either omega-6-derived or omega-3-derived. Omega-6 and omega-3 compete for the enzymes that convert PUFA into eicosanoids. Therefore, the ratio of omega-6 to omega-3 in tissues (related to the ratio in the diet) determines the ratio of omega-6-derived eicosanoids to omega-3-derived eicosanoids. Omega-6 eicosanoids are very potent and play a central role in inflammation. They aren't "bad", in fact they're essential, but an excess of them is probably not good. Omega-3 eicosanoids are generally less potent, less inflammatory, and tend to participate in long-term repair processes. So in sum, the ratio of omega-6 to omega-3 in the diet will determine the potency and quality of eicosanoid signaling, which will determine an animal's susceptibility to inflammation-mediated disorders. One of the key enzymes in the pathway from PUFA to eicosanoids (specifically, a subset of them called prostanoids) is cyclooxygenase (COX). COX-1 is expressed all the time and serves a "housekeeping" function, while COX-2 is induced by cellular stressors and contributes to the the formation of inflammatory eicosanoids. Non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen inhibit COX enzymes, which is why they are effective against inflammatory problems like pain and fever. They are also used as a preventive measure against cardiovascular disease. Basically, they reduce the excessive inflammatory signaling promoted by a diet with a poor omega-6:3 balance. You wouldn't need to inhibit COX if it were producing the proper balance of eicosanoids to begin with. Dr. Kuang-Chung Shih's group at the Department of Internal Medicine in Taipei placed rats on five different diets:- A control diet, eating normal low-fat rat chow.
- A "high-fat diet", in which 45% of calories came from a combination of industrial lard and soybean oil, and 17% of calories came from sucrose*.
- A "high-fat diet" (same as above), plus the COX-2 inhibitor celecoxib (Celebrex).
- A "high-fat diet" (same as above), plus the COX-2 inhibitor mesulid.
- An energy-restricted "high-fat diet".
The "high-fat diets", besides being high in sucrose (table sugar), also presumably had a poor omega-6:3 ratio, in the neighborhood of 10:1 or possibly higher. Weight and fat mass in rats and humans increases with increasing omega-6 in the diet, and also increases with a high 6:3 ratio. I wrote about that here. Rats eating the high-fat diets (groups 2- 4) gained weight as expected**.Rats in group 2 not only gained weight, they also experienced increased fasting glucose, leptin, insulin, triglycerides, blood pressure and a massive decline in insulin sensitivity (seven-fold relative to group 1). Rats in groups 3 and 4 gained weight, but saw much less of a deterioration in insulin and leptin sensitivity, and blood pressure. Group 2 also developed fatty liver, which was attenuated in groups 3 and 4. If you're interested, group 5 (energy restricted high-fat) was similar to groups 3 and 4 on pretty much everything, including insulin sensitivity. So there you have it folks: direct evidence that insulin resistance, leptin resistance, high blood pressure and fatty liver are mediated by excessive inflammatory eicosanoid signaling. I wrote about something similar before when I reviewed a paper showing that fish oil reverses many of the consequences of a high-vegetable oil, high-sugar diet in rats. I also reviewed two papers showing that in pigs and rats, a high omega-6:3 ratio promotes inflammation (mediated by COX-2) and lipid peroxidation in the heart. Are you going to quench the fire by taking drugs, or by reducing your intake of omega-6 and ensuring an adequate intake of omega-3? *Of course, they didn't mention the sucrose in the methods section. I had to go digging around for the diet's composition. This is typical of papers on "high-fat diets". They load them up with sugar, and blame everything on the fat. **Rats gain fat mass when fed a high-fat diet (even if it's not loaded with sugar). But humans don't necessarily gain weight on a high-fat diet (i.e. low-carb weight loss diet). What's the difference? Low-carbohydrate diet trials indicate that humans spontaneously reduce their caloric intake when eating low carbohydrate, high-fat food.
In the last post, I reviewed some of the factors that I believe could have contributed to the epidemic of heart attacks that began in the 1920s and 1930s in the U.S. and U.K., and continues today. I ended on smoking, which appears to be a major player. But even smoking is clearly trumped by another factor or combination of factors, judging by the unusually low incidence of heart attacks in France, Japan and on Kitava.One of the major changes in diet that I didn't mention in the last post was the rise of industrial liquid vegetable oils over the course of the 20th century. In the U.S. in 1900, the primary cooking fats were lard, beef tallow and butter. The following data only include cooking fats and spreads, because the USDA does not track the fats that naturally occur in milk and meat (source):
Animal fat is off the hook. This is the type of information that makes mainstream nutrition advice ring hollow. Let's see what happened to industrial vegetable oils in the early 1900s:
I do believe we're getting warmer. Now let's consider the composition of traditional American animal fats and industrial vegetable oils:
It's not hard to see that the two classes of fats (animal and industrial vegetable) are quite different. Animal fats are more saturated (blue). However, the biggest difference is that industrial vegetable oils contain a massive amount of omega-6 (yellow), far more than animal fats. If you accept that humans evolved eating primarily animal fats, which is well supported by the archaeological and anthropological literature, then you can begin to see the nature of the problem.Omega-6 and omega-3 fats are polyunsaturated fatty acids that are precursors to a very important class of signaling molecules called eicosanoids, which have a hand in virtually every bodily process. Omega-6 and omega-3 fats compete with one another for the enzymes (desaturases and elongases) that convert them into eicosanoid precursors. Omega-6-derived eicosanoids and omega-3-derived eicosanoids have different functions. Therefore, the balance of omega-6 to omega-3 fats in the diet influences the function of the body on virtually every level. Omega-6 eicosanoids tend to be more inflammatory, although the eicosanoid system is extraordinarily complex and poorly understood.What's better understood is the fact that our current omega-6 consumption is well outside of our ecological niche. In other words, we evolved in an environment that did not provide large amounts of omega-6 all year round. Industrial vegetable oils are a product of food processing techniques that have been widespread for about 100 years, not enough time for even the slightest genetic adaptation. Our current level of omega-6 intake, and our current balance between omega-6 and omega-3, are therefore unnatural.
The ideal ratio is probably very roughly 2:1 omega-6:omega-3. Leaf lard is 6.8, beef tallow is 2.4, good quality butter is 1.4, corn oil is 45, cottonseed oil is 260. It's clear that a large qualitative change in our fat consumption occurred over the course of the 20th century. I believe this was a major factor in the rise of heart attacks from an obscure condition to the primary cause of death. I'll be reviewing the data that convinced me in the next few posts.
The Coronary Heart Disease Epidemic
The Coronary Heart Disease Epidemic: Possible Culprits Part I
The Omega Ratio
A Practical Approach to Omega Fats
Polyunsaturated Fat Intake: Effects on the Heart and Brain
Polyunsaturated Fat Intake: What About Humans?
Vegetable Oil and Homicide
Dr. William Lands, one of the pioneers of the eicosanoid field, compiled this graph. It may be the single most important clue we have about the relationship between diet and ischemic heart disease (heart attacks).
