The Diet-Heart Hypothesis: Oxidized LDL, Part II

In the last post, I presented the evidence that oxidized LDL (oxLDL) is a dominant factor in the arterial disease known as atherosclerosis, although probably not the only factor. In this post, I'll describe some of the major contributors to oxLDL.

Polyunsaturated Fats Increase LDL Oxidation

The serum concentration of oxLDL is strongly influcenced by diet. One dietary determinant of oxLDL is dietary polyunsaturated fat (PUFA). PUFA are inherently susceptible to oxidative damage, compared to monounsaturated and saturated fats. The predominant PUFA in the modern diet is linoleic acid, found excessively in industrial seed oils like corn oil, sunflower oil, safflower oil, cottonseed oil and soy oil. LDL is naturally rich in linoleic acid, even in cultures such as the Kitavans who have a very low dietary intake of it. However, LDL content of linoleic acid does correlate with dietary intake, and the Kitavans have a comparatively small amount of linoleic acid in their LDL, relative to industrial cultures.

There have been a number of media reports in the last few years proclaiming that monounsaturated fat reduces LDL oxidation compared to saturated and polyunsaturated fat. This is rather implausible on the surface, so let's take a closer look. There are two ways to measure oxLDL:

1. Measure it directly from the blood
   
2. Take normal LDL from the blood, expose it to copper in a test tube, and see how fast it oxidizes

The first reflects actual oxLDL in the blood, whereas the second reflects "susceptibility to oxidation" and has a dubious relationship with actual oxidized LDL in the bloodstream. This results in statements like the following (ref):

LDL resistance to copper-induced oxidation, expressed as lag time, was highest during the MUFA-rich diet (55.1±7.3 minutes) and lowest during the PUFA(n-3)– (45.3±7 minutes) and SFA- (45.3±6.4 minutes) rich diets.

This was published in a paper by P. Mata and colleagues in 1996. They fed 42 volunteers one of four different diets for 5 weeks each: one rich in saturated fat, one rich in monounsaturated fat, one rich in linoleic acid PUFA, and one rich in linoleic acid plus omega-3 PUFA. They emphasized the finding quoted above, as did the media. But there's an embarrassing piece of data buried in the paper that the authors, and the media, ignored (thanks to Chris Masterjohn for pointing this out). Here's what they saw when they looked directly at LDL oxidation in their volunteers:

Oops! LDL oxidation in the two PUFA groups was increased by more than 31%. The difference between the leftmost two groups and the rightmost two was statistically significant. As one would expect, oxidized LDL is proportional to the amount of PUFA in LDL, which is proportional to dietary PUFA. This somehow got left out of the abstract and media reports. The same investigators published a similar report a year later.

In another diet trial, participants were placed on one of two diets for 5 weeks: a low-fat, high PUFA diet low in vegetables; or a low-fat, high PUFA diet high in vegetables. The authors were forthright about their findings, so I'll let them summarize:

The median plasma OxLDL-EO6 increased by 27% (P less than 0.01) in response to the low-fat, low-vegetable diet and 19% (P less than 0.01) in response to the low-fat, high-vegetable diet. Also, the Lp(a) concentration was increased by 7% (P less than 0.01) and 9% (P=0.01), respectively.

This is the diet mainstream cardiologists have been prescribing to heart attack patients for 40 years. The trials I mentioned above are the only three I'm aware of in which fat quality was manipulated and oxLDL was directly measured (the first two were based on subsets of the same data). They all suggest that replacing saturated fat with PUFA increases oxLDL.

I suspect that the effect has less to do with the decrease in saturated fat and more to do with the increase in PUFA, although there's no way to know for sure. In the Lyon Diet-Heart trial, which I believe was the most successful diet trial of all time, linoleic acid was reduced to 3.6% of calories, but saturated fat was also reduced. Another reason is that there are numerous low-fat, low PUFA, high-carbohydrate cultures that have low levels of atherosclerosis and heart attacks. The Kitavans, for example, don't seem to have heart attacks or strokes (although no autopsies have been done so we don't know how much atherosclerosis they have).

They get 69% of their calories from high-glycemic starchy tubers, and their 21% fat comes mostly from coconut so it's highly saturated. Their blood lipids are low in omega-6 linoleic acid and very saturated. But there's a little surprise in the data: their lipids are full of palmitic acid (saturated), despite the fact that their diet contains very little of it. The reason is that their livers are turning all that carbohydrate into saturated fat, which is what happens when you eat more carbohydrate than you can burn immediately or store as glycogen. The moral of the story is that you don't need to eat saturated fat to have saturated LDL: a high-carbohydrate diet can accomplish the same thing, especially if it has a high glycemic index.

Fat-Soluble Antioxidants Decrease LDL Oxidation

LDL carries fat-soluble antioxidants, predominantly vitamin E and coenzyme Q10 (CoQ10). One form of vitamin E, alpha-tocopherol, slows atherosclerosis in most animal models but has shown equivocal results in human trials. There is even the suggestion that it may increase LDL oxidation under some circumstances. I don't recommend supplementing with vitamin E. However, the first line of antioxidant defense in LDL is provided by CoQ10. CoQ10 unequivocally reduces LDL oxidation in human subjects, and potently reduces atherosclerosis in animal models.

CoQ10 has a special relationship with cardiovascular health. Levels are reduced in individuals with cardiovascular disease and high oxLDL. Whether this is cause or effect, it's difficult to say. However, supplementing with CoQ10 has been repeatedly shown to be effective for high blood pressure and congestive heart failure. There has been one controlled trial of CoQ10 (120 mg/day) supplementation for the prevention of heart attacks, which reduced cardiac events including deaths by 45%, compared to a group receiving B vitamins. The CoQ10 group showed a large reduction in plasma lipid oxidation. This is a promising result and the experiment should be repeated.

CoQ10 is not an essential nutrient, although food does contribute a small portion of our total CoQ10 use. The large majority of CoQ10 is synthesized by the body itself, and this is dependent on a number of essential nutrients, including vitamin B2, B3, B5, B6, B12, vitamin C and folic acid. Thus, the body's synthesis of CoQ10 is dependent on overall nutritional status. Sub-clinical deficiency of any of these vitamins can hypothetically contribute to reduced CoQ10 production and thus oxLDL. This is potentially a big problem since modern Americans get more than half their calories from nutrient-poor refined foods. Liver is the single best source of many of these vitamins, and also holds the title of Most Nutritious Food on the Planet. It's also rich in CoQ10.

CoQ10 synthesis declines with age and is reduced in people with disorders involving oxidative stress, like cardiovascular disease. It's also greatly reduced by the cholesterol-lowering drugs statins. I'm not generally in favor of supplements, but CoQ10 seems to have a lot of promise and nothing but positive side effects that I'm aware of. CoQ10 deficiency may be a common theme in a number of modern disorders.

Excess Blood Sugar and Fructose Increase LDL Oxidation

Both type I and type II diabetes are associated with higher levels of oxLDL, therefore, prolonged high blood glucose may contribute to LDL oxidation due to glycosylation of the LDL protein ApoB. Fructose consumption increases oxLDL relative to glucose. Fructose is a very powerful glycosylating agent (binds non-specifically to other molecules, causing damage). Although it isn't present at high levels in the general circulation, it does interact with blood lipids in the hepatic portal vein as it moves from the digestive tract to the liver to be turned into fat (palmitic acid). Peter at Hyperlipid has written extensively about the role of glycosylation in LDL oxidation.

The Diet-Heart Hypothesis: The Verdict

The diet-heart hypothesis, the idea that dietary saturated fat and cholesterol raise blood cholesterol and thus increase heart attack risk, is a half-century embarrassment to the international scientific community. It requires willful ignorance of the fact that saturated fat does not increase total cholesterol or LDL in humans, in the long term. It requires a simplistic view of blood lipids that ignores the potentially harmful effects of replacing animal fats with carbohydrate or industrial seed oils. Worst of all, it requires selective citation of the literature on diet modification trials.

I have to conclude that if dietary saturated fat and cholesterol play any role whatsoever in cardiovascular disease, it's a minor one that's trumped by other factors. Industrial seed oils and sugar are likely to play an important role in cardiovascular disease.

The Diet-Heart Hypothesis: Oxidized LDL, Part I

In my reading about lipoprotein particles (LDL, HDL, etc.) and how they associate with cardiac risk, I've come across three LDL-related markers that associate with risk: LDL cholesterol, LDL particle number, and LDL size/density. Is this a coincidence, or is there a reason for it?

The first marker, LDL cholesterol, is probably nothing more than a crude approximation of particle number. But LDL particle number and size/density are related to something else, that probably actually causes atherosclerosis rather than simply being associated with it: oxidized LDL (oxLDL).

oxLDL is formed when the lipids in LDL particles react with oxygen and break down. This happens specifically to the unsaturated fats in LDL, because saturated fats, by their chemical nature, are very resistant to oxidative damage. Polyunsaturated fats are much more susceptible to oxidative damage than saturated or monounsaturated fats. Linoleic acid (the omega-6 fatty acid found abundantly in industrial seed oils) is the main polyunsaturated fatty acid in LDL.

LDL is packaged with antioxidants in the liver, primarily vitamin E and coenzyme Q10 (CoQ10), to prevent its oxidation*. However, the more time it spends in the blood, the more likely it is to exhaust its antioxidant store and become oxidized. Also, the smaller the LDL particle, the more likely it is to become trapped in the vessel wall and become oxidized there.

Oxidized LDL Correlates Tightly with Cardiac Risk

oxLDL has turned out to be a very sensitive marker of cardiac risk, surpassing traditional markers like LDL, HDL, and triglycerides in most studies to date. Since the discovery of sensitive assays that detect oxidized LDL drawn directly from patient blood, a number of studies have been published supporting its ability to detect atherosclerosis (plaque buildup in the arteries), heart attack risk and even the metabolic syndrome.

Holovet and colleagues published a study comparing the ability of oxLDL and a traditional risk factor assessment to detect coronary artery disease. The traditional method is called the Global Risk Factor Assessment Score (GRAS), and includes age, total cholesterol, HDL, blood pressure, diabetes and smoking status. It's similar to the commonly used Framingham risk score (which, interestingly enough, doesn't include LDL).

GRAS was able to correctly differentiate a healthy person from a person with coronary artery disease 49% of the time, while oxLDL was correct 82% of the time. Thus, oxLDL by itself was far more accurate than a whole battery of traditional cholesterol and cardiac markers. Coronary patients had more than twice the level of circulating oxLDL than the healthy comparison group.

In a large prospective study by Meisinger and colleagues, participants with high oxLDL had a 4.25 higher risk of heart attack than patients with lower oxLDL. oxLDL blew away all other blood lipid markers by nearly a factor of two. From the abstract:

Plasma oxLDL was the strongest predictor of CHD events compared with a conventional lipoprotein profile and other traditional risk factors for CHD.

Oxidized LDL Makes Sense

It's time to cross the threshold from markers of heart attack risk to causes of atherosclerosis. Regular, non-oxidized LDL has few properties that would make it a suspect in atherosclerosis. It's just a little particle carrying cholesterol and fats from the liver to other organs. As soon as it oxidizes, however, it becomes pro-inflammatory, immunogenic, damaging to the vessel wall, and most importantly, capable of transforming immune cells called macrophages into foam cells, a major constituent of arterial plaque.

Researchers have been interested in the plaque-generating properties of oxLDL for over three decades, and quite a bit of data have accumulated. They've identified cellular receptors that allow macrophages to ingest oxLDL (CD36 and SR-A). These receptors are specific for oxLDL and do not recognize normal LDL to a significant degree. Mice whose macrophages lack either of these two receptors have the same amount of circulating LDL as normal mice, yet have 60 to 70 percent less atherosclerosis when fed a plaque-forming diet (1, 2). Shorter-term studies have not always been consistent however, suggesting that there are alternative mechanisms. I'll expand on this more later.

Another line of evidence comes from the ability of LDL-borne antioxidants to prevent atherosclerosis in animal models. The powerful synthetic antioxidant probucol greatly reduces atherosclerosis in a number of animal models. It also reduces the extremely high cholesterol rodents and herbivorous animals get when they eat a high-cholesterol "atherogenic diet", but several studies have concluded that the majority of probucol's effect is due to its antioxidant ability rather than its ability to reduce cholesterol (ref).

Vitamin E and CoQ10 are two other LDL-borne antioxidants that can reduce atherosclerosis in animal models, particularly in combination with one another. Vitamin E alone is not as effective, and in some studies totally ineffective, which is one possible explanation for the equivocal results of vitamin E cardiovascular trials in humans. The most effective combination of antioxidants is probably the one provided by a nutrient-dense diet.

In Summary

Multiple lines of evidence suggest that oxidized LDL plays a dominant role in atherosclerosis. Not only is it associated with cardiovascular risk, there's also a large body of evidence suggesting it actually directly contributes to it. In the next post, I'll describe how you can modify your level of oxidized LDL using diet.

* People often think of colorful fruits and vegetables when they think of antioxidants, but vitamin E and CoQ10 are found in both plant and animal foods. Fruits and vegetables are generally not good sources of these fat-soluble antioxidants. Good sources include organ meats, nuts, pastured butter, avocados and red palm oil. The body also manufactures CoQ10 itself.