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We know that those with diets higher in plant foods (and lower in animal foods) have a decreased risk of heart attack and stroke (as well as less diabetes and even cancer).

The favorite culprit is the high fat and cholesterol content of animal meats. A second is high-temperature cooking (pan-frying, grilling), producing cancer causing and other harmful intermediary compounds. 

Although both may be contributing factors, more and more evidence is pointing to two proteins found in all red meat - carnitine and the cell sugar-protein molecule Neu5Ac - as the root cause of severe blood vessel disease.

Carnitine is a muscle protein. After digestion, carnitine is absorbed and processed in the liver into trimethylamine (TMA) which is then modified by colon bacteria (our microbiome) into trimethyl n-oxide (TMAO).

In lab experiments, TMAO is directly toxic to blood vessel lining cells. The injured cells absorb fats from the circulation, which results in the formation of blood vessel plaques. In the presence of high blood fat and cholesterol levels, the process is accelerated. 

A clinical parallel has been identified in studies on patients seen in hospital ERs for chest discomfort. Those with the highest blood levels of TMAO (compared to the lowest) are six times as likely to die within the following month and twice as likely to die within seven years.

Vegetarians, as a group, have the lowest average blood levels of TMAO. Interestingly, when given a single dose of carnitine (a piece of steak) their blood TMAO levels barely budge. The reason? A person's microbiome responds to their diet. Expose the colon bacteria to more of a specific food and those that thrive on it multiply. Being exposed to only small amounts of carnitine in a no-meat diet, there are very few carnitine metabolizing bacteria available to process that occasional steak. This suggests that an occasional meal containing meat should be less harmful to your blood vessels than when it is part of your daily diet.

The second meat protein is Neu5Gc, a cell surface protein found on all non-human mammalian cells (but not chicken or fish). Eons ago a genetic mutation in humans led to its modification to a similar molecule (Neu5Ac). As this new protein provided some protection against malaria, it became the dominant form.

Our immune system recognizes invading germs by their cell surface proteins. When it detects proteins that are different from our own, antibodies to kill the invading germs are produced.

With this change in our cell sugar-protein, the immune system now sees all non-human mammal meat as “different,” the immune system revs up, and antibodies are formed. The result is that those who regularly eat beef (a mammal meat) have blood markers reflecting a state of chronic inflammation.

A side effect of the overactive immune system is collateral damage to cells throughout the body. In the blood vessels, this means more atherosclerosis with an increased risk of heart attacks and strokes, and in other cells the damage increases the risk of cancer.

Supportive evidence once again comes from the laboratory where mice, genetically altered with the human gene mutation and placed on a meat diet, have twice the heart attack risk of genetically unmodified mice on a similar diet.

These two harmful effects of a mammal meat diet are moderated to a degree by small molecules produced from the metabolism of fiber by our microbiome. But with most red meat containing diets being lower in fruits, vegetables and whole grains, this protection is weakened. The effect is observed in those on a strict paleo diet (high red meat and no whole grain) who have much higher blood TMAO levels than those who eat even a small amount of whole grain.

What does this body of work suggest as far as healthy diet changes?

First, cut red meat intake and decrease exposure to the toxic effects of TMAO. In one large study, there was a 10% (one in ten) decrease in heart disease deaths in those who cut their red meat intake to half a serving per day!  Alternatives to be considered are poultry, fish, nuts, legumes, low-fat dairy, and whole grains. 

Second is to take advantage of the protective effects of fiber. The benefits of additional servings of fruits and vegetables have been shown to increase up to 10 servings per day.

Finally, it is never too late to make a change. Health professionals who began eating more whole grains, fruits, vegetables, and fish were able to significantly lower their risk for death in as few as eight years.

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