A prospective study from nine European countries (European Heart Journal Trial) followed for 12.6 years showed that heart attacks are strongly associated with eating mammal meat and processed meats (Circulation, April 22, 2019). Many previous studies have shown that a vegetarian diet is associated with reduced heart attack risk (Am J of Clin Nutr, July 1, 2014;100(suppl 1):320S–328S). For more than 70 years, scientists have blamed dietary saturated fat and cholesterol for causing heart attacks, but studies have failed to show that heart attacks are caused by either dietary saturated fats or dietary cholesterol. I reported recently on new research on TMAO as a possible culprit, and this week’s Journal of the American Medical Association contains a detailed article on the same studies written by Jennifer Abbasi (JAMA, published online May 22, 2019). She notes that, “Researchers at Stanford University are already studying the effects of plant-based meat alternatives on TMAO levels in a clinical trial. But short of going full-on herbivore, people can substantially reduce TMAO levels within as little as a month by eliminating or reducing red meat, according to the European Heart Journal Trial that compared protein sources.”
In my opinion, we now have two leading theories to explain the association between meat and the increased risk of heart attacks: TMAO and Neu5Gc. These two theories are non-contradictory; that is, they may both be risk factors at the same time.
TMAO (TriMethylAmine Oxide)
When you eat foods that contain choline or carnitine, these components pass to your colon where bacteria there convert them to a gas called trimethylamine (TMA) that is absorbed into the bloodstream and passes to the liver where liver enzymes convert TMA to TMAO. Choline and carnitine are found in large amounts in meat, poultry, fish, dairy, and egg yolks, and in far lower amounts in plants. Mammal meat raises blood levels of TMAO much higher than poultry, and it also changes the bacteria in your colon to the ones that make TMA. When you switch from eating mammal meat to eating primarily chicken and plants, blood levels of TMAO drop markedly as do the concentrations of colon bacteria that make TMA (Eur Heart J, Feb 14, 2019;40(7):583–594). In the European Heart Journal Trial, the amount of saturated fat eaten had no effect whatever on blood levels of TMAO.
TMAO may increase risk of heart attacks by:
• reducing cholesterol clearance from the bloodstream,
• increasing cells that deposit cholesterol in plaques,
• increasing the cytokines that promote inflammation to form plaques, and
• increasing clotting that is the ultimate cause of heart attacks (Cell, March 24, 2016;165(1):111-124).
Other foods and supplements that contain the chemicals that can form TMAO include:
• Processed foods that contain phosphatidylcholine, also known as lecithin
• Dietary supplements that include choline or carnitine
• Energy drinks and protein supplements that contain lecithin or choline
Fish also contain carnitine and choline, and may raise blood levels of TMAO. However, eating fish is not associated with increased risk for heart attacks (Microbial Ecology in Health and Disease, May 19, 2017:28(1)), possibly because the omega-3 fatty acids in deep-water fish help to reduce inflammation and clotting that increase heart attack risk.
In 1982, Professor Ajit Varki of the University of California San Diego discovered a sugar-protein molecule (called Neu5Gc) that appears in the tissues of almost every mammal except humans (Proc Nat Acad of Sciences, Sept 29, 2003). When you eat mammal meat, the Neu5Gc from the meat is absorbed into your bloodstream and your immune system treats this sugar-protein in the same way that it treats germs that try to get into your cells. Your immune system recognizes germs by the sugar-proteins on the surface of their cells. If the surface proteins are the same as those on your own cells, your immune system lets the protein enter you cells, but if the surface proteins on membranes are different from those on your cells, your immune system tries to kill the invading germs. Eating mammal meat turns on your immune system just as germs do, and if you eat mammal meat regularly, your immune system will stay active all the time. This is called inflammation. The same cells and chemicals that attack and kill germs can punch holes in your arteries to form plaques that can break off to lead to a heart attack. People who have the most markers of an overactive immunity are the ones most likely to suffer, and die from, heart attacks (J of Nutr, May 22, 2019).
Researchers have identified a gene (CMAH) that produces Neu5Gc, the potentially harmful component in most mammal meat (Genome Biol Evol, Jan 1, 2018;10(1):207-219).
For many years I have recommended that you limit or avoid red meat (meat from mammals), even though I could only show its association with heart attacks rather than cause-and-effect. With the continuing research on TMAO and Neu5Gc, there is still no clear cause-and-effect, but the evidence of association has become much stronger. Red meat and processed meats have also been linked to increased risk for diabetes, some types of cancers and other diseases. See my recent report, Even Occasional Meat May Be Harmful