Egg Yolks, Cholesterol and TMAO

By Dr. Gabe Mirkin


The NIH-AARP Diet and Health Study followed 521,120 U.S. adults, average age 62.2 years, for an average of 16 years and found that eating half an egg per day was associated with increased risk for death from heart attacks, cancer, and all causes ( Note #1). Each egg yolk contains approximately 200 mg of cholesterol, and each additional 300 mg of dietary cholesterol eaten daily increased risk for death from all causes by 19 percent, heart attacks by 16 percent and cancer by 24 percent. A previous study found similar results (Note #2). However, a review of 40 studies published between 1979 and 2013 found some controversy about harm from eating eggs (Note #3). Researchers analyzed 211 different papers and found that more than 85 percent reported that eating eggs raised blood cholesterol (Note #4), but there is enough controversy about whether dietary cholesterol is harmful that the American Heart Association and American College of Cardiology (2013) and the U.S. Dietary Guidelines (2015-2020) did not mention lowering dietary cholesterol as a strategy to help prevent heart attacks.


TMAO, Rather than Cholesterol, May Be the Culprit
For many years, researchers have recommended that preventing heart attacks involves restricting egg yolks and all other foods that are rich in cholesterol, but there is now extensive evidence that egg yolks may increase heart attack risk by a different mechanism. Egg yolks contain lecithin and choline, two chemicals that are converted by some of the more than 100 trillion bacteria in your colon into a chemical called TriMethylAmine (TMA). TMA is absorbed into your bloodstream and travels to your liver where it is converted to TriMethylAmine Oxide (TMAO). TMAO can damage blood vessels to start plaques forming in your arteries, can increase clotting that causes heart attacks and strokes (Cell, March 24, 2016;165(1):111–124), and can damage DNA in your cells to increase cancer risk. A review of 17 clinical studies covering 26,167 subjects, followed for an average 4.3 years, found that high blood levels of TMAO are associated with an almost double increased risk for an early death (Note #5). The chances for dying increased by 7.6 percent for each 10 micromoles/L increase in blood levels of TMAO. Lowering blood levels of TMAO helps to prevent heart attacks (Note #6)


Evidence that Diabetics Should Restrict Eggs
• People who regularly consumed one or more eggs per day (equivalent to 50 grams) increased their risk of diabetes by 60 per cent (Note #7).
• In healthy men and women, no association was found between eating one egg per day regularly and risk for heart attacks and strokes, but for diabetics eating one egg per day was associated with increased risk for heart attacks (Note #8).
• In healthy men, three or more eggs per week was linked to higher levels of sugar stuck on cells (HbA1C) that measures cell damage from high blood sugar levels; and in diabetics, eating three eggs per week was associated with higher blood sugar levels and increased risk of stroke (European Journal of Nutrition, Nov 2, 2017).
• In healthy men and women, six eggs per week increased risk for diabetes (Diabetes Care, Feb 2009;32(2):295–300).
• Healthy North Americans who eat more than two eggs per week appear to be at increased risk for diabetes, but studies from Spain, France, Finland and Japan showed no increased risk for diabetes (Journal of Clinical Nutrition, Jan 6, 2016).
• A review of 17 studies failed to show increased risk for heart attacks in people who eat eggs (BMJ, January 2013). However, regular egg eaters who are diabetic suffered 150 percent more heart attacks than diabetics who ate eggs sporadically.
• Ultrasound tests showed that people who ate more than three eggs a week had increased plaques in their arteries when compared to those who ate two or fewer eggs a week, even after other risks such as smoking were ruled out (Note #9).


Comparing Eggs to Other Breakfast Foods
Sugar and other refined carbohydrates may put you at higher risk for heart attacks, diabetes and premature death than eating cholesterol and TMAO in meat and eggs. It makes no sense to replace eggs with:
• pancakes, waffles or French toast covered with sugary syrup
• many dry breakfast cereals that are made by grinding whole grains into flour, removing most of the fiber and adding sugar
• bakery products such as bagels and muffins
• sausages, bacon and other processed meats that can increase risk for cancers as well as heart attacks


My Recommendations
Your overall diet is far more important than whether or not you eat eggs on occasion(Note #10 ). I believe that most North Americans should restrict eggs to not more than three or four a week, and they should not be eaten with red or processed meats (Note #11). If your LDL cholesterol is over 100, or you have heart problems or diabetes, I think that you should restrict eating eggs (Am J Clin Nutr, 2013;98:146-59). A more healthful breakfast can contain oatmeal or other whole grains, with nuts and fruits added for flavoring. Nuts are not fattening, even though they contain lots of fat, and fruits contain healthful soluble fiber.



#1 PLoS Med, Feb 9, 2021;18(2):e1003508
#2 JAMA, 2019;321(11):1081–95
#3 Am J Clin Nutr, 2015;102(2):276–94)
#4 American J of Lifestyle Med, Dec 11, 2019
#5 European Heart Journal, July 19, 2017;38(39):2948–2956
#6 Cell, Dec 17, 2015;163(7):1585-95)
#7 Br J Nutr, Oct 8, 2020;1-8
#8 Am J Clin Nutr, 2013;98:146–159; BMJ, 2013;346:e8539
#9 Atherosclerosis, 2012 Oct;224(2):469-73
#10 (Nutrients, 2015 Sep 3;7(9):7399-420
#11 JAMA, 1999;281:1387-94