Nobody really knows whether or not eating eggs is safe. We have studies showing that people who eat more than five eggs a week have increased risk for diabetes and breast and colon cancer, but the studies show only that eating eggs is associated with these conditions. We have no studies that show that eggs cause disease in humans.

Researchers recently reviewed almost 2,000 studies on eggs and health (American Journal of Clinical Nutrition, Feb 2016;103.2:474-80). They threw out more than 1,100 of the studies because they could not obtain the full text of the articles. From the remaining studies, they selected only those that followed people over the years and compared what happened to those who ate eggs with those who didn’t. They ended up with eight studies that were dependable and met their criteria. These eight studies covered 219,979 participants from various countries, of whom 8,911 developed diabetes (defined as fasting glucose >126 mg/dL, non-fasting glucose >200 mg/dL, or hemoglobin A1c >6.5 percent). Studies on non-Americans showed no association between eating eggs and diabetes, but the studies done on North Americans showed a 39 percent increased risk of developing diabetes among subjects with high egg consumption (more than three eggs per week) compared to those with low egg consumption.

Eggs and Heart Attacks
Another review covering 17 studies failed to show increased risk for heart attacks in people who ate eggs (British Medical Journal, January 2013). However, regular egg eaters who were 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 compared to those who ate two or fewer eggs a week, even after other risks such as smoking were ruled out (Atherosclerosis, Oct 2012;224(2):469-73). The more eggs a person ate, the greater the formation of plaques. The same authors showed that smoking increased plaque formation in the same way.

Egg and Cancers of the Breast, Ovary or Prostate
Researchers at Harvard found in prospective observational studies that eating more than five eggs per week was associated with increased risk for breast cancer and that risk increased at more than nine eggs per week (Br J Nutr, Oct 14, 2015;114(7):1099-107). However, they found no relationship between eating eggs and prostate or ovarian cancer, although eating more than five eggs per week was associated with increased risk for fatal prostate cancer. Another review of 13 studies showed increased risk for breast cancer in Europeans, Asians, and postmenopausal women who ate more than two eggs per week (Breast Cancer, May 2014;21(3):251-61).

Eggs and Gastrointestinal Cancers
A review of 44 studies of 424,867 participants who developed 18,852 gastrointestinal cancers showed increasing risk for these cancers with increasing intake of eggs, with a stronger association with colon cancer (Eur J Nutr, Oct 2014;53(7):1581-90).

Why Eggs May Harm You
All of these studies show only associations, not cause-and-effect. The research that has caused me to limit my own consumption of eggs explains how lecithin (found in eggs, meat and milk) leads to the formation of a chemical called TMAO. Lecithin is broken down in your intestines into another chemical called choline. Your intestinal bacteria use choline as a source for their energy and then release a breakdown product that is converted by your liver to TMAO (trimethylamine oxide). People with high blood levels of TMAO appear to be at increased risk for heart attacks.

Multiple animal studies show that TMAO punches holes in arteries. The holes bleed, clot and then start to form plaques which can eventually cause strokes and heart attacks. Researchers at the Cleveland Clinic have shown that TMAO increases the formation of plaques in human arteries (Nature Medicine, April 7, 2013). They also showed that in more than 4,000 patients who had had heart catheterizations, those with the highest TMAO levels had the highest rate of heart attacks, strokes, and dying over the next three years. High levels of TMAO also have been shown to raise blood pressure (Canadian Journal of Cardiology, December 2014;30(12):1700–1705). Vegetarians and vegans have much lower concentrations of blood TMAO than meat eaters (Nat Med, 2013;19:576-585). People with high levels of TMAO are at increased risk for colon cancer (Cancer Res, Dec 15, 2014;74(24):7442-52).

Two minutes after eating two hard-boiled eggs, people develop a high rise in blood TMAO levels because lecithin in eggs is converted to TMAO very quickly (N Engl J Med, April 25, 2013;368:1575-1584). The authors proved that the intestinal bacteria produced the TMAO by showing that giving antibiotics to people and animals before they ate an egg prevented the TMAO from being formed. More on TMAO

My Recommendations
Scientists have not shown that eggs cause heart attacks or cancers, but they have shown that eating eggs is associated with increased risk for these diseases. The data show that the risks are non-existent or very low at fewer than three eggs per week, but as people eat more eggs, their risk for cancers and heart attacks appears to increase also. I think the research on TMAO justifies my recommendation to restrict eggs to no more than a few a week. Note:  All of the potential concerns about eggs are from the yolks; there is no known problem with egg whites or products made from them such as EggBeaters.