DEAR DOCTOR K:
I hope you can answer this question once and for all: Are eggs bad for your health or not?
I don’t think any medical issue is ever settled “once and for all.” New knowledge sometimes modifies or even replaces old knowledge. This surely has happened with the question of whether eggs are bad for your health.
I was taught, first by my parents and then in medical school, that you should eat eggs infrequently. The reason? A single egg yolk contains a whopping 213 milligrams of cholesterol. The thinking, then, went that eggs would raise your blood cholesterol levels. That, in turn, would increase your chance of developing atherosclerosis — cholesterol-filled plaques on the walls of arteries. And that in turn would increase the risk of heart attacks and strokes.
It seemed reasonable. So I reserved eggs for Sunday mornings only.
Eggs also contain some heart-healthy substances: protein, vitamins B12 and D, riboflavin, folate, antioxidants and unsaturated fats. But the negative effects of cholesterol were thought to trump the positive effects of these substances.
Then research discovered something unexpected: Saturated fats in your diet raise your blood levels of total cholesterol and “bad” LDL cholesterol a lot more than does cholesterol in your diet. Why? Because most of the cholesterol in your body does not come from your diet. Rather, it is made by your liver. And saturated fats in your diet cause your liver to make lots of cholesterol. (A large egg contains only about 1.5 grams of saturated fat.)
The next discovery came from large observational studies, many conducted here at Harvard Medical School. Hundreds of thousands of people in these studies have been followed for decades. What they eat, and the diseases they develop, are carefully recorded.
These studies show that the average healthy person suffers no negative health effects from eating an egg a day. In particular, the studies did not find a greater risk of heart attacks or strokes in people who ate an egg a day. Plus, some, but not all, studies have shown that eating eggs regularly increases “good” HDL cholesterol and decreases triglycerides.
However, the studies did find some evidence that people with diabetes or people who already have heart disease probably should eat no more than three egg yolks per week. (Eating just the whites is fine.)
Be sure to cook the eggs until both the whites and the yolks are firm; under-cooked eggs can transmit a bacterial infection called salmonella.
Finally, my colleagues in the Department of Nutrition at Harvard’s School of Public Health point out that what you eat with your eggs is an important consideration. They remind us that eggs with salsa and a whole-wheat English muffin are a far different meal than eggs with cheese, sausages, home fries and white toast. It’s one of those obvious pieces of advice that’s easy to forget as you make, or order, breakfast.