Eggs have long been recognized as a source of high-quality protein. The World Health Organization (WHO) and other public health authorities actually use eggs as their reference standard for evaluating the protein quality in all other foods. Egg protein is usually referred to as "HBV" protein, meaning protein with High Biological Value. Since eggs are used as the reference standard for food protein, they score 100% on the HBV chart. The high quality of egg protein is based on the mixture of amino acids it contains. (Amino acids are the building blocks for making proteins.) Eggs provide a complete range of amino acids, including branched chain amino acids (leucine, isoleucine, valine), sulfur-containing amino acids (methionine, cysteine), lysine, tryptophan, and all other essential amino acids. Their protein is sometimes referred to as a "complete protein" for this reason.
All B vitamins are found in eggs, including vitamins B1, B2, B3, B5, B6, B12, choline, biotin, and folic acid. Choline is a standout among these B vitamins. In fact, eggs rank higher in choline than any other food. In the U.S., an average diet provides about 300 milligrams of choline per day - less than the recommended amount for an adult woman (425 milligrams) or an adult man (550 milligrams). Since one egg provides over 100 milligrams of choline and only 75-80 calories, it provides far more choline for far less calories than most other choline-rich foods.
The mineral content of eggs also deserves special mention here--not because eggs are a rich source of most minerals but because they are a rich source of certain minerals that can sometimes be difficult to obtain from other foods. Eggs are a very good source of both selenium and iodine. While many fish, shellfish, and mushrooms can be rich sources of selenium, persons who avoid these foods may sometimes have difficulty getting an adequate amount of this important antioxidant mineral from food. For persons who do not use iodized salt in recipes or at the table and who do not consume either yogurt or cow's milk, this mineral can also sometimes be challenging to obtain from food.
The nutrients found in an egg are distributed fairly evenly between the yolk and the white. This distribution of nutrients is a common characteristic of whole, natural foods and it is one of the reasons that we recommend consumption of whole eggs (except, of course, when only the yolk or the white is called for in a recipe). The chart below explains what approximate percent of the total nutrient amount is found in the yolk and the white of an egg. You will notice that the first four nutrient groupings are those that are found predominately in the egg white, while those that follow are found predominately in the egg yolk (all except for the last nutrient, selenium, which is divided fairly evenly between the egg white and yolk).
As a group, research studies on the health benefits of eggs have shown mixed results. Part of the difficulty that researchers encounter when trying to determine the pros and cons of egg intake is the tendency of participants to consume other foods high in fat and/or saturated fat along with eggs. For example, at breakfast meals in the U.S., eggs are often consumed together with bacon, sausage, or ham. This simultaneous consumption of eggs with other foods can make it difficult for researchers to separate out the specific influence of the eggs. In addition, from a calorie standpoint, two eggs typically provide only 150-175 calories—only 7-8% of a 2,000-calorie diet. This small amount can make it more difficult for researchers to pinpoint the role played by the eggs.
Another complicating factor in egg research is the fiber-free nature of eggs. Since fiber typically has a risk-lowering affect for cardiovascular disease, diabetes, and cancer, egg intake might show up as problematic in a diet that was otherwise very low in fiber, yet helpful in a diet that was otherwise rich in fiber.
These factors described above do not change our view of eggs as an unusually nutrient-rich food that can provide a unique combination of nutrients for a very small number of calories. But they do underscore the importance of integrating eggs into an otherwise healthy meal plan.
In the area of cardiovascular disease, recent studies have shown no increased risk of either heart attack or stroke in conjunction with egg intake of one to six eggs per week. Interestingly, these studies have also shown the ability of egg intake to increase levels of HDL cholesterol (the "good" cholesterol). Not only did egg intake increase the number of HDL molecules, it also improved their composition and allowed them to function more effectively. This improved function may have been the result of more phosphatidylethanolamine being added to the HDL molecules. (The addition of phosphatidylethanolamine, in turn, might have been related to the rich initial choline content of the eggs.)
Not all egg studies show potential cardiovascular benefits, however, and in some studies, egg intake has been related to some increased mortality risk. However, as mentioned previously, it's been difficult for researchers to separate out the possible role of other foods in many studies. Particularly in mortality studies, which often examine diet in very general terms, they are unable to look closely at specific egg amounts in the diet.
One further note about the relationship between egg intake and cardiovascular risk: some persons with type 2 diabetes may be more susceptible to unwanted cardiovascular problems in relationship to egg intake if their type 2 diabetes has also created problems with cholesterol transport through the bloodstream. (These transport problems often correspond to low levels of apolipoprotein E and high levels of apolipoprotein C-III in the blood, which can be determined by lab testing.) Given this connection, persons with type 2 diabetes are encouraged to consult with their healthcare provider when making decisions about eggs in their meal plan.
Like studies on eggs and cardiovascular risk, studies on eggs and cancer risk have been mixed. We have seen large-scale studies in which egg intake was associated with a decreased risk of breast cancer and included along with vegetables, fruits, and legumes as a desirable factor in a risk-lowering meal plan. Yet we have also seen studies in which risk of colon and rectal cancer was increased by egg intake. For us, the mixed nature of these cancer studies underscores the need to consider health benefits of eggs as being conditional upon the overall quality of the diet; we should not be assuming that eggs will automatically lower or raise cancer risk regardless of an overall meal plan. As noted in the world’s healthiest foods.