Eggs have been a staple in the human diet for thousands of years. From hunter-gatherers collecting eggs from the nests of wild birds, to the domestication of fowl for more reliable access to a supply of eggs, to today’s genetically selected birds and modern production facilities, eggs have long been recognized as a source of high-quality protein and other important nutrients.
Over the years, eggs have become an essential ingredient in many cuisines, owing to their many functional properties, such as water holding, emulsifying, and foaming. An egg is a self-contained and self-sufficient embryonic development chamber. At adequate temperature, the developing embryo uses the extensive range of essential nutrients in the egg for its growth and development. The necessary proteins, lipids, carbohydrates, vitamins, minerals, and functional nutrients are all present in sufficient quantities for the transition from fertilized cell to newborn chick, and the nutrient needs of an avian species are similar enough to human needs to make eggs an ideal source of nutrients for us. (The one essential human nutrient that eggs do not contain is ascorbic acid (vitamin C), because non-passerine birds have active gulonolactone oxidase and synthesize ascorbic acid as needed.) This article summarizes the varied nutrient contributions eggs make to the human diet.
Macro and Micro Nutrient in Eggs
The levels of many nutrients in an egg are influenced by the age and breed or strain of hen as well as the season of the year and the composition of the feed provided to the hen. While most variations in nutrients are relatively minor, the fatty acid composition of egg lipids can be significantly altered by changes in the hen’s diet. The exact quantities of many vitamins and minerals in an egg are determined, in part, by the nutrients provided in the hen’s diet. Hen eggs contain 75.8% water, 12.6% protein, 9.9% lipid, and 1.7% vitamins, minerals, and a small amount of carbohydrates. Eggs are classified in the protein food group, and egg protein is one of the highest quality proteins available. Virtually all lipids found in eggs are contained in the yolk, along with most of the vitamins and minerals. Of the small amount of carbohydrate (less than 1% by weight), half is found in the form of glycoprotein and the remainder as free glucose.
Egg proteins, which are distributed in both yolk and white (albumen), are nutritionally complete proteins containing all the essential amino-acids (EAA). Egg protein has a chemical score (EAA level in a protein food divided by the level found in an ‘ideal’ protein food) of 100, a biological value (a measure of how efficiently dietary protein is turned into body tissue) of 94, and the highest protein efficiency ratio (ratio of weight gain to protein ingested in young rats) of any dietary protein. The major proteins found in egg yolk include low density lipoprotein (LDL), which constitutes 65%, high density lipoprotein (HDL), phosvitin, and livetin. These proteins exist in a homogeneously emulsified fluid. Egg white is made up of some 40 different kinds of proteins. Ovalbumin is the major protein (54%) along with ovotransferrin (12%) and ovomucoid (11%). Other proteins of interest include flavoprotein, which binds riboflavin, avidin, which can bind and inactivate biotin, and lysozyme, which has lytic action against bacteria.
A large egg yolk contains 4.5 g of lipid, consisting of triacylglycerides (65%), phospholipids (31%), and cholesterol (4%). Of the total phospholipids, phosphatidylcholine (lecithin) is the largest fraction and accounts for 26%. Phosphatidylethanolamine contributes another 4%. The fatty-acid composition of eggyolk lipids depends on the fatty-acid profile of the diet. The reported fatty-acid profile of commercial eggs indicates that a large egg contains 1.55 g of saturated fatty acids, 1.91 g of monounsaturated fat, and 0.68 g of polyunsaturated fatty acids. (Total fatty acids (4.14 g) does not equal total lipid (4.5 g) because of the glycerol moiety of triacylglycerides and phospholipids and the phosphorylated moieties of the phospholipids). It has been reported that eggs contain less than 0.05 g of trans-fatty acids. Egg yolks also contain cholesterol (211mg per large egg) and the xanthophylls lutein and zeaxanthin.
Eggs contain all the essential vitamins except vitamin C, because the developing chick does not have a dietary requirement for this vitamin. The yolk contains the majority of the water-soluble vitamins and 100% of the fat-soluble vitamins. Riboflavin and niacin are concentrated in the albumen. The riboflavin in the egg albumin is bound to flavoprotein in a 1:1 molar ratio. Eggs are one of the few natural sources of vitamins D and B12. Egg vitamin E levels can be increased up to tenfold through dietary changes. While no single vitamin is found in very high quantity relative to its DRI value, it is the wide spectrum of vitamins present that makes eggs nutritionally rich.
Eggs contain small amounts of all the minerals essential for life. Of particular importance is the iron found in egg yolks. Research evaluating the plasma iron and transferrin saturation in 6-12-month-old children indicated that infants who ate egg yolks had a better iron status than infants who did not. The study indicated that egg yolks can be a source of iron in a weaning diet for breast-fed and formula-fed infants without increasing blood antibodies to egg-yolk proteins. Dietary iron absorption from a specific food is determined by iron status, heme- and nonheme-iron contents, and amounts of various dietary factors that influence iron absorption present in the whole meal. Limited information is available about the net effect of these factors as related to egg iron bioavailability. In addition to iron, eggs contain calcium, phosphorus, sodium, potassium, magnesium, zinc, copper, and manganese. Egg yolks also contain iodine (25 mg per large egg), and this can be increased twofold to threefold by the inclusion of an iodine source in the feed. Egg selenium content can also be increased up to ninefold by dietary manipulations.
Choline was established as an essential nutrient in 1999 with recommended daily intakes (RDIs) of 550mg for men and 450mg for women. The RDI for choline increases during pregnancy and lactation owing to the high rate of choline transfer from the mother to the fetus and into breast milk. Animal studies indicate that choline plays an essential role in brain development, especially in the development of the memory centers of the fetus and newborn. Egg-yolk lecithin (phosphatidylcholine) is an excellent source of dietary choline, providing 125mg of choline per large egg.
Egg yolk contains two xanthophylls (carotenes that contain an alcohol group) that have important health benefits – lutein and zeaxanthin. It is estimated that a large egg contains 0.33 mg of lutein and zeaxanthin; however, the content of these xanthophylls is totally dependent on the type of feed provided to the hens. Egg-yolk lutein levels can be increased up to tenfold through modification of the feed with marigold extract or purified lutein.
An indicator of the luteinþzeaxanthin content is the color of the yolk; the darker yellow-orange the yolk, the higher the xanthophyll content. Studies have shown that egg-yolk xanthophylls have a higher bioavailablity than those from plant sources, probably because the lipid matrix of the egg yolk facilitates greater absorption. This increased bioavailability results in significant increases in plasma levels of lutein and zeaxanthin as well as increased macular pigment densities with egg feeding.
Eggs are one of the richest sources of dietary cholesterol, providing 215 mg per large egg. In the 1960s and 1970s the simplistic view that dietary cholesterol equals blood cholesterol resulted in the belief that eggs were a major contributor to hypercholesterolemia and the associated risk of cardiovascular disease. While there remains some controversy regarding the role of dietary cholesterol in determining blood cholesterol levels, the majority of studies have shown that saturated fat, not dietary cholesterol, is the major dietary determinant of plasma cholesterol levels (and eggs contain 1.5 g of saturated fat) and that neither dietary cholesterol nor egg consumption are significantly related to the incidence of cardiovascular disease. Across cultures, those countries with the highest egg consumption actually have the lowest rates of mortality from cardiovascular disease, and within-population studies have not shown a correlation between egg intake and either plasma cholesterol levels or the incidence of heart disease. A 1999 study of over 117 000 men and women followed for 8-14 years showed that the risk of coronary heart disease was the same whether the study subjects consumed less than one egg a week or more than one egg a day. Clinical studies show that dietary cholesterol does have a small influence on plasma cholesterol levels. Adding one egg per day to the diet would, on average, increase plasma total cholesterol levels by approximately 5mg dl_1 (0.13mmol/L). It is important to note, however, that the increase occurs in both the atherogenic LDL cholesterol fraction (4mg dl_1(0.10mmol/L)) and the antiatherogenic HDL cholesterol fraction (1 mg dl_1(0.03mmol/L)), resulting in virtually no change in the LDL:HDL ratio, a major determinant of cardiovascular disease risk. The plasma lipoprotein cholesterol response to egg feeding, especially any changes in the LDL:HDL ratio, vary according to the individual and the baseline plasma lipoprotein cholesterol profile. Adding one egg a day to the diets of three hypothetical patients with different plasma lipid profiles results in very different effects on the LDL:HDL ratio. For the individual at low risk there is a greater effect than for the person at high risk, yet in all cases the effect is quantitatively minor and would have little impact on their heart-disease risk profile.
Overall, results from clinical studies indicate that egg feeding has little if any effect on cardiovascular disease risk. This is consistent with the results from a number of epidemiological studies. A common consumer misperception is that eggs from some breeds of bird have low or no cholesterol. For example, eggs from Araucana chickens, a South American breed that lays a blue-green egg, have been promoted as low-cholesterol eggs when, in fact, the cholesterol content of these eggs is 25% higher than that of commercial eggs. The amount of cholesterol in an egg is set by the developmental needs of the embryo and has proven very difficult to change substantially without resorting to hypocholesterolemic drug usage. Undue concerns regarding egg cholesterol content resulted in a steady decline in egg consumption during the 1970s, 1980s, and early 1990s, and restriction of this important and affordable source of high-quality protein and other nutrients could have had negative effects on the well-being of many nutritionally ‘at risk’ populations. Per capita egg consumption has been increasing over the past decade in North America, Central America, and Asia, has remained relatively steady in South America and Africa, and has been falling in Europe and Oceania. Overall, world per capita egg consumption has been slowly increasing over the past decade, in part owing to the change in attitude regarding dietary cholesterol health concerns.