Eggs have been a staple food 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 a 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 due 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 utilizes 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 sufficient for the transition from a fertilized cell to a newborn chick. The nutrient needs for an avian species are similar 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 an active gulonolactone oxidase and synthesize ascorbic acid as needed.) This article summarizes the varied nutrient contributions eggs make to the human diet.
Although the majority of eggs consumed today are chicken eggs, a variety of eggs from different species of birds are commercially available in different parts of the world, from the petite quail egg to the very large ostrich egg. Eggs from commercial chickens differ from wild breeds in that they have a lower cholesterol and lipid content. This difference is thought to be the result of many years of genetic selection of breeds with increased feed to egg conversion ratios as well as faster rates of lay.
The commercial hen utilized in today’s egg production has been selected for optimal feed conversion and egg production along with overall health, disease resistance, livability, and temperament. The majority of egg production is carried out using a battery cage system, which offers a high degree of control over environment, feed, water, hygiene, biosecurity, and egg collection. This system also facilitates mechanization. Other production systems include barn and free range (including organic production methods), which offer more freedom to birds, but often lead to higher disease and mortality rates in birds, and potentially increased susceptibility to bacterial contamination of the eggs.
Shifting dietary patterns in the population have resulted in compensatory changes in the egg industry. A major change has been the increased use of eggs going to egg products for the pre-prepared packaged food industry. In the USA more than 30% of the total egg production is used to make egg products, and egg product usage has been the most rapidly growing part of the industry accounting for the majority of the increased per capita egg consumption over the past decade. Another area of growth has been the special egg market. As consumers become more health conscience there has been an emphasis on functional components of foods that contribute to health and well-being. Eggs with enhanced nutrient benefits, especially with increased content of omega-3 fatty acids, are available worldwide. Eggs enriched with vitamin E, vitamin D, vitamin B12, riboflavin, folate, selenium, and lutein are also available in limited markets. These nutrient enhancements are all achieved by modification of the hens’ feed.
Egg Macronutrient and Micronutrient Content and Distribution
The levels of many nutrients in an egg are influenced by the age, and breed or strain of the hen as well as the season of the year and composition of the feed provided to the hen. Although 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. As noted above, 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 of the essential amino acids. Egg protein has a ‘chemical score’ (essential amino acid 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’ (PER: ratio of grams of weight gain to grams of 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.
As shown in Table 3, egg protein contains substantial amounts of essential and nonessential amino acids. The first column shows the amount, in grams, of each of the amino acids in one large egg. The second column indicates the amount of each amino acid per 100 g of whole egg. The third column shows the dietary reference intake (DRI) for all of the essential amino acids (EAAs) per 50 g of total dietary protein, and the last column indicates the percentage of the DRI for each essential amino acid provided by one large egg. Although a large egg provides only some 3% of the energy in a 2000-kcal (8394 kJ) diet, it provides 11% of the protein needs. The EAAs in an egg contribute between 12% and 31% of the DRI for the various EAAs.
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 which accounts for 26%. Phosphotidylethanolamine contributes another 4%. The fatty acid composition of egg yolk lipids is dependent 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 saturated fatty acids, 1.91 g monounsaturated fat, and 0.68 g polyunsaturated fatty acids. (Total fatty acids (4.14 g) does not equal total lipid (4.5 g) due to 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. Eggs yolks also contain cholesterol (211 mg per large egg) and the xanthophylls lutein and zeaxanthin. The lipid profile of a large egg is presented in Table 4.
Eggs contain all essential vitamins except vitamin C, because the developing chick does not have a dietary requirement for this vitamin. As shown in Table 5, the yolk contains the majority of the water-soluble vitamins and 100% of the fatsoluble 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 by seven- to 10-fold through dietary changes. Other than choline (see the section on Egg Choline), no single vitamin is found in very high quantity relative to its DRI value, however, it is the wide spectrum of vitamins present that make eggs nutritionally rich.
Eggs contain small amounts of all minerals essential for life. Of particular importance is iron found in egg yolks. Research evaluating the plasma iron and transferrin saturation in 6- to 12-month-old infants indicated that those who ate egg yolks had a better iron status than those who did not eat egg yolks. 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.
In addition to iron, eggs also contain calcium, phosphorus, sodium, potassium, magnesium, zinc, copper, and manganese (Table 6). Egg yolks also contain iodine (25 mg per large egg), which can be increased two- to three-fold by inclusion of an iodine source in the feed. Egg selenium content can also be increased seven- to nine-fold by dietary manipulations.
Choline was established as an essential nutrient in 1999 with Recommended Daily Intake (RDI) of 550 mg d1 for men and 450 mg d1 for women. The RDI for choline increases during pregnancy and lactation due 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 development of the memory centers, of the fetus and newborn. Recent studies have indicated that higher intakes of choline are associated with reduced risk of neural tube defects, reductions in plasma markers of inflammation, and decreased breast cancer risk. Studies have also shown that in the USA less than 10% of adult men and women attain the recommended AI for choline, including only 1 in 10 pregnant females. Egg yolk lecithin (phosphotidylcholine) is an excellent source of dietary choline, providing 125 mg of choline per large egg, and adding an egg a day to the diet can significantly increase the number of adults with adequate intakes.
Egg yolk contains two important xanthophylls (carotenes that contain an alcohol group) which have important health benefits – lutein and zeaxanthin. It is estimated that a large egg has 0.33 mg of lutein plus 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 five- to 10-fold through modification of the feed with marigold extract or purified lutein. An indicator of the lutein plus zeaxanthin content is the color of the yolk, the darker yellow– orange the yolk, the higher is the xanthophil content. Studies have shown that egg yolk xanthophylls have a higher bioavailability as compared with those from plant sources, probably due to the lipid matrix of the egg yolk facilitating 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. Although there remains some controversy regarding the role of dietary cholesterol in determining blood cholesterol levels, the majority of studies have shown that (1) saturated fat, not dietary cholesterol, is the major dietary determinant of plasma cholesterol levels, and eggs contain 1.5 g of saturated fat and (2) neither dietary cholesterol nor egg consumption are significantly related to cardiovascular disease incidence. Across cultures, those countries with the highest egg consumption actually have the lowest rates of cardiovascular disease mortality; and within-population studies have not shown a correlation between egg intake and either plasma cholesterol levels or heart disease incidence. A 1999 study of more than 117 000 men and women followed for 8–14 years showed that the risk for coronary heart disease was the same whether the study subjects consumed less than one egg a week or more that one egg a day. Numerous other epidemiological studies have failed to find a significant relationship between egg intake and cardiovascular disease risk.
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 approximately to 5 mg dl1. It is important to note, however, that the increase occurs in both the atherogenic LDL cholesterol fraction (4 mg dl1) and the antiatherogenic HDL cholesterol fraction (1 mg dl1) 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, varies depending on the individual and the baseline plasma lipoprotein cholesterol profile. As shown in Table 7, 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 a 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 birds 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 a low cholesterol egg 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 to be very difficult to substantially change 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 nutritional 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, per capita world egg consumption has been slowly increasing over the past decade in part due to the change in attitude regarding dietary cholesterol health concerns.
Allergenic Aspects of Egg Proteins
Eggs are one of the most common causes of food allergies in infants and young children. Although the majority of egg allergies are caused by egg white protein, proteins in both the egg white and yolk are associated with allergies. The egg white contains 50% ovalbumin, which is the major allergen. Other egg white allergenic proteins are ovomucoid, ovotransferrin, and lysozyme. Most egg allergies in young children are outgrown by age 5 years following an elimination diet.
Owing to the allergenicity of egg proteins, it is advised not to feed egg yolks to infants younger than 6 months of age and to wait until children are 12 months of age to feed them egg whites. When feeding egg yolks to children between the ages of 6 and 12 months, the eggs should be prepared so that the egg white can be completely removed such as in hard-cooked eggs.
There is an increasing interest worldwide in production and marketing of speciality eggs with enhanced nutrient benefits. The nutrient composition of an egg can be significantly modified by the composition of the feed. Commercially available nutrient-enhanced eggs contain increased amounts of omega-3 fatty acids, vitamin E, selenium, and lutein. Other enhancements include increased content of vitamin D and the B vitamins as well as incorporation of conjugated linoleic acid (CLA).
Omega-3 fatty acids: The fatty acid content of eggs is easily and significantly affected by the fatty acid profile of the hen’s feed. The omega-3 fatty acid content can be increased in the egg by feeding the hens a source of omega-3 fatty acids. In some countries, fish meal is used as a source of omega-3 fatty acids but this can result in eggs with a fishy odor and taste. Marine algae is another source of omega-3 fatty acid resulting in higher concentrations of eicosapentanoic acid (EPA) and docosahexanoic acid (DHA) in egg yolks. Flax-seed oil is also used as a source of omega-3 fatty acids and results in increased levels of a-linolenic acid in egg yolks. The relative proportions of DHA to a-linolenic acid can be controlled by feeding a mixture of flax seed and marine algae. It is possible to attain levels as high as 200 mg of omega-3 fatty acids per large egg.
Although omega-3 fatty acid levels in eggs are well below levels found in fish, such as salmon or tuna, eggs can still be an important source of omega-3 fatty acids to the diet. For people who cannot eat fish, eggs with higher levels of omega-3 fatty acids can be an important way to include these beneficial fatty acids in the diet.
Other nutrients in specialty eggs: By altering the content in the feed, other nutrients in eggs can be enhanced such as lutein, vitamin E, and selenium. Vitamin E is usually added to the feed to serve as an antioxidant when the polyunsaturated fatty acids are increased. Vitamin E levels in eggs have been increased as much as 25-fold. The increased vitamin E in these eggs can provide an additional natural source of this important fat-soluble vitamin. Lutein (a xanthophyll) can also be increased in eggs by increasing the amount in feed, usually as marigold extract. Lutein is deposited in the egg yolk at levels as high as 2 mg per large egg and the human body readily absorbs lutein from the egg phopholipid matrix. Nutritional needs for selenium vary widely due to differences in the selenium content of regional soils. Egg selenium levels can be increased five- to eight-fold with addition of an organoselenium source to the feed.
Egg Food Safety
Eggs pose a unique food safety problem because eggs can be contaminated internally with the pathogenic bacteria Salmonella enterica serovar Enteritidis (SE). If SE infects the reproductive tract of laying hens, it can be deposited in the egg during formation. In addition to internal egg contamination by SE, egg shells can be contaminated with a number of microorganisms. Caution is required when selecting eggs for consumption. Only clean eggs should be consumed. Vaccination of hens against S. enterica, temperature control, proper handling, and cooking are all important control measures to reduce the incidence of SE illness.
When SE internally contaminates an egg, it is thought that the SE is deposited at the yolk membrane in the egg white. The integrity of the vitelline membrane is very important to prevent SE from entering the yolk where it could grow very rapidly due to the nutrient-rich environment. The egg white has natural antimicrobial compounds, such as lysozyme, that help prevent SE from growing in the egg white.
In naturally contaminated eggs, scientists have documented that approximately 10–100 cells of SE may be deposited in an egg. The bacterial cell count will remain low unless the egg is exposed to temperatures that would allow rapid growth of SE or the vitelline membrane breaks down. Even when flocks are infected with SE, only a small percentage of the eggs produced will contain SE. Properly cooking the eggs to a temperature of 63 1C for 3 min, 65 1C for 1 min, or 70 1C for 1 s will destroy SE if present in an egg.
Role of Eggs in the Diet
The nutritional contributions of eggs to a diet are determined by the per capita consumption profile of a given country. In countries like Japan, with the highest per capita egg consumption, eggs play an important role as a source of nutrients whereas in countries like India, with very low per capita consumption values, its role is minor. Worldwide there are many different misperceptions and myths regarding eggs that influence consumption patterns.
Eggs are a nutrient-dense source of many essential amino acids, vitamins, and minerals and, as shown in Figure 1, eggs contribute a number of nutrients to the American diet in amounts proportionally greater than their caloric contributions. While providing only 1.3% of the calories, they provide nine different nutrients ranging in amounts from 2 to 6% of the DRI. Such nutrient-dense foods can play an important role in diets of seniors who have decreased caloric intakes as well as weight-reduction/weight-maintenance diets. Studies have shown that egg intake has a significant effect on satiety beyond what would be predicted from its protein and fat content. Egg intake slows the rate of gastric emptying resulting in a flatter blood glucose response as well as a lower insulin response. The effects on gastric emptying appear to be related to effects of egg yolk (not white) intake on secretion of cholecystokinin and gastric inhibitory peptide.
For nutritionally vulnerable populations including the poor, the very young, the very old, pregnant women, and those suffering from chronic diseases, eggs are an affordable, nutrient-dense source of high-quality protein important for maintaining health and facilitating recovery. Pregnancy is an especially important time to optimize intake of high-quality protein as well as other essential nutrients to reduce the risk of low birth weight and the associated development of chronic diseases and other health problems during the future adult years of the infant. Eggs also serve as an important dietary source of choline during pregnancy and lactation to provide choline to the fetus and newborn for brain development. In addition, eggs provide a satiety effect which, in view of the global problem of obesity, can be a valuable addition to weight-loss and weight-maintenance programs. For these various populations, from infant to aged, there are a multitude of health reasons to include nutrient-dense eggs as part of the diet, and for many of these groups it can be economically feasible.
The high-quality protein, many nutritional components, low caloric content, affordability, blandness, ease of digestibility, and satiety response are all characteristics that make eggs ideal for inclusion in the diet across the lifespan, from very young to very old, and under all conditions, health or convalescence. As noted in the Dietary Guidelines 2010, ‘‘Independent of other dietary factors, evidence suggests that one egg (i.e., egg yolk) per day does not result in increased blood cholesterol levels, nor does it increase the risk of cardiovascular disease in healthy people.’’
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