boiled egg diet

The egg diet

Egg is a source of macro and micronutrients that meet all requirements to support embryonic development until hatching 1). Eggs, an inexpensive but highly nutritious food, provide balanced nutrients that impact human health 2). The average consumption of eggs/year/capita in the world ranges from 62 (India) to more than 358 (Mexico) 3) and is even less in African Countries (36 eggs/year/capita) 4). Table eggs that are commercialized are not fertilized and are produced by about 3 billion hens, specifically bred throughout the world for human consumption. Eggs contain ample essential proteins, fats, vitamins, minerals, and bioactive compounds, and their compositions and net amount could be influenced by strain, age, hen diet, and environmental conditions. The nutrients to energy density ratio of one egg is high with many essential nutrients as shown in Table 1 5). A medium-sized boiled egg (50 g) contains 78 kcal energy, 6.29 g protein, 0.56 g carbohydrate, and 5.3 g total fat, of which 1.6 g is saturated, 2.0 g is monounsaturated, 0.7g is polyunsaturated, and 186 mg is cholesterol 6). As for micronutrients, egg contains a variety of minerals (calcium, iron, magnesium, phosphorus, potassium, sodium, and Zinc) and most vitamins (thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, vitamin A, vitamin E, vitamin D, and vitamin K) except for vitamin C. Some of these nutrients, such as zinc, vitamin A, vitamin D, and vitamin E, may not be enough in a western diet. Egg remains a food product of high nutritional quality for adults including elderly people and children and is extensively consumed worldwide. A total of 550 distinct proteins were identified so far in egg-white and yolk/vitelline membranes and the physiological function of only 20 of them is characterized to date 7). In parallel, there is compelling evidence that egg also contains many and still-unexplored bioactive compounds, which may be of high interest in preventing/curing diseases 8). A small amount of egg proteins is not assimilated 9), especially when egg is consumed as a raw ingredient 10). The higher digestibility of cooked egg proteins results from structural protein denaturation induced by heating, thereby facilitating hydrolytic action of digestive enzymes.

Eggs have been identified to represent the lowest-cost animal source for proteins, vitamin A, iron, vitamin B12, riboflavin, choline, and the second lowest-cost source for zinc and calcium 11). In addition to providing well-balanced nutrients for infants and adults, egg contains a myriad of biologically active components 12). These components are allocated in the various internal egg components (Figures 1 and 2). It has to be mentioned that eggshell and its tightly associated eggshell membranes are usually not consumed, although eggshell membranes are edible 13).

Daily nutrient intake of egg consumers was significantly greater than that of nonconsumers for all nutrients studied (except dietary fiber and vitamin B6). Eggs contributed < 10% of daily intake of energy and vitamin B6, 10% to 20% of folate and total, saturated and polyunsaturated fat, and 20% to 30% of vitamins A, E and B12 in egg consumers 14). Compared to people who consume egg, nonconsumers had higher rates of inadequate intake (defined by Estimated Average Requirements (EAR) or < 70% Recommended Dietary Allowance [RDA]) for vitamin B12 (10% vs. 21%), vitamin A (16% vs. 21%), vitamin E (14% vs. 22%) and vitamin C (15% vs. 20%) 15).

Table 1. Nutrient values of a medium-size boiled egg, whole milk with added vitamin D, and boiled manufacturing beef

CategoryBoiled eggWhole milk with added vitamin DBoiled manufacturing beef

NutrientUnit1 large egg
(50.0 g)
Value
(100 g)
Value
(100 g)
Value
(100 g)

Approximates
Energykcal7815561126
Waterg37.3174.6288.1373.1
Proteing6.2912.583.1524.21
Total lipid (fat)g5.310.613.253.26
Carbohydrate, by differenceg0.561.124.80
 Fiber, total dietaryg0000
 Sugars, totalg0.561.125.050

Minerals
 Calcium, Camg25501136
 Iron, Femg0.591.190.031.78
 Magnesium, Mgmg5101016
 Phosphorus, Pmg8617284129
 Potassium, Kmg63126132183
 Sodium, Namg621244332
 Zinc, Znmg0.531.050.375.02

Vitamins
 Vitamin C, total ascorbic acidmg0000
 Thiaminmg0.0330.0660.0460.042
 Riboflavinmg0.2570.5130.1690.096
 Niacinmg0.0320.0640.0891.759
 Vitamin B6mg0.060.1210.0360.16
 Folate, DFEμg224450
 Vitamin B12μg0.561.110.451.02
 Vitamin A, RAEμg74149468
 Vitamin A, IUIU26052016227
 Vitamin E (α-tocopherol)mg0.521.030.070.57
 Vitamin D (D2 + D3)μg1.12.21.30.1
 Vitamin DIU4487515
 Vitamin K (phylloquinone)μg0.10.30.30

Lipids
 SFAsbg1.6333.2671.8651.154
 MUFAsbg2.0384.0770.8120.897
 PUFAsbg0.7071.4140.1950.246
 Trans fatty acidsg0000.078
 Cholesterolmg1863731067

Footnote: a) Nutrient values and weights are for edible portion

Abbreviations: SFAs = saturated fatty acids, MUFAs = monounsaturated fatty acids, and PUFAs = polyunsaturated fatty acids.

[Source 16) ]

Figure 1. Egg nutrients

Egg nutrients

Footnote: Basic composition of edible parts of the egg. (a) Egg white; (b) Egg yolk. Note that for (b), results refer to egg yolk/vitelline membrane complex.

[Source 17) ]

Figure 2. Egg composition

egg composition

Egg Nutrients

Egg proteins are distributed equally between egg white and egg yolk, while lipids, vitamins, and minerals are essentially concentrated in egg yolk (Figure 1). Water constitutes the major part of egg (Figure 1) and it is noteworthy that the egg is devoid of fibers. The relative content of egg minerals, vitamins, or specific fatty acids may vary from one national reference to another 18) but remains globally comparable when considering major constituents such as water, proteins, lipids, and carbohydrates. The major egg nutrients are, indeed, very stable and depend on the ratio of egg white to yolk in contrast to minor components, which are affected by several factors including hen nutrition. In a whole, raw, and freshly laid egg, water, protein, fat, carbohydrates, and ash represent about 76.1%, 12.6%, 9.5%, 0.7%, and 1.1%, respectively 19).

Egg macronutrients

Egg Proteins

Egg white and egg yolk are highly concentrated in proteins. The concentration of proteins is, on average, 12.5 g per 100 g of whole raw fresh egg, while egg yolk with its vitelline membrane and egg white contain 15.9 g protein and 10.90 g protein per 100 g, respectively. These values are slightly modified by hen genetics and age. Thanks to complementary proteomic approaches, nearly 1000 different proteins have been identified in the chicken egg, including the eggshell 20). Hundreds of different proteins have been identified and are associated with specific physiological functions to fulfill time-specific requirements during embryo development. The compartment-specificity of some of these proteins can be explained by the fact that egg yolk and egg white are formed by distinct tissues. Egg yolk has essentially a hepatic origin, while egg white is synthesized and secreted after ovulation of the mature yolk in the hen’s oviduct 21).

For humans, eggs are one of the best sources of high quality protein, only inferior to breast milk. Egg proteins have been proved to possess antioxidants, such as phosvitin which contains large amount of phosphoserines, ovotransferrin that can chelate with Fe3+, and ovalbumin that can covalently bind to polysaccharide to enhance its antioxidant activity 22). These proteins can inhibit lipid oxidation by binding to metal or scavenging free radical. Eggs may be used as a potential natural source of antioxidant, which can be further used in food or cosmetics industry. The antioxidant function of eggs could prevent humans from a large number of degenerative processes, such as cardiovascular diseases occurring 23). Besides, egg protein, especially egg yolk protein, has a significantly greater satiety effect compared with other protein sources 24). Some research results have shown that there was reduced energy intake after the breakfast with an egg and a bagel 25), larger weight loss after 8 weeks with the breakfast containing an egg and a bagel as part of a hypocaloric diet 26), and significant change in satiety hormones after an egg in the breakfast 27). Proteins in egg yolk can be classified into apolipoproteins, phosvitin, egg yolk globulin, and riboflavin binding protein 28).

Egg yolk is a complex milieu containing 68% low-density lipoproteins (LDL) “bad” cholesterol, 16% high-density lipoproteins (HDLs) “good” cholesterol, 10% livetins and other soluble proteins, and 4% phosvitins. These components are distributed between non-soluble protein aggregates called granules (19–23% of dry matter), which account for about 50% of yolk proteins, and a clear yellow fluid or plasma, that corresponds to 77–81% of dry matter 29). Apolipoprotein B, apovitellenin-1, vitellogenins, serum albumin, immunoglobulins, ovalbumin, and ovotransferrin are the most abundant proteins of egg yolk, representing more than 80% of total egg-yolk proteins 30). Yolk is tightly associated with the vitelline membranes, which consist of two distinct layers 31) that form an extracellular protein matrix embracing the yolk. These membranes provide to the yolk a physical separation from other egg compartments and prevents subsequent leakage of egg yolk towards egg white.

The egg white is a gel-like structure that lacks lipids and is composed mainly of water (about 88%) 32), fibrous structural proteins (ovomucins), glycoproteins (ovalbumin, protease inhibitors), antibacterial proteins (lysozyme), and peptides 33). The average volume of egg white is estimated to be 30 mL (for an egg weighting 60 g, eggshell included) and protein concentration is about 110 mg/mL of egg white. In total, 150 distinct proteins have been identified in egg white 34), knowing that the very abundant ovalbumin accounts for 50% of the total egg-white proteins. The physiological function of this protein in egg remains unknown but ovalbumin is assumed to provide essential amino-acids for the chicken embryo growth. Egg-white ovalbumin thus represents a valuable source of amino-acids for human nutrition. Besides ovalbumin, egg white is concentrated in antibacterial lysozyme that is currently used as an anti-infectious agent in many pharmaceuticals and as food preservative. The viscous aspect of egg white is essentially due to ovomucin 35). Remarkably, egg white is also characterized by the presence of four highly abundant protease inhibitors 36) that may delay digestion of egg components, especially when egg white is used as a raw ingredient in some food preparations.

Egg Lipids

In addition to protein, eggs also contain a large number of active lipid components, such as unsaturated fatty acids, phospholipids, choline, and carotenoids. As shown in Table 1, monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) are, respectively, 2.0 g and 0.7 g, and content of saturated fatty acids is 1.6 g in one medium-size egg. The total lipid content is relatively stable in the egg ranging from 8.7 to 11.2 per 100 g of whole egg, when considering various EU countries and USA egg composition tables 37). These lipids are only concentrated in the egg yolk (Figure 1 and Table 1) and a small part may remain tightly associated with vitelline membranes 38).

Phospholipids account for approximately 10% of the wet weight of egg yolk 39), which mainly includes phosphatidylcholine (PC), phosphatidylethanolamine (PE), lysophosphatidylcholine (lysoPC), sphingomyelin (SM), and some neutral lipids in minor quantities. Dietary phospholipids, a potential source of bioactive lipids, may have broad effects on cholesterol metabolism, HDL functions, and inflammation 40). Yang et al. 41) reported that dietary polyunsaturated fatty acids (PUFAs), phosphatidylcholine, and sphingomyelin significantly inhibit the uptake of cholesterol in Caco-2 monolayer, which may have potential therapeutic effect on reducing cholesterol absorption as functional food ingredients. As a component of lecithin, choline also exists in egg in a larger amount 42). Choline may play a particularly useful role in fetal and neonatal brain development, as inadequate choline intake during pregnancy increases neural tube defects risk of infant 43). Other functional components from egg are the carotenoids, which are natural pigments in hen egg yolks and account for less than 1% of yolk lipids 44). The two major carotenoids in eggs are carotene and xanthophylls (lutein, cryptoxanthin, and zeaxanthin) which are highly bioavailable in egg yolk and are associated with the reduced risk of age-related macular degeneration and cataracts, cancer, and carotid artery atherosclerosis 45).

Carbohydrates

Egg does not contain any fibers and its content in carbohydrates is low (0.7%). Egg carbohydrates are distributed between egg yolk and egg white (Figure 1). Glucose is the dominant free sugar in the egg (about 0.37 g per 100 g of whole egg) and is essentially present in egg white (0.34 g per 100 g of egg white versus 0.18 g per 100 g of egg yolk) 46). Trace amounts of fructose, lactose, maltose, and galactose have been detected in raw egg white and raw egg yolk 47). Carbohydrates are also highly represented in egg proteins, knowing that many of them are glycoproteins undergoing post-translational glycosylations prior to secretion by reproductive tissues of the hen to form yolk, membranes, and egg white.

Egg Micronutrients

Egg Vitamins and Choline

The egg and, more precisely, the egg yolk, is a vitamin-rich food that contains all vitamins except vitamin C (ascorbic acid). The absence of vitamin C in the egg may result from the fact that birds are capable of satisfying their own vitamin C requirements, by de novo synthesis from glucose 48). The ability to produce vitamin C has been lost during the process of evolution in several animal species including guinea pigs, monkeys, flying mammals, humans, and some evolved passerine birds 49). Consequently, these latter species, but not domestic birds, are dependent on dietary sources of vitamin C (fruits and vegetables). The egg yolk contains high amount of vitamin A, D, E, K, B1, B2, B5, B6, B9, and B12, while egg white possesses high amounts of vitamins B2, B3, and B5 but also significant amounts of vitamins B1, B6, B8, B9, and B12 (Table 2). Eating two eggs per day covers 10% to 30% of the vitamin requirements for humans. It is noteworthy that the content of liposoluble vitamins (vitamins A, D, E, K) in egg yolk is highly dependent on the hen’s diet. In addition to these vitamins, eggs represent a major source of choline, which is essentially concentrated in the yolk (680 mg/100 g in the egg yolk versus 1 mg/100 g in the egg white) 50). It has been reported that hard-boiled egg represents the second major source of choline after beef liver 51) and the first source of choline in the US diet 52). In foods, choline is found as both water-soluble (free choline, phosphocholine, and glycerophosphocholine) and lipid-soluble forms (phosphatidylcholine and sphingomyelin) and has important and diverse functions in both cellular maintenance and growth across all life stages. It plays some roles in neurotransmission, brain development, and bone integrity 53).

Table 2. Egg vitamins (average content; µg/100g).

NameEgg, Whole, RawEgg Yolk, RawEgg White, Raw
Vitamin A or Retinol1603710
Vitamin A precursor or Beta-carotene0880
Vitamin D or Cholecalciferol2.05.40
Vitamin E or Alpha-tocopherol105025800
Vitamin K or Phylloquinone0.30.70
Vitamin C000
Vitamin B1 or Thiamin401764
Vitamin B2 or Riboflavin457528439
Vitamin B3 or Niacin7524105
Vitamin B5 or Pantothenic acid15332990190
Vitamin B61703505
Vitamin B8 or Biotin16.5–53.827.2–49.45.7–7.9
Vitamin B9 or Folate471464
Vitamin B12 or Colabamin0.891.950.09
[Source 54) ]

Minerals and Trace Elements

Egg is rich in phosphorus, calcium, potassium, and contains moderate amounts of sodium (142 mg per 100 g of whole egg) (Table 3). It also contains all essential trace elements including copper, iron, magnesium, manganese, selenium, and zinc (Table 3), with egg yolk being the major contributor to iron and zinc supply. The presence of such minerals and micronutrients in egg is quite interesting as a deficiency in some of these (Zn, Mg, and Se) has been associated with depression and fatigue 55) and development of pathological diseases. The concentration of some of those trace elements (selenium, iodine) may be significantly increased depending on hen’s diet.

Table 3. Egg minerals and trace elements (average content; mg/100g)

NameEgg, Whole, RawEgg Yolk, RawEgg White, Raw
Calcium561297
Copper0.0720.0770.023
Iodine0.0210.180.002
Iron1.752.730.08
Magnesium12511
Manganese0.0280.0550.011
Phosphorus19839015
Potassium138109163
Selenium0.0300.0560.020
Sodium14248166
Zinc1.292.300.03
[Source 56) ]

Antinutritional Factors

As mentioned above, major proteins of egg include protease inhibitors that may delay the proper degradation of egg proteins by inhibiting digestive enzymes including pepsin, trypsin, and chymotrypsin. Egg white is a major source of ovostatin, ovomucoid, ovoinhibitor, and cystatin 57). Moreover, some of these molecules (ovoinhibitor, ovomucoid, cystatin) possess many disulfide bonds that are likely to confer moderate resistance to denaturation by proteases and gastric juices. Some of these antinutritional factors may be partly denatured by heat 58) during the process of cooking, thus facilitating protein access to digestive proteases. Moreover, some vitamin-binding proteins highly concentrated in egg may also limit some vitamin access—avidin that binds vitamin B12 (biotin) exhibits the highest known affinity in nature between a ligand and a protein 59). The bioavailability of biotin for consumers may be compromised by the tight complex formed between avidin and its bound vitamin B8.

Egg Nutraceuticals

There is increasing evidence that egg is not solely a basic food of high nutritional value but that it also contains many bioactive compounds (lipids, vitamins, proteins, and derived hydrolytic peptides) 60) of major interest for human health. In vitro analyses performed on purified proteins have revealed a great potential in egg proteins as they exhibit a diversity of biological activities. Various tools combining physicochemical, analytical, and in silico approaches 61) can be used to identify hydrolytic peptides with potential bioactivities. It is remarkable that a lot of egg proteins have no identified physiological function described yet, besides providing essential amino-acids for the embryo but also for egg-eating species including humans. In addition to egg proteins displaying a wide spectrum of antimicrobial activities that could contribute to intestine health, many efforts have been made in the last decades to further characterize biological activities of egg-derived hydrolytic peptides that may naturally occur during the digestive process 62). Interestingly, some of these bioactive peptides are specifically generated after limited proteolysis of denatured egg proteins 63), after boiling. Most of these studies were performed in vitro, but this finding opens many fields of research. To date, little is known on how egg proteins resist acidic pH of the stomach, digestive proteases, and intestinal microbiote, and how the presence of egg protease inhibitors in the diet can interfere with the degradation of egg proteins by digestive proteases. The kinetics of protein digestion is sequential, starting with the hydrolysis of proteins into peptides until complete degradation into dipeptides and, finally, free amino acids. But it is known that some egg proteins (ovalbumin, ovomucoid) are only partly digested 64) suggesting that some bioactive peptides may be generated naturally without undergoing complete degradation into amino acids.

Antimicrobials

Egg antimicrobials in edible parts are essentially concentrated in egg white and the vitelline membrane. Depending on the protein considered, these antimicrobials may exhibit antibacterial, antiviral, antifungal, or antiparasitic activities (Table 4).

Their antibacterial effect relies on several bactericidal or bacteriostatic mechanisms. Some of them have a powerful activity via interaction with bacterial walls that further triggers permeabilization and bacterial death (lysozyme, avian beta-defensins, etc.). The effects of the other molecules are rather indirect by decreasing the bioavailability of iron (ovotransferrin) and vitamins (avidin) that are required for some microbial growth, and by inhibiting microbial proteases that are virulent factors of infection (ovoinhibitor, cystatin) 65). The various egg antimicrobial molecules that have been described so far in literature are listed in Table 4. Interestingly, some of them (AvBD11, OVAX, avidin, beta-microseminoprotein) are not expressed in the human genome 66), suggesting that they may constitute powerful anti-infectious agents against human enteric pathogens, to reinforce intestinal host immunity.

In addition to these egg proteins and peptides, there are increasing data reporting the antimicrobial activity of egg-derived peptides that may be released after partial hydrolysis by exogenous proteases. Such hydrolytic peptides obtained from lysozyme 67), from ovotransferrin 68), from ovomucin 69), and from cystatin 70) have shown a broad range of antibacterial activities.

Table 4. Major egg antimicrobial proteins

Gene ID/Gene SymbolTarget OrganismsLocalization 1
Avian beta-defensin 11414876/AVBD11BacteriaEW, VM
Avidin396260/AVDBacteriaEW, VM, EY
Beta-microseminoprotein-like101750704BacteriaEW
Cystatin396497/CST3Bacteria, viruses, fungi, parasitesEW, VM, EY
Gallin422030/OvoDA1BacteriaEW
Immunoglobulin YEY
Lysozyme396218/LYZBacteria, viruses, fungiEW, VM, EY
Ovalbumin-related protein X420898/OVALXBacteriaEW, VM, EY
OvoglobulinG2/TENP395882/BPIFB2BacteriaEW, VM, EY
Ovoinhibitor416235/SPIK5BacteriaEW, EY
Ovomucin (alpha and beta subunits)395381/LOC395381 (alpha)
414878/MUC6 (beta)
Bacteria, virusesEW, VM
Ovotransferrin396241/TFBacteria, virusesEW, VM, EY
Phosvitin424547/VTG1BacteriaEY
Pleiotrophin418125/PTNBacteriaEW
Vitelline membrane outer layer protein 1418974/VMO1BacteriaEW, VM, EY

Footnote: 1) Edible parts

Abbreviations: EW = egg white; EY = egg yolk; VM = vitelline membrane.

[Source 71) ]

Antioxidant Activities

Long-term oxidative stress in the gastrointestinal tract can lead to chronic intestinal disorders and there is increasing interest in investigating the potential of food-derived antioxidants, including egg antioxidants, in intestinal health. Chicken egg contains many antioxidant compounds that encompass vitamins, carotenoids, minerals, and trace elements but also major egg-white proteins 72) such as ovotransferrin, in its native form or as hydrolytic peptides 73), ovomucoid and ovomucoid hydrolysates 74), ovomucin hydrolysates and derived peptides 75), and egg yolk-proteins including phosvitin 76). Most of these molecules have been generated in vitro but some assays performed in a porcine model have revealed the beneficial effect of proteins derived from egg yolk in reducing the production of pro-inflammatory cytokines 77). The authors concluded that supplementation of the diet with egg yolk-proteins may be a novel strategy to reduce intestinal oxidative stress 78).

Anti-Cancerous Molecules

There are only few data showing that food-derived proteins and peptides can also be beneficial to prevent and to cure cancer diseases 79). Several studies have confirmed the tumor-inhibitory activity of egg white lysozyme using experimental tumors. Its effect essentially relies on immunopotentiation 80). Ovomucin (beta subunit) and ovomucin-derived peptides also showed anti-tumor activities via cytotoxic effects and activation of the immune system 81). The anticancerous effect of egg tripeptides 82) and hydrolytic peptides from ovotransferrin 83) have also been published. Information in this field is quite scarce, but it may be worth continuing to investigate such activities. Some interesting data may arise from studies on egg protease inhibitors 84) since similar molecules existing in other food product, including legumes like pea, have been described as potential colorectal chemopreventive agents 85).

Immunomodulatory Activities

Several egg proteins have potential immunomodulatory activities. Among these, egg-white lysozyme is a promising agent for the treatment of inflammatory bowel disease. In a colitis porcine model, lysozyme was demonstrated to significantly protect animals from colitis and reduce the local expression of pro-inflammatory cytokines while increasing the expression of the anti-inflammatory mediators 86). Sulfated glycopeptides generated by proteolysis from ovomucin, chalazae, and yolk membrane can exhibit macrophage-stimulating activities in vitro 87). Cytokines, such as egg-white pleiotrophin, play a pivotal role in the generation and resolution of inflammatory responses. In human, pleiotrophin have been shown to promote lymphocyte survival, and to drive immune cell chemotaxis 88). But, the biological significance of the potential immunomodulatory activity of egg white pleiotrophin in human intestine remains very speculative. In contrast, some valuable immunomodulatory activities might emerge from ovotransferrin and egg yolk vitellogenin hydrolysates 89) after partial degradation by digestive proteases.

Antihypertensive Activities

Considering the prevalence and importance of hypertension worldwide (over 1.2 billion individuals) 90), there is increasing ongoing research to find ways to regulate this multifactorial disease. At the population level, the most important factors of long-term control of blood pressure are sodium and potassium intakes and the importance of the renin-angiotensin-aldosterone system. Most egg-derived peptides with anti-hypertensive activities exhibit inhibitory activities against the angiotensin-converting enzyme (ACE). This enzyme triggers the processing and activation of angiotensin I into the active vasoconstrictor angiotensin II. Several yolk-derived peptides bearing antihypertensive activities have been described in the literature 91) along with ovotransferrin and egg white hydrolysates [125,126]. Some of these peptides contain only three amino-acids 92). Some of these tripeptides were demonstrated to be active in vivo—the oral administration of these peptides that have been administrated orally in hypertensive rats contributed to significantly reduce blood pressure 93) and thus, may help in diminishing the occurrence of cardiovascular diseases 94).

Egg Storage and Heat Treatment

Shell eggs are stored at room temperature or preferably in the fridge prior to be used by consumers (eggs are considered as “fresh” up to 28 days after laying). The conditions of egg storage can induce deep internal changes including physicochemical modifications that may increase some technological properties that are useful for the food industry, and alteration of antibacterial properties of the egg white 95). These alterations result from water exchange between yolk and the egg white and from water and carbon dioxide loss through the eggshell pores, which elicited an increase in the air cell that develops between the two eggshell membranes. Albumen height decreases with time of storage while albumen pH and whipping volume increase 96). In parallel, the strength of vitelline membrane decreases upon egg storage due to loosening thereby impacting yolk shape/index (the yolk becomes flat and its diameter is higher) 97). These latter modifications favor egg white/egg yolk exchanges of components such as carbohydrates and glucose 98), proteins 99), vitamins, and trace elements 100). In addition, storage duration and conditions are associated with protein degradation 101) and a decrease in its antibacterial potential 102). However, except for proteins, there is only little information available that describes the changes/denaturation in lipids, vitamins, and minerals composing both egg white and egg yolk during storage. It will be interesting to further investigate how these modifications impact the respective functional, nutritional, and technological properties of egg yolk and egg white (foaming, emulsifying properties, etc.). Recent data has demonstrated that the antioxidant activity of egg yolk was globally unchanged during six weeks of retail storage 103). All these alterations of freshness criteria are accelerated at room temperature when compared to refrigerated conditions.

In addition to storage, one would expect that egg nutrients may also be modified during cooking. No clear evidence of minerals or vitamins denaturation could be observed when comparing fresh, soft-boiled, and hard-boiled egg (Table 5).

Some data appear contradictory from one reference source to another (CIQUAL versus USDA, Table 5). Anyhow, it seems that the amount of polyunsaturated fatty acids, selenium, and vitamin A 104) tends to decrease upon cooking, especially in hard-boiled eggs (Table 5).

Noticeably, proteins undergo major conformational modifications upon cooking, even though their relative amount is not impacted by cooking (Table 5). This protein denaturation may be beneficial to inactivate antinutritional factors such as egg-white antiproteases but also to denature highly resistant proteins, thereby facilitating protease activity in the digestive tract. A higher digestibility of egg proteins may also contribute to limiting hypersensitivity to eggs in children 105). Meanwhile, it has been shown that cooking significantly reduces the oxygen radical scavenging capacity (antioxidant potential) of egg yolk associated with free aromatic amino acids, lutein, and zeaxanthin 106), and also impacts lipids of yolk 107). These observations corroborate that taking into account the food matrix and the way eggs are prepared is of major importance to appreciate egg digestibility and its associated nutritional and nutraceutical quality 108). To conclude, it is fairly difficult to assess the beneficial/risk balance of cooking egg for human health as many molecules may be affected by cooking, while, in parallel, the heating process may increase the digestibility of egg proteins and potentially reveal potential new bioactive peptides 109); but, it is worth mentioning that cooking eggs also allows for the elimination potential pathogens responsible for toxi-infections in consumers. In conclusion, taking into account all these data, the advice to retain most nutritional and nutraceutical benefits associated with egg would be to foster the consumption of poached or soft-boiled eggs, where the egg white is cooked (to inactivate antinutritional factors and potential pathogenic bacteria) while the egg yolk remains essentially raw (to preserve most vitamins, lipids, micronutrients, and some bioactive (antioxydant) molecules).

Table 5. List of egg characteristics and major components that vary upon cooking

NameEgg, Whole, Raw, FreshEgg, Whole, Soft-BoiledEgg, Whole, Hard-Boiled
Energy (kcal/100g)140; 143142; 143134; 155
Protein (g/100g)12.7; 12.5612.2; 12.5113.5; 12.58
Carbohydrate (g/100g)0.27; 0.721.08; 0.710.52; 1.12
Fat (g/100g)9.83; 9.519.82; 9.478.62; 10.61
FA saturated (g/100g)2.64; 3.1263.11; 3.112.55; 3.267
FA monounsaturated (g/100g)3.66; 3.6584.42; 3.6433.57; 4.077
FA polyunsaturated (g/100g)1.65; 1.9111.28; 1.9041.03; 1.414
Cholesterol (mg/100g)398; 372222; 370355; 373
Salt (g/100g)0.310.20.31
Calcium (mg/100g)76.8; 56150; 5641; 50
Potassium (mg/100g)134; 138164; 138120; 126
Selenium (µg/100g)3023.87.01
Vitamin A, Retinol (µg/100g)182; 160132; 16061.5; 149
Vitamin D (µg/100g)1.88; 2.01.28; 2.01.12; 2.2
Vitamin E (mg/100g)1.43; 1.052.17; 1.041.03; 1.03
Choline (mg/100g)250; 293.8230; 293.8
[Source 110) ]

Effect of egg intake on blood cholesterol and cardiovascular diseases

The effects of egg intake on blood cholesterol and cardiovascular diseases have been discussed in several meta-analysis studies using research data collected over 60 years 111), 112). Most experimental, clinical, and epidemiologic studies concluded that there was no evidence of a correlation between dietary cholesterol by eggs consumption and an increase in plasma total-cholesterol in healthy people 113). Large epidemiological works have been conducted to investigate the effect of egg intake on blood cholesterol levels and risk of cardiovascular diseases in children 114), young people 115), women 116), men 117), and older adults 118). Some have shown that egg consumption did influence the blood cholesterol level but did not increase the risk of cardiovascular diseases in healthy people 119). Meanwhile, other studies reported that high dietary cholesterol intake due to egg consumption is a risk factor for cardiovascular diseases and diabetes 120), 121), 122). The results of epidemiological studies and human intervention studies on the relationship of dietary egg intake and cardiovascular diseases risks are summarized in Table 6. Even though American Heart Association 123) and 2015-2020 US Dietary Guidelines Advisory Committee 124) have removed the restriction of dietary cholesterol for healthy people in USA, there still are different conclusions due to differences in race, genetic makeup, physical fitness, and especially physiological status 125).

Among the 19 prospective studies investigating the effect of dietary egg intake on cardiovascular diseases risks, 6 studies reported positive correlation between egg consumption and different types of cardiovascular diseases incidents or mortality in healthy people 126). Pang (2017) reported the positive correlation with total cholesterol 127), and Spence (2012) reported the positive correlation with plaque area 128). However, other studies (11 out of 19) reported no difference on the cardiovascular diseases risks affected by the amount of egg intake 129). The adverse effect of egg consumption is observed in population with high risk of cardiovascular diseases, including people with diabetes or hypercholesterolemia, and who are sensitive to dietary cholesterol 130). Diabetic populations are in the high risk of cardiovascular diseases with two to four folds higher than healthy people. These studies also showed that diabetic people are more vulnerable to cardiovascular diseases after egg consumption 131), with a doubling of coronary risk with an egg per day in US population 132), and 5-fold risk in Greece population 133). Meanwhile, some studies found that high egg consumption increased the risk of gestational diabetes mellitus 134), insulin resistance 135), and the risk of diabetes 136). Therefore, the effect of egg consumption on cardiovascular diseases might be mediated by diabetes.

Almost all human intervention studies showed the serum LDL “bad” cholesterol and HDL “good” cholesterol levels increased in high egg consuming groups (1 to 3 eggs per day comparing to no egg or with egg substitute), while the ratio of serum LDL “bad” cholesterol to HDL “good” cholesterol (LDL/HDL) is unchanged (see Table 6). Most of these papers concluded that egg consumption is not a risk factor for cardiovascular diseases, based on the fact that the LDL/HDL ratio is unchanged because this ratio is thought to be a stronger risk factor for cardiovascular diseases. However, serum LDL “bad” cholesterol level alone should still be considered as a risk factor for cardiovascular diseases. This is especially true for those people whose blood cholesterol level is more sensitive to dietary cholesterol consumption. There are good reasons for the recommendation that persons at risk of vascular disease limit cholesterol to 200 mg/day 137). The very high cholesterol content of egg yolk (237 mg in a 65-gram egg) is a problem in itself, and even one large egg yolk exceeds that limit. Other studies reported the high cholesterol and high lipid diet could induce the inflammation in plasma, which is thought to contribute to atherosclerosis 138), and the susceptibility of LDL “bad” cholesterol to be oxidized could be increased by dietary cholesterol 139).

Table 6. Epidemiological and human intervention studies on the effect of dietary egg intake and cardiovascular diseases risks

(a) Prospective studies

ReferenceParticipantsAgeFollow-up (years)OutcomeResult b
MaleFemale
Bernstein 2011 140)43,15084,01030-7526Incident stroke(-)
Burke 2007 141)25625815-8814CHD, mortality
Dawber 1982 142)91230-5924Incident CHD and blood cholesterol level(-)
Djoussé 2008 143)21,327040-8520Incident MI and stroke(-)
Mortality
Goldberg 2014 144)57285757-7511Incident stroke(-)
Carotid atherosclerosis
Haring 2014 145)12,06645-6422Incident CHD(-)
Houston 2011 146)864107770-799Incident CVD↑ especially in diabetic people
Hu 1999 147)37,85180,08234-7514Incident stroke and CHD(-) while in diabetic people may have ↑ effect
Mann 1997 148)4,1026,70016-7913.3Ischemic heart disease mortality
Nakamura 2004 149)5,1864,07730-7014Stroke and CHD mortality↑ in women
Nakamura 2006 150)43,31947,41640-6910.2Incident CHD(-)
Qureshi 2006 151)3,7565,97825-7415.9All stroke, CAD(-) while in diabetic people may have ↑ effect
Sauvaget 2003 152)15,35024,99934-10316Stroke mortality(-)
Scrafford 2011 153)14,946>178.8CHD and Stroke mortality(-)
Zazpe 2011 154)6,1708,01520-905.8Incident CVD(-)
Voutilainen 2013 155)1,019051.9 (Mean)18.8Carotid atherosclerosis, incident MI(-)
Pang 2017 156)8,1318,463>60N/ASerum LDL and total cholesterol
Spence 2012 157)66959346-77N/ACarotid plaque area
Trichopoulou 2006 158)42458950-80 (Adult diabetics)4.5 (mean)Mortality

(b) Human intervention

ReferenceParticipantsAgeIntervention time (weeks)Intervention methodOutcomeResult a
MaleFemale
Missimer 2017 159)242618-3011c2 eggs/day vs. oatmealSerum LDL and HDL
Serum LDL/HDL(-)
Serum ghrelin↑ satiety

Lemos 2018 160)161418-20133 eggs/day vs. choline bitartrate supplementSerum LDL and HDL
Serum LDL/HDL(-)
SREBPs and HMG-CoA reductase level↓ cholesterol biosynthesis

Herron 2002 161)05118-49 (pre-menopausal)11c1 egg/day vs.
0 egg/day
Serum LDL and HDL
Serum LDL/HDL(-)
CETP level↑ reverse cholesterol transport

Herron 2003 162)40018-5711c1 egg/day vs.
0 egg/day
Serum LDL and HDL
Serum LDL/HDL↑ only in hyper-respondersd
CETP, LCAT level↑ reverse cholesterol transport

Mutungi 2008 163)28040-70 (overweight/obese)12CRD:
3 eggs/day vs. SUB
Serum LDL/HDL(-)
Serum HDL

Greene 2005 164)1329>6011c3 eggs/day vs. SUBSerum LDL and HDL
Serum LDL/HDL(-)

Ballesteros 2004 165)25298-1211c2 eggs/day vs. SUBSerum LDL/HDL(-)

Knopp 2003 166)7811943-6740, 2 and 4 eggs/daySerum LDL and HDL

Knopp 1997 167)864541-68 (HC or HL)122 eggs/day vs. SUBSerum LDL↑ in HC
Serum HDL↑ in both HL and HC

Abbreviations: CHD = coronary heart disease; CVD = cardiovascular disease; MI = myocardial infarction; LDL = low-density lipoprotein; HDL = high-density lipoprotein; SREBP = sterol regulatory element-binding protein; CETP = cholesteryl ester transfer protein; LCAT = lecithin-cholesterol acyltransferase; CRD = Carbohydrate-restricted diets; SUB = cholesterol-free, fat-free egg substitute; HC = hypercholesterolemia; HL = hyperlipidemia;

Footnote: b) ↑ increase, ↓ decrease, (-) no influence; c) Intervention time contain a 3-weeks washout time within the intervention period; dhyperresponders: increase in total cholesterol of ≥0.06 mmol/L for each additional 100 mg of dietary cholesterol consumed.

[Source 168) ]

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