EGG

In 1968, the American Heart Association (AHA) recommended eating less than 3 eggs a week. It argued that increased cholesterol intake increases blood cholesterol levels and also risk of cardiovascular disease [1].

Half a century has passed, and AHA currently recommends to eat 1 egg (or 2 egg whites) a day as part of a healthy diet [2].

Let’s summarize what we actually know about eggs, except that they contain cholesterol and they are made up of egg white, yolk, and egg shell (in fact, even more parts contribute to their composition, but we will not bother with that unless it will not absolutely necessary).

Eggs are classified as protein foods with an average content of 12 g protein / 100 g. So far, 550 different proteins have been identified in eggs. Proteins are not only in egg white but also in egg yolk or vitelline membrane. This membrane separates the yolk from the egg white, preventing them from being mixed. Egg proteins are complete, it means that they contain all the essential amino acids in optimal amounts [3].

Eat poached eggs, soft-boiled eggs, eggs Sunny Side Up, and eggs over easy, where the egg yolk remains almost raw, and egg white is cooked. And another recommendation is: eat whole eggs rather than egg whites alone.

Egg white only cooked, why?

Egg components are highly digestible, although a small amount of protein contained in egg white is not absorbable, especially when egg is consumed as raw.

  • However, heat treatment can increase the digestibility of proteins through the process called denaturation, thereby facilitating their accessibility to digestive enzymes.
  • Heat treatment also inactivates antinutritional factors in egg white (protease inhibitors = substances that delay the proper degradation of egg proteins by inhibiting digestive enzymes), thereby facilitating digestion and increasing the usefulness of the proteins contained [4, 5, 6].

Since some proteins and protease inhibitors (such as ovomucoid) are resistant to heating [5, 6], the digestibility of proteins, even in cooked egg white, is not 100%, but partial (91-94%) [7, 8].

  • Heat treatment of the egg white is important not only for increasing the digestibility of the proteins contained, but also for eliminating the potential risk of infection by various pathogens that may be present in the egg white, such as Salmonella enterica Enteritidis.

Salmonella survives mostly on the surface of the eggshell. It penetrates the hen’s eggs through a cracked shell or if the eggs are washed with water, which breaks the oily coating on the surface of the egg to protect it.  Salmonella occurs in the egg white, not in the yolk [9, 10].

There are also various antimicrobial substances in the egg white and vitelline membrane, such as antibacterial lysozyme, ovotransferrin which indirectly reduces the bioavailability of iron, and avidin which reduces the biavailability of vitamin B12. These substances thus prevent the growth of some microorganisms that need these nutrients [11].

  • Intake of great amounts of raw egg white, including avidin, can cause a deficiency of biotin (vitamin H) in the human body. Biotin deficiency is manifested by a red rash, most commonly around the mouth, and a red itchy skin. However, avidin is partially denatured and its binding to biotin is thus reduced in cooked eggs. The incidence of this condition is rare in humans [12].

People suffering from egg allergy may have a problem with egg white because major allergens are mainly found here, including ovomucoid (major egg allergen), ovalbumin, ovotransferrin and lysozyme. This allergy is the second most common allergy in children (with a prevalence estimated to by between 1.8 to 2% in children younger than five years). It occurs most frequently during the first 5 years of life, but in most cases the prevalence of egg allergy decrease with age, and usually, it resolves by school age [13, 14, 15, 16, 17, 18, 19].

Eat raw egg yolk, why?

Egg yolk is a true treasure of nutrients in the egg, not only in eggs, but overall in nutrition. Therefore, it is important to cook it carefully, not to throw it in a hot pan, so that we lose all its wealth in seconds.

The yellowish-orange color of the yolk can tell us about its nutritional wealth. The color saturation depends on the amount of caroteinoids and fat-soluble vitamins [3].

  • The high temperature decreases the antioxidant potential of the yolk by reducing the amount of caroteinoids, especially xanthophylls, and other substances.
    • The egg yolk contains lutein, zeaxanthin and xanthophylls. These substances have been shown to reduce the risk of developing macular degeneration. It is a visual disorder that is the most common cause of blindness in people over the age of 50 [20].
  • At high temperatures, the amounts of polyunsaturated fatty acids, selenium and vitamin A tend to decrease (this has been shown for hard-boiled eggs) [5].
  • The high temperature leads to the oxidation of the contained fats.
    • Oxidation or rancidity of fats occurs when fats (especially those rich in unsaturated fatty acids) are exposed to heat, moisture or oxygen. Rancid fats contain free radicals that have negative effects on human health.

Diet of laying hens plays an important role in the fatty acids profile. In general, the ratio of unsaturated fatty acids to saturated is particularly high in yolk compared to other foods of animal origin [21, 22].

Egg white definitely cannot compete with egg yolk due to micronutrient content. Egg yolk is the main source of iron and zinc in the egg. The amount of selenium and iodine in the yolk may be significantly increased by laying hen’s diet (at lower magnitudes iron, zinc, fluoride, or magnesium can also be increased by dietary supply for hens) [23]. The average selenium content (around 5 μg / egg) can be increased 3- to 6- fold and reach 30-40 μg / egg when hens are supplied with 0.3 to 0.5 mg selenium / kg of the total diet (via selenomethionine or selenium-enriched) yeast). Such egg enrichment can cover up to 50-70% of daily human requirements [24]. Similarly, hen nutrition is a way to enrich egg in lipophilic vitamins (A, D3, E, K2) or water-soluble vitamins (B complex)  [3].

The egg contains all vitamins except vitamin C. Maybe you ask, don’t birds need vitamin C? Actually, they can synthesize it from glucose, which is contained mainly in egg white (0.34 g / kg).Guinea pigs, monkeys and some species of another animals, as well as humans, have been lost the ability to synthesize vitamin C during evolution [25], which make us dependent on its dietary intake.

Eggs, especially egg yolks (680 mg / 100 g versus 1 mg / 100 g in egg white) are the second best source of choline after bovine liver. Higher intake of choline in humans is associated with better cognitive function and memory. Choline also plays a very important role in brain development and should not be neglected especially in the diet of pregnant and breastfeeding women [3, 26].

There is no doubt that the composition of free range eggs differs from caged eggs, but we will talk about it again another time 😉

Eat whole eggs, not only egg white, why?

Studies show that eating whole egg, not only egg whites, activates the rapamycin complex (called mTOR), a signaling cascade that maintains skeletal muscle mass and induces muscle protein synthesis [27]. 

SUMMARY

In conclusion, if we want to preserve as many nutrients as possible in the egg and increase its digestibility, the best cooking methods for eggs seems to be:

  • poached eggs
  • Sunny Side Up eggs
  • eggs over easy
  • soft-boiled eggs…

…when the egg white is heat-treated completely (inactivation of antinutritional substances, potential pathogens and protein denaturation leading to better digestibility), while the yolk remains almost raw, thereby most of the vitamins, fats, micronutrients and bioactive ingredients are retained.

SOURCES

[1] Committee on Nutrition; American Heart Association. Diet and Heart Disease; American Heart Association: Dallas, TX, USA, 1968.

[2] Gallagher, D., Heymsfield, S. B., Heo, M., Jebb, S. A., Murgatroyd, P. R., & Sakamoto, Y. (2000). Major dietary patterns are related to plasma concentrations ofmarkers of inflammation and endothelial dysfunction. The American Journal of Clinical Nutrition, 72(3), 694–701. https://doi.org/10.1093/AJCN

[3] Réhault-Godbert, S., Guyot, N., & Nys, Y. (2019). The golden egg: Nutritional value, bioactivities, and emerging benefits for human health. Nutrients, 11(3), 1–26. https://doi.org/10.3390/nu11030684

[4] Evenepoel, P.; Claus, D.; Geypens, B.; Hiele, M.; Geboes, K.; Rutgeerts, P.; Ghoos, Y. Amount and fate of egg protein escaping assimilation in the small intestine of humans. Am. J. Physiol. 1999, 277, G935–G943.

[5] Ramalho, H.M.; Santos, V.V.; Medeiros, V.P.; Silva, K.H.; Dimenstein, R. Effect of thermal processing on retinol levels of free-range and caged hen eggs. Int. J. Food Sci. Nutr. 2006, 57, 244–248.

[6] Evenepoel, P.; Geypens, B.; Luypaerts, A.; Hiele, M.; Ghoos, Y.; Rutgeerts, P. Digestibility of cooked and raw egg protein in humans as assessed by stable isotope techniques. J. Nutr. 1998, 128, 1716–1722.

[7] Stanciuc, N.; Cretu, A.A.; Banu, I.; Aprodu, I. Advances on the impact of thermal processing on structure and antigenicity of chicken ovomucoid. J. Sci. Food Agric. 2018, 98, 3119–3128. 

[8] Van der Plancken, I.; Van Remoortere, M.; Van Loey, A.; Hendrickx, M.E. Trypsin inhibition activity of heat-denatured ovomucoid: A kinetic study. Biotechnol. Prog. 2004, 20, 82–86.

[9] De Vylder, J.; Raspoet, R.; Dewulf, J.; Haesebrouck, F.; Ducatelle, R.; Van Immerseel, F. Salmonella Enteritidis is superior in egg white survival compared with other Salmonella serotypes. Poult. Sci. 2013, 92, 842–845.

[10] Lublin, A.; Sela, S. The Impact of Temperature During the Storage of Table Eggs on the Viability of Salmonella enterica Serovars Enteritidis and Virchow in the Eggs. Poult. Sci. 2008, 87, 2208–2214. 

[11] Rehault-Godbert, S.; Herve-Grepinet, V.; Gautron, J.; Cabau, C.; Nys, Y.; Hincke, M. Molecules involved in chemical defence of the chicken egg. In Improving the Safety and Quality of Eggs and Egg Products, Vol 1: Egg Chemistry, Production and Consumption; Nys, Y., Bain, M., Van Immerseel, F., Eds.;Woodhead Publ Ltd.: Cambridge, UK, 2011; Volume 1, pp. 183–208.

[12] Enders, G. (2015). Střevo není tabu – O trávicím traktu vesele i vážně – Giulia Endersová | Knihy Dobrovský. Ikar.

[13] Mine, Y.; Yang, M. Recent advances in the understanding of egg allergens: Basic, industrial, and clinical perspectives. J. Agric. Food Chem. 2008, 56, 4874–4900.

[14] Venter, C.; Pereira, B.; Voigt, K.; Grundy, J.; Clayton, C.B.; Higgins, B.; Arshad, S.H.; Dean, T. Prevalence and cumulative incidence of food hypersensitivity in the first 3 years of life. Allergy 2008, 63, 354–359. 

[15] Osterballe, M.; Hansen, T.K.; Mortz, C.G.; Host, A.; Bindslev-Jensen, C. The prevalence of food hypersensitivity in an unselected population of children and adults. Pediatric Allergy Immunol. Off. Publ. Eur. Soc. Pediatric Allergy Immunol. 2005, 16, 567–573. 

[16] Peters, R.L.; Koplin, J.J.; Gurrin, L.C.; Dharmage, S.C.;Wake, M.; Ponsonby, A.L.; Tang, M.L.K.; Lowe, A.J.; Matheson, M.; Dwyer, T.; et al. The prevalence of food allergy and other allergic diseases in early childhood in a population-based study: HealthNuts age 4-year follow-up. J. Allergy Clin. Immunol. 2017, 140, 145–153.e148.

[17] Kovacs-Nolan, J.; Phillips, M.; Mine, Y. Advances in the value of eggs and egg components for human health. J. Agric. Food Chem. 2005, 53, 8421–8431. 

[18] Anton, M.; Nau, F.; Guerin-Dubiard, C. Bioactive fractions of eggs for human and animal health. In Improving the Safety and Quality of Eggs and Egg Products, Vol 2: Egg Safety and Nutritional Quality; VanImmerseel, F., Nys, Y., Bain, M., Eds.;Woodhead Publ Ltd.: Cambridge, UK, 2011; pp. 321–345.

[19] Windhorst, H.-W.; Grabkowsky, B.; Wilke, A. Atlas of the Global Egg Industry; International Egg Commission: London, UK, 2015.

[20] Ranard, K.M.; Jeon, S.; Mohn, E.S.; Griffiths, J.C.; Johnson, E.J.; Erdman, J.W., Jr. Dietary guidance for lutein: Consideration for intake recommendations is scientifically supported. Eur. J. Nutr. 2017, 56, 37–42.

[21] Lopez Sobaler, A.M.; Aparicio Vizuete, A.; Ortega, R.M. Role of the egg in the diet of athletes and physically active people. Nutr. Hosp. 2017, 34, 31–35.

[22] Kim, J.E.; Campbell,W.W. Dietary Cholesterol Contained in Whole Eggs Is NotWell Absorbed and Does Not Acutely Affect Plasma Total Cholesterol Concentration in Men and Women: Results from 2 Randomized Controlled Crossover Studies. Nutrients 2018, 10, 1272.

[23] Schiavone, A.; Barroeta, A.C. Egg enrichment with vitamins and trace minerals. In Improving the Safety and Quality of Eggs and Egg Products; Van Immerseel, F., Nys, Y., Bain, M., Eds.;Woodhead Publishing Limited: Cambridge, UK, 2011; Volume 2, pp. 289–320.

[24] Nys, Y.; Schlegel, P.; Durosoy, S.; Jondreville, C.; Narcy, A. Adapting trace mineral nutrition of birds for optimising the environment and poultry product quality. Worlds Poult. Sci. J. 2018, 74, 225–238. 

[25] Chatterjee, I.B. Evolution and the biosynthesis of ascorbic acid. Science 1973, 182, 1271–1272.

[26] Fallon, S. (2017). Nourishing fats : why we need animal fats for health and happiness. Grand Central Life & Style. 2017. 272 p.

[27] Abou Sawan, S., van Vliet, S., West, D. W. D., Beals, J. W., Paluska, S. A., Burd, N. A., & Moore, D. R. (2018). Whole egg, but not egg white, ingestion induces mTOR colocalization with the lysosome after resistance exercise. American Journal of Physiology – Cell Physiology, 315(4), C537–C543. https://doi.org/10.1152/ajpcell.00225.2018

 

 

 

 

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