Dear Professor Cordain,
I had some questions about the lysozyme section. Lysozyme is an enzyme that breaks down peptidoglycan. Humans obviously do not have peptidoglycan. It makes up the bacterial cell wall. I think someone got lost on this one?
Lysozyme has no eukaryotic “membranolytic activity.”
Dr. Cordain’s response:
Many thanks for your response to Maelan’s answer to Barbara’s question about eggs in the Paleo Diet. Let me first address my feelings about egg consumption for the general population and then I will respond to your point, which is technically valid.
In all three of my books, I have advocated egg consumption, particularly eggs that are produced with high omega 3 fatty acid contents. Chicken eggs are generally a nutritious food and are a good source of selenium, vitamin A, vitamin D and the B vitamins, some minerals and lutein as Barbara indicated. Additionally, numerous recent experimental and epidemiological studies (reviewed in references 1, 2) indicate that regular egg consumption (7 per week) does not increase the risk for coronary heart disease (CHD). As I have previously noted, although eggs are one of the most concentrated sources of dietary cholesterol (212 mg per egg), dietary cholesterol has a minimal effect upon blood cholesterol concentrations in most people.1,3 Further, high cholesterol egg diets cause an increase in blood HDL particles (the good particles that remove cholesterol from the body)1 and reduce the highly atherogenic small dense LDL particles while simultaneously increasing the less atherogenic large, “fluffy” LDL particles.4, 5
So, should everybody include eggs in their diet on a daily basis? Not necessarily, particularly if we examine the evolutionary template. Without question our pre-agricultural ancestors would have collected and consumed eggs from birds’ nests whenever possible. However, in the wild, bird eggs only appear seasonally. Hence, pre-agricultural humans could have never consumed two eggs for breakfast every morning of the year similar to some westernized people, but rather only occasionally for a few brief weeks or months.
If we follow up on the clue from the evolutionary template and examine eggs more closely, they maintain certain nutritional shortcomings that may be problematic, particularly for people suffering from autoimmune diseases and allergies. Although eggs are classified as animal food sources and are lumped together with meats in the USDA My Pyramid, eggs are uniquely different from meats in that they represent the reproductive endpoints of adult birds which exist outside their mother’s body in a semi permeable, warm compartment. As such, all eggs are particularly vulnerable to invasion, attack and destruction by microorganisms such as fungi, bacteria and viruses present in their nesting environment.
The innermost yolk of a chicken egg represents the growing embryo which is anchored to the albumen or egg white by structures called chalazae. Outward from the egg white are the inner and outer membranes and then the shell, all of which provide physical barriers to infection from pathogens and microorganisms. The egg white makes up about 58% of the total egg volume and contains about 50% of the total egg protein and is composed of 88.5% water, 10.5% protein and 0.5% carbohydrate.6 The function of the egg white is threefold: 1) storage of nutrients for the growing embryo (yolk), 2) protection of the egg from microbial attack, and 3) transport of nutrients into the growing embryo.
As I have previously mentioned, a chicken egg is the reproductive endpoint for adult birds and survives by living outside its mother’s body in a semi permeable compartment that is essentially immovable. Accordingly, it has no means of protecting itself from microorganisms or predation by physical escape or avoidance. For this reason, the evolutionary strategy eggs have taken to protect themselves from microbial invaders is to select for toxic substances in the egg white; mainly in the form of antimicrobial proteins. Except for ovalbumen which comprises 54% of the total protein in egg white, virtually all the other major proteins (~33% of the total) maintain one form or another of antimicrobial activity.
Egg white allergy in the general population varies between 1.6 – 3.2% and is the second most common cause of food allergy in children next to milk.8 For both adults and children one or more of the following symptoms may occur: hives, atopic dermatitis (red, flaky itchy skin), asthma, runny nose, diarrhea, abdominal pain, rapid swelling of the skin and mucosa, and anaphylactic shock which may be life threatening. The major allergens in egg white are ovomucoid, ovalbumen, ovotransferrin and lysozyme8 (Table 1). So for the vast majority of children and adults (98.4 – 96.8% of the population), egg white allergy is not a problem, and except for anaphylactic shock is not a debilitating or life threatening condition.
The same conclusion may not be true for people suffering from an autoimmune disease (e.g. multiple sclerosis, rheumatoid arthritis, ankylosing spondylitis, lupus erythematosus and others), since egg white consumption may contribute to the origin and progression of these diseases via a process of molecular mimicry that I have previously outlined for dietary lectins found in wheat, peanuts and other grains and legumes.11
Egg white protein is no simple protein, but rather a conglomeration of multiple proteins which have been designed by natural selection to cause toxic and lethal effects in bacteria and microorganisms reminiscent of food lectins found in grains and legumes.11 In order for any food proteins to potentially cause or promote an autoimmune disease, it must:
- Survive the human digestive processes intact
- Cross the gut barrier intact either alone or with other attached proteins
- Interact with the immune system in a manner suspected of causing an autoimmune disease.
A number of egg white proteins fulfill these necessary steps. Most problematic is the egg white protein, lysozyme which is actually an enzyme known as N-acetylhexosaminodase that is also found in many human tissues, including tears.12 The function of lysozyme in both egg whites and in human tears is to act as a potent bacterialcidal agent by binding and dissolving bacterial cell walls.12
Now let me answer Eric’s specific point. Bacterial cell walls are called the murein or peptidoglycan layer which is a gigantic polymer of (N-acetylglucosamine and N-acetylmuyramic acid) polysaccharide strands cross linked through short peptide bridges at the lactyl groups of the muramic acid residues. Lysozyme degrades bacterial cell walls by catalyzing hydrolysis of the beta (-1,4-) linkage between N-acetylglucosamine and N-acetylmuramic acid. As Eric correctly pointed out human cells do not maintain a murein (peptidoglycan) exterior lining. Consequently, dietary lysozyme from egg whites do no increase intestinal permeability by breakdown of intestinal cell membranes as suggested by Maelan, but rather lysozyme increases intestinal permeability by other means that I will explain. It is this increase in intestinal permeability or “leaky gut” that makes egg white consumption problematic for people with egg allergies or autoimmune disease.
Lysozyme is unusual among the major egg white proteins in that it has an alkaline isoelectric point, which means that it can form strong complexes with other egg white proteins including ovomucin, ovalbumen and ovotransferrin.6, 12 Hence, even though lysozyme is a benign enzyme produced in our own bodies, when we eat egg white lysozyme, it comes as a compound attached to other egg white proteins or to gut borne bacterial proteins foreign to our bodies.
In the human digestive tract, enzymes called proteases normally break down proteins into their constituent amino acids so that the amino acids can be absorbed across the intestines. Because egg white protein contains high concentrations of protease inhibitors (ovomucoid, ovoinhibitor, ovostatin, cystatin) (Table 1), the human gut proteases (trypsin and chymotrypsin primarily) are less effective in degrading egg white proteins, and lysozyme/egg white protein complexes. Additionally lysozyme is stable in the acidic gut environment12 and therefore arrives intact in the lower gastrointestinal tract.
So what’s wrong if lysozyme/egg white protein/bacterial complexes aren’t dissolved by normal digestive processes? Normally, large multifaceted proteins such as these complexes don’t have a prayer of getting across the intestinal barrier and into the bloodstream where they can interact with the immune system. Once again, lysozyme is an unusual protein because it rapidly breeches the gut barrier and enters human circulation,14 due to its unusual chemical properties (a positively charged isoelectric point).15 Because lysozyme maintains a positive charge, it results in an electrostatic attraction to the negatively charged proteoglycans (the glycocalyx) of intestinal epithelial cells which in turn allows lysozyme to be absorbed rapidly into circulation.15
Absorption of pure egg white lysozyme by itself into circulation is likely not problematic because lysozyme is an enzyme that the body naturally produces. Rather it is the complexes that lysozyme forms with other egg white proteins, gut borne bacterial peptides and food peptides that may adversely stimulate the immune system. When these chimeric (monster) molecules breech the gut barrier via their net positive charge and enter circulation, they have the capacity to promote allergy and autoimmune disease.
Just how autoimmune diseases are triggered by gut borne antigens from egg white lysozyme and other common foods (cereal grains, legumes, dairy and certain saponin containing foods) is another good story that our research group has been working on for the past decade. We hope to publish this seminal paper in 2010.
Loren Cordain, Ph.D. Professor Emeritus
- Fernandez ML. Dietary cholesterol provided by eggs and plasma lipoproteins in healthy populations. Curr Opin Clin Nutr Metab Care. 2006 Jan;9(1):8-12.
- Kritchevsky SB. A review of scientific research and recommendations regarding eggs. J Am Coll Nutr. 2004 Dec;23(6 Suppl):596S-600S.
- Howell WH, McNamara DJ, Tosca MA, Smith BT, Gaines JA. Plasma lipid and lipoprotein responses to dietary fat and cholesterol: a meta-analysis. Am J Clin Nutr. 1997 Jun;65(6):1747-64.
- Herron KL, Lofgren IE, Sharman M, Volek JS, Fernandez ML.High intake of cholesterol results in less atherogenic low-density lipoprotein particles in men and women independent of response classification. Metabolism. 2004 Jun;53(6):823-30.
- Maki KC, Van Elswyk ME, McCarthy D, Seeley MA, Veith PE, Hess SP, Ingram KA, Halvorson JJ, Calaguas EM, Davidson MH.Lipid responses in mildly hypertriglyceridemic men and women to consumption of docosahexaenoic acid-enriched eggs. Int J Vitam Nutr Res. 2003 Oct;73(5):357-68.
- Stevens L. Egg white proteins. Comp Biochem Physiol B 1991;100:1-9.
- Szxena I, Tayyab S. Protein proteinases inhibitors from avian egg whites. Cell Mol Life Sci 1997;53:13-23.
- 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.
- Wellman-Labadie O, Picman J, Hincke MT. Comparative antibacterial activity of avian egg white protein extracts. Br Poult Sci. 2008 Mar;49(2):125-32.
- Takahashi K.G., Nakamura A., Mori K. Inhibitory effects of ovoglobulins on bacillary necrosis in larvae of the pacific oyster, Crassostrea gigas. J Invert Pathol 2000;75:212-217.
- Cordain L, Toohey L, Smith MJ, Hickey MS. Modulation of immune function by dietary lectins in rheumatoid arthritis. Br J Nutr. 2000 Mar;83(3):207-17.
- Proctor VA, Cunningham FE. The chemistry of lysozyme and its use as a food preservative and a pharmaceutical. Crit Rev Food Sci Nutr 1988;26:359-395.
- Holtje JV. Lysozyme substrates. EXS 1996;75:105-10.
- Hashida S, Ishikawa E, Nakamichi N, Sekino H. Concentration of egg white lysozyme in the serum of healthy subjects after oral administration. Clin Exp Pharmacol Physiol. 2002 Jan-Feb;29(1-2):79-83.
- Nishikawa M, Hasegawa S, Yamashita F, Takakura Y, Hashida M. Electrical charge on protein regulates its absorption from the rat small intestine. Am J Physiol Gastrointest Liver Physiol. 2002 Apr;282(4):G711-9.