Designer Eggs: Altering Mother Nature’s Most Perfect Food

Why does man want to alter mother nature’s most perfect food? After all, ‘if it ain’t broke, don’t fix it.’ The desire to alter the composition of an egg is not due to an innate flaw, but rather to produce an egg with unique features. Such features are an increased mineral and (or) vitamin content, lower lipid and cholesterol concentration and altered fatty acid profile.

One of the first researchers to document the ability to change the nutrient profile of the egg was Cruickshank (1934). The polyunsaturated fatty acid profile of the egg was changed through dietary manipulation. Van Elswyk (1997), however, pointed out that the nutritional importance of Cruickshank’s discovery was not fully appreciated until the early 1970s. At that time healthconscious Americans turned away from eating eggs because they believed if they consumed eggs then the cholesterol and saturated fatty acids contained in the egg would immediately put them at death’s door. Because of this belief, modification of egg quality to reduce the amount of cholesterol and to decrease the ratio of saturated to unsaturated fatty acid content became the goal of numerous research efforts.

Modifying egg quality can be accomplished by inducing metabolic changes in the hen that can result in synthesis of compounds that essentially end up in her egg. Another way is to change the characteristics of membrane transport to facilitate movement of compounds into the egg. One of the more common acceptable ways to modify egg quality is to manipulate the diet of the hen such that the desired compounds eventually find their way into the egg. If a food, such as the egg, is nutritionally modified it is referred to as a designer or functional food (American Dietetic Association, 1995).

Nabor (1979) reviewed the transfer of nutrients into the egg. Pigments are not considered nutrients and this is probably why he did not include them in his review. The type of pigment and its concentration are directly influenced by the dietary concentration of any particular pigment. Numerous natural and synthetic pigment sources have been used successfully to pigment eggs. The high protein blue-green alga Spirulina platensis has also been shown to be an efficient pigment source for poultry and eggs (Ross and Dorminy, 1990; Anderson et al., 1991). Newman (1997) reviewed the nutritional benefits of supplementing various species of Spirulina to the animal’s diet.

Nabor’s classification of nutrients found in the egg and their responsiveness to dietary influence has remained unchanged (Table 1). Since his review more has been learned about how dietary changes influence the egg’s nutrient content. Scientific research costing hundreds of millions of dollars has been conducted throughout the world on nutrition and how it influences human health. Since eggs are a rich source of dietary cholesterol, the egg has been the subject of a considerable amount of this nutritional research because of the known relationship between cholesterol and atherosclerosis.

Poultry researchers have been dedicating a considerable amount of their efforts in recent decades to studies with hens in an attempt to lower egg yolk cholesterol to satisfy concerns of health-conscious consumers (Van Elswyk, 1997). Research has shown that not only cholesterol, but the egg content of several other nutrients can be manipulated to produce today’s so-called ‘designer’ eggs. These eggs are marketed as specialty eggs of added value because of their unique features and usually command a higher price than conventional eggs in the marketplace.




INCREASED MINERALS

The mineral content of feed, age of the hen and environmental conditions all have an influence on the mineral content of the egg (Cunningham et al., 1960). Much of the research that has been conducted in mineral metabolism as related to the mineral content of the egg has been concentrated on the egg shell and its associated membranes. There is no question that the dietary mineral content, especially calcium and phosphorus, plays a very important role in maintaining egg shell integrity.With regard to the yolk and albumen, however, there has been little success in changing the total calcium and phosphorus concentration. There has been little interest, with the exception of selenium, in increasing the overall mineral content of the egg for the designer market.

Animal nutritionists have a good idea as to how much selenium should be in an animal’s diet for achieving maximum production and to maintain health. There is far more controversy about the amount of selenium needed in the human diet and the related health benefits (Latshaw, 1987). Crops produced in certain regions of the United States are very low in selenium content. Animals eating plants or cereals grown in selenium-deficient soils require supplemental selenium in their diets. Poultry diets having the nutritionally correct amount of selenium have been shown to prevent exudative diathesis and to improve egg production and hatchability. Cantor and Scott (1974) established that laying hens had a nutritional requirement for selenium for normal egg production, hatchability and performance of progeny.

Humans in certain parts of the world can also suffer from a selenium deficiency. In China, a condition known as Keshan disease is caused by a selenium deficiency. It affects mainly the heart causing muscle degeneration. In similar situations in poultry, it is beneficial to supplement a hen’s diet with a highly available selenium source which will result in higher selenium levels in both her body tissues and eggs. Usually, the rise in selenium concentration of the egg albumen following dietary supplementation will continue for 6 to 8 days and in egg yolk for 10 to 14 days before the level in each stabilizes (Latshaw, 1987).

Cantor (1997) was able to increase the selenium concentration of eggs from commercial egg-type laying hens by feeding either an inorganic selenium source (sodium selenite, Na2SeO3) or two organic sources (selenium yeast as Sel-Plex 50, and high selenium soybean meal). Supplementing the diet with 0.3 mg/kg selenium as selenium yeast did not affect overall egg production, feed intake or egg weight (Table 2). The average selenium concentration of eggs from hens fed the control diet was 0.044 μg/g compared to 0.243μg/g for eggs from hens fed the selenium yeast product. When high selenium soybean meal was used in the diet the selenium concentration of eggs was approximately twice that of eggs from hens fed diets containing low selenium soybean meal (Table 3). The average value of egg selenium concentration (0.243 μg/g, Table 2) due to feeding the selenium yeast product providing 0.3 mg Se/kg diet was similar to the egg selenium concentrations obtained from feeding laying hens high selenium soybean meal with or without inorganic selenium supplementation (Table 3).




 


DIFFERENT FATTY-ACID PROFILE

The importance of the omega-3 fatty acids for human health and the potential for the egg to serve as the vehicle for supplying these fatty acids to the human diet is well established. It is further accepted that dietary manipulation has potential for modification of an egg’s lipid fatty acid profile. There are numerous literature reviews and research articles documenting the importance of eggs as the vehicle for supplying omega-3 fatty acids in the human diet. Thus, this particular type of designer egg has received much publicity and attention in the media.

Hargis andVan Elswyk (1993) reviewed the various sources of omega-3 fatty acids and discussed their importance in human diets. Eggs, enriched with omega-3 fatty acids, are used as a viable alternative to the direct consumption of fish and fish by-products as a source of the omega-3 fatty acids.

The use of flax seed (linseed) in the diets of laying hens as a source of omega-3 fatty acids was reviewed by Van Elswyk (1997). A linear effect on omega-3 fatty acid composition in egg yolk as the concentration of flax seed or fish oil in the diet increased has been reported (Scheideler and Froning, 1966; Castor and Leeson, 1990; Jiang et al., 1991). Scheideler et al. (1997) tested the effects of flax seed or menhaden fish oil with supplemented vitamin E on consumer acceptance, oxidative products and the yolk color of fresh and stored eggs. In general, they found that consumer acceptability of eggs from hens fed flax seed was similar to eggs from diets enriched with menhaden fish oil and not greatly different from eggs produced by hens fed standard diets.

Egg yolk contains sterols, phospholipids and triglycerides. When hens are fed flax seed the parent omega-3 fatty acid, linolenic, is incorporated in the yolk triglyceride, whereas the longer chain omega-3 fatty acids, such as eicosapentaenoic, dososapentaenoic and docosahexaenoic acids are deposited exclusively in the phospholipids (Jiang et al., 1991). Therefore, the egg becomes an important vehicle to provide the omega-3 fatty acids necessary for neural development and visual acuity in infants as well as adults (Van Elswyk, 1993; Simopoulos and Salem, 1992).


LOWERED EGG CHOLESTEROL

The poultry industry has continually strived to supply the consumer with the highest quality fresh and processed product. During the last 20 years the egg industry has been under intense attack by anti-cholesterol advocates. The egg has been blamed, blasted and defamed as one of the major culprits in coronary heart disease (CHD). Yaffee et al. (1991) concluded that the public’s perception of eggs as a major source of dietary cholesterol was a significant contributing factor to the overall decline in egg consumption since the late 1960s. Today, the annual per capita consumption of eggs is 237 compared to 320 in 1967. The public’s negative perception about eggs, however, is changing and the egg industry will soon see a long awaited and deserved comeback. Eggs are now being accepted as the nutritious food they are.

Today, we know that the unwarranted anxiety about eating eggs is unjustified. The egg industry has never tried to hide the fact that eggs are a rich source of cholesterol. Yet, because eggs do indeed contain cholesterol, they have, mostly by the news media, been singled out as a food which ‘everyone’ should omit from their diet. The public should realize that such ‘blanket’ recommendations do not apply to everyone and such recommendations should be applied on a per case basis.

The poultry industry is well aware of the cholesterol health issues. Product quality and safety have always been of primary importance. In fact, research interest in the poultry industry has continued to deal with quantification and reduction of cholesterol in eggs so that a low-cholesterol product will be available to those consumers who need to lower their dietary cholesterol intake.

Primitive man, in order to survive, learned to consume animal products and simultaneously developed regulatory mechanisms. Man has had to cope with cholesterol in his diet ever since he began eating animals and their products. The genetic selection of animals for meat production coupled with vast supplies of grain resulting from farm mechanization since the 1920s brought about a change in the composition of meat. It contained a greater fat content than that of wild animals or grass-fed domestic livestock. The consumption of these meats and high energy convenience foods containing high concentrations of saturated fats and refined sugars, along with a sedentary life-style, smoking, stress and high blood pressure, contribute to an increased risk of CHD. It is very difficult for people to reduce the portion sizes of meat in order to decrease fat consumption because most people simply enjoy eating meat as well as eggs.

The cholesterol in an egg is found only in the yolk. Over 95% of the yolk cholesterol is associated with yolk triglyceride-rich lipoproteins (Griffin, 1992). Eggs from commercial egg-type laying hens typically contain about 200 mg cholesterol per egg (Beyer and Jensen, 1989; Van Elswyk et al., 1991).

Designer eggs containing lower cholesterol and saturated fat concentrations are available to the general public. Efforts to reduce egg yolk cholesterol content have involved the investigation of the effects of diet, pharmacological intervention and genetic selection (Griffin, 1992). Cholesterol concentration in the yolk is not easily changed. It has been suggested that the relative resistance of egg composition to alterations in diet apparently reflects the nutritional and structural requirements for avian embryonic development (Kuksis, 1992).

Egg yolk cholesterol can be modified in various ways. Hargis (1988) reviewed these modifications and concluded that the cholesterol content of egg yolk could be altered by 25% or more through high dietary cholesterol and fat supplementation. The resulting increase in excretion of cholesterol into the egg by the hen following high dietary cholesterol supplementation seems to be a major regulatory mechanism for blood cholesterol. Recently, Lin and Lin (personal communication,1998) found that an organic chromium source (BioChrome chromium yeast) reduced cholesterol and improved egg hatchability. Genetic selection studies have indicated sufficient genetic variability in yolk cholesterol levels to make a genetic selection program feasible. Selection for lower egg cholesterol has not, however, been successful by this means and has resulted in only a slight reduction in egg cholesterol concentration. Unfortunately, this reduction is associated with a decline in egg production (Hargis, 1988).

Dietary fiber and administration of several drugs have been shown to have only moderate efficacy for the reduction of egg yolk cholesterol concentrations (commonly 5–10%). The use of various pharmacological agents to lower egg yolk cholesterol will be limited if such agents or their metabolites are excreted into the egg (Hargis, 1988).

Chromium is an essential micromineral which has been shown to be necessary for the metabolism of lipids and carbohydrates. Anderson (1987) reviewed the major functions of chromium metabolism in humans and laboratory animals and emphasized the importance of this trace element in regulation of blood glucose concentration. Chromium is an indispensable component of the glucose tolerance factor which functions to potentiate the activity of insulin.Adeficiency of chromium results in retarded growth, insulin resistance, impaired glucose tolerance, hyperlipidemia and hypercholesterolemia. Supplementing chromium to the diet will ameliorate the above conditions (Stoecker et al., 1987).

The biological activity and metabolism of chromium in animals is dependent on its chelated form (Mertz, 1975). An organic form of chromium found in brewer’s yeast was designated as a glucose tolerance factor (Mertz, 1975). Brewer’s yeast must derive its chromium from the barley malt liquor in which the yeast multiplies. Naismith et al. (1991) and Mahdi and Naismith (1991) hypothesized that barley must be a rich natural source of chromium.

These authors conducted a series of experiments to test their hypothesis that barley’s therapeutic action in the management of diabetes mellitus in Iraq was due to its chromium content. They found that in adult diabetic rats a diet containing barley did indeed have a modulating effect on the symptoms of diabetes (blood glucose concentration and water consumption) when compared with a starch or sucrose-based diet. It was postulated that the beneficial effect of barley might be explained by its very high chromium content of 5.7 μg/g. Gibson (1989) reported that many plant products contain only low levels of chromium and milling removes about 83% of a grain’s original concentration of chromium. The milling of grain products may decrease the dietary concentration of chromium when these products are fed to animals.

Studies by McCarty (1991) and Page (1991) have shown that the organic form of chromium is superior to the inorganic form. Not only has dietary chromium supplementation been shown to reduce serum concentrations of glucose and lipid, it has a serum cholesterol lowering effect as well (Evans, 1989; Page, 1991; Lien et al., 1993). Lien et al. (1996) fed commercial eggtype White Leghorn hens 100 g/day of a corn-soybean meal based diet supplemented with either 0, 200, 400 or 800 ppb organic chromium (picolinate) These authors reported a 39% reduction in serum cholesterol with the 800 ppb level of chromium of supplementation. Compared to egg yolks from hens fed the control ration, yolks from hens fed 200, 400 or 800 ppb chromium contained 14, 29 and 34% less cholesterol, respectively (Table 4).

Nakaue and Hu (1997) used 22- and 75-week old commercial egg-type laying hens fed diets supplemented with 0, 200 or 800 ppb chromium in an experiment designed to evaluate the cholesterol lowering ability of chromium. They reported no differences in egg production, feed conversion, egg interior quality or blood triglycerides in either age group. The younger group of hens laid eggs with significantly lower yolk cholesterol levels at both levels of chromium supplementation.Anumerical decrease in yolk cholesterol occurred in the older group of hens fed the chromium supplemented diets.

Chromium has been reported to improve egg interior quality. Jensen and Maurice (1980) identified chromium as the factor in distiller’s grains that improved albumen quality. When chromium is supplemented to the diet there is a direct relationship between dietary concentration and yolk chromium accumulation (Anderson, 1989).




Researchers continue to search for pharmacological ways to efficiently produce low cholesterol eggs. Recently, Dr Robert Elkin, a pioneer in this field, was the first to show that the amount of cholesterol in an egg can be reduced by as much as 45–50% by the drug atorvastatin without seriously impairing egg production (Elkin et al., 1997). Follow-up studies are planned by these investigators to determine if the egg contains any drug residues and if potential nutrient compositional changes occurred in the low cholesterol eggs. In addition to attaining cholesterol lowering effects of a magnitude never before achieved, these researchers were also the first to establish a direct effect of cholesterol on fertility in birds. When the low cholesterol eggs collected from artificially inseminated, atorvastatin-fed hens were incubated, only 20% hatched. Among the remaining 80% of the set eggs approximately 70% were infertile.

Dr Elkin believes (personal communication) that egg cholesterol will be significantly lowered (by at least 50%) through one of two mechanisms: 1) reduction of very low density lipoprotein (VLDL) synthesis, VLDL secretion and/or cholesterol richness of the VLDL particles, or 2) interference with the uptake of VLDL at the level of the oocyte plasma membrane (the oocyte vitellogenesis receptor).

The reason for this is because almost all of the cholesterol and most of the lipid destined for oocytic (future egg yolks) uptake is synthesized in the hen’s liver and packaged in the form of a VLDL and transported in the plasma. Thus, anything that alters either VLDL synthesis, composition or secretion by the liver will be expected to alter egg cholesterol. The plasma VLDL and vitellogenin which account for approximately 60 and 24% of egg yolk dry matter, respectively, are taken up into growing oocytes by the process of receptor-mediated endocytosis. Anything that could selectively block VLDL uptake by the oocyte would be expected to markedly alter egg yolk cholesterol and fat content.

In September, 1997, a US patent was issued to Albert J. Meier and John M. Wilson (Meier and Wilson, 1977) for a method of altering the cholesterol content of eggs. These researchers have found that when n-dihydroxyphenylalanine (L-DOPA) in the bloodstream of laying hens is elevated, the hens produce eggs with reduced cholesterol content, a lower ratio of saturated to unsaturated fatty acids and increased total protein. The L-DOPA is administered to the hens as a dietary supplement.


INCREASED VITAMINS

The successful attempts to modify vitamin composition of eggs by dietary vitamin supplementation was reviewed by Nabor (1993). This review highlighted the factors influencing egg vitamin composition, variability in egg vitamin composition, relationship of dietary vitamin content to egg content and the efficiency of vitamin transfer to the egg. He concluded that egg vitamin content is highly variable and dependent primarily on the vitamin concentration of the hen’s diet. Vitamin A content of eggs responds slowly to changes in dietary vitaminAcontent, whereas the riboflavin content responds rapidly to dietary changes in riboflavin concentration. Vitamin D, pantothenic acid, folic acid, biotin and B₁₂ respond greatly to increases in the dietary levels of these vitamins. Transfer efficiency to the egg is very high for vitamin A and high for riboflavin, pantothenic acid, biotin and B₁₂. Transfer efficiency is medium for vitamin D₃ and vitamin E and low for vitamin K, thiamin and folic acid.

The review by Nabor (1993) showed that the vitamin content of eggs can be increased over certain ranges of diet fortification and with varying efficiency of vitamin transfer. If development of designer eggs containing higher concentrations of certain vitamins is ever the objective of a commercial enterprise, vitamin transfer efficiency and vitamin cost would be two of the major considerations used in determining the economic feasibility of marketing such eggs (Nabor, 1993).

The development of designer eggs containing increased concentrations of vitamin E, ß-carotene and vitaminAhas recently been of interest to researchers due to their proposed strong protective association with certain human cancers (Jiang et al., 1994). These three nutrients are known to function as antioxidants, quenching singlet oxygen and free radicals (Bendich and Olson, 1989; Krinsky, 1989; Knekt et al., 1991; Stahelin et al., 1991).


ANTIBODIES

All types of antigens (bacteria, viruses, foreign proteins, etc.) are constantly invading animals. Animals in turn develop specific immunoglobulins against the antigens. These specific immunoglobulins, more commonly called antibodies, bind to their specific antigens and by doing so prevent harm to the host animal. This is commonly referred to as neutralization of the antigen.

The chicken is no exception when it comes to antibody production. The hen produces antibodies to neutralize the antigens to which she is exposed. These antibodies circulate throughout her body and are transferred to her egg as protection to the developing chick.

Immunologists are taking advantage of the fact that the hen can develop antibodies against a large array of antigens. Specific antigens are selected and injected into the hen, who develops antibodies against them along with a resulting transfer of antibodies to her egg yolk. The egg yolk is collected and a specific antibody is isolated and purified. This type of designer egg which serves as a source of specific antibodies is still in its developmental infancy. As new biotechnological knowledge is gained, this type of designer egg will most likely gain in popularity and use, resulting in a range of antibodies for treatment of snake venoms to the countering of microorganisms which cause tooth decay (Yamamoto et al., 1997).

Gene transfer will eventually make it possible to use the laying hen as a‘bioreactor’ for the production of pharmaceuticals and other proteins (Shuman, 1991). One possible way to achieve this product would be to express the pharmaceutical gene in the oviduct of the hen in order to have the resulting protein incorporated into the egg albumen. Another way would be to express the gene in the liver and manipulate the resulting protein so as to have it incorporated in the egg yolk. The ultimate goal would be to provide a new manufacturing system that can produce bioproteins at a cost which would be less than the current mammalian or bacterial cell culture techniques. The high rate of egg production, relatively short generation interval of hens and high protein ratio in eggs make laying hens a more advantageous bioreactor than their mammalian counterparts such as, rabbits, mice, goats, sheep and cattle. It can safely be said that the future for designer eggs is exciting.


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