Currently, eggs have become one of the most important sources of animal protein, due to their nutritional qualities as well as their social acceptance. For this reason, its production is widespread and, in order to meet the needs of the global population, it is carried out intensively in many countries, which means that egg laying hens are making a huge metabolic effort.

Intestinal health and performance of egg laying hens are closely related, since the absorption of nutrients involved in the egg production process occurs in the intestine. It is essential that the intestine is in optimal physiological conditions since, in egg laying hens, the basis is the efficient transformation of nutrients from the diet into eggs, and the intestine is the first barrier that these nutrients must cross to finally become a part of the eggs.

An adequate intestinal health and physiology guarantee an optimal absorption of nutrients and minerals, which improve productivity, internal  and external egg quality, as well as the production of clean eggs, important in terms of food quality.

1. Intestinal physiology

Enterocytes are the most abundant epithelial cells of the intestinal mucosa. Their functions are, on one hand, to act as a selective barrier that allows nutrients to pass into the organism and, on the other hand, to prevent the entry of pathogens or toxins. Therefore, the intestine has two functions: a nutritional and a defensive one.

• Nutritional function

Importance for the general functioning of the organism

The proper inclusion of nutrients in the diet is essential for the organism, since the correct development and functioning of all animal organs and systems depend on the nutrient supply. In addition, certain nutrients provide energy for the metabolic processes that maintain these animal systems.

However, providing nutrients in the diet is not enough. It is also essential that they are absorbed and assimilated by the organism and, thus, nutrients can be used at different levels. Therefore, the optimal physiology of the intestine becomes especially relevant, since it is the organ responsible for the absorption of nutrients from the diet

Importance for egg production

For egg production, nutritional needs are very high due to the metabolic demand of the process, as well as the amounts of the different nutrients and minerals that will form the eggs. For example, for the formation of the eggshell, two grams of calcium are needed, which means 10% of the total calcium from the diet.

The gut is under stress as it needs to absorb big quantities of this mineral, together with other nutrients. In addition, the metabolic needs of the egg laying process must be met. All of this explains why it is so important to maintain the intestine in an optimal physiologically state.

Behaviour of the different nutrients in the intestine in relation to egg production

The surface of the intestine is formed by villi and microvilli to provide a wider absorption surface. The small intestine is the first organ of absorption and digestion and, in its different segments, there are numerous enzymes adapted to the multiple substrates, to transform proteins, carbohydrates and complex lipids into simple molecules that are absorbed.

• Carbohydrates: Mostly digested and absorbed in the small intestine, where enzymes transform them into monosaccharides. Although eggs have a very low carbohydrate content, feeding the hen with adequate dietary levels is essential to provide the amount of energy needed for egg laying.
• Lipids: Once emulsified by bile salts and in contact with duodenal lipases, they are transformed into monoglycerides and fatty acids. In the egg, these lipids will be located in the yolk, mostly as complex lipoproteins. In the egg, the saturated/unsaturated fatty acids ratio is considered the most appropriate for consumers among animal products for human nutrition.
• Proteins: Their digestion begins in the proventriculus and the gizzard, but occurs in greater proportion in the small intestine, where they are also absorbed. Different pancreatic and intestinal enzymes transform them into aminoacids that are subsequently absorbed through an active transport mechanism. Proteins will form an important part of the egg white, although, they can be found in the yolk, too, but in a moderate proportion.
• Minerals: They are absorbed in both large and small intestine depending on factors such as pH and transport molecules. In egg laying hens, the two essential minerals for egg production are calcium and phosphorus. These should be administered at appropriate levels to form the eggshell, this way, the bird does not have to use the calcium stored in the bones to form the shell. There is a Ca: P ratio in the diet that will vary according to the productive stage of the hens, but it should always be higher than 1. Other important minerals that will be part of the egg are potassium, selenium, iron, sodium and magnesium.
• Vitamins: Some are absorbed by passive diffusion mechanisms and many others thanks to the presence of transporters. Their absorption is very important, since not many vitamins are stored in the organism. In egg production, vitamins D and C are especially important. The former is essential for the absorption of calcium in the intestine. The latter is very important in commercial egg laying hens to minimize heat stress that may affect layers during specific periods.  Vitamins of the B group are also important, because they will become part of the egg, as well as vitamins A and E.

Some of these dietary nutrients absorbed in the intestine will supply the hen with the basal nutritional requirements defined for each genetic line; while another significant amount of nutrients will form the eggs. Components such as lipids, cholesterol, vitamins and xanthophylls will be part of the yolk, minerals will mainly be part of the eggshell, and the egg white’s main components are proteins (albumin).

• Defensive function

Importance for the general functioning of the organism

As described above, the defensive function of the intestinal cells is to prevent the passage of microorganisms or toxins from the intestinal lumen to the organism. It is a very important function, since the intestine is the main route of entry of many microorganisms, from where they can reach different locations such as the liver, the kidneys, the joints or any other organ and cause different disorders. For this, the intestinal defensive function has a significant impact on a general organic level, since it contributes to health maintenance by preventing the penetration of microorganisms through this route.

The defensive barrier of the intestines is composed of the glycocalyx, the mucus, and the microvilli.

• On the one hand, glycocalyx is characterized by its high content of negative charges that hinder the contact of pathogenic microorganisms with the cell membrane.
• The mucus produced in the gut covers the surface of the intestine, facilitating the passage of nutrients, but not that of the pathogens .
• Finally, microvilli are extensions of the intestinal epithelium composed of enterocytes. The integrity of this epithelium depends on the tight junctions that are established between adjacent enterocytes, which allows only the passage to nutrients.

Importance for egg production

In the case of egg laying hens, the aforementioned defense mechanisms of enterocytes prevent the proliferation of pathogenic microorganisms and, therefore, the damage or destruction of microvilli. This ensures, the absorption of those nutrients necessary for egg production and prevents flora imbalances that would lead to an increase in the percentage of dirty eggs and negatively impact the final consumer in terms of food safety.

2. Alterations in egg laying hens derived from inadequate intestinal physiology

As described in the previous points, intestinal functions are of a great importance, to make sure the organism is working properly and for egg production. When the physiology is altered, different disorders may appear, such as the ones described in the following points:

Shell quality problems

When intestinal physiology is in a poor condition, there may be a deficiency in mineral absorption, especially calcium. When this occurs, initially, hens use the calcium they have stored in their bones, but, if the problem continues, these reserves may get depleted and, gradually, thin-shelled eggs appear, which will lead to an increase in the percentage of broken eggs. In extreme cases, even shell-less eggs can be observed.

Bone fractures

In continuation with what was described in the previous point, the bird skeletal system weakens due to the use of calcium reserves of the bones, which leads to bone fractures, such as the head of the femur and other locomotor problems of similar pathogenesis.

Increase in the percentage of dirty egg

Another indirect consequence of a deficient intestinal physiology is that it increases the possibility of an imbalance of the intestinal flora. When this occurs, certain microorganisms grow disproportionately, damaging the mucosa, which causes problems in the consistency of faeces. This increases the percentage of dirty eggs, which has a negative impact on the final consumer, since the probability of cross-contamination increases.

Changes in egg internal quality

Due to the alteration of the nutritional function of the intestine, feed digestion and nutrient absorption are affected, and this leads to a decrease in their concentration in the egg. This deficiency causes yolk pigmentation problems, poorer egg nutritional value, worsening of the Haugh Units, among other issues.

Decrease in the egg laying percentage and egg size

Related to the previous point, the alteration of the nutritional functions of the intestine will also decrease the percentage of egg laying. This is because the bird will not absorb enough nutrients and minerals to cover the needs for egg production (both for the metabolic process and to form the egg).

The mentioned problems, derived from an inadequate intestinal physiology, lead to poor qualitative and quantitative egg production, which is, in most cases, very difficult to reverse in the short term, and that leads to significant economic losses.

3. Maintenance of adequate intestinal physiology

In order to meet the high productive objectives of egg laying hens, feed must be strictly formulated to contain all the nutrients in the necessary proportion. In addition, as described in the previous points, maintaining intestinal physiology in optimal conditions is essential to maximize nutrient utilization and, at the same time, to avoid alterations that have a negative impact on the egg laying process.

Intestinal physiology can be altered by several factors, such as the following:

1. Presence of microorganisms, such as viruses, bacteria, fungi and parasites, or their toxins. Their action in the gastrointestinal tract is very varied, from the degradation of the mucus layer to the destruction of enterocytes. Any of these actions has consequences that negatively impact egg laying parameters, both quantitative and qualitative.
2. Poor quality of the diet components, such as recycled fats that contain residues or with high levels of acidity that cause irritation of the villi and prevent adequate nutrient absorption in the intestine. This also promotes adequate conditions for microorganisms to multiply in the mucosa.
3. Poorly balanced diets with excess of any of the components, which may exceed the absorption capacity of the intestine and become available as an ideal substrate for the growth of microorganisms and the consequent development of related pathologies.
4. Enzyme deficiencies that transform carbohydrates or proteins into simple absorbable components. As in the previous point, non-absorbed components will become available as a substrate for pathogenic microorganisms.
5. Excessive use of antibiotics, which causes an imbalance of the intestinal flora, the proliferation of microorganisms that can damage the mucosa, either due to their multiplication, or due to the production of toxins.
6. Imbalances of the intestinal flora that can lead to an increase in bacterial enzymatic activity and the production of ammonia or other irritating gases for the intestinal mucosa. On the other hand, they can lead to the production of enterotoxins that cause intestinal transit disorders and diarrhea and, therefore, poor nutrient absorption and an increase in the percentage of dirty eggs.

For the maintenance of intestinal physiology, it is essential to optimize the condition of the intestine and the enterocyte. For this, the addition of intestinal conditioner pronutrients in the diet is essential. These are active natural molecules that have a mechanism of action with a metagenetic basis and whose target cells are enterocytes.

In these intestinal cells, pronutrients stimulate the synthesis of multiple specific functional proteins that have different effects, such as those detailed below:

• Increase in the regeneration rate of enterocytes. Thanks to this, the epithelium will be renewed faster and, at the same time, microorganisms and other debris attached to old enterocytes will detach along with them, so that bacterial growth is limited.
• Improvement of the status of tight junctions, which improves selective paracellular permeability and allows the passage of nutrients, but not that of the pathogens.
• Increase in enterocyte metabolic rate for a better nutrient absorption (vitamins and aminoacids) and to prevent the absorption of toxins.

Stimulation of the immune response of the enterocytes, improving their resistance to infections.

4. Conclusions

• The nutrition of the egg laying hens is a very important factor for the production of eggs in sufficient quantity and quality and to obtain good economic results.
• The intestine is the organ that plays a fundamental role in nutrient absorption. It is the first barrier of the organism and absorbs the nutrients that finally form the egg. Therefore, intestinal physiology must be in good condition to optimize nutrient utilization.
• Alterations of intestinal physiology in egg laying hens cause more economic losses than alterations in any other system. This is because the intestine is a key organ in productive performance, and any intestinal damage will have a negative impact on egg production.
• Maintaining the intestinal condition is essential to guarantee the nutritional and defensive functions of the intestine and, thus, optimize the egg laying process.
• To maintain the intestinal condition, it is essential to add pronutrients in the diet, which improve the physiology of the intestine through the production of functional proteins that optimize enterocyte physiology.

You can read more about this topic at the following links:

• Physiology of egg laying hens
• The broodiness
• Egg shell formation
• Egg shell quality problems
• Eggshell problems in layers: causes and solutions