INTRODUCTION
The stress caused after early weaning in piglets favors a decrease in voluntary feed intake, especially in the first week after weaning, and may additionally affect the morphometry, intestinal function and immune system by diminishing the digestive capacity with a consequent deficit in absorptive potential [
1]. One important tool to minimize the non-desirable effects of weaning is the quality of pre-initial and initial diets. In turn, dairy products stand out for their nutritional qualities and excellent acceptability, and can contribute to improving feed digestibility and maintaining mucosal integrity throughout the small intestine [
2]. Furthermore, it has been suggested that the indiscriminate use of antibiotics in animal feed will lead to resistance against pathogenic bacteria which affect animals and humans [
3]. As a consequence, several countries have prohibited or will prohibit the use of antibiotics as growth promoters in animal production. Thus, nutritional strategies such as the addition of functional amino-acids to diets such as arginine, glycine, proline, tryptophan, glutamic acid and glutamine have been used to diminish intestinal degeneration after weaning and to promote adequate growth performance [
2].
The addition of glutamine to animal diets can improve small intestine morphology, especially in stress situations like weaning. Among other functions, glutamine acts as metabolic fuel for fast renewing cells, and glutamic acid substitutes glutamine in many of its roles, including those for energy generation and amino-acid synthesis [
4]. Piglets fed with diets containing 1% glutamine presented higher daily weight gains, feed conversion and villus height (VH) in the jejunum [
5].
Although most studies have shown that glutamine and glutamate provide benefits to growth performance index and to intestinal structure, there are few studies on the influence of these amino acids on diets all-grain feeding.
In view of the above, the present study had the objective to evaluate including L-glutamine along with L-glutamic acid in weaned piglets’ diets, with or without whey powder, by measuring the effects on growth performance, incidence of diarrhea, morphometry and intestinal integrity, hepatic glycogen index and nutrient digestibility.
DISCUSSION
Animals that consumed the diet all-grain feeding (G) mainly composed of corn and soybean meal presented worsening in the performance variables in initial and whole periods when compared with the animals that received the diets with the L-glutamine and glutamic acid supplementation containing (or not) whey powder. This can be explained by the fact that the piglets’ digestive system is adapted from birth to weaning to secrete the digestive enzymes which digest lactose in milk in greater quantity, but not other ingredients, especially those of plant origin [
12].
In addition, ingredients of grain origin (such as soy naturally) present compounds which act as natural plant protection known as anti-nutritional factors, such as trypsin and chymotrypsin inhibitors, which inhibit protein digestion; lectins, whose main action mode combine with intestinal wall cells and thereby cause non-specific interference in the absorption of nutrients; allergic factors (glycinin and β-conglycinin), which reduce nutrient absorption and cause deleterious effects on the microvilli of the small intestine; and there are also soluble non-starch polysaccharides, which cause a decrease in animal performance, and which can remain present even after thermal processing [
13].
In turn, the inclusion of the product containing L-glutamine associated with glutamic acid in diets containing (or not) whey powder reduced the deleterious effect of the diet all-grain feeding. This was probably due to the fact that glutamine has important functions, especially in the gastrointestinal tract, being used as an energy fuel by rapidly dividing cells such as intestinal mucosa and immune system cells [
14].
Likewise, glutamic acid from the diet is an important fuel for epithelial cells, in addition to being involved in the excitatory neurotransmission of efferent vagal stimulation and in nutrient detection [
15].
In this context, results found by Cabrera et al [
16] showed that supplementation with 1% L-Glutamine or 0.88% aminoGut (commercial product with at least 10% glutamine and 10% glutamic acid) improved the feed conversion rate in the first three weeks after weaning; however, no difference was found for other performance variables. When supplemented with glutamine and glutamic acid at levels of 0.1% glutamine + 0.9% glutamic acid; 0.2% glutamine + 0.8% glutamic acid; 1% glutamine and 1% glutamic acid in the diet of piglets with an average weight of 9.22 kg, He et al [
5] observed that piglets weaned at 28 days consumed the diet with 1% glutamine more and showed better weight gain in the 28 days of the experiment when compared with the animals of the control diet.
In an experiment lasting 14 days which analyzed the performance of piglets weaned at 18 days and exposed to a 12-hour transport simulation, 0.20% glutamine supplementation provided an increase in feed intake after the 10th day and greater body weight after the 13th day when compared to treatments with or without antibiotics [
17].
In contrast to these results, Rodrigues et al [
18] did not observe an improvement in the performance of piglets recently weaned and fed diets containing 0.8% L-glutamine associated with L-glutamic acid with or without the addition of valine in the period from 24 to 46 days of age.
Regarding diarrhea incidence, there was no effect of the treatments tested in this study. Weaning causes a harmful effect on the intestinal barrier function, often observed by breaking it, providing increased permeability to toxins, bacteria and antigen-associated foods, which penetrate the intestinal epithelium and result in inflammation, malabsorption, diarrhea and reduced growth [
19]. Post-weaning diarrheal syndrome is related to the exacerbated multiplication of pathogenic bacterial strains in the piglet intestine and is associated with nutritional, management and environmental factors [
20].
In addition, diets for piglets in the post-weaning period have a high crude protein level, which can undergo microbial fermentation when not fully digested, favoring the occurrence of diarrhea due to increased proliferation of pathogenic bacteria [
20]. Therefore, the fact that all animals consumed isoproteic diets and were subjected to similar handling and facilities may have influenced the results of the present study. These results corroborate those found by Rodrigues et al [
18] who did not observe the influence of diets supplemented with L-glutamine associated with L-glutamic acid with or without the addition of valine on diarrhea incidence in piglets from 24 to 46 days of age.
However, supplementation with 1% L-glutamine and glutamic acid in the diet of piglets weaned at 21 days of age caused lower diarrhea incidence when compared to animals which consumed the control diet [
21].
The diet all-grain feeding mainly composed of corn and soybean meal without the addition of the product containing glutamine and glutamic acid negatively affected the parameters related to intestinal integrity and cell proliferation rate (cell mitosis). One of the probable causes of this to explain this result is that the piglet digestive system undergoes modifications in weaning until it is prepared for digesting ingredients of plant origin [
12]. Thus, it is essential that the gastrointestinal tract identify the changes that the new diet causes so that it is able to promote changes in pH, enzyme secretion and motility, with consequent improvement of digestive and absorptive processes [
22].
Another factor which can compromise intestinal integrity is the feed consumption. Low feed consumption can cause inhibitory morphometric changes, since the intestinal segments need to obtain nutrients in sufficient quantity to meet the demands for mucosal protein synthesis and growth. These modifications include shortening and modification of the villus structure, hyperplasia of the crypt cells, in addition to functional changes in the intestine [
23] decreased VH/CD, increased goblet cell count, which is closely related to epithelium protection, and an increase in the mitosis rate in the intestinal epithelium through cell turnover due to a higher desquamation rate.
However, it was found that supplementation with the product containing L-glutamine and glutamic acid on diet all-grain feeding promoted an increase in VH, a decrease in CD and an improvement in the VH/CD, which provides greater contact area with nutrients and consequently greater absorption of them. Glutamine is the main source of energy for intestinal tissue, with 67% of dietary glutamine being used by mucous cells, intestinal cells and bacteria. This amino acid provides essential energy for cell multiplication and favors biosynthesis of nucleic acids such as purines and pyrimidines, which play a fundamental role in maintaining intestinal integrity [
14;
24]. Likewise, glutamic acid has been shown to exert numerous functions in the metabolism of nutrients. Its importance is due to the fact that it is the main oxidative fuel for intestinal cells, and a precursor for the synthesis of important cellular antioxidants such as glutathione, arginine and proline which compose glycoproteins present in intestinal mucus [
24].
In this context, in a study carried out with 1% glutamine supplementation in the diet of piglets weaned at 21 days of age and slaughtered at 28 days of age, the VH and the VH/CD (p<0.05) of the pigs increased in the jejunum when compared with those that were not supplemented [
25]. In the same sense, in evaluating the supplementation of 0.20% glutamine, there was an increase (p<0.05) in the VH in the duodenum and the VH/CD in the duodenum and jejunum of piglets when compared to the treatment without the use of antibiotics [
17].
On the other hand, in evaluating glutamine supplementation associated with L-glutamic acid at levels of 0.1% glutamine + 0.9% glutamic acid; 0.2% glutamine + 0.8% glutamic acid; 1% glutamine and 1% glutamic acid, He et al [
5] found that they did not affect (p>0.05) VH, CD, or VH/CD of duodenum, jejunum or ileum in piglets slaughtered at 28 days. In another study testing different diets (control; with 1% glutamic acid; 1% glutamine and 1% nucleotides) for piglets, Amorim et al [
26] did not observe an effect (p>0.01) on VH or VH/CD; however, it was found that there was a decrease (p<0.01) in the CD in the group supplemented with 1% glutamine when compared to the group that received 1% nucleotide.
There was a higher apoptosis rate in the duodenum in the animals that received the totally diet all-grain feeding when compared to those which consumed the diet containing the whey powder, which may have been responsible for the changes in the morphometric parameters. However, there was a reduction in the apoptosis rate due to the inclusion of dairy products in the diet. Lactose present in dairy products acts by maintaining the integrity of the intestinal mucosa [
2] and in turn may have an inhibitory role on the cell apoptosis rate.
The higher deposition of hepatic glycogen in animals that consumed diets supplemented with L-glutamine and glutamic acid may be related to the participation of this amino acid in energy metabolism, as evidenced by the participation in the synthesis of amino sugars, purines and pyrimidines which are part of A, ATP, and NADH coenzyme molecules which are direct participants in the Krebs Cycle. Linked to this, glycogen synthesis is stimulated by glutamine, which has a direct role in glycogenesis due to its ability to increase glycogen synthetase enzyme activity, which favors forming glycogen in the liver [
29]. In addition, glutamic acid is used by succinic, fumaric, malic, oxaloacetic and other acids in its metabolism for the synthesis of glycogen which can be stored (in most cases) in the liver tissue [
30].
The consumption of a totally diet all-grain feeding constrained the digestibility coefficients of nutrients, probably due to the low activity of specific enzymes for degrading ingredients of plant origin. The complete development of the enzymatic system usually takes place until the eighth week of life in piglets, so the gastrointestinal tract produces insufficient carbohydrates, proteases and other enzymes which act in digesting ingredients which are usually used in starter diets in the post-weaning period, especially those of plant origin [
31].
In this sense, lactose present in dairy products is widely used in composing diets for piglets right after the weaning period, as it acts in improving the feed acceptability [
32] and increases consumption of dry matter, nitrogen digestibility and promptly provides energy to the tissues [
33] in addition to favoring animal feed consumption.
The improvement in the digestibility coefficients of the diets containing whey powder (GW) varied from 1.02% to 1.76%, which may explain the non-improvement in the weight gain and feed conversion of the animals in the period from 24 to 55 days, receiving GW diets compared to grain-based (G) diets, another aspect that may be related to the method used to determine the digestibility of diets, in which the animal receives a restricted amount of feed, based on its metabolic weight, which it enhances the use of nutrients [
34] and there is a reduction in energy losses [
35] differently from animals that receive feed at will, which increases their maintenance needs due to increased consumption and is necessarily associated with compensatory gain [
35].
The highest digestibility coefficient of crude protein observed was when supplementation of the product containing L-glutamine and glutamic acid occurred; this can be explained by the great importance that glutamine has in metabolic processes. It is an indispensable amino acid for the growth of most cells and tissues, and glutamine can act as a metabolic regulator to increase synthesis and reduce protein catabolism under conditions of high protein degradation which include periods of stress (such as weaning), as well as periods of rapid tissue growth and diseases [
36].
These results indicate that supplementation of 1% commercial product containing L-glutamine + glutamic acid in diets for piglets from 24 to 55 days of age, dispenses with the use of whey powder when evaluating growth performance. Amino acid supplementation alone or associated with whey powder affects (p<0.05) positively the indicators of the intestinal integrity.