The current study showed no significant differences on the performance parameters of pigs fed with different levels of supplementation with IGF-1. There was also no significant differences on digestibility across treatments. A direct contrast against growth performance parameters could not be made because, most of the IGF-1 supplementation studies directly measured the capacity and development of the gastro intestinal tract (GIT). In an attempt to explain the results of IGF-1 supplementation, a number of factors have been listed and supported by earlier literature. These factors are the age of the pig used, the influence of IGF-1 Binding Protein (IGFBP) and receptors, the effect of digestion on oral IGF-1 administration, the feed as well the quality of supplement incorporation, the variability of the treatments, experimental error as well as the total number of pigs used on this study.
Age of the Piglet and IGF-1
The data of Dunshea and Walton in 1995, strongly suggested that the subcutaneous infusion of IGF-1 analogue on pigs aged 0-27 days expressed not only increased piglet growth rate (ADG) in late lactation, but also increased the growth of small intestine, spleen and pancreas. In this instance, the authors suggested that the spleen size seen may reflect an enhanced immune response. In 1996, Burin et al., conducted a study involving oral supplementation of human recombinant IGF-1. Their results demonstrated that oral administration of human recombinant IGF-I during the first 4 days after birth significantly increased small intestinal mucosal growth in formula-fed neonatal pigs. Beneficial effects of orally administered IGF-I have been reported in neonatal piglets by Houle et al., (2000) and Burin et al., (2001). However, no effects of oral IGF-1 on serum concentration, overall body weight, or growth of tissues other than the intestine were reported. Most recently, Donovan and Monaco, in 2004 concluded that, in general, there is no effect of oral IGF-1 on serum concentration, overall body weight or growth of tissues other than the intestine in neonatal pigs. On the current study, overall average age of pigs used at the beginning of the experiment was 50.6 days, the youngest pig recorded was 44 days while the oldest was 52 days. The average weight was 9.48 kg, with the lightest recorded at 5.60 kg and heaviest at 12.78 kg. The age of the pig is a major difference on from the studies cited, which were conducted on neonatal pigs, 27 days being the oldest animal. Age has a linear effect towards intestinal development (Efird et al., 1982) thereby directly proportional or translated into growth performance of the pigs.
IGF-1 Binding Protein and Receptors
To be able to express its effect, IGF-1 needs binding protein. Insulin-like growth factor (IGF) action is influenced by the insulin-like growth factor binding proteins (IGFBPs). Binding Proteins (BP) control the distribution, function and activity of IGF-1 in vivo and are produced and expressed in at least one form in almost all tissues. It is thought that mechanisms controlling the expression of BPs during various physiological states will determine whether IGFs, both IGF-1 and IGF-2, have effects on IGF- mediated growth (Hossner et al., 1997). According to Lee et al., 1993, IGF-I and IGFBP-3 concentrations in porcine serum are low during fetal life, but they increase in postnatal life. On the other hand, IGFBP-2 is present in porcine follicular fluid (Monschein et al. 1990), it decreases in postnatal life (Russell and Van Wyk 1995; Lee et al., 1993). IGF receptors (Fernandez-Moreno et al., 1987) types I and II (Schober et al., 1990; Young et al., 1990 ) have been detected along the entire neonatal intestine. Schober et al. (1990) concluded that in the piglet, binding of IGF-I to intestinal receptors was highest at birth, declined at 3 and 5 day postpartum, but recovered by 21 day postpartum. This may very well explain the potency of IGF-1 on the neonatal pigs. In addition, receptor binding is higher in newborn animals and declines with age (Li et al., 2006 and Donovan et al., 2004). On this current study, however, these binding proteins and receptors were not studied nor quantified, such that no direct conclusion can be made on the its role on the magnitude of effect during IGF-1 supplementation. It could only be said, based on the above publications that, IGF-1 binding proteins (IGFBP) and receptors mitigates the effects of IGF-1. On this study, however, it is impossible to say whether or not that there is sufficient amount of serum binding proteins and receptors or even their total absence or presence in the circulation during the phase of production and age of the pig utilized. It could only be strongly suggested, based on the above literature, that, binding protein’s availability as well as the receptors, will dictate the site and mechanism of action of IGF-1. Thereby understanding that binding proteins and receptors are paramount to IGF-1 to cause an effect, it will nonetheless assist on explaining the results of this study.
Effect on Digestion of Orally Administered IGF-1
Burrin et al., in 1992 concluded that orally administered IGF-1 is stable to digestion during neonatal stage of the piglet. According to Schoknecht et al. (1993), exogenous IGF-1 (or analogue) treatment given to pigs does not feedback in a negative manner to inhibit endogenous IGF-1 secretion, thereby allowing the growth-promoting properties of this hormone to be expressed, his conclusion formed the basis of dose dependent IGF-1 feeding trials. However IGF-1 is rapidly degraded within the adult gastrointestinal tract (Xian et al., 1995; Rao et al., 1998), but it appears to survive digestion and exert effects within the neonatal intestine. On the current study, we cannot entirely conclude whether or not the orally supplemented IGF-1 was affected by gastro intestinal secretions. We can only refer to the above-mentioned studies that digestion will exert an effect on the availability of circulating IGF-1, in particular with older pigs.
Other Factors
The health and nutritional status of the animal may very well affect the magnitude of response. The absence of severe diarrhea is suggestive of a mature GIT, absence of pathogenic challenge or stable immune system, thus none or minimum effect of supplementation can be achieved on an apparently healthy animals. The average fecal score of Group C (IGF-1 at 0.5kg/ton) which is statistically significant has the value of 1.77 which leaned towards moist feces, thereby negating the ability of the treatment to reduce cases of diarrhea. The overall significance of the fecal score is with the time factor. When the analysis was done per week, no significant difference among treatments were observed. However, when day 1 to 28 was treated as one period, statistical difference was observed. Interestingly, the other treatments are statistically the same. However, this could be taken as an important factor for further studies for the optimum inclusion rate for the free IGF-1 on diets. Again, it must be noted that the experiment was conducted in weaned pigs that were apparently healthy and that pathogenic infection is low or none at all, with no other known disease challenge that manifested at the start of the feeding trial. The cause for these findings were, the least to say, unclear, but may be attributed to the timing, the quantity of diet obtained and amount consumed, the quality of IGF-1 incorporation in the diet, dietary composition, and age at weaning, that constitute differences on absorptive function in the GIT. Some of these are clearly the limitations of this study. In general, it is well-accepted that response to feed additives is dependent on age of the pig, disease level, environmental factors, and type of diet. It may be possible that at this stage of production, the pig has been well acclimatized on the diet showing consistent feed intake and weight gain. Healthy and well-fed animals with appropriate management, favorable environment, and minimal stress are generally less responsive to a feed additive.
CBC and TNF-α Concentration
Piglets are known to be susceptible to enteric infections due to not fully developed immunity and by contact to pathogens naturally present in the environment. (Stokes et al., 2004; Petrovic et al., 2009). IGF-1 has been proven effective in enhancing immunity of piglets by stimulating T-lymphocytes and natural killer cells (Brocardo et al., 2001; Smith, 2010), gastrointestinal maturation (Xu et al., 1994; Hartke et al., 2005), and nutrient absorption (Alexander and Carey, 1999). Enhancement of immunity can also be related to previous studies on dietary supplementation of free IGF-1 in sows which resulted to increase in survivability of piglets (Reyes et al., 2015; Song et al., 2014a and 2015b; Reyes et al., 2016). In the present study, nursery pigs at day 15 that were not supplemented with free IGF-1 have WBC levels higher than the normal values for pigs which is (24.86 vs. 11-22 x 103/µL; The Merck Veterinary Manual, 2016). This demonstrates the potential of supplementing free IGF-1 in enhancing immunity of nursery pigs during challenged conditions. The involvement of IGF-1 on red blood cell production has already been established (Miyagawa et al., 2000; Kling et al., 2006). The increased rate of RBC production leads to better oxygen transport in blood. In a study that involved cardiopulmonary bypass of piglets, intravenous infusion of IGF-1 improved oxygen transport by reducing oxygen consumption as well as increasing cardiac output and oxygen delivery during the first 6 hours post-operation (Li et al., 2004). Better oxygen transport promotes anabolism such as muscle tissue growth. Improvement in RBC indices has potential to improve growth rate and health of pigs even during times of stress. Exposure to micro-organisms commonly elicit the production of cytokines. These soluble factors enhance several innate immune functions that aim to limit the spread of infection. The onset of production of these cytokines is rapid, and several of them may reach systemic levels during a short period after infection. Thus, cytokines can serve as markers for ongoing infections and be used to discriminate between infections of bacterial or viral origin. Several bacterial compounds (peptidoglycan, lipotechoic acid, polysaccharide, various toxins, heat shock proteins, and super antigens) can induce the production of pro-inflammatory cytokines (Degre, 1996) and consequently also infections with Gram-positive bacteria can result in measurable blood levels of TNF-α , IL-6, IL-8, and G-CSF (Kragsbjerg et al., 1996). One study on porcine intestinal ion transport shows that TNF- α may be involved in the stimulation of NaCl secretion in the ileum (Alexander et al., 2001, 2002 and Kandil et al., 1994). Their data suggest that the increased expression of TNF- α may be implicated in the development of post-weaning diarrhea. Pro-inflammatory cytokines, such as TNFα, play a vital role in the normal host resistance to infection, serving as immunomodulators and as mediators of inflammatory signs (Kiarie et al., 2009) thus, identifying the changes in serum concentration of which could exhibit the effect of free IGF-1 supplementation on inflammatory response. The results illustrated that there is no statistical difference between the treated and non-treated groups with regards to the TNFα (pg/ml) serum concentration. This could imply that, since there are no severe clinical signs of diarrhea within the control and treatment groups, an immunomodulatory response through the pro-inflammatory cytokine, TNF-α, was demonstrated at lower values. A study reported by Nyachoti et al. (2012) reported that increased TNF-α increased from 111.4 to 162.1 pg/ml upon ETEC challenge which proves that during inflammatory encounter, TNFα serum concentration will be on upsurge. In a study done by Kruse et al. (2008), TNF-α increased in all animals after inoculation of Brachyspira hyodysenteriae, with a peak at 300 pg/ml. It must be emphasized that the release of TNF- α, and other inflammatory cytokines, are rapid and induced early in a disease or injury process. However, it’s not exclusive to viral and bacterial infections. A study by Pie et al. (2004) suggested that Cytokines play a central role in immune cell response, but they also participate in the maintenance of tissue integrity. Changes in the cytokine network of the pig gut may be expected at weaning, because abrupt changes in dietary and environmental factors lead to important morphological and functional adaptations in the gut. The current results, could suggest that the TNF-a levels present on the serum could indicate an inflammatory response or could be attributed to weaning. Stress brought by weaning commonly induce production of pro-inflammatory cytokines, such as TNF-α (Kiarie et al., 2009). Upon the presence of inflammation, nursery pigs supplemented with free IGF-1 could be able to immunomodulate or decrease the level of serum TNF-α , therefore reducing the signs of inflammation. However, attributing these values entirely to infection could not be determined due to the numerous factors affecting TNF-a release.
Concluding, the dietary supplementation of insulin growth factor-1 (IGF-1) had no significant effect on the production performance, serum concentration of tumor necrosis factor alpha and nutrient digestibility in weaned pigs. Collectively, results indicated that supplementation of free-IGF-1 in piglets decreased WBC and serum TNF-α levels while increasing RBC indices in piglets 2 weeks after weaning. We can also measure earlier (e.g. day 7) to see possible greater response. It can be concluded that supplementation of free-IGF-1 can lessen the negative impact of post-weaning stress particularly response to pathogen infection and inflammation. The study offered suggestive evidence for the role of dietary supplementation of IGF-1 on fecal consistency. No diarrhea was observed on the entire trial period, although pigs fed with control diet yielded the same fecal score. Further study should be focused on this potential. The study appears to support the argument that age of the pig is an important factor to consider. This would suggest that future research may done on neonatal pigs or at pre weaning period with a considerable population. It is relevant to relate levels of IGF-1 in the circulation, thus, blood profiling can be done periodically during the trial period. Future research into IGF-1 supplementation can also be done by measuring the weight and length of the small intestine, furthermore a histopathological examination can be performed to characterized the villous height and crypt depth. Modification of the basal diet can also be suggested. Other inflammatory cytokines can also be included as a measure of inflammation.