Copper plays an vital role in the synthesis of hemoglobin and the activation of several oxidative enzymes necessary for normal metabolism of pigs[7]. Several studies have revealed that the positive effects of high-grade copper in the feed of piglets are mainly growth promotion [15, 16] and antibacterial activity [4], higher nutritional levels of Cu (as CuSO4) at concentrations of 100 to 250 mg·kg-1 improved growth performance in young pigs [4, 17]. In our study, LC (6 mg·kg-1 Cu) seems unable to meet the nutritional needs of nursing piglets when antibiotics are withdrawn from creep feed because the G:F value decreased compared with the CON group (20 mg·kg-1 Cu), and the diarrhea rate significantly increased compared with the HC group (300 mg·kg-1 Cu) group. When piglets were fed a 20 mg·kg-1 Cu diet, they tended to gain more and eat less among the three groups across the entire experimental period, which would be adequate to meet the nursing piglets’ requirements. We also observed that 300 mg·kg-1 of dietary Cu enhanced ADG and ADFI during days 14 to 28, but ADG and the G:F value decreased during days 29 to 40. Taken together, these results suggest that high levels of dietary copper can promote short-term growth.
Human hair as an excretory system for trace metals, and considering that concentrations of these elements in hair are reported to be correlated with the diagnosis of various diseases [11, 18], hair samples have demonstrated long-term nutritional status compared to biofluids, which often serve as important indicators for mineral content monitoring and toxic metal accumulation in human [19], hair mineral analysis has become an interesting diagnostic tool in the assessment of health and nutritional status [20, 21]. In our study, of the 13 elements tested, concentrations of eight elements in hair were affected by levels of dietary copper, with hair Na, K, and Cu increasing in the HC group, suggesting that Na-K balance changes and reabsorption decreased. Concentrations in hair Fe, Mn, and Al increased in the CON group than in the HC group, and these elements share the same uptake mechanism (transferrin), and Fe, Mn, and Al are related to inflammation in humans and animals [22], corroborated by an observed increase in TNF-α in the CON group (see Additional file1, Table S2). Noteworthy changes were seen in the correlation pattern between macro and micro or toxic elements with increased copper levels in the diet (Fig. 2). The number of positive correlations between macro-micro elements in hair and serum increased in the CON group compared with other groups (Fig. 2b, e); further, the absorption, utilization, and excretion of many trace elements in animals are greatly affected by other trace elements [23], so these results suggest that dietary 20 mg·kg-1 Cu can maintain homeostasis in piglets due to maintenance of the interactions between macro and micro elements [2]. Correlations between hair macro (Na, K) and micro (Cu, Mn, Zn) elements appear in the HC group (Fig. 2c). As is well known, trace elements such as Cu, Zn, and Mn are essential for normal growth, disease resistance, production and reproduction in farm animals [2], suggesting that a dietary dose of 300 mg·kg-1 Cu changed the balance of Na-K and affected the absorption and utilization of Cu, Mn, and Zn. We also observed that a negative correlation of toxic (Pb and Al) elements appeared in the HC group. These changes could reflect imbalance or adverse status in these elements [22], and marginal or severe element imbalances can be considered risk factors for several diseases [24]. These results suggest that a dietary dose of 300 mg·kg-1 Cu might have adverse effects on nursing piglets’ health.
To understand the relationship between the ion balance and nursing piglets’ health status, correlations between ionomics profiles and serum biochemical parameters were analyzed (Fig. 3). Our previous works on serum biochemical parameters suggested that high dietary levels of copper may have a potential anti-inflammatory effect when antibiotics were withdrawn from the diet, which also had an obvious effect on the antioxidative status of nursing piglets. However, the liver dysfunction might occur in piglets when they fed with 6 and 300 mg·kg-1 Cu in diet [10]. Hair Na and K were positively correlated with IGF-1 and T-AOC and negatively correlated with TNF-α. These results suggested that changes in Na-K balance induced by 300 mg·kg-1 of dietary Cu were related to the inflammatory response and antioxidant capacity. The concentrations of Fe in hair, serum, and feces were positively correlated with MDA, ALT, and albumin, a relationship between Fe concentrations and hepatic function has been reported [25], and 300 mg·kg-1 of dietary Cu may affect the absorption of Fe and further damage the hepatic function, inducing oxidative stress in nursing piglets. Concentrations of Cu in hair, serum, and feces were negatively correlated with MDA, TBA, and BUN, suggesting that changes in Cu induced by 300 mg·kg-1 of dietary Cu affect the hepatic function and cause oxidative damage [26]. Dietary Cu at a concentration of 300 mg·kg-1 increased GH in nursing piglets was demonstrated by the correlation between fecal Cu and GH. The concentrations of Zn in hair and feces were positively correlated with MDA, T-AOC, and creatinine. Zn has the function of stabilizing cell membrane structure and protecting free radicals from oxidative damage [27, 28], and participates in the regulation of liver function and urea production. In general, high-level dietary copper affected the ionomics profiles, which adversely affects the health of nursing piglets.
As we know, fecal metabolites reflect the final status of animal digestion, absorption, and metabolism of feed nutrients [10]. Our previous analysis of significant fecal metabolites showed that the capacity of dietary monosaccharide and protein absorption decreased, and the level of organic acids was increased in nursing piglets fed with 6 mg·kg-1 Cu diet (see Additional file1, Fig. S1), these suggest that 6 mg·kg-1 Cu supplementation harms the health of piglets when antibiotic withdrawn from the feed [10]. In this study, we are concerned about changes in the composition of elements in hair, which reflect the body’s metabolism changes [24]. The correlation and enrichment analysis showed that the Na and K in hair which significantly affected by dietary copper, were negatively correlated with fecal inosine (Fig. 4), consistent with the result that hair Na and K were negatively correlated with serum TNF-α (Fig. 3), due to the inosine is a purine metabolite and has a systemic anti-inflammatory effect [29]. The hair Cu was significantly increased in the HC group than the LC group and negatively correlated with phenylalanine and tyrosine metabolism pathways (Fig. 5), dopamine β-hydroxylase (DBH) and phenylalanine hydrolase are two key enzymes in these pathways which its cofactor is copper [30, 31]. The hair Cu was negatively correlated with the mitochondrial electron transport chain pathway (Fig. 5). Various enzymes in this pathway use copper as a cofactor, such as a cytochrome c oxidase and nicotinamide adenine dinucleotide (NADH) dehydrogenase [30-32]. The activity of the electron transport chain is related to the generation of reactive oxygen species (ROS) and the body's redox state [33, 34]. In this study, dietary 300 mg·kg-1 Cu enhanced the mitochondrial electron transport chain pathway, which promoted the formation of ROS and affecting the redox status of piglets, these verified by the results of negative correlation between hair Cu and serum MDA (Fig. 4).
The fecal Cu content mainly comes from the accumulation of unabsorbed copper in the diet. In our study, fecal Cu was negatively correlated with the betaine metabolism pathway. Betaine is an important methyl donor in the process of homocysteine synthesis of methionine and an important intermediate in the process of energy metabolism [35, 36], the process by which betaine provides methyl groups is mainly catalyzed by betaine homocysteine transferase (BHMT), a cytoplasmic enzyme that relies on zinc activation [37], due to the antagonism between copper and zinc [2], suggested that high-level dietary copper hinder the absorption of zinc to a certain extent, and inhibit the activity of BHMT as well as the function of betaine methyl donor. In the process of betaine producing methionine, homocysteine is also a substrate for the enzyme action of BHMT. When the activity of BHMT decreases, the level of homocysteine in the blood rises, which has a certain relationship with vascular disease, thrombosis, and renal dysfunction [38, 39], these suggested that 300 mg·kg-1 Cu inhibited the methyl supply capacity of betaine and further affected protein biosynthesis and the health of nursing piglets. Our previous study found that the level of pantothenic acid was decreased in the HC group [10]. The enrichment analysis in this study showed that the fecal Cu was negatively correlated with pantothenate and CoA biosynthesis pathway, pantothenic acid plays an important role in the process of decomposing carbohydrates, fatty acids, and amino acids to produce energy [40]. These results suggest that dietary high-level copper affected the digestion and absorption of diets by piglets, which affected the biosynthesis of pantothenic acid and CoA, further affecting carbohydrate, fat, amino acid, and energy metabolism of nursing piglets, leading to changes in metabolic homeostasis.