No abstract

Medium‐chain fatty acids (MCFA) are often included in neonatal milk formulas as a more readily oxidized source of energy compared with long‐chain fatty acids (LCFA). This rapid oxidation of MCFA is thought to enhance growth. Thus, the objectives of this study were to evaluate the effect of energy supplementation and fat source on growth and body composition, and determine the rate of Krebs Cycle substrate utilization in neonatal pigs. Eleven three‐day‐old pigs were allocated to one of three sow milk replacer formulas: a control (CONT) and diets rich in either LCFA or MCFA. The CONT formula provided 80%, whereas the MCFA and LCFA formulas provided 120%, of the metabolizable energy requirements of neonatal pigs. Pigs were fed for 20 days and body composition measured by DXA before initiation and on day 20 of feeding. At the end of the study, pigs were euthanized and muscles and organs collected and weighed. Livers were digested with collagenase and isolated hepatocytes were incubated in media containing all amino acids, glucose, and short‐chain fatty acids in the presence of [U13C] labeled alanine, glucose, propionate, and glutamate for 90 min. The enrichment of Krebs Cycle intermediates was determined using gas chromatography/mass spectrometry. Body weight of pigs in the LCFA group was greater than those in the MCFA and CONT groups on days 18 and 20. In addition, percent fat measured by DXA was greater for MCFA (8%) and LCFA (7%) than those in the CONT (4%) group, and this increase in fat deposition occurred at the expense of lean tissue (P < 0.05). Longissimus dorsi and heart weights as percent of body weight were similar for all groups; however, soleus weight was less for MCFA compared with those in the CONT and LCFA groups. In addition, liver and kidney weights as a percentage of body weight were greater for pigs in the MCFA group compared with those in the CONT group with LCFA being intermediate (P< 0.05). There was no effect of formula feeding on glucose and alanine contribution to lactate synthesis in isolated hepatocytes, but this contribution was greater for glucose (36%) than for alanine (15%) (P<0.05). Conversely, the contribution of alanine (35%) carbon to pyruvate was greater than the contribution of glucose (7%) (P<0.05). Furthermore, in hepatocytes of pigs fed CONT formula 39% of pyruvate carbon was derived from alanine and 31–34% for pigs in the LCFA and MCFA groups (P<0.05). Lastly, glutamate contributed 64% of α‐ketoglutarate carbon in hepatocytes of CONT and MCFA and only 40% for LCFA pigs. These data suggest that dietary MCFA increase body fat without increasing lean tissue accretion compared with CONT. In addition, anaplerosis from glutamate was greater for pigs fed MCFA and CONT than for those fed LCFA.

Medium (MCFA) and long (LCFA) chain fatty acids are often added to enhance the caloric value of infant formulas. Evidence suggests that MCFA promote growth, and are preferred over LCFA due to greater digestibility and ease of absorption. Our hypothesis was that MCFA supplementation would enhance neonatal pig growth to a greater extent than LCFA. Neonatal pigs (n=4) were fed a low energy control (CONT), or two isocaloric high energy formulas containing fat either from LCFA, or MCFA for 20 d. Pigs fed the LCFA had greater body weight compared to CONT and MCFA fed pigs (P < 0.05). In addition, pigs fed the LCFA and MCFA had more body fat than those in the CONT group. Liver and kidney weights as a percentage of body weight were greater (P ≤ 0.05) for pigs fed the MCFA than those fed the CONT formula with those fed LCFA intermediate (P ≤ 0.05). Pigs in the CONT and LCFA groups had less liver fat (12%) compared with those in the MCFA (26%) group (P ≤ 0.05). Isolated hepatocytes from these pigs where incubated in media containing [13C]tracers of alanine, glucose, glutamate and propionate. Our data suggest alanine contribution to pyruvate is less in hepatocytes from LCFA and MCFA pigs than those in the CONT group (P < 0.05). These data suggest that a formula rich in MCFA caused steatosis compared with an isocaloric LCFA formula. In addition, MCFA feeding can alter hepatocyte metabolism and increase total body fat without increasing lean deposition.

Lean growth is impaired in low birth‐weight (LBWT) neonatal pigs and correlates with a reduction in protein synthesis rate in skeletal muscle. Translation initiation via mTOR signaling pathway is the rate limiting step in protein synthesis. Leucine binds and inactivates Sestrin2, an inhibitor of the mTORC1 complex. The objective of this study was to determine whether dietary branched‐chain amino acids (BCAA) stimulate translation initiation in skeletal muscle of LBWT pigs. LBWT and normal birth weight (NBWT) were defined when weight at birth was 2 SD below or within 0.5 of the corresponding litter average. Three day old LBWT and NBWT pigs (n=11‐13/group) were assigned to a Control or BCAA sow milk‐replacer formula in a 2×2 factorial arrangement. Formulas were isonitrogenous and contained either 1% BCAA (1:0.68:0.5 leucine:isoleucine:valine) or 0.7% alanine, and were offered at 250 mL·kg body weight−1·day−1 in 5 equal meals. Carotid and jugular catheters were surgically placed on day 21. On day 29, blood samples were taken over 4 h following a meal. Pigs were euthanized 1 h after a final meal. Data were analyzed using MIXED procedure of SAS version 9.4. BCAA supplementation increased BCAA concentration in plasma (P<0.0001) and in muscle free amino acid pool (P<0.001) compared to the Control diet, which similarly increased alanine concentration in plasma and muscle (P<0.0001). Although BCAA supplementation decreased circulating Pro, Ser, Asp, and Lys (P<0.001, 0.05, 0.05, 0.06, respectively), concentrations were not different in skeletal muscle. LBWT pigs expressed less BCAT2 mRNA in the longissimus dorsi than NBWT pigs, but expression was enhanced to normal levels with BCAA supplementation (P<0.05). BCKDHA expression was greater in the liver of LBWT compared to NBWT pigs irrespective of diet (P<0.05). Circulating branched‐chain α‐keto acids increased only in response to BCAA supplementation (P<0.0001). LBWT pigs expressed less Sestrin2 in skeletal muscle (P<0.01), suggesting enhanced sensitivity to leucine. However, despite lower expression of Sestrin2 and increased intramuscular BCAA concentrations, 4EBP1 and S6K1 phosphorylation were not affected by BCAA supplementation. These data suggest that branched‐chain amino acid supplementation did not enhance translation initiation in muscles of neonatal pigs regardless of birth weight.

Postnatal muscle growth is impaired in low birth weight (LBWT) neonatal pigs. Dietary leucine supplementation has been established as a dietary intervention to enhance muscle growth in growing animals. The aim of this study was to investigate the efficacy of supplementing LBWT neonatal pig diets with branched-chain amino acids (BCAA) to enhance the rate of protein accretion. Twenty-four pigs were housed in individual cages starting at 1 d of age. LBWT and normal birth weight (NBWT) were defined when birth weight was 2 SD below or within 0.5 of the litter average. Each LBWT pig was matched with a NBWT same sex littermate and assigned to either the control diet or supplemented with 1% BCAA in a 2×2 factorial arrangement. Diets were isocaloric, isonitrogenous and met NRC requirements. Pigs were fed 250 ml/kg BW/day of control or BCAA sow milk replacers. Carotid and jugular catheters were surgically placed on day 21 for blood sampling. Body composition was measured by dual x-ray absorptiometry on day 0 and 28. On day 29, blood samples were taken over 4 h following a meal. Data were analyzed using MIXED procedure of SAS version 9.4. Plasma leucine, isoleucine, and valine concentrations were greater in pigs fed BCAA (P &lt; 0.001). LBWT pigs had lower weight gain (P &lt; 0.001), average daily gain (P &lt; 0.01), and feed conversion (P &lt; 0.0001), and accreted protein more efficiently (P &lt; 0.0001) than NBWT pigs regardless of diet. Although BCAA supplementation had no effect, these data suggest that LBWT pigs grow leaner and more efficiently than NBWT pigs when limit-fed.