Dietary glycine supplementation enhances postweaning growth and meat quality of pigs with intrauterine growth restriction

Abstract

Pigs with intrauterine growth restriction (IUGR) have suboptimum growth performance and impaired synthesis of glycine (the most abundant amino acid in the body). Conventional corn- and soybean meal-based diets for postweaning pigs contain relatively low amounts of glycine and may not provide sufficient glycine to meet requirements for IUGR pigs. This hypothesis was tested using 52 IUGR pigs and 52 litter mates with normal birth weights (NBW). At weaning (21 d of age), IUGR or NBW pigs were assigned randomly to one of two nutritional groups: supplementation of a corn–soybean meal-based diet with either 1% glycine plus 0.19% cornstarch or 1.19% L-alanine (isonitrogenous control). Feed consumption and body weight (BW) of pigs were recorded daily and every 2 or 4 wks, respectively. All pigs had free access to their respective diets and clean drinking water. Within 1 wk after the feeding trial ended at 188 d of age, blood and other tissue samples were obtained from pigs to determine concentrations of amino acids and meat quality. Neither IUGR nor glycine supplementation affected (P > 0.05) feed intakes of pigs per kg BW. The final BW, gain:feed ratio, carcass dressing percentages, and four-lean-cuts percentages of IUGR pigs were 13.4 kg, 4.4%, 2%, and 15% lower (P < 0.05) for IUGR pigs than NBW pigs, respectively. Compared with pigs in the alanine group, dietary glycine supplementation increased (P < 0.05) final BW, gain:feed ratio, and meat a* value (a redness score) by 3.8 kg, 11%, and 10%, respectively, while reducing (P < 0.05) backfat thickness by 18%. IUGR pigs had lower (P < 0.05) concentrations of glycine in plasma (−45%), liver (−25%), jejunum (−19%), longissimus dorsi muscle (−23%), gastrocnemius muscle (−26%), kidney (−15%), and pancreas (−6%), as compared to NBW pigs. In addition, dietary glycine supplementation increased (P < 0.05) concentrations of glycine in plasma and all analyzed tissues. Thus, supplementing 1% of glycine to corn–soybean meal-based diets improves the growth performance, feed efficiency, and meat quality of IUGR pigs.

Pigs with intrauterine growth restriction require supplemental glycine for improvements in growth, feed efficiency, and meta quality, when fed conventional corn- and soybean meal-based diets. Our results support the notion that these pigs do not synthesize sufficient glycine during the period from weaning to market weight. This finding has important implications for developing new nutritional strategies to raise low-birth weight pigs and improve the efficiency of pork production.

Introduction

Approximately 15–20% of newborn pigs exhibit intrauterine growth restriction, with birth weights being less than 1.1 kg for breeds with Duroc, Hampshire, Landrace, and Yorkshire genetic backgrounds (Quiniou et al., 2002; Wu et al., 2006; Ali et al., 2021). Without proper nutritional intervention, 76% of intrauterine growth restriction (IUGR) pigs will die before weaning (Wu et al., 2010), primarily due to physical weakness (Vallet et al., 2013), intestinal dysfunction or abnormality (Wang et al., 2005, 2010; D’Inca et al., 2010), malnutrition (Santos et al., 2022), and hyperammonemia (Ji et al., 2017). IUGR pigs that survive weaning have lower growth rates and diminished feed efficiency (Rodrigues et al., 2020; Posey and Davis, 2023), often requiring an additional wk of feeding to reach market weight, compared to their litter mates with normal birth weights (NBW; Ji et al., 2017). In addition, the carcass yields, total areas of desired lean cuts, and meat quality of IUGR pigs at market weight are ­generally reduced, while percentages of undesirable fat depots are greater than those for NBW littermates (Nissen and Oksbjerg, 2011). Thus, IUGR pigs are usually culled at birth by producers (Geiping et al., 2022). This represents a substantial loss in the productivity of sows and economic returns, as well as a considerable waste of pigs that could otherwise be beneficial to the swine enterprise.

We have reported that IUGR pigs have an impaired ability to synthesize glycine (Hu et al., 2023b), which is the most abundant amino acid (AA) in the plasma and tissue proteins of swine (Flynn et al., 2000; Wu and Li, 2022), while also being a nutritionally essential AA for young pigs (Wang et al., 2014). Furthermore, glycine stimulates protein synthesis and inhibits proteolysis in skeletal muscle cells through the activation of the mechanistic target of rapamycin (MTOR) cell signaling via yet unknown mechanisms (Sun et al., 2016; Caldow et al., 2019), while reducing whole-body white fat mass (El Hafidi et al., 2004). In the United States and many regions of the world, soybean meal (SBM) is used widely as the primary protein source in diets for postweaning pigs due to its desirable price, high-protein content, and balanced profiles for most AAs (ASA, 2022). However, SBM contains a relatively low amount of glycine (only 2.3% on an as-fed basis; Li and Wu, 2020), which may be a limiting factor for the maximum growth potential of postweaning pigs (Powell et al., 2011; Wang et al., 2014; Li and Wu, 2018; Silva et al., 2020), particularly IUGR pigs (Hu, 2017). In support of this view, compared with the isonitrogenous control (2.37% L-alanine), supplementing 2% glycine to an SBM-based diet [containing 21% crude protein (CP)] for 7 d improved the growth performance of weanling NBW pigs (Ji et al., 2022). In addition, supplementing 0.52% glycine to an SBM-based low-protein (13% CP) diet with a 0.34% mixture of Lys, Met, Thr, Trp, Ile, and Val for 28 d restored the growth performance of postweaning NBW pigs to that for NBW pigs fed an SBM-based diet (containing 18% CP) with no Lys supplementation (Powell et al., 2011). However, there is a paucity of information regarding effects of long-term glycine supplementation on the growth and meat quality of IUGR or NBW pigs between weaning and market weight, an important issue in swine nutrition. The objective of the present study was to test the hypothesis that an SBM-based diet without supplementation may not provide sufficient glycine to meet requirements for postweaning IUGR pigs.

Materials and Methods

This study was approved by The Institutional Animal Care and Use Committee of Texas A&M University (Animal Use Protocol No. 2016-0273) in accordance with the Animal Welfare Act and Regulations of the United States Department of Agriculture.

Animals and diets

Pigs were the offspring of Yorkshire × Landrace sows bred to Duroc × Hampshire boars and housed in the Animal Science Teaching, Research and Extension Complex (ASTREC) at Texas A&M University. Sows (first parity) were fed conventional corn- and SBM-based diets containing 12% CP and 18% CP during gestation and lactation periods, respectively, as described previously (Li et al., 2014; Rezaei et al., 2022). The birth weights of IUGR and NBW pigs ranged from 0.77 to 1.09 kg and 1.39 to 1.50 kg, respectively. At 21 d of age, IUGR pigs were weaned and fed corn- and SBM-based diets (Table 1). Based on their birth weights and litter of origin, 21-d-old IUGR and NBW pigs were assigned randomly into one of the following four treatment groups: 1) IUGR pigs supplemented with 1% glycine + 0.19% cornstarch (IUGR-Gly); 2) IUGR pigs supplemented with 1.19% L-alanine as the isonitrogenous control (IUGR-Ala); 3) NBW pigs supplemented with 1% glycine + 0.19% cornstarch (NBW-Gly); and 4) NBW pigs supplemented with 1.19% L-alanine (NBW-Ala). The treatments were arranged in a 2 × 2 factorial design. There were 2 to 4 pigs per pen according to the availability of IUGR piglets at 21 d of age. The corresponding birth weights (kg) of pigs in the treatment groups were: NBW-Ala, 1.45; NBW-Gly, 1.43; IUGR-Ala, 0.99; and IUGR-Gly, 0.99. Because 10 sows farrowed within a 24-h period in our swine facilities, 8 to 16 IUGR piglets and 8 to 16 NBW pigs were available at weaning (21 d of age) on the same day (therefore providing 2 to 4 IUGR pigs per pen in either the Ala or the Gly group). The number and sex of 21-d-old pigs in the treatment groups were: NBW-Ala, 30 (17 barrows and 13 gilts); NBW-Gly, 30 (15 barrows and 15 gilts; IUGR-Ala, 27 (14 barrows and 13 gilts); and IUGR-Gly, 27 (13 barrows and 14 gilts). The preweaning growth of NBW and IUGR pigs occurred as expected in our swine facilities (Rezaei et al., 2011). At 21 d of age (the initial day of AA supplementation), the mean body weights (kg) of pigs in the IUGR-Ala and IUGR-Gly groups were 5.12 and 5.26 kg, respectively (P = 0.335). The presence of an imbalanced distribution of animals per pen may inherently impact the variability among the experimental units. There were three pens for each of the four treatment groups in one replicate of the feeding trial. In this study, there were a total of 12 pens (4 replicates) for each treatment group, with the number of IUGR pigs being the same as the number of NBW pigs at 21 d of age.

Table 1.

Content of nutrients and energy in the basal diets for growing and finishing pigs¹

Nutrients, as-fed basis	Age of pigs (days)
	21–642	64–923	92–1484	148–1885
Dry matter, %	90.13	90.22	89.37	89.40
Crude protein, %	19.95	17.93	16.02	14.01
Starch, %	37.02	41.13	46.93	49.91
Crude fiber, %	8.59	9.20	10.32	10.40
Crude fat, %	7.12	4.67	4.04	4.10
Minerals, %	17.31	17.20	11.89	10.82
Vitamins plus choline	0.14	0.15	0.17	0.16
Metabolizable energy, kcal/kg diet	3408	3394	3299	3262

¹Pigs were weaned at 21 d of age. The basal diet of each feeding phase (prepared by Producer’s Co-Operative Feed Mill in Bryan, TX) was supplemented with either 1% glycine plus 0.19% corn starch (treatment group) or 1.19% L-alanine (isonitrogenous control). The content of nutrients in the basal diets was determined using the AOAC (1990) methods, and the content of metabolizable energy was calculated according to NRC (2012) values for the ingredients used.

²The ingredients of the 20%-crude protein (CP) basal diet (%) were: corn grain, 55.545; soybean meal (48% CP), 26.5; fishmeal (menhaden), 2; flaxseed, 4; NUTRI-SURE, 1; soybean oil, 2; coconut oil, 0.5; L-lysine (80%), 0.33; DL-methionine (98%), 0.03; L-threonine, 0.1; dicalcium phosphate, 1.35; ground limestone, 1; NaCl, 0.25; trace-mineral premix, 0.15; vitamin premix, 0.5; Verxite (a magnesium-aluminum-iron silicate), 0.5; sodium bentonite (a mineral clay), 0.5; NUPRO-2000 (a yeast extract product), 0.2; yeast culture, 0.5; yeast extract, 0.1; citric acid, 0.15; propionic acid, 1; fumaric acid, 1; benzoic acid, 0.5; dried fermentation products of probiotics (Enterococcus faecium, Lactobacillus acidophilus, Aspergillus niger, Trichoderma longibrachiatum, and Bacillus subtillis), 0.05 (0.01 for each product); neohesperidin dihydrochalcone (an artificial sweetener derived from citrus), 0.003; butylated hydroxytoluene (an antioxidant preservative), 0.01; butylated hydroxyanisole (an antioxidant preservative), 0.01; L-carnitine, 0.01; and Yucca schidigera extract (an anti-glucocorticoid and anti-oxidative substance), 0.012. The basal diet contained the following amino acids (%): Ala, 1.07; Arg, 1.31; Asn, 0.87; Asp, 1.27; Cys, 0.33; Gln, 1.79; Glu, 1.77; Gly, 1.02; His, 0.51; Ile, 0.86; Leu, 1.73; Lys, 1.36; Met, 0.36; Phe, 0.98; Pro, 1.34; Ser, 0.96; Thr, 0.87; Trp, 0.25; Tyr, 0.78; and Val, 0.95; the following macro-minerals (%): Ca, 0.99%; P, 0.74; Na, 0.16; Cl, 0.21; K, 0.81; S, 0.24; and Mg, 0.19; the following micro-minerals (mg/kg diet): Mn, 74.9; Fe, 529; Cu, 37.9; Co, 1.13; Zn, 206; I, 1.03; and Se, 1.05; and the following vitamins (mg/kg diet): retinyl acetate, 3.96; D-α-tocopherol, 68.6; menadione, 2.20; cholecalciferol, 0.069; riboflavin, 10.1; niacin, 54.8; pantothenic acid, 31.1; vitamin B₁₂, 0.30; D-biotin, 0.48; pyridoxine, 7.20; thiamin, 5.57; folate, 3.63; and choline, 1198.

³The ingredients of the 18%-CP basal diet (%) were: corn grain, 61.735; soybean meal (48% CP), 23; fishmeal (menhaden), 1; flaxseed, 2; soybean oil, 1; coconut oil, 0.5; rice bran, 2; wheat middlings, 1; L-lysine (80%), 0.28; DL-methionine (98%), 0.02; L-threonine, 0.07; dicalcium phosphate, 1.25; ground limestone, 0.9; NaCl, 0.35; trace-mineral premix, 0.15; vitamin premix, 0.5; Verxite, 0.5; sodium bentonite, 0.5; NUPRO-2000, 0.2; yeast culture, 0.25; yeast extract, 0.05; citric acid, 0.15; propionic acid, 1; fumaric acid, 1; benzoic acid, 0.5; dried fermentation products of probiotics (Enterococcus faecium, Lactobacillus acidophilus, Aspergillus niger, Trichoderma longibrachiatum, and Bacillus subtillis), 0.05 (0.01 for each product); neohesperidin dihydrochalcone, 0.003; butylated hydroxytoluene, 0.01; butylated hydroxyanisole, 0.01; L-carnitine, 0.01; and Yucca schidigera extract, 0.012. The basal diet contained the following amino acids (%): Ala, 0.98; Arg, 1.14; Asn, 0.78; Asp, 1.11; Cys, 0.31; Gln, 1.65; Glu, 1.55; Gly, 0.89; His, 0.46; Ile, 0.77; Leu, 1.61; Lys, 1.16; Met, 0.31; Phe, 0.88; Pro, 1.26; Ser, 0.85; Thr, 0.76; Trp, 0.22; Tyr, 0.71; and Val, 0.85; the following macro-minerals (%): Ca, 0.86; P, 0.71; Na, 0.19; Cl, 0.26; K, 0.75; S, 0.22; and Mg, 0.18; the following micro-minerals (mg/kg diet): Mn, 75.4; Fe, 508; Cu, 35.1; Co, 1.14; Zn, 201; I, 1.02; and Se, 1.07; and the following vitamins (mg/kg diet): retinyl acetate, 4.08; D-α-tocopherol, 71.3; menadione, 2.64; cholecalciferol, 0.069; riboflavin, 10.7; niacin, 53.7; pantothenic acid, 31.9; vitamin B₁₂, 0.31; D-biotin, 0.45; pyridoxine, 6.61; thiamin, 5.88; folate, 3.72; and choline, 1328.

⁴The ingredients of the 16%-CP basal diet (%) were: corn grain, 69.98; soybean meal (48% CP), 19.2; soybean oil, 0.5; rice bran, 5; wheat middlings, 2; L-lysine (80%), 0.23; dicalcium phosphate, 1.15; ground limestone, 0.85; NaCl, 0.47; trace-mineral premix, 0.15; and vitamin premix, 0.5. The basal diet contained the following amino acids (%): Ala, 0.91; Arg, 0.98; Asn, 0.69; Asp, 0.97; Cys, 0.29; Gln, 1.53; Glu, 1.35; Gly, 0.78; His, 0.41; Ile, 0.68; Leu, 1.50; Lys, 0.96; Met, 0.26; Phe, 0.79; Pro, 1.20; Ser, 0.77; Thr, 0.62; Trp, 0.19; Tyr, 0.65; and Val, 0.77; the following macro-minerals (%): Ca, 0.75; P, 0.69; Na, 0.21; Cl, 0.33; K, 0.73; S, 0.21; and Mg, 0.22; the following micro-minerals (mg/kg diet): Mn, 69.6; Fe, 218; Cu, 26.4; Co, 0.13; Zn, 158; I, 0.62; and Se, 0.44; and the following vitamins (mg/kg diet): retinyl acetate, 3.92; D-α-tocopherol, 73.8; menadione, 1.10; cholecalciferol, 0.041; riboflavin, 7.22; niacin, 81.3; pantothenic acid, 32.4; vitamin B₁₂, 0.033; D-biotin, 0.27; pyridoxine, 9.12; thiamin, 6.65; folate, 1.45; and choline, 1467.

⁵The ingredients of the 14%-CP basal diet (%) were: corn grain, 75.1; soybean meal (48% CP), 14.5; rice bran, 5; wheat middlings, 2; dicalcium phosphate, 1; ground limestone, 0.75; NaCl, 0.5; trace-mineral premix, 0.15; and vitamin premix, 0.5. The basal diet contained the following amino acids (%): Ala, 0.85; Arg, 0.83; Asn, 0.60; Asp, 0.82; Cys, 0.26; Gln, 1.38; Glu, 1.16; Gly, 0.68; His, 0.37; Ile, 0.59; Leu, 1.38; Lys, 0.66; Met, 0.24; Phe, 0.70; Pro, 1.12; Ser, 0.68; Thr, 0.53; Trp, 0.16; Tyr, 0.59; and Val, 0.68; the following macro-minerals (%): Ca, 0.65; P, 0.64; Na, 0.24; Cl, 0.35; K, 0.56; S, 0.18; and Mg, 0.16; the following micro-minerals (mg/kg diet): Mn, 50.7; Fe, 192; Cu, 23.1; Co, 0.057; Zn, 154; I, 0.61; and Se, 1.09; and the following vitamins (mg/kg diet): retinyl acetate, 4.1; D-α-tocopherol, 72.5; menadione, 1.10; cholecalciferol, 0.041; riboflavin, 7.11; niacin, 65.9; pantothenic acid, 30.6; vitamin B₁₂, 0.037; D-biotin, 0.23; pyridoxine, 7.91; thiamin, 5.43; folate, 1.43; and choline, 1412.

All pigs were maintained in pens (1.5 m × 2 m/pen) in a climate-controlled nursery room from d 21 to d 92 of age at ASTREC’s Swine Center, in a climate-controlled grower room with bigger pens (2.5 m × 3 m/pen) in ASTREC’s Nutrition & Physiology Center from d 92 to d 127 of age, and then in outside pens (2.5 m × 5 m/pen) of ASTREC’s Swine Center from d 127 to d 188 of age. The basal diet for each growing phase met the NRC (2012)-recommended requirements of swine for all nutrients, including total AAs. The basal 20%-CP, 18%-CP, and 16%-CP diets contained one to three added crystalline AAs, as indicated in Table 1. The CP content of diets for the pigs were as follows: 20% from d 21 to d 64 of age; 18% from d 64 to d 108 of age; 16% from d 108 to d 146 of age; and 14% from d 146 to d 188 of age. Supplemental glycine accounted for 98%, 112%, 128%, and 147% of the glycine present in the basal diets containing 20%, 18%, 16%, and 14% CP, respectively.

Beginning at 21 d of age, pigs had free access to clean drinking water and their respective diets (provided once daily at 8:00 AM) and were weighed individually at 10:00 AM every 2 or 4 wks. The feed intake of pigs in each pen was recorded daily. At 138 d of age, each pig was vaccinated with 2 mL Suvaxyn RespiFend MH/HPS (Zoetis, Parsippany-Troy Hills, NJ) via intramuscular injection against Hemophilus parasuis and Mycoplasma hyopneumoniae bacteria). At 159 d of age, each pig was vaccinated with 2 mL Suvaxyn Parvo/E via intramuscular injection against Erysipelothrix rhusiopathiae-induced erysipelas.

During the entire study, the numbers of deaths were: one barrow in one pen and one gilt in another pen in the ­NBW-Ala group, one barrow in one pen in the NBW-Gly group, one barrow in one pen and one gilt in another pen in the IUGR-Ala group, and one barrow in one pen and one barrow in another pen in the IUGR-Gly group; and the numbers of injured pigs were: one barrow in one pen in the NBW-Ala group, one gilt in one pen in the NBW-Gly group, one gilt in one pen in the IUGR-Ala group, and one gilt in one pen in the IUGR-Gly group. Thus, the numbers of pigs that were either dead or injured in the treatment groups were: three in the NBW-Ala group, two in the NBW-Gly group, three in the IUGR-Ala group, and three in the IUGR-Gly group. The dead or injured pigs were not present in the same pen and were removed from the study when the incidences occurred, without affecting the number of pens per treatment group. On the last day of the feeding trial, all pigs (188 d of age) were weighed.

Analysis of meat quality

Because of high costs, only those pigs in the last three replicate pens per treatment group were used for the analysis of meat quality. The number of pigs in the treatment groups was: IUGR-Gly, 6; IUGR-Ala, 7; NBW-Gly, 10; and NBW-Ala, 9. These 32 pigs continued to be fed their respective diets before they were slaughtered on 2 days (d 189 and d 196 of age; 16 pigs/d) according to hazard analysis and critical control points (HAACP) protocols at the Texas A&M University’s Rosenthal Meat Science and Technology Center for the collection of tissues, as described by Maddock et al. (2002). On the first day of slaughter (d 189 of age), 1 to 2 pigs from each pen were selected randomly (a total of 16 pigs), depending on the number of pigs in the pens, and the remaining pigs were slaughtered on the second day (d 196 of age). Briefly, 1 h before slaughter, pigs were transported from ASTREC to the Rosenthal Meat Science and Technology Center at Texas A&M University (about a 5-min drive). Approximately 2 h after food removal, each pig was electrically stunned and bled by the puncture of the jugular vein for the collection of 10 mL blood samples into heparinized tubes. Thereafter, tissues (~10 g each), including liver, jejunum, pancreas, kidney, longissimus dorsi muscle (the 10th rib of the back-loin), and gastrocnemius muscle were immediately obtained, placed in zip-locked bags, and immersed in liquid nitrogen. Aliquots of blood samples (1 mL) were immediately centrifuged at 10,000 × g for 1 min, and plasma (the supernatant fluid) was harvested. Frozen tissues were stored at −80°C.

After the tissue collection and evisceration, hot (unchilled) carcass was weighed, and its weight was divided by live weight to calculate dressing percentage. The carcass was then split through the center of the vertebral column and down the midline and was chilled at 4°C in a cooler for the determination of right-side carcass and meat quality traits, as described previously (Maddock et al., 2002; Savell and Smith, 2021). Briefly, muscle pH was measured in the M. longissimus thoracis at 45 min and at 24 h after chilling with the use of a digital pH meter (Model IQ 150; Spectrum Technologies, Aurora, IL). At 24 h after chilling, carcasses were evaluated for the following traits: 1) the weight of boneless four lean cuts (ham, loin, picnic shoulder, and Boston butt); 2) USDA muscle score (thick = 3.0, intermediate = 2.0, and thin = 1); 3) backfat thickness over the 10th rib; 4) the loineye muscle area of the M. longissimus thoracis between the 10th and 11th ribs, using a dot grid; 5) marbling score (1.0 to 10.0, corresponding to intramuscular lipid content), which was determined in the M. longissimus thoracis; (6) meat color score (1.0 = pale pinkish gray to white; 3.0 = reddish pink; 6.0 = dark purplish red), which was measured in three different locations of the M. longissimus thoracis using a Hunter MiniScan EZ (Model 4500L; Hunter Labs, Inc. Reston, VA; 31.8 mm aperture, Illuminant D65, 10° observer) colorimeter and averaged to represent the value for each loineye; (7) drip loss of the M. longissimus thoracis; and (8) Commission Internationale de l’Eclairage (CIE) L*, a*, and b* color space values in the M. longissimus thoracis. To ensure the accuracy of the CIE values, the Hunter MiniScan EZ was calibrated after every 60^th measurement using manufacturer’s white and black reference tiles.

Analysis of Amino acids in plasma and tissues

AAs in plasma and tissues (liver, jejunum, pancreas, kidney, longissimus dorsi muscle, and gastrocnemius muscle) were analyzed using high-performance liquid chromatography (HPLC) as described previously (Wu and Knabe, 1994). Briefly, 100 µL plasma was deproteinized with 100 µL of 1.5 M HClO₄, followed by the addition of 2.25 mL of HPLC-grade H₂O and 50 µL of 2 M K₂CO₃. In addition, 100 mg tissue was homogenized in 2 mL of 1.5 M HClO₄, followed by the addition of 1 mL of 2 M K₂CO₃. The neutralized solution was centrifuged at 600 × g for 10 min to obtain the supernatant fluid. The latter was used for precolumn derivatization with o-phthaldialdehyde in the HPLC’s autosampler, and the fluorescent derivatives of AAs (except for cysteine) were separated in the HPLC column and detected at excitation and emission wavelengths of 340 nm and 450 nm, respectively. For the analysis of cysteine, a 50 µL of neutralized sample was mixed with 50 µL of 25 mM iodoacetic acid (an alkylating agent) for 10 min at 25°C to convert cysteine into S-carboxymethylcysteine. The latter was then analyzed through precolumn derivatization with o-phthaldialdehyde as described above.

Composition of nutrients and collagen in meat

The content of water, crude protein (including collagen), crude fat, minerals (ash), and carbohydrates in meat (the M. longissimus thoracis) was determined using the proximate analysis method (AOAC, 1990). The specific method numbers for the determination of nutrients are: 934.01, dry matter; 954.01, crude protein; 920.39, crude fat; and 942.05, ash. The content of carbohydrate was calculated as the difference between dry matter and the sum of crude protein plus crude fat plus ash. The content of collagen in meat was determined on the basis of the content of 4-hydroxyproline, as described previously (Wu et al., 2016).

Statistical analyses

Results are expressed as means ± SEM. All data were first tested for normality using the Shapiro-Wilk W Test in the JMP 15 Pro software (Cary, NC), and their normal distribution was confirmed by a probability of > 0.05. Neither litter origin nor the number of pigs per pen was used as a block in the statistical analysis of data. A completely randomized design was not used for data analysis. Data on the growth performance of pigs were analyzed with pen as the experimental unit using the JMP 15 Pro software for the three-way analysis of variance (ANOVA), according to least squares procedures and testing for main effects [age, birth weight status (“IUGR” vs. “normal”), dietary treatment (“glycine” vs, “alanine”)] and their interactions. Data on biochemical metabolites in plasma and tissues, as well as carcass and meat qualities, were analyzed with pig as the experimental unit using the JMP 15 Pro software for the two-way analysis of variance, according to least squares procedures and testing for main effects [birth weight status (“IUGR” vs. “normal”) and dietary treatment (“glycine” vs. “alanine”)] and their interactions. In the present study (2 × 2 factorial design), the fixed (glycine and IUGR) and random effects model is: Y_ijk  = µ + α_i  + β_j  + (αβ)_ij  + ɛ_ijk (JMP 15 Pro software), where Y_ijk is k’th observation at the i’th level of factor A (glycine) and the j’th level of factor B (IUGR), µ is the overall mean, α_i is the random effect for the i’th level of factor A (glycine), β_j is the random effect for the j’th level of factor B (IUGR), (αβ)_ij is the interaction effect of the i’th level of factor A (glycine) and the j’th level of factor B (IUGR), and ɛ_ijk is the random error effect. Differences among treatment means were determined using the Student–Newman–Keuls multiple comparison test. A probability value of P ≤ 0.05 was taken to indicate statistical significance.

Results

Effects of dietary glycine supplementation on the growth performance of pigs

All pigs had increased (P = 0.001) body weight with age, but IUGR reduced (P = 0.001) the body weight of pigs (Table 2). At 188 d of age, IUGR pigs weighed 13.4 kg less (P = 0.001) than NBW pigs, and dietary glycine supplementation increased (P = 0.001) the final body weight of pigs by 3.8 kg. At 188 d of age, the mean body weights (kg) of pigs in the IUGR-Gly group were 5.5 kg heavier (P = 0.004) than those in the IUGR-Ala group. There were interactions in the effects of Gly × IUGR (P = 0.036), Gly × Day (P = 0.011), and IUGR × Day (P = 0.001) on the body weights of pigs. Neither IUGR nor dietary glycine supplementation affected (P > 0.05) the feed intake of pigs per kg BW during the nursery, growing, or finishing periods (Table 2). The feed efficiency (gain:feed ratio) of pigs decreased (P = 0.001) with age. Between d 21 and d 188 of age, IUGR reduced (P = 0.043) the feed efficiency of pigs by 4.4% as compared with NBW pigs, but dietary glycine supplementation increased (P = 0.001) this variable by 11% as compared with Ala pigs (Table 2).

Table 2.

Growth performance of pigs from weaning (day 21 of age) to market weight (day 188 of age)¹

Age of pigs (d)	NBW pigs			IUGR pigs
	Alanine		Glycine	Alanine		Glycine
Body weight, kg/pig
21	6.47 ± 0.09		6.51 ± 0.09	5.12 ± 0.09		5.26 ± 0.11
35	8.53 ± 0.08		8.69 ± 0.11	6.62 ± 0.08		7.04 ± 0.12
50	15.1 ± 0.26		15.6 ± 0.25	11.9 ± 0.20		13.0 ± 0.30
64	23.6 ± 0.41		24.3 ± 0.30	19.0 ± 0.40		21.0 ± 0.37
92	45.6 ± 1.3		47.3 ± 1.1	38.9 ± 0.91		40.8 ± 1.01
120	74.8 ± 1.2		76.5 ± 1.8	65.4 ± 0.7		69.8 ± 1.0
148	97.9 ± 1.7		100.9 ± 2.4	88.5 ± 1.4		93.1 ± 1.3
176	124.5 ± 1.4		126.4 ± 1.8	110.7 ± 0.85		116.1 ± 1.6
188	133.9 ± 1.6		136.0 ± 1.9	118.8 ± 0.64		124.3 ± 1.6

P-values
Gly	IUGR	Day	Gly × IUGR	Gly × day	IUGR × day	Gly × IUGR × day
0.001	0.001	0.001	0.036	0.011	0.001	0.909

Feed intake, kg/pig
d 21 to 35	5.48 ± 0.56		4.99 ± 0.48	3.83 ± 0.21		3.95 ± 0.11
d 35 to 50	10.4 ± 0.64		10.2 ± 0.74	8.03 ± 0.24		8.84 ± 0.37
d 50 to 64	15.0 ± 0.70		14.9 ± 0.90	11.8 ± 0.50		13.0 ± 0.40
d 64 to 92	44.7 ± 3.0		43.8 ± 3.1	39.0 ± 1.8		37.3 ± 2.4
d 92 to 120	69.0 ± 4.5		66.5 ± 6.6	65.7 ± 2.7		67.4 ± 3.5
d 120 to148	68.0 ± 5.7		66.2 ± 6.1	72.3 ± 3.9		70.2 ± 5.5
d 148 to 176	73.1 ± 5.8		69.8 ± 8.2	70.5 ± 5.9		68.0 ± 5.9
d 176 to 188	27.9 ± 2.2		25.9 ± 1.7	24.9 ± 1.3		25.9 ± 2.4
d 21 to 188	313 ± 19		302 ± 22	296 ± 13		295 ± 17

P-values
Gly	IUGR	Day	Gly × IUGR	Gly × Day	IUGR × Day	Gly × IUGR × Day
0.536	0.221	0.001	0.636	0.998	0.747	0.999

Feed efficiency (gain:feed ratio, g/g)2
d 21 to 35	0.402 ± 0.015		0.462 ± 0.014	0.398 ± 0.012		0.450 ± 0.023
d 35 to 50	0.661 ± 0.014		0.714 ± 0.036	0.657 ± 0.012		0.678 ± 0.014
d 50 to 64	0.591 ± 0.018		0.616 ± 0.020	0.605 ± 0.015		0.619 ± 0.018
d 64 to 92	0.511 ± 0.015		0.557 ± 0.018	0.504 ± 0.010		0.551 ± 0.023
d 92 to 120	0.412 ± 0.019		0.481 ± 0.033	0.406 ± 0.016		0.469 ± 0.025
d 120 to 148	0.340 ± 0.038		0.417 ± 0.044	0.329 ± 0.032		0.369 ± 0.033
d 148 to 176	0.356 ± 0.015		0.405 ± 0.028	0.328 ± 0.012		0.384 ± 0.025
d 176 to 188	0.356 ± 0.011		0.396 ± 0.023	0.321 ± 0.012		0.351 ± 0.020
d 21 to 188	0.399 ± 0.018		0.457 ± 0.032	0.395 ± 0.015		0.423 ± 0.023

P-values
Gly	IUGR	Day	Gly × IUGR	Gly × Day	IUGR × Day	Gly × IUGR × Day
0.001	0.043	0.001	0.463	0.874	0.861	0.998

¹Data are means ± SEM, n = 12 pens. Based on their birth weights and litter of origin, 21-d-old IUGR and NBW pigs were assigned randomly into one of the following four treatment groups: (1) IUGR pigs supplemented with 1% glycine + 0.19% cornstarch (IUGR-Gly); (2) IUGR pigs supplemented with 1.19% L-alanine as the isonitrogenous control (IUGR-Ala); (3) NBW pigs supplemented with 1% glycine + 0.19% cornstarch (NBW-Gly); and (4) NBW pigs supplemented with 1.19% L-alanine as the isonitrogenous control (NBW-Ala). The treatments were arranged in a 2 × 2 factorial design with measurements at different days. There were 2 to 4 pigs per pen. Pigs of 21–64, 64–92, 92–148, and 148–188 d of age had free access to corn- and soybean meal-based diets containing 20%, 18%, 16%, and 14% crude protein, respectively.

²The feed efficiency of pigs is expressed as the ratio of body weight gain/feed consumption; the greater the value, the higher the feed efficiency for growth.

Effects of dietary glycine supplementation on tissue weights of pigs

The liver and kidneys of IUGR pigs were 9.8% (P = 0.001) and 4.3% (P = 0.043) heavier than those of NBW pigs, respectively, but the weights of other analyzed tissues (abdominal fat, heart, lungs, spleen, and small intestine) did not differ (P > 0.05) between IUGR and NBW pigs (Table 3). Dietary glycine supplementation decreased (P = 0.001) liver weights of pigs by 6.2%, increased (P = 0.018) the weights of the small intestine of pigs by 6.7% and had no effect (P > 0.05) on the weights of other analyzed tissues (Table 3). There were no interactions (P > 0.05) for effects of Gly × IUGR on weights of any of the analyzed tissues.

Table 3.

Effects of intrauterine growth restriction and dietary glycine supplementation on the weights of tissues in market-weight pigs¹

Tissue weight (% of body weight)	NBW pigs		IUGR pigs		P-values
	Alanine (n = 9)	Glycine (n = 10)	Alanine (n = 7)	Glycine (n = 6)	Gly	IUGR	Gly × IUGR
Abdominal fat	1.190 ± 0.097	1.038 ± 0.100	1.162 ± 0.156	1.006 ± 0.085	0.206	0.739	0.994
Liver	1.359 ± 0.019	1.299 ± 0.022	1.521 ± 0.037	1.399 ± 0.018	0.001	0.001	0.226
Heart	0.320 ± 0.006	0.330 ± 0.009	0.329 ± 0.014	0.336 ± 0.007	0.428	0.394	0.949
Lungs	1.019 ± 0.029	1.092 ± 0.016	1.057 ± 0.051	1.120 ± 0.084	0.132	0.481	0.915
Spleen	0.198 ± 0.005	0.192 ± 0.010	0.203 ± 0.008	0.206 ± 0.014	0.982	0.275	0.557
Kidneys	0.350 ± 0.006	0.346 ± 0.005	0.362 ± 0.009	0.364 ± 0.005	0.934	0.043	0.711
Small intestine	0.911 ± 0.023	0.946 ± 0.014	0.858 ± 0.024	0.941 ± 0.032	0.018	0.202	0.305

¹Data are means ± SEM, with the numbers of pigs indicated in the parentheses. Pigs with normal birth weights (NBW) or intrauterine growth restriction (IUGR) received dietary supplementation with either 1% glycine + 0.19% cornstarch or 1.19% L-alanine (isonitrogenous control) beginning at weaning (21 d of age). Tissue weights were determined within 1 wk after the feeding trial ended at 188 d of age.

Effects of dietary glycine supplementation on concentrations of AAs in plasma and tissues

Among all the measured AAs in the plasma of pigs, only glycine and serine were affected by IUGR or dietary glycine supplementation (Table 4). Specifically, IUGR reduced (P = 0.001), but dietary glycine supplementation increased (P = 0.001), concentrations of glycine in the plasma of pigs by 35% and 46%, respectively. In addition, IUGR reduced (P = 0.005), but dietary glycine supplementation increased (P = 0.001), concentrations of serine in the plasma of pigs by 12% and 17%, respectively. Concentrations of alanine in the plasma were 21% greater (P = 0.001) in pigs receiving supplemental alanine than those for pigs receiving supplemental glycine but did not differ (P > 0.05) between NBW and IUGR pigs. Concentrations of other AAs in plasma were not affected (P > 0.05) by either IUGR or dietary glycine supplementation (Supplementary Table S1).

Table 4.

Concentrations of glycine (Gly) and serine (Ser) in the plasma and tissues of pigs at market weight¹

Amino	NBW pigs		IUGR pigs		P-values
Acid	Alanine (n = 9)	Glycine (n = 10)	Alanine (n = 7)	Glycine (n = 6)	Gly	IUGR	Gly × IUGR
	Plasma (nmol/ml)
Gly	940 ± 31	1242 ± 77	521 ± 17	898 ± 30	0.001	0.001	0.496
Ser	267 ± 12	315 ± 16	239 ± 8.0	275 ± 14	0.001	0.005	0.605
Ala	718 ± 24	594 ± 14	710 ± 32	586 ± 14	0.001	0.717	0.997
	Liver (nmol/g tissue)
Gly	6167 ± 204	7245 ± 222	4620 ± 117	6223 ± 213	0.001	0.001	0.208
Ser	1209 ± 62	1398 ± 76	942 ± 74	1233 ± 37	0.002	0.004	0.464
Ala	3160 ± 174	3263 ± 207	3255 ± 257	3335 ± 194	0.545	0.801	0.830
	Longissimus dorsi muscle (nmol/g tissue)
Gly	1638 ± 73	1802 ± 37	1255 ± 56	1607 ± 43	0.001	0.003	0.492
Ser	198 ± 14	219 ± 17	199 ± 18	206 ± 15	0.403	0.734	0.689
Ala	1249 ± 87	1197 ± 74	1124 ± 70	1246 ± 86	0.674	0.649	0.299
	Gastrocnemius muscle (nmol/g tissue)
Gly	1820 ± 75	1974 ± 98	1349 ± 80	1787 ± 29	0.002	0.001	0.106
Ser	191 ± 16	216 ± 9.4	185 ± 11	217 ± 13	0.039	0.829	0.792
Ala	1334 ± 91	1343 ± 75	1214 ± 78	1232 ± 49	0.872	0.167	0.962
	Jejunum (nmol/g tissue)
Gly	7747 ± 369	8883 ± 271	6276 ± 252	7810 ± 249	0.001	0.001	0.529
Ser	810 ± 26	911 ± 35	706 ± 32	815 ± 55	0.009	0.012	0.926
Ala	1805 ± 78	1879 ± 76	1776 ± 82	1812 ± 82	0.505	0.559	0.812
	Kidney (nmol/g tissue)
Gly	9008 ± 326	9995 ± 157	7655 ± 133	8968 ± 203	0.001	0.001	0.507
Ser	824 ± 22	893 ± 31	790 ± 20	823 ± 41	0.096	0.091	0.555
Ala	2757 ± 142	2910 ± 145	2848 ± 120	2836 ± 123	0.632	0.953	0.573
	Pancreas (nmol/g tissue)
Gly	15316 ± 215	15998 ± 279	14103 ± 244	15598 ± 237	0.001	0.005	0.134
Ser	1205 ± 39	1267 ± 25	1191 ± 49	1178 ± 62	0.569	0.236	0.391
Ala	5508 ± 272	4133 ± 111	5812 ± 229	3892 ± 84	0.001	0.881	0.206

¹Data, expressed as µM, are means ± SEM, with the numbers of pigs indicated in the parentheses. Pigs with normal birth weights (NBW) or intrauterine growth restriction (IUGR) received dietary supplementation with either 1% glycine + 0.19% cornstarch or 1.19% L-alanine (isonitrogenous control) beginning at weaning (21 d of age). Blood and other tissue samples were obtained from the pigs within 1 wk after the feeding trial ended at 188 d of age.

Concentrations of both glycine and serine in the liver of pigs were affected by IUGR and dietary glycine supplementation (Table 4). Specifically, IUGR reduced concentrations of glycine (P = 0.001) and serine (P = 0.004) in livers by 19% and 17%, respectively. In contrast, dietary glycine supplementation increased the hepatic concentrations of glycine (P = 0.001) and serine (P = 0.002) by 25% and 22%, respectively. Concentrations of other AAs (including alanine) in the liver were not affected (P > 0.05) by either IUGR or dietary glycine supplementation (Supplementary Table S2).

Among all the measured AAs in the longissimus dorsi muscle of pigs, only glycine was affected by IUGR and dietary glycine supplementation (Table 4). Specifically, IUGR reduced (P = 0.003), but dietary glycine supplementation increased (P = 0.001), concentrations of glycine in this muscle by 17% and 18%, respectively (Table 4). Concentrations of other AAs (including alanine and serine) in the longissimus dorsi muscle were not affected (P > 0.05) by either IUGR or dietary glycine supplementation (Supplementary Table S3). Similar results were obtained for the gastrocnemius muscle, except that IUGR decreased (P = 0.030) the intramuscular concentrations of methionine by 12% and dietary glycine supplementation increased (P = 0.039) the intramuscular concentrations of serine by 15% (Supplementary Table S4).

Concentrations of both glycine and serine in the jejunum of pigs were affected by IUGR and dietary glycine supplementation (Table 4). Specifically, IUGR reduced concentrations of glycine (P = 0.001) and serine (P = 0.012) in the jejunum by 15% and 12%, respectively. In contrast, dietary glycine supplementation increased concentrations of glycine (P = 0.001) and serine (P = 0.009) in the jejunum by 19% and 14%, respectively. Concentrations of other AAs (including alanine) in the jejunum were not affected (P > 0.05) by either IUGR or dietary glycine supplementation (Supplementary Table S5).

Among all the measured AAs in the kidneys of pigs, only glycine was affected by IUGR and dietary glycine supplementation (Table 4). Specifically, IUGR reduced (P = 0.001), but dietary glycine supplementation increased (P = 0.001), concentrations of glycine in the kidneys of pigs by 13% and 14%, respectively. Concentrations of other AAs (including alanine) in the kidneys were not affected (P > 0.05) by either IUGR or dietary glycine supplementation (Supplementary Table S6). Similar results were obtained for the pancreas (Table 4 and Supplementary Table S7), except that pancreatic concentrations of alanine were 41% greater (P = 0.001) in pigs receiving supplemental alanine than those for pigs receiving supplemental glycine.

Effects of dietary glycine supplementation on the nutrient composition and quality of meat

Neither IUGR nor dietary glycine supplementation affected (P > 0.05) the composition of nutrients [water, crude protein (including collagen), crude fat, minerals, and carbohydrates] in the meat of pigs (Supplementary Table S8). However, dietary glycine supplementation tended to decrease (P = 0.081) the content of crude fat in meat by 6%. There were no interactions of IUGR × Gly (P > 0.05) on the content of all analyzed nutrients in the meat of the pigs.

Compared with NBW pigs, IUGR pigs had a lower carcass dressing percentage (–2%; P = 0.026) and a lower four-lean-cuts percentage (–15%; P = 0.001), but a greater a* value (an indicator of meat redness; +13%, P = 0.004; Table 5). Compared with pigs in the alanine group, dietary supplementation with glycine decreased (P = 0.046) back fat thickness by 18%, while increasing (P = 0.016) the a* value by 10%. The backfat thickness of glycine-supplemented IUGR pigs was 30% less (P < 0.05) than that for alanine-supplemented IUGR pigs. Interestingly, the interaction between IUGR and glycine affected (P = 0.041) muscle scores. Additional measures of meat quality (including muscle score, drip loss, four lean cuts, marbling score, meat color score, L* value, b* value, and muscle pH) did not indicate negative effects (P > 0.05) of dietary supplementation with glycine, compared with pigs in the alanine control group.

Table 5.

Effects of intrauterine growth restriction and dietary glycine supplementation on the carcass and meat qualities of pigs at market weight¹

Variable	NBW pigs		IUGR pigs		P-values
	Alanine (n = 9)	Glycine (n = 10)	Alanine (n = 7)	Glycine (n = 6)	Gly	IUGR	Gly × IUGR
Carcass dressing, %	75.0 ± 0.43	75.0 ± 0.73	73.5 ± 0.59	73.7 ± 0.64	0.756	0.026	0.921
Carcass weight, kg	99.1 ± 0.44	99.6 ± 0.22	99.9 ± 0.38	99.0 ± 0.82	0.412	0.981	0.273
BF thickness2, cm	2.32 ± 0.20	2.18 ± 0.18	2.54 ± 0.48	1.78 ± 0.13	0.046	0.972	0.084
Loineye area3, cm2	51.7 ± 1.7	46.5 ± 1.8	47.2 ± 2.8	49.9 ± 2.7	0.785	0.939	0.142
Muscle score4	2.29 ± 0.16	2.00 ± 0.00	2.14 ± 0.14	2.50 ± 0.22	0.621	0.130	0.041
Drip loss,3 %	2.87 ± 0.41	2.61 ± 0.82	1.73 ± 0.25	2.03 ± 0.60	0.938	0.265	0.605
FLC (bone-less), %	54.5 ± 2.0	49.7 ± 0.85	43.8 ± 1.1	44.4 ± 1.7	0.383	0.001	0.236
Marbling score3	2.00 ± 0.00	1.90 ± 0.10	1.86 ± 0.14	2.17 ± 0.17	0.348	0.577	0.073
Meat color score3	2.89 ± 0.20	3.00 ± 0.26	2.57 ± 0.20	3.17 ± 0.17	0.137	0.746	0.303
L* value5	62.4 ± 1.1	59.7 ± 1.4	59.8 ± 1.9	57.8 ± 1.6	0.130	0.149	0.845
a* value5	10.8 ± 0.51	12.0 ± 0.33	12.3 ± 0.45	13.5 ± 0.64	0.016	0.004	0.980
b* value5	19.2 ± 0.30	19.5 ± 0.26	19.7 ± 0.36	20.1 ± 0.47	0.277	0.156	0.954
Muscle pH at 24 h6	5.48 ± 0.04	5.45 ± 0.03	5.43 ± 0.03	5.46 ± 0.03	0.158	0.778	0.060

¹Data are means ± SEM, with the numbers of pigs indicated in the parentheses. Pigs with normal birth weights (NBW) or intrauterine growth restriction (IUGR) received dietary supplementation with either 1% glycine + 0.19% cornstarch or 1.19% L-alanine (isonitrogenous control) beginning at weaning (21 d of age). Carcass and meat qualities were determined within 1 wk after the feeding trial ended at 188 d of age.

²Measured over the 10th rib.

³Measured in the M. longissimus thoracis between the 10th and 11th ribs.

⁴Score used in pork carcass grading to denote the muscularity of a pork carcass. The values were determined in the M. longissimus thoracis.

⁵Objective evaluation of meat color: L* for the lightness from black (0) to white (100), a* from green (−) to red (+), and b* from blue (−) to yellow (+). The higher the L* value, the paler the meat. The higher the a* value, the redder the meat. The higher the b* value, the yellower the meat. These values were determined in the M. longissimus thoracis.

⁶Measured in the M. longissimus thoracis at 24 h after the chilling of carcasses at 4^°C.

Abbreviations: BF: backfat; FLC: four lean cuts.

Discussion

Much research has shown that IUGR has adverse impacts on nutrient metabolism (Hughson et al., 2003; Shen et al., 2018; Amdi et al., 2020; Cheng et al., 2021; Tang and Xiong, 2022) and growth performance (Hegarty and Allen, 1978; Zhang et al., 2015; Reynolds et al., 2022) of animals including pigs. We reported that concentrations of glycine in the plasma of IUGR pigs were about one-half of those for NBW litter mates due, in part, to reduced synthesis of glycine from 4-hydroxyproline (Hu et al., 2023b). The major findings of this study are that dietary glycine supplementation to IUGR pigs for 167 d increased glycine availability in plasma and tissues, as well as the growth performance of pigs; reduced backfat thickness; and improved meat quality. Thus, IUGR pigs fed an SBM-based diet required supplemental glycine for their optimum growth and development. To our knowledge, the present work is the first to assess the responses of both IUGR and NBW pigs to long-term glycine supplementation between weaning and market weight. These results indicate that IUGR pigs can be successfully and economically reared for meat production when dietary requirements for glycine are met.

Glycine is chemically stable in the stomach of pigs, and its catabolism in the small intestine is limited (Wu, 2022). Thus, unencapsulated glycine can be provided in diets to effectively increase its concentrations in the systemic circulation of swine. Results of a previous study showed that oral administration of 0.02 to 0.08 g glycine/kg BW increased concentrations of glycine in the plasma of preweaning IUGR pigs in a dose-dependent manner (Hu, 2017). Interestingly, despite similar feed intake between IUGR and NBW pigs during the growing-finishing period, dietary glycine supplementation increased the concentration of glycine in the plasma of IUGR pigs by 72% but only by 32% in the plasma of NBW pigs. This may be due to a greater demand of NBW pigs for glycine for protein deposition in tissues (particularly skeletal muscle) during growth, as well as a greater rate of whole-body glycine catabolism, compared with IUGR pigs. The greater ­concentrations of serine in the plasma of glycine-supplemented pigs compared with alanine-supplemented pigs is consistent with the evidence for serine hydroxymethyltransferase activity in tissues of pigs (Hu et al., 2023a). Our finding that dietary supplementation with 1% glycine did not reduce concentrations of other AAs in plasma suggests a lack of imbalances or antagonisms among dietary AAs under conditions of our study.

Growth of animals depends primarily on the accretion of skeletal muscle (Wu, 2018). IUGR pigs are born with less skeletal-muscle mass and have a disproportionate reduction in muscle weight that persists during the postnatal period (Cadaret et al., 2019). IUGR pigs have fewer muscle fibers that are also smaller due to the downregulation of myogenic factors, leading to a compromised rate of postnatal growth (Felicioni et al., 2020). Interestingly, among all measured AAs, only glycine was present in lower concentrations in the longissimus dorsi muscle of IUGR pigs at market weight than in NBW pigs, suggesting a selective impairment of glycine uptake by this skeletal muscle of IUGR pigs. This was also true for both the kidneys and the pancreas in which glycine was also the only AA with reduced concentrations in IUGR pigs, compared with NBW pigs. Similarly, concentrations of both glycine and serine were reduced in the gastrocnemius muscle of IUGR pigs than those for NBW pigs. Of note, concentrations of both glycine and serine were less in the liver and small intestine of IUGR pigs, compared with values for NBW pigs, suggesting IUGR-linked impairment in the intestinal absorption and/or hepatic uptake of both AAs. Further research is warranted to determine the differential expression of glycine transporters in tissues of IUGR and NBW pigs.

The number of skeletal muscle fibers in pigs is fixed at birth (Ji et al., 2017). However, dietary glycine supplementation increased muscle gain in both IUGR and NBW pigs. This is likely due to a positive effect of glycine on the accretion of protein within the muscle. An underlying mechanism may be the activation of the mechanistic target of rapamycin (MTOR) cell signaling pathway by glycine that increases protein synthesis and decreases proteolysis in muscle cells (Sun et al., 2016). We surmise that growing pigs, particularly IUGR pigs, cannot synthesize sufficient glycine to meet metabolic needs when fed a conventional corn-SBM-based diet alone. Based on the whole-body metabolism of ¹⁵N-glycine in preterm infants, Jackson et al. (1981) suggested that these neonates do not synthesize sufficient glycine. Likewise, in a recent clinical study with patients under chronic hemodialysis, oral glycine supplementation at 14 g/d for 4 mo increased skeletal muscle mass (Genton et al., 2021). Thus, under compromised conditions such as IUGR, preterm birth, and organ dysfunction, dietary glycine supplementation is likely necessary to improve whole-body growth or protein balance. This is consistent with glycine being the most abundant AA in the body and supports the view that glycine is truly a functional AA in swine nutrition (Wu, 2010). Based on the number of IUGR pigs (~145 x 10⁶) born per year worldwide and the net value ($17.34) of saving one postweaning IUGR pig as analyzed in August 2020, an increase in net income for dietary supplementation with glycine to postweaning pigs is estimated to be $2.51 × 10⁹/year (i.e., 145 × 10⁶ × 17.34 = 2.51 × 10⁹; Zhang et al., 2021).

Another important finding from the present work is that dietary glycine supplementation reduced backfat thickness and hepatic fats in pigs, suggesting an important role for ­glycine in regulating fat metabolism in the body. The underlying biochemical mechanisms are unknown, but glycine alleviates fat accretion and obesity in rats by improving antioxidative and anti-inflammatory responses (Chen et al., 2021). Glycine can stimulate lipolysis in white adipocytes of obese animals (El Hafdi et al., 2004) and the oxidation of long-chain fatty acids in the liver (Rom et al., 2020), thereby reducing amounts of triacylglycerols in the liver and the whole body. Interestingly, glycine supplementation had no effect on the total weight of abdominal fat, indicating a depot site-specific response in pigs, as reported for the nutritional regulation of fat metabolism in rodents and humans (Chusyd et al., 2016).

Glycine is an essential precursor for the synthesis of glutathione and creatine (McCarty et al., 2018; Wu, 2022), both of which can improve pork quality (Maddock et al., 2002; Liu and Eady, 2005). Accordingly, the carcasses of glycine-supplemented IUGR pigs scored higher on key aspects of meat quality grading. Specifically, the positive effects of glycine supplementation on meat quality included a decrease in backfat thickness and an increase in the a* value (a scoring system for meat redness) without affecting the composition of nutrients in the meat, compared with pigs in the alanine group. Similar results have been reported for NBW mini-pigs fed an SBM-based diet supplemented with 0.16% glycine between d 70 and d 130 of age (Zhong et al., 2021). Likewise, Matyba et al. (2021) reported that the composition of nutrients in meat did not differ between IUGR and NBW pigs. The greater the a* value, the greater the myoglobin content in the meat (Mancini and Hunt, 2005). Thus, consumers generally prefer redder pork (meat with a higher a* value) to whiter pork. Our finding on the beneficial effects of glycine supplementation on increasing the a* value of pork may be explained by the fact that glycine is required for the synthesis of heme [an essential component of myoglobin (Wu, 2022)]. At present, the mechanisms responsible for the unexpected increase in the a* value in the meat from IUGR pigs compared with NBW pigs are unknown but may be the result of adaptationally lower rates of intramuscular myoglobin degradation in IUGR pigs. Importantly, other measures of carcass and meat qualities [including dressing percentage, side weight percentage, loineye area, muscle score, drip loss (an indicator of water-holding capacity), four-lean-cuts percentages, marbling score, meat color score, L* value, b* value, and muscle pH] did not reveal negative effects due to IUGR or dietary glycine supplementation. Considering that carcass quality from IUGR pigs is usually inferior due to reduced meat yields and increased fat deposition (Mancini and Hunt, 2005; Nissen and Oksbjerg, 2011), our results indicated positive effects of dietary glycine supplementation on meat quality. Improvements in the quality of pork meat can further enhance the benefits of dietary glycine supplementation to IUGR pigs following weaning. The ease and inexpensive strategy of dietary glycine supplementation makes this an attractive option for pork producers to utilize in managing IUGR pigs.

In conclusion, although glycine has been traditionally classified as a nutritionally nonessential AA in pigs, it must be considered conditionally essential for their optimal growth and development, particularly in IUGR pigs that do not adequately synthesize glycine. IUGR pigs fed conventional corn-SBM-based diets had lower concentrations of glycine in plasma and tissues (including skeletal muscle, liver, small intestine, kidney, and pancreas), compared with NBW litter mates. Concentrations of serine were also less in the plasma, liver and small intestine of IUGR pigs than NBW pigs. In all the analyzed tissues, the concentrations of other AAs (except for methionine in the gastrocnemius muscle) were not different between IUGR and NBW pigs. Dietary supplementation with 1% glycine improved the growth performance, feed efficiency, and meat quality of IUGR pigs. Collectively, these results indicate that growing-finishing pigs do not synthesize sufficient amounts of glycine for metabolic utilization and that supplemental dietary glycine is essential for the optimum growth and carcass quality of postweaning pigs, especially IUGR pigs. Thus, glycine supplementation is a cost-effective means to enhance productivity in the swine industry.

Glossary

Abbreviations

AA

amino acid

ASTREC

Animal Science Teaching, Research and Extension Complex

CP

crude protein

HPLC

high-performance liquid chromatography

IUGR

intrauterine growth restriction

NBW

normal birth weights

SBM

soybean meal

Conflict of interest statement

The authors declare that they have no conflict of interest.