Intramuscular fat (IMF) plays an important role in various aspects of meat quality, and it is also critical for the nutritional value of meat (1). Higher content of IMF can increase the meat quality as it contributes to pork tenderness, flavor, and juiciness (2). Furthermore, muscle fatty acid profiles also play a key role in meat quality because of their high value for human health. Polyunsaturated fatty acids (PUFA), especially n-3 fatty acids, are considered as functional ingredients to prevent cardiovascular disease in humans (3). The n-3 PUFA deficiency and excessive content of n-6 PUFA are associated with the development of insulin resistance and metabolic disorders (4). Therefore, abnormal lipid metabolism could inhibit the growth and decrease the meat quality of animals. In addition, a recent study has reported that changes in fatty acid composition due to different feeding strategies are associated with the changes in the mRNA expression of genes related to fatty acid metabolism (5). For example, fatty acid synthase (FAS) and stearoyl coenzyme A desaturase (SCD) genes are associated with the regulation of fatty acid biosynthesis, while lipoprotein lipase (LPL) can regulate the fatty acid transportation from blood to fat cell (6, 7). Thus, the regulation of IMF content and fatty acid composition in muscles has been of great interest in recent years. Furthermore, antibiotics are commonly used in the feed industry to promote the growth of animals. Owing to the ban on in-feed antibiotics in the livestock industry, a number of alternative feed additives with similar effects to in-feed antibiotics in livestock production have attracted increased attention.

Betaine is a common term for trimethylglycine, a substrate for betaine-homocysteine methyltransferase (HMT) in liver and kidney (8). As a promising antioxidant agent, betaine can prevent lipid peroxidation and regulate lipid metabolism to improve growth performance and meat quality. Gheisari et al. (9) reported that the betaine addition (1 g/kg) to the methionine-deficient diet could significantly improve antioxidant defense and meat quality by decreasing lipid peroxidation in breast muscles of broilers. In addition, dietary betaine could improve the meat quality of lambs, and rumen-protected betaine exhibited better effects than that of the unprotected-betaine (10). However, there are limited studies about maternal betaine addition on offspring's growth performance and lipid metabolism.

Bama mini-pig is a high-quality local pig breed in China because of its characteristics of delicious meat, high roughage tolerance, and good adaptability. However, the lower dietary nutrient level and extensive management methods have resulted in lower growth, feed conversion, and lean meat rate of this mini-pig. Our previous studies showed that 3.50 kg/t betaine addition to sow and sow-offspring diets could increase the carcass weight, carcass yield, lean meat rate, meat color, shear force, and crude protein content of muscles at different stages by altering the plasma biochemical parameters and amino acid composition, and regulating the expression level of genes related to myosin heavy-chain (MyHC) isoform and myogenic regulatory factors (MRFs) in skeletal muscle (11). Moreover, dietary betaine hydrochloride addition (3.50 kg/t) to sows from gestation to lactation could improve suckling piglet's health by enhancing intestinal morphology and immunity and altering intestinal microbiota (12). Therefore, we hypothesized that dietary betaine might influence the growth performance and meat quality of pigs by regulating the lipid content, IMF content, and mRNA expression of genes related to fatty acid metabolism in skeletal muscle. Thus, the current study was conducted to test our hypothesis that the addition of dietary betaine to sow and sow-offspring would improve the offspring's growth performance and thereby influence the lipid metabolism. Furthermore, to explain the mechanism of dietary betaine addition on growth performance and lipid metabolism of Bama mini-pigs by evaluating plasma hormones and the fatty acid metabolism-related genes.

As an osmotic regulator, dietary betaine has been widely studied as a methyl donor on protein synthesis and nitrogen metabolism. Moreover, there are lots of creditability evidence showing that dietary betaine can influence growth performance and meat quality through its different functional properties. Previous studies have found that dietary betaine addition during the growing and finishing stages improved the growth performance and meat quality of animals (21, 22). In addition, maternal nutrition also influences the offspring's growth later in life (23). Therefore, the present study evaluated the effects of betaine addition in sow and sow-offspring diets on the growth performance and meat quality-associated parameters of pigs at different stages. The results showed that the betaine addition in sow and sow-offspring diets influenced the growth performance, plasma hormone, and lipid metabolism of Bama mini-pigs.

Betaine is widely used in animal production because of its positive effects on animal performance. In the present study, betaine addition in sow-offspring diets increased the BW of 125-day-old pigs, as well as the ADG and ADFI at 35–65 and 65–95 day-old. Therefore, the results indicate that betaine addition in sow-offspring diets can improve the growth of the piglets by enhancing the feed intake at an earlier age. Huang et al. (24) also found that dietary addition of 1.25 kg/t betaine for 42 days could increase the ADG of finishing pigs by 5.50%, but not the ADFI and F:G. In a sub-optimal methionine diet (15 g Met/kg CP), dietary betaine addition (0.80 kg/t) could improve the BW gain of broilers at 21-day-old, as well as the breast yield and F:G of broilers at 42-day-old (25). Furthermore, juvenile freshwater prawn Macrobrachium rosenbergii fed with 5 g/kg glycine betaine presented higher BW, food intake, and F:G (26). Therefore, these findings suggested that dietary betaine addition might increase the protein synthesis and decomposition through its methyl donor properties, thus increasing the growth and development of pigs.

Plasma hormone levels can reflect the physiological and health status of pigs. Growth hormone can regulate the growth and development of animals directly or indirectly by stimulating IGF-1 (27). Huang et al. (28) reported that dietary betaine addition could stimulate the release of GH and promote the growth of finishing pigs. However, the present study showed that betaine addition in sow and sow-offspring diets did not affect the plasma GH level, which might be related to the measured time of the plasma GH level. Kraetzl et al. (29) reported that the GH secretion is pulsatile, and its level is related to the measured time in pigs, that GH levels were higher during the night (20:00–08:00) than during the day (08:00–20:00). Therefore, the inconsistencies of these results might be related to the measuring time, as the present study only measured the GH level during the daytime. Gastrin and growth hormone-releasing factors could jointly promote GH secretion, regulate protein and sugar metabolism, and thus promote the growth of animals (30). As a type of gastrointestinal-derived hormone, PYY can inhibit feed intake, delay gastric emptying, increase the absorption of electrolytes in the ileum, and reduce the secretion of starch (31). The LEP is a protein hormone that can suppress the appetite and play a major role in regulating body fat, BW, and energy balance in animals (32). In the present study, betaine addition in sow diets decreased plasma PYY and LEP levels of 65-day-old pigs, while betaine addition in sow-offspring diets increased the Gas level of 95-day-old pigs, which are beneficial to promote the feed intake and growth of pigs.

The IMF mainly reflects muscle tenderness and flavor, and is a key factor to the formation of muscle taste. Existing studies showed that the IMF content between 2 and 3% is the best (33). In the present study, the IMF content was significantly decreased in the PM muscles of 125-day-old pigs but not in LD and BF muscles. However, Martins et al. (34) reported that 1.00 kg/t betaine addition in sow diets for 20 weeks could increase the IMF content in LD and BF muscles of Alentejano pigs. Huang et al. (35) found that 1.20 kg/t betaine addition in corn-soybean meal diets for 42 days increased the IMF content in LD muscle of finishing pigs. Therefore, these inconsistencies of IMF content might be related to the age of pigs. Moreover, the results suggested that dietary betaine mainly increased the IMF deposition in the earlier age of pigs, while could inhibit the IMF deposition in the later age.

The composition of fatty acid in the muscle plays an important role in the formation of pork flavor. High levels of unsaturated lipids may contribute to desirable flavors, like oxidized off-flavors (36). In the present study, dietary betaine addition tends to decrease the contents of PUFA, such as C18:1n9t, C18:3n6, C18:2n6c, and C20:4n6. These findings are in agreement with those of Madeira et al. (37), who reported that dietary betaine addition mainly decreases the percentage of monounsaturated fatty acid (including C16:1cis-9 and C18:1cis-11) proportions in the muscles of pigs. These findings might be resulted by the methyl donor properties of betaine. The principal physiologic role of betaine is as a methyl donor, which means transmethylation of betaine participates in the carnitine and phosphatidylcholine synthesis and fatty acid oxidation. Therefore, dietary betaine addition in sow diets may enhance the oxidation of PUFA and then reduce fatty acid and IMF contents (38, 39).

The SREBP1 is a membrane-bound protein that can regulate the most enzymes involved in fatty acid synthesis, including acetyl-CoA carboxylase (ACC), FAS, and SCD (40, 41). The LPL is an important lipid regulatory enzyme involved in hydrolyzing triglycerides of plasma lipoproteins and supplying free fatty acids for storage in adipocytes or oxidation in other tissues (42). Moreover, FAS is a multifunctional protein, which main functional role is to catalyze the synthesis of palmitate from the acetyl-CoA and malonyl-CoA, in the presence of nicotinamide adenine dinucleotide phosphate (NADPH), into long-chain saturated fatty acid (43). In the present study, betaine addition in sow-offspring diets down-regulated the level of the FAS in the LD muscle of 65-day-old pigs, as well as in the BF and PM muscle of 95-day-old pigs, while dietary betaine addition in sow and sow-offspring diets up-regulated the level of FAS in the PM muscle of 65-day-old pigs. Moreover, betaine addition in sow diets up-regulated the level of LPL in the BF muscle of 125-day-old pigs while down-regulated in the BF muscle of 95-day-old pigs. Therefore, these findings suggest that dietary betaine can regulate fatty acid synthesis and fat deposition by enhancing lipogenesis and reducing lipolysis. Meanwhile, dietary betaine addition in sow and sow-offspring diets up-regulated the expression level of SREBP1 in the PM muscle of 65-day-old pigs, while dietary betaine addition in sow-offspring diets down-regulated the SREBP1 level in the LD and PM muscles of 125-day-old pigs. Thus, the present study showed that dietary betaine could improve the synthesis of fatty acids in the earlier age, while inhibiting the process in the later age, which is also consistent with the changes of the IMF content in the muscles of pigs.

In summary, betaine addition in sow-offspring diets could improve the growth performance of piglets by increasing body weight, average daily gain, and average daily feed intake, decreasing the contents of several fatty acids, and inhibiting the fat deposition in muscles. Betaine addition in both sow and sow-offspring diets could influence the plasma hormones related to ingestion. Moreover, betaine addition in sow-offspring diets had more distinct effects than the addition in sow diets. These findings provide a reference for regulating lipid metabolism and promoting the growth of pigs in practical production by dietary betaine addition.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

The present study was conducted following Chinese guidelines for animal welfare and experimental protocols and approved by the Animal Care and Use Committee of the Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha, China (ISA-2018-071).

YC, MS, QZ, and MA performed sampling and nutrient measurements, analyzed data, interpreted the data, and drafted the manuscript. MS and QG conducted animal feeding and sampling. XK contributed to experimental concepts and design, provided scientific direction, and together with MA finalized the manuscript. All authors read and approved the final manuscript.

This work was supported by the National Natural Science Foundation of China (No. 31772613), Special Funds for Construction of Innovative Provinces in Hunan Province (2019RS3022), and Production and Research Talent Support Project of the CAS Wang Kuancheng Initiative Talent Program.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.