1. Introduction
Fat is an important tissue for storing energy, and is closely related to livestock and poultry production and meat quality [
1]. Excessive fat deposition will decrease the lean meat percentage, the meat quality and the ratio of feed conversion, and even cause serious health problems such as fatty liver [
2].
Ningxiang pig, as a local pig breed of China, occupies an important position in Chinese pork consumption with its delicious taste—as a fat-type pig, it has strong ability to deposit fat [
3]. Lei et al. [
4] reports that Ningxiang pigs exhibit higher back fat thickness than Large White pigs (4.54% vs. 2.61%). He et al. [
5] conclude that Ningxiang pigs have higher fat mass (42.3% vs. 21.9%) and lower muscle mass (35.4% vs. 58.9%) than Duroc × Landrace × Large Yorkshire pigs. Therefore, it is necessary to find a way to decrease the fat deposition in Ningxiang pigs for high feed conversion efficiency with lower cost. As an intermediate product of metabolism process, betaine generally exists in animals and plants. With the further maturity of betaine chemical synthesis method and the continuous reduction of production cost, betaine has been widely used in livestock and poultry production [
6]. Fernández et al. [
7] found that betaine supplementation increased lean deposition in growing Iberian gilts. Therefore, it was hypothesized that betaine supplementation may reduce fat deposition and improve lean meat percentage in finishing Ningxiang pigs. The study was conducted to explore the effect of betaine supplementation on growth performance, carcass quality and fat metabolism in finishing Ningxiang pigs, which provide theoretical basis for the development and utilization of betaine in Ningxiang pigs.
4. Discussion
There were studies reporting that diet with betaine supplementation might upregulate the growth performance in pigs [
13]. However, there were no differences of ADG, ADFI and FCR between the control and test group in this study, which were in line with Albuquerque et al. [
14] and Sale et al. [
15].
Meat quality is an important actor for livestock and poultry production and used to be evaluated by meat color, shearing force, drip loss, cooking loss and others. In general, meat color is the first consideration for people to make a purchase decision, although meat color is not the most reliable forecaster for its safety and quality [
16]. Besides, the percentage of intramuscular fat (IMF) is also the important actor of meat quality. Additionally, there were researches reporting the positive effect of IMF on meat quality [
17,
18,
19]. Meat color depends on the content of meat pigment, which is mainly composed of myoglobin, hemoglobin and trace-colored metabolites. Yu et al. [
20] found that meat color scores were correlated to the proportion of oxymyoglobin, and negatively correlated with deoxymyoglobin and metmyoglobin content. Therefore, the meat color has a close relationship with the quality of meat. This study presented no marked change on a* and L* value of meat, which was consistent with the result of other research [
21]. However, b*
24h value of the BET diet group was markedly decreased. Thus, it indicated that betaine diet has a positive effect on the meat color. Besides, the study found that diet with betaine supplementation decreased the drip loss, cooking loss and shear force of the LT muscle, while increasing the water-holding capacity and tenderness of the muscles. These were consistent with other studies that betaine can maintain the balance of water in the cell by the special structure, so that betaine diet can greatly affect water-holding capacity in meat [
22,
23]. As a result of the less exudated water distributing on the surface of the muscle fiber, the ability to reflect light was weakened, thus b*
24h value of meat with betaine diet decreased [
24]. The present study indicated that the expression of FABP4 in LT muscles of the BET diet group was significantly upregulated. However, it was found that there was no difference in ether extract on longissimus thoracis between the control and test group. As a candidate gene for IMF deposition [
25], FABP4 affects metabolic pathways by regulating transport of fatty acids and other ligands [
26,
27]. The present results indicated that diets supplemented with betaine upregulated the expression of FABP4. However, it had no negative effect on ether extract of longissimus thoracis. In summary, dietary with betaine supplementation could affect the meat quality via decreasing b*
24h value of meat, increasing the water-holding capacity and tenderness of the muscles. Additionally, diet with betaine supplementation has no negative effect on ether extract of muscle.
Present results indicated that diet with betaine supplementation decreased the levels of EE and TC in liver, which was consistent with previous studies [
28]. Fat is an important part of the body, playing a key role in storing and supplying energy, maintaining body temperature, fixing internal organs and promoting the absorption of fat-soluble vitamins. However, excessive fat deposition makes negative influence on growth and even causes illness [
29]. As the main metabolic organ of the body, the liver is an important center of fat synthesis and decomposition. Betaine is involved in the fat oxidation in the liver. In addition, betaine can supply methyl for the body to produce lecithin and carnitine, which can promote the metabolization of fat in liver [
30,
31]. Thus, dietary with betaine supplementation decreased the level of TC in the liver.
Fat metabolism includes the deposition, synthesis, decomposition and transport. Fat deposition is actually the proliferation and growth of adipocytes. The key to control the proliferation and growth of adipocytes is to regulate the synthesis and decomposition of fat. ACC is the rate-limiting enzyme for fatty acid de novo synthesis [
32,
33]. Previous study finds that betaine can decrease fat deposition and improve the lean meat percentage of animals [
34]. Present results indicated that dietary with betaine supplementation decreased fat percentage and back fat thickness. The reason for the decrease of fat deposition might be related to genes expression. The present study indicated that dietary with betaine supplementation regulated the genes mRNA expression in subcutaneous back fat (downregulated the ACC and FAS mRNA expression and upregulated the ATGL and Sirt1 mRNA expression), and the results were in line with the findings previously described in literature [
35]. Therefore, the mechanism might be related to the function of modification of DNA methylation with betaine. Besides, there are reports found that choline diet could mitigate excessive fat deposition, while betaine could change into choline in body [
36,
37]. Kim et al. [
38] reported that the ACC mRNA expression in viral-induced mice downregulated after diet with betaine was supplemented. Another study indicated gene methylation levels of the ACC in diet with betaine supplementation group mice was higher than control group, which verified betaine may regulate the expression of ACC by supplying methyl [
39]. Besides, diet with betaine supplementation can also regulate the expression of ACC by regulating other genes [
30,
31]. FAS, the finally rate-limiting enzyme for fatty acid de novo synthesis, mainly exist in the liver and fat tissues, which can catalyze the synthesis of acetyl-CoA and malonyl-CoA into long-chain fatty acids [
40]. High-fat diet promotes high expression of FAS [
41]. Current results had verified betaine dietary downregulated the FAS and ACC mRNA expression in fat by Yang et al. [
42] and Dong et al. [
43]. ATGL can regulate the deposition of fat by regulating the process of triglyceride decomposition [
44]. It is reported that systemic and adipocyte-specific ATGL knockout mice exhibit moderate obesity [
45]. In addition, ATGL can promote the expression of Sirt1 [
46]. Sathyanarayan et al. [
47] found the high expression of ATGL on mice showed excessive fat deposition, and the situation was found to alleviate after the knockdown of Sirt1, which proved that the function of ATGL on fat metabolism appears to be mediated by Sirt1. Zhao et al. [
48] found that betaine diet could upregulate the ATGL mRNA expression in fat by modification of DNA methylation. In summary, the present study suggested that betaine could mitigate excessive fat deposition by downregulating the expression of ACC and FAS and upregulating the expression of ATGL and Sirt1.