Discussion
We found in this study that the administration of VD or DAG could significantly reduce the body weight gain and the body fat percentage in rats fed a high fat diet and those effect were much stronger when a combination of VD and DAG was applied. The results showed that duck oil DAG and VD may have synergistic effects on reducing body weight gain and fat deposit in obese rats. Murase et al. (2001) showed that body mass and visceral fat were significantly decreased in mice fed a DAG diet compared with a triacylglycerol (TAG) diet, confirming DAG can reduce fat deposit in mice. Guo et al. (2018) found that VD supplementation to obese people could help to reduce the body mass index.
We also found in the present study that duck oil DAG and VD administrations could significantly reduce the circulating lipids, increase AST and ALT enzyme activity in rats fed the high fat diet, and had best effects compare with DAG or VD administration. Ciantti et al. (2007) found that bone marrow stromal cells in the VD receptor gene knockout mice were more likely to differentiate into adipocytes. Other studies have shown that a low serum 25 hydroxyvitamin D concentration is associated with higher blood lipid concentration (Bouillon, 2018), and VD supplementation could reduce blood lipids. Liu et al. (2018) found that the circulating TG level was significantly reduced in response to 1, 25(OH)2D3 intervention in diabetes rats induced by feeding a high-fat diet. Meng et al. (2004) showed that feeding a high DAG diet significantly reduced the plasma levels of TC, TG, and apolipoprotein b, increased the level of HDL-C, and inhibited the production of neutral fat in rats.
The activities of AST and ALT are important indicators to the liver function (Yan, 2007). He et al. (2016) found that the AST and ALT activity in blood increased in obese rats treated with DAG microcapsule. Piao (2013) found that VD3 had a protective effect on spot necrosis of liver tissue in mice when acute liver injury was induced by administration of carbon tetrachloride. Our results are basically consistent with above results, and also found that duck oil DAG and VD administrations had higher value compare with DAG or VD administration. The results showed that duck oil DAG and VD had the synergistic effects on reducing serum lipids and enhancing AST and ALT activity.
Disorder of lipid metabolism can lead to the development of obesity (Gao, 2011; Jin, 2016; Yang, 2017). The principle of promoting fat mobilization should be followed in the treatment of obesity. FAS is a key enzyme to control the synthesis of fatty acids, a process that leads to the accumulation of TG and the risk of hyperlipidemia (Liu, 2013; Lu, 2012). CPT1 is a rate-limiting enzyme for the oxidation of fatty acids (Wang, 2017). In obese people, the increasing level of FAS will inhibit the decomposition of body fat and reduce the activity of CPT1 enzyme, so fat deposit could increase. In this study, we found that duck oil DAG combined with VD and DAG alone or VD alone could significantly down-regulate the FAS expression and up-regulate the CPT 1, and the expression of both genes was highly correlated with the TC, TG, LDL, and HDL concentrations, and the AST and ALT activity, counteracting the fat deposit. Wang et al. (2013) and Xiang et al. (2007) found that DAG decreased the FAS expression and increased the CPT1 expression and opioid melanocortin progenes, thus regulated fatty acid metabolism. Hao et al. (2018) found the AMP-dependent protein kinase (AMPK) regulates acetyl-CoA carboxylase (ACC)/CPT1 signaling pathway and promotes fatty acids metabolism and reduce fat accumulation in hypertensive rats. Wu (2012) found that when the expression of lipid metabolism genes in the liver was altered by administration of taurine, the activity of AST and ALT changed. The results of the current experiment are basically consistent with above results. The results showed that duck oil DAG and VD changed the FAS and CPT1 mRNA expression, leading to the corresponding changes in the circulating lipids and the enzymes involved in lipid metabolism in liver, unfavored for fat deposit in obese rats.
Previous reports have provided direct evidence that there is a close relationship between intestinal microflora and the development of obesity. Intestinal microflora disorder can cause chronic and persistent low-grade inflammatory response, lipid metabolism, and intestinal hormone secretion, thus promote the formation of obesity (Cheng et al., 2020). Our results in the present study showed that the obesity significantly changed the composition of the microbiota in the ceacum, and the administration of VD or DAG or a combination of VD and DAG increased Bacteroidales and Lachtobacillus , decreasedLachnospiraceae , Allprevotella , Desulfvibrio, andAllobaculum abundance. Interestingly, Lachnospiraceae andBacteroidales in the DG10VD12.5 group had higher effects compared with those in the VD12.5 or DG10 group, and Desulfvibrio ,Lactobacillus, Allprevotella , and Allobaculumabundances in the DG10VD12.5 group were no significant difference compared with those in the NO group. It has been suggested that the abundance of Lachtobacillus and Bacteroidales decreases in obesity patients (Doris et al., 2017; Wang et al., 2020) .Lactobacillus exerts a significantly beneficial effect on the gut by inhibiting the growth of pathogenic bacteria, modulating the community of intestinal microflora (Kuugbee et al., 2016; Ou et al., 2011). Desulfovibrionaceae , which is a family of gram-negative sulfate-reducing intestinal bacteria, induces the reduction of sulfate. Researchers have reported that the abundance ofDesulfovibrionaceae is increased in the feces of IBD patients (Berry & Reinisch, 2013). These results indicate that administration of the DAG and VD could improve the change of microbiota in obese rats.