As metabolomics can provide useful information at a global system level, in the present study, the differences in the plasma metabolite profiles of dairy cows with two levels of CBCS were identified by LC-MS. In fact, seven important metabolites were identified and involved in four KEGG pathways. For our interest, the concentrations of classic metabolic indexes were also measured. A link between BCS and metabolic indicators has been discussed in obvious studies [15,16,19]. It is well known that leptin, the intake satiety signal, regulates the whole-body metabolism by stimulating energy expenditure and inhibiting food intake [3]. Adiponectin helps to enhance fatty acid oxidation and energy expenditure and inhibit proinflammatory cytokines production [20,21]. In our study, circulating leptin levels were up-regulated, and adiponectin levels were down-regulated in the HBCS group, which is similar to the results in humans [22] and rodents [23]. In addition, the concentration of TNF-α tended to be greater in HBCS cows, indicating higher calving BCS was associated with a proinflammatory response [17]. Pires et al. [16] reported that cows calving with high BCS presented elevated plasma NEFA and BHB concentrations during the early lactation (up to 7 weeks postpartum), as explained by metabolically, due to the intense mobilization of body fat. Surprisingly, the concentrations of NEFA and BHB were not affected by CBCS in this study. This may be because during the early lactation, BCS loss associated mobilization of body fat is more important in concentration of metabolic indicators rather than BCS per se [24]. It may also be due to the fact that the plasma samples were collected at the special occasion of calving, and the hormone stimulation may affect the lipid metabolism. However, the Spearman’s correlation analysis demonstrates a significant correlation between the traditional indicators and the seven metabolites, indicating these differential metabolites may be important molecules related to metabolic balance in over-condition cows.

In plasma, significant amounts of LysoPCs are formed by lecithin:cholesterol acyltransferase that catalyzes the transfer of the fatty acids of position sn-2 of PC to the free cholesterol [25,26]. LysoPCs have various species because they have different combinations of fatty acids with varying lengths and saturation. In this study, all species of LysoPCs in plasma concentrations decreased (saturated and unsaturated) in the HBCS group, which is in line with the results in humans [13], mice [12], and dogs [9]. Barber et al. [12] discovered that a high-fat diet caused a reduction in plasma LysoPC levels in mice. In addition, obese humans showed a reduction in plasma LysoPCs, indicating that adiposity plays an important role in altering the plasma LysoPC profile [12]. Moreover, the decrease of LysoPC in obesity was negatively correlated with body mass index [7]. In this study, the abundance of LysoPCs were negatively correlated with elevated energy metabolism values; therefore, the reduction of plasma LysoPCs in this study might be partly due to the slight accumulation of NEFA that could induce metabolic stress in the HBCS cows [13]. However, inconsistent with the results of our study, Pietiläinen et al. [22] reported that concentrations of LysoPCs were higher in obese co-twins compared to non-obese co-twins. To sum up, body condition changes LysoPCs metabolism profile, which is an important indicator.

As important signaling molecules, LysoPCs have diverse biological functions on cellular signaling processes, apoptosis induction, and anti-infective effects, especially on the inflammation process [27,28]. At present, the evidence is emerging to indicate that LysoPCs serve as important mediators in inflammatory diseases [28]. In this study, there was a negative correlation between LysoPCs and proinflammatory cytokines (TNF-α and IL-6), similar to the results of Heimerl et al. [7], who reported that LysoPCs correlated significantly with a proinflammatory indicator (e.g., C-reactive protein), and the decreased concentration of LysoPC was associated with its protective and anti-inflammatory effects. Thus, given fat cows being in serious proinflammatory conditions, it is reasonable to speculate that alterations of LysoPCs profile may be related to the role of LysoPCs in neutralizing the inflammation. Additionally, it was reported that LysoPCs may regulate inflammation through their fatty acyl chain [29]. For example, the unsaturated LysoPCs (20:4 and 22:6) could counteract proinflammatory effects [13,30]. Whereas saturated LysoPC (16:0), acting as an inflammatory inducer, could promote proinflammatory cytokine activity, cause migration of immune cells, and cause plasma leakage [30]. However, the reason for the decrease in saturated LysoPC(15:0) in HBCS requires further study.

In mammalian membranes, phosphatidylethanolamine (PE) is the second most abundant phospholipid, accounting for about 15–25% of the total lipids of mammalian cells [31]. In this study, PE(14:0/22:2(13Z,16Z)) differs significantly between the NBCS and HBCS groups, which not only participates in autophagy, but also in retrograde endocannabinoid signaling, glycerophospholipid metabolism, and GPI-anchor biosynthesis. Among them, autophagy, and retrograde endocannabinoid signaling pathways are closely related to obesity-associated metabolic disorders [32,33,34].

As an important component for autophagy, PE is covalently attached to the microtubule-associated protein light chain 3 to trigger autophagosome formation in mammalian cells [35]. Autophagy has been shown to regulate cellular lipid storage and lipid droplets biogenesis/breakdown [33,36]. Decreased autophagy contributes to hepatic lipid accumulation and then further inhibits autophagic function, thereby increasing lipid retention [36,37]. In addition, excessive lipid infiltration in the liver of cows impairs autophagic activity [34]. Since PE(14:0/22:2(13Z,16Z)) was negatively correlated with NEFA and BHB, the decrease of PE(14:0/22:2(13Z,16Z)) concentration in the HBCS group in this study may contribute to the suppression of autophagy and then promote excessive lipids accumulation. In support of this, Ghaffari et al. [38] have shown that autophagy in HBCS cows may be suppressed, because they found that the mRNA abundance of mammalian target of rapamycin (mTOR) in the skeletal muscle was greater in high BCS cows than in normal BCS cows on d + 21 after calving, and mTOR was considered to be a potent suppressor of autophagy [39,40].

PE participates in the synthesis of anandamide, which is an endogenous ligands of cannabinoid receptors in the retrograde endocannabinoid signaling pathway [41]. It has been reported that the endocannabinoid system is involved in regulating energy balance, such as feed intake, energy expenditure, and lipid metabolism [42]. In addition, the formation of anandamide is accompanied by the production of ethanolamides of other fatty acids, which are collectively referred to as N-acylethanolamines (NAEs) [43]. However, major NAEs do not show ligand activity for cannabinoid receptors, but they participate in a variety of biological activities. In particular, oleoyl-ethanolamide (OEA, a non-endocannabinoid NAE) mediates the satiety and the bodyweight through activation of the peroxisome proliferator-activated receptor-α [44], a nuclear receptor that promotes fatty acid oxidation and reduces triglyceride level. The dysfunction of OEA signaling could contribute to overweight and obesity [45]. In this study, PE(14:0/22:2(13Z,16Z)) was involved in the retrograde endocannabinoid signaling pathway, but its exact role in this pathway remains to be further clarified. In the present study, the number of cows used was limited. Thus, studies with more individuals should be conducted to better understand the plasma metabolic changes in cows with different body conditions.