Ovine blood parameters concentrations of specific physiological states, such as late gestation and lactation, are not as available on literature as there is for other farm animals. Although many of the substances analyzed in the present study are somehow associated with insulin activity, but were not affected by Cr supplementation, the data obtained by this experiment is scientific and clinically relevant for comparations.
During late gestation and early lactation, ruminants have a large and expected drop in lipogenesis and increase in lipolysis, which can lead to accumulation of triglycerides in the liver and abnormal production of ketone bodies [29,30]. In the present study, blood parameters indicate no compromise of liver functions [31]. However, there were responses to Cr supplementation that suggests an interference in nutrient partition at elevated metabolic demands.
Blood NEFA level have a direct relation with fat mobilization, and, therefore, with negative energy balance. Reduction in serum NEFA concentrations is one of the most relevant effects of Cr supplementation in ruminants during prepartum and early lactation periods [17,22,29,30,32]. An improvement on glucose uptake by insulin-dependent organs, modulated by more Cr available, decreases the reliance on body reserves and reduces fat mobilization and circulating NEFA [29]. Moreover, whereas increased supply of Cr improves insulin sensitivity in adipose tissue, it may also increase lipogenesis and, consequently, decrease net fatty-acid release from the cell [32]. Dairy cows receiving Cr methionine had lower serum NEFA concentration in prepartum period than cows not supplemented, with similar insulin concentrations [22]. In the present study, at late gestation, only the dose of 0.5 mg of Cr lowered NEFA concentration. This result agrees with the founds of lactating cows fed Cr at 0.05 and 0.10 g/kg BW0.75 where only the lower dose decreased NEFA levels, compared to control [29].
Kafilzadeh et. al [22], supplementing dairy cows Cr methionine during gestation, partially attributed the lower NEFA levels in prepartum to the reduced blood cortisol levels, as insulin did not change in comparison to the control group. In the present study, cortisol level was not affected by Cr supplementation neither in late gestation, nor in lactation. However, 0.5 mg of Cr lowered blood insulin level at lactation with no change in glucose level, reenforcing that there is a non-linear dose-dependent improvement in insulin sensitivity.
Increase in insulin sensitivity is one of the most documented responses to Cr supplementation [17]. Cr is the active compound of chromodulin, a cofactor for insulin action, which has an intrinsic role in insulin sensitivity and cellular signaling pathway. The increment of available Cr upregulates intracellular mRNA levels of the glucose transporter type 4, increasing membrane associated GLUT4, boosting the rate of glucose uptake [33].
Unchanged glucose levels during lactation with lower secretion of insulin by CR0.5 in comparison with CR0 suggest that the lower dose of Cr supplemented was capable of enhance insulin efficiency. Glucose homeostasis is strictly controlled and do not oscillate in ruminants as much as in monogastric animals [34]. Thus, to sustain the same basal glucose concentration of the non-supplemented treatment, CR0.5 ewes required a smaller amount of insulin. Non-linear insulin responsiveness to increased Cr supply were observed in post-partum dairy cows receiving 0.03, 0.06 and 0.12 mg of Cr/kg of metabolic BW [30].
In accordance with blood insulin level at lactation, BHB level was also reduced by 0.5 mg of Cr supplementation, suggesting that the referred dose enabled a more effective utilization of energy provided by the diet. The lack of benefits on energy partition with higher Cr doses compared to CR0 might suggest an alleviated Cr mediation of improved insulin sensitivity and signaling.
The relatively low concentrations of blood BHB between treatments and the absence of BCS change over time during trial indicates moderate or even low negative energy balance. There was no evidence of triglycerides accumulation on liver, for its synthesis functions were not compromised, as seen on blood protein concentrations. However, blood creatinine levels during lactation may suggest that 0.5 mg of Cr increase the effectiveness of aminoacidic turnover and protein synthesis in comparison to 1.5 mg of Cr, although there were no differences of indicators of protein metabolism between CR0.5 and CR0.
There was a notable interaction between Cr and serum levels of phosphorus (P) in the present study. As seen during lactation, serum P concentration increased linearly according to the amount of Cr provided, with CR1.5 treatment exceeding the reference range [35]. Calcium (Ca), on the other hand, was unaffected. Mineral intake at higher concentrations may have antagonist functions on the retention of other minerals. Sankaramaniel et al. [36], administering Cr intraperitoneally in rats, found that Cr inhibited both bone mineralization and resorption, while the concentrations of Ca and P in bone remained unaltered. Furthermore, Cr treatment had a 1.5-fold increase in serum P level, with serum Ca unchanged. Authors suggested that there may be increased absorption and/or decreased excretion of P, as there is less chance for the mobilization of P from the bone [36]. However, in a previous study [26], the femur density of lambs born from ewes that received 1 mg of Cr during gestation was negatively affected by the supplement [26]. Like other tissues, the bone has the ability to accumulate Cr when supplemented. It is expected that accumulated Cr might alter the metabolic activities of these sites, leading to altered bone turnover [36,37]. Further studies are needed to stablish the exact mechanism of Cr action on the bone and whereas it is mediated by insulin or not.
Summarizing, the correlation of the results of the presented study with the previous results of literature shows indications that Cr has the capacity to modulate insulin sensitivity in ruminants and interfere positively on the management of energy metabolism during ovine gestation and lactation. However, limitations of benefits of Cr supplementation were found regarding higher doses, implying why there are controversial conclusions towards the literature. It was also concluded that Cr supplementation doses used interfered progressively in serum phosphorus concentration. Moreover, the connection between Cr and mineral metabolism is a subject to be more investigated and better comprehended.