Vitamin E but not selenium alleviates heat stress compromised metabolism in growing pigs

Pigs raised in a hot environment have an increased carcass fat to protein ratio (Christon 1988). As heat stress (HS) attenuates lipid mobilisation in growing pigs (Pearce et al. 2013), it is hypothesised that the increased adiposity is due to elevated insulin, an anti-lipolytic hormone. We have previously observed that supra-nutritional supplementation of selenium (Se) or Vitamin E (VE) could alleviate the physiological response to HS in growing pigs (Liu et al. 2014). Therefore, the aim of this study was to investigate the effects of HS and antioxidant supplementation on insulin related metabolism.

Thirty-six gilts (Large White × Landrace) weighing 28.0 ± 4.1 kg (mean ± SD) were fed a Control diet (0.24 ppm Se, 17 IU/kg VE; NRC 2012), a high Se diet (1.0 ppm Se yeast, 17 IU/kg VE), or a high VE diet (0.24 ppm Se, 200 IU/kg α-tocopherol) diet for 14 days. Pigs were then exposed to either a thermoneutral (TN; 20°C) or cyclic HS (35°C from 09:00–17:00 h; 28°C overnight) for 8 d (n = 6 per treatment group). All pigs were restrictedly fed 80% of ad libitum over the trial. Pigs were fasted for 18 h from 1800 h on d 7 of thermal exposure and received a simplified oral glucose tolerance test (OGTT; 2 g/kg dextrose) on d 8. Plasma samples were obtained at –1, 30, 60 and 120 min in relative to dextrose intake for insulin, glucose and non-esterified fatty acids (NEFA) measurement. Data were analysed by REML in Genstat (15th edn.).

Feed intake was not different (P > 0.05) amongst treatment groups. During the OGTT the HS pigs had a higher glucose concentration at 30 min (6.9 vs 7.9 mM for TN vs HS, P < 0.01) and lower insulin concentrations at 30 min and 60 min (101 vs 93 and 23 vs 13 μU/mL for TN vs HS at 30 and 60 min, respectively; P < 0.05 for both comparisons) compared to TN conditions. This suggests that HS might have compromised insulin secretion. While glucose levels were not affected by dietary antioxidants, pigs fed on higher Se exhibited higher insulin concentrations (P = 0.01) 60 min after the OGTT during TN conditions, suggesting that overloaded Se might potentiate insulin resistance in normal condition. During HS pigs had lower fasting NEFA levels than those measured under TN condition (250 vs 141 μM, P < 0.05), indicating HS reduced lipid mobilisation. The pigs fed high VE had higher NEFA concentrations at 120 min than Control pigs in HS (P < 0.05), suggesting high VE facilitated lipid mobilisation in HS (Table 1).

Table 1.  Effects of heat stress and Se or VE on plasma metabolites in growing pigs during an OGTT^A

In conclusion, the observed decrease in lipid mobilisation during HS was not due to hyperinsulinemia, but rather a decrease in insulin secretion was noted. The attenuated lipid metabolism during HS needs to be further explored. Reduced lipid mobilisation during HS was ameliorated by dietary VE, but not by Se, suggesting that VE may prevent increased adiposity experienced during hot seasons.