Development of non-invasive methods to monitor the transfer of dietary volatile compounds in pigs

Abstract

The transfer of volatile compounds from the maternal diet to amniotic fluid and milk has been previously evaluated using gas chromatography–mass spectrometry (GC/MS) (Palou et al. 2015). Both of these maternal fluids have disadvantages associated with the sampling procedures (i.e. sow monitoring at farrowing and oxytocin injection). Blood sampling is another method used to evaluate the transfer of dietary volatile compounds; however, this procedure is invasive, which causes stress and discomfort in the animals. The aim of this study was to develop a non-invasive user friendly methodology to assess the transfer of essential oil (EO) volatile compounds from feed to saliva and carpal gland secretion in pigs. We hypothesised that the quantification of volatile dietary compounds to saliva and carpal gland secretions will be indicative of the amount consumed and the transfer rate in the pre-absorptive and post-absorptive stages, respectively. This trial was conducted at the Herston Medical Research Centre. Twelve post-weaning piglets were individually penned and allocated into one of three treatments: (1) single dose of a mix of EO in feed (EOF, 0.45 mg/kg BW of the principal compound of each EO (EOPC)); (2) single dose of a mix of EO intravenously (IV) injected (EOIV, 32 μg/kg bodyweight (BW) of each EOPC); and (3) a control consisting of standard feed without EO. Treatments were provided with the morning meal. The EO comprised lemon ironbark, peppermint gum, nerolina, clove, thyme, cinnamon, oregano, geraniol and anethole. Saliva, carpal gland secretion and serum were collected at different time points: 5, 15, 30, 45, 60, 120 and 180 min after administration of the treatments. Saliva was collected by approaching the mouth of the pig with a sea sponge attached to tweezers. While piglets were distracted chewing on the sponge, carpal gland secretion was collected by a procedure consisting of a gauze sponge impregnated in distilled water to gently wipe the skin in two directions (top to bottom and left to right) repeated three times (‘skin washing’). In order to collect multiple blood samples a catheter was surgically implanted in the external jugular vein of all piglets. Data was statistically analysed using ANOVA in Minitab 16 (Minitab Inc., State College, PA, USA), to evaluate the transfer of EOPC to saliva, skin washing and serum between the three treatments at each time point. Paired t-test was performed to evaluate differences in the concentration of EOPC between serum with saliva and skin washing in treated pigs. The EOPC were analysed by headspace–solid phase micro extraction-CG/MS. Overall there were significantly higher (P < 0.05) concentrations of EOPC in saliva compared to serum in EOF piglets while significantly lower (P < 0.05) concentrations were found in saliva compared to serum in EOIV piglets. Additionally, the concentrations of volatile compounds in skin washings were significantly lower (P < 0.05) than in serum in both EOF and EOIV treatment groups. These results indicated that the high levels of dietary EOPC found in saliva of piglets in EOF treatment are explained by direct contact of saliva with the volatile compounds present in feed. The result suggests that the levels of dietary volatile compounds found in saliva after oral consumption of EO are indicative primarily of aerial transfer from feed contents in the gastrointestinal tract, which, in turn, may be related to recent intake. On the other hand, the EOPC levels found in skin washings were absorbed from the gastrointestinal tract first, transferred to blood (or lymph) and then secreted through the carpal glands. Thus, EOPC in carpal gland secretions would indicate post-absorptive transfer efficiency and stability in biological tissues. To the best of our knowledge, this is the first report that monitors the transfer of dietary volatile compounds to saliva and carpal gland secretions. It was concluded that the data offered two new approaches of monitoring the transfer of dietary volatile compounds to maternal fluids: one related to pre-absorptive (saliva) and the other to post-absorptive (carpal gland secretion) events. These results are relevant to the understanding of maternal-offspring imprinting through maternal diets in mammalian species including humans. Further investigation is required to evaluate the correlation between levels of dietary volatile compounds found in saliva and skin washing with those found in milk and amniotic fluid in pigs.