Effects of dietary menthol-rich bioactive lipid compounds on zootechnical traits, blood variables and gastrointestinal function in growing sheep

BACKGROUND: The present study aimed at investigating the influence of 90% menthol-containing plant bioactive lipid compounds (PBLC, essential oils) on growth performance, blood haematological and biochemical profile, and nutrient absorption in sheep. Twenty-four growing Suffolk sheep were allotted into three dietary treatments: Control (without PBLC), lower dose of PBLC (PBLC-L; 80 mg/d) and higher dose of PBLC (PBLC-H; 160 mg/d). Sheep in all groups were fed meadow hay ad libitum plus 600 g/d of concentrate pellets for 28 d. RESULTS: Average daily gain was not affected by treatment. Feeding of PBLC increased hay and total feed intake per kg body weight (P < 0.05). Counts of total leucocytes, lymphocytes and monocytes were not different among treatments. However, neutrophil count decreased (P < 0.05) in PBLC-H with a similar trend in PBLC-L (P < 0.10). Concentrations of glucose, bilirubin, triglycerides, cholesterol, urea and magnesium in serum were not different among sheep fed different doses of PBLC. However, serum calcium concentration tended to increase in PBLC-H (P < 0.10) and serum concentrations of aspartate & asparagine (P < 0.01) and glutamate & glutamine (P < 0.05) increased linearly with increasing PBLC dose. In ruminal epithelia isolated from the rumen after killing, baseline conductance (G(t); P < 0.05) and short-circuit current (I(sc); P < 0.01) increased in both PBLC groups. Ruminal uptakes of glucose and methionine in the presence of Na(+) were not affected by the dietary PBLC supplementation. In the absence of Na(+), however, glucose and methionine uptakes increased (P < 0.05) in PBLC-H. In the jejunum, I(sc) tended to increase in PBLC-H (P < 0.10), but baseline G(t) was not affected. Intestinal uptakes of glucose and methionine were not influenced by PBLC in the presence or absence of Na(+). CONCLUSION: The results suggest that menthol-rich PBLC increase feed intake, and passive ion and nutrient transport, the latter specifically in the rumen. They also increased serum concentrations of urea precursor amino acids and tended to increase serum calcium concentrations. Future studies will have to show whether some of these findings might be commonly linked to a stimulation of transient receptor potential (TRP) channels in the gastrointestinal tract.