Three-time feeding does not influence insulin daily rhythm in horses

The aim of the present study was to investigate the influence of three-time feeding on insulin daily rhythm in horses. At this purpose, five clinically healthy Italian saddle horses were housed under natural photoperiod (L/D) followed by constant darkness (D/D). Horses were fed three times a day (6:00; 12:00; 18:00) with hay and a mix of cereals. Blood samples were collected every 3 h over 48-h period (24 h during L/D and 24 h during D/D). On each serum sample insulin concentration was assessed. Two-way for repeated measures analysis of variance (ANOVA) was applied to investigate statistically differences due to time of day and experimental conditions. Cosinor rhythmometry was applied to investigate daily rhythmicity in the two experimental periods. Our results showed a significant effect of time of day and experimental conditions on serum insulin concentrations. Daily rhythmicity of insulin serum concentrations was observed during L/D and D/D. Statistically differences in MESOR (Midline-Estimating Statistic of Rhythm), amplitude and acrophases were observed between L/D and D/D. MESOR and amplitude statistically decreased in D/D respect to L/D, and acrophase was postponed in L/D respect to D/D. Robustness of rhythm did not change between the two experimental period. In conclusion, we can hypothesised that the daily rhythm of insulin in horses is endogenously generated; and that during the routine management of the stabled horses providing feed three times a day favourite the physiological secretion of insulin in this species.