Effects of feeding frequency and voluntary salt intake on fluid and electrolyte regulation in athletic horses

The effect of feeding frequency and voluntary sodium intake (VSI) on fluid shifts and plasma aldosterone concentration (PAC) were studied at rest and after exercise in six athletic horses. The horses were fed twice a day (2TD ) and six times a day (6TD) for 25 days for each protocol, according to a c hangeover design. VSI was measured by weighing each horse's salt block dail y. Feeding 2TD or 6TD caused no major alterations in fluid shifts, but in t he 2TD treatment there was a postprandial increase in plasma protein concen tration and osmolality that lasted <1 h. PAC and VSI were not affected by f eeding frequency. VSI ranged from 0 to 62 mg.kg body weight(-1).day(-1) and caused significant alterations in PAC. At VSI <26 mg.kg body weight(-1.)da y(-l), a diurnal rhythm for PAC was noted. Water intake, fecal concentratio ns of sodium and potassium, and packed cell volume during exercise were inf luenced by VSI. The response to exercise did not differ between treatments. In conclusion, VSI, but not feeding frequency, has significant effects on fluid and electrolyte regulation in athletic horses.