Plasma concentrations of leptin, insulin-like growth factor-I, and insulin in relation to changes in body condition score in heifers

The objective of this study was to determine the relationships among plasma concentrations of leptin, insulin, and IGF-I with dynamic changes in body condition scores (BCS) in heifers. Nineteen Zebu-Brown Swiss crossbred heifers, 24 to 30 mo old, weighing 322 9 kg, and with an initial BCS of 2.6 +/- 0.11 (range = 1 to 9) were used. Heifers were fed 60% of their maintenance requirements until they reached a BCS of less than or equal to2. Heifers were then maintained at that level for 25 d, after which they were fed to gain 1 kg of body weight daily until a BCS of 6 was reached. Heifers were weighed weekly and BCS was measured every 2 wk. Plasma samples were collected twice weekly, and leptin and insulin were determined by RIA. An immunoradiometric assay was used to measure IGF-I from one sample every 2 wk. Plasma concentrations of leptin were positively correlated during nutritional restriction (NR) and weight gain (WG) periods with BCS (r = 0.47 for NR, and r = 0.83 for WG; P < 0.01) and body weight (r = 0.40 for NR, and r = 0.78 for WG; P < 0.01). Plasma concentrations of leptin decreased during nutritional restriction (P < 0.01) as BCS decreased. During weight gain, leptin concentration increased at BCS 3 and there- after for each integer change in the BCS. Regression analysis showed that changes in body weight affect leptin concentrations within a given BCS. There was a decrease in IGF-I as BCS declined (P < 0.01). During weight gain, by contrast, IGF-I increased significantly (P < 0.01) with every unit change in body condition up to BCS of 4 and plateaued thereafter. Insulin concentrations did not change during nutritional restriction when BCS decreased from 3 to 1. However, once the diet was improved, there was a large increase in insulin concentrations in heifers with BCS 1 (P < 0.01). Among heifers of BCS 2 and 3, insulin did not differ and was lower than in heifers of BCS 1 (P < 0.01). Insulin increased (P < 0.01) among heifers at BCS 4 to 6. Leptin was positively correlated (P < 0.01) with both IGF-I (r = 0.34 for NR, and r = 0.36 for WG) and insulin (r = 0.18 for WG). Insulin was correlated with IGF-I (r = 0.60; P < 0.01). During nutritional restriction, insulin did not correlate with leptin (r = -0.05), BCS (r = -0.03), or IGF-I (r = 0.07). It was concluded that leptin serves as a dynamic indicator of body condition in heifers, as well as an indicator of nutritional status.