ORIGINAL ARTICLE Insulin-like growth factor I in growing thoroughbreds W. B. Staniar 1 , D. S. Kronfeld 1 , R. M. Akers 2 and P. A. Harris 3 1 Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, 2 Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, and 3 Equine Studies Group, WALTHAM Centre for Pet Nutrition, Melton Mowbray, UK Introduction Growth rate is regulated mainly by the somatotropic axis, a major component in the chemical communi- cation network that links metabolism to nutrient intakes (Le Roith et al., 2001; Yakar et al., 2002). Principle components in the somatotropic axis are insulin, growth hormone (GH) and insulin-like growth factor I (IGF-I). Circulating IGF-I is an espe- cially important, but broadly acting, regulatory factor in skeletal development (Yakar et al., 2002), and healthy skeletal development an important objective for those raising equine athletes. Previous research efforts have examined plasma IGF-I in relation to diet, dam parity, weaning and age (Cymbaluk and Laarveld, 1996; Malinowski et al., 1996; Ropp et al., 2003). Nutrition and envi- ronment influence circulating IGF-I, although vari- able results require careful attention to differences in experimental constraints and applicability of the results (Ketelslegers et al., 1995; Cymbaluk and Laarveld, 1996). The role of the somatotropic axis in facilitating physiological changes in response to the environment has been studied in other species (Sarko et al., 1994; Owens et al., 1999; Lincoln et al., 2001; Gabillard et al., 2003), but similar data for the horse are incomplete and its application unclear. We hypothesize that meals that result in glycaemic and insulinaemic responses significantly greater than Keywords dietary energy sources, environment, growth, somatotropic axis, thoroughbred Correspondence W. B. Staniar, Middleburg Agricultural Research and Extension Center, 5527 Sullivans Mill Road, Middleburg, VA 20117, USA. Tel: (540) 687 3521; Fax: (540) 687 5362; E-mail: wstaniar@vt.edu Received: 29 May 2006; accepted: 5 September 2006 First published online: 15 March 2007 Summary The objective of this longitudinal study was to characterize growth and plasma insulin-like growth factor I (IGF-I) concentrations in pasture- raised thoroughbreds fed two sources of dietary energy. Mares and foals were randomly assigned to either a sugar and starch (SS) or fat and fibre (FF)-rich feed, and plasma IGF-I and growth were measured once a month from 1 to 16 months of age. These dependent variables were also compared with day length and ambient temperature. There was an association between plasma IGF-I concentration and average daily gain (ADG) (r ¼ 0.32, p < 0.001). There were also clear seasonal patterns in both ADG and plasma IGF-I, with high values in June and May, and a low value in March. Plasma IGF-I and ADG were positively associated with day length and temperature. Plasma IGF-I was never higher (p > 0.10) in the FF group when compared with the SS group, and was higher in the SS group during a rapid growth phase in the spring of year 2 (p < 0.10). The results establish an association between ADG and IGF- I in the horse and indicate that environment and age may influence this relationship. In addition, plasma IGF-I is influenced by dietary energy source at particular times of year. This link has important implications in designing feeding management strategies that are aimed at addressing skeletal development. DOI: 10.1111/j.1439-0396.2006.00666.x 390 Journal of Animal Physiology and Animal Nutrition 91 (2007) 390–399 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd