Effects of dietary starch and energy levels on maximum feed intake, growth and metabolism of Nile tilapia, Oreochromis niloticus An Tran-Duy a, ⁎, Ben Smit a , Anne A. van Dam b , Johan W. Schrama a a Wageningen University, Department of Animal Sciences, Aquaculture and Fisheries Group, P.O. Box 338, 6700 AH Wageningen, The Netherlands b UNESCO-IHE Institute for Water Education, Department of Environmental Resources, P.O. Box 3015, 2601 DA Delft, The Netherlands article info abstract Article history: Received 29 July 2007 Received in revised form 3 March 2008 Accepted 4 March 2008 The aim of this study was to gain insight into how Nile tilapia (Oreochromis niloticus) regulate feed and energy intake in response to diets low and high in starch and cellulose. It was hypothesized that high-starch diets would reduce feed intake due to the effect of high blood glucose level, and that stomach volume may limit feed intake of fish fed diets low in energy. Four experimental diets, low starch–no cellulose inclusion, high starch–no cellulose inclusion, low starch–with cellulose inclusion, and high starch–with cellulose inclusion, were formulated. The high-starch diets and diets with cellulose inclusion were 17.5% more energy-diluted than the low-starch diets and diets without cellulose inclusion, respectively. Male tilapia were fed to apparent satiation for six weeks. Feed and digestible energy intake of fish fed diets with cellulose inclusion increased and decreased by 8.3% and 5.5%, respectively, compared to fish fed diets without cellulose inclusion. This suggests the role of stomach volume in restricting feed consumption. Fish fed high-starch diets achieved only 0.5% more feed intake and 13.9% less digestible energy intake than fish fed low-starch diets. The lower increase in feed intake and higher decrease in digestible energy intake of fish fed high-starch diets than of fish fed diets with cellulose inclusion suggests that high blood glucose suppresses feed intake in Nile tilapia. An alternative explanation for the differences in feed and digestible energy intake of fish fed different diets was based on the fact that heat production was not influenced by starch nor cellulose-inclusion levels. Thus, under satiation feeding, oxygen uptake capacity may determine feed and digestible energy intake in fish rather than blood glucose or stomach volume. © 2008 Elsevier B.V. All rights reserved. Keywords: Feed intake Carbohydrate Glucostatic Energy metabolism Tilapia Oreochromis niloticus 1. Introduction Feeding fish to satiation while avoiding feed waste can help maximize production (Brett and Groves, 1979; Elliott, 1994; Sæther and Jobling, 1999; Sun et al., 2006) and mitigate environmental pollution (Persson, 1991; Naylor et al., 2000). This requires knowledge about feed intake regulation. Among a wide range of factors affecting feed intake, nutrient and energy supply (and therefore diet composi- tion) have been shown to be important (Fletcher, 1984; Forbes, 1988; Blundell and Halford, 1994; Woods et al., 1998). It is likely that the composition of aquaculture feeds will change considerably in the future. Due to the decline in supply of fish meal and fish oil from wild fish (Naylor et al., 2000), plant protein and carbohydrate have been considered as important alternative ingre- dients for formulated fish feeds. Due to plant characteristics, the replacement of fish meal by plant protein leads to an increase in dietary carbohydrate content. Regardless of the protein source, the use of carbohydrate-rich diets has been considered economical as fish would utilize the inexpensive carbohydrate as a source of energy, thus sparing the absorbed protein for growth. Despite the large number of studies on the effects of dietary carbohydrate levels on fish growth (e.g. Furuichi and Yone, 1980; Shiau and Peng, 1993; Brauge et al., 1994; Peragon et al.,1999; Ali and Al-Asgah, 2001; Keshavanath et al., 2002), information on the relationship between dietary carbohydrate levels and maximum feed intake is scant. One of the consequences of increasing the carbohydrate content of the diet is an increase in dietary volume, i.e. a decrease in dietary energy concentration. Assuming that fish eat to meet their energy requirement (Rozin and Mayer, 1961; Boujard and Médale, 1994; Morales et al., 1994; Dias et al., 1998), they would compensate a low dietary energy concentration in high-volume, high-carbohydrate diets by ingesting more food. This could lead to a situation in which stomach volume restricts the feed intake before the energy require- ment is fulfilled. Another consequence of higher carbohydrate concentration in the diet may be that blood glucose level increases (Cowey et al., 1977; Brauge et al., 1994). This may also have an effect on feed intake. The glucostatic theory states that glucose receptors in the ventromedial hypothalamus produce hunger or satiation signals in response to below- or above-normal plasma glucose levels (Mayer, 1953, 1955). In mammals, there is ample evidence supporting this role of glucose in feed intake regulation and energetic homeostasis (Russek, 1963; Booth, 1972, 1979; Booth and Jarman, 1976; Louis-Sylvestre and Le Aquaculture 277 (2008) 213–219 ⁎ Corresponding author. E-mail address: an.tranduy@gmail.com (A. Tran-Duy). 0044-8486/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2008.03.004 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online