Dynamics of protein and lipid intake regulation of rainbow trout studied with a wide lipid range of encapsulated diets and self-feeders.

Feed intake in fish is examined extensively but there is not much information about the dynamics of regulation i.e. how fish react to different diets, and how these reactions change, over a longer period of time. The present study was designed to evaluate the dynamics of food intake regulation in rainbow trout over a very wide range of dietary protein and lipid levels; from a very low lipid level (5%) to an extremely high level (55%). The study was conducted with three subsequent 40-day blocks of 20 fish and the intake dynamics of the lipid effect were studied by splitting the 40-day experimental period to shorter periods of 10 days. Depending on a diet the rainbow trout were more willing to ingest larger surpluses of both protein and lipid during the periods 0-10, 10-20 and 20-30 days if compared with the anticipated nutritional intake target of rainbow trout. A strong regulatory response against high lipid intake was seen during the last period (days 30-40) leading not only to a decrease in lipid intake but much more drastic decrease in protein intake. Thus, a significant nonlinear interaction between time and dietary protein and lipid was found indicating that the effect of protein and lipid was dynamic.

Plasma ghrelin levels in rainbow trout in response to fasting, feeding and food composition, and effects of ghrelin on voluntary food intake.

Ghrelin, a peptide hormone which stimulates growth hormone (GH) release, appetite and adiposity in mammals, was recently identified in fish. In this study, the roles of ghrelin in regulating food intake and the growth hormone (GH)-insulin-like growth factor I (IGF-I) system of rainbow trout (Oncorhynchus mykiss) were investigated in three experiments: 1) Pre- and postprandial plasma levels of ghrelin were measured in relation to dietary composition and food intake through dietary inclusion of radio-dense lead-glass beads, 2) the effect of a single intraperitoneal (i.p.) injection with rainbow trout ghrelin on short-term voluntary food intake was examined and 3) the effect of one to three weeks fasting on circulating ghrelin levels and the correlation with plasma GH and IGF-I levels, growth and lipid content in the liver and muscle was studied. There was no postprandial change in plasma ghrelin levels. Fish fed a normal-protein/high-lipid (31.4%) diet tended to have higher plasma ghrelin levels than those fed a high-protein/low-lipid (14.1%) diet. Plasma ghrelin levels decreased during fasting and correlated positively with specific growth rates, condition factor, liver and muscle lipid content, and negatively with plasma GH and IGF-I levels. An i.p. ghrelin injection did not affect food intake during 12-hours post-injection. It is concluded that ghrelin release in rainbow trout may be influenced by long-term energy status, and possibly by diet composition. Further, in rainbow trout, ghrelin seems to be linked to growth and metabolism, but does not seem to stimulate short-term appetite through a peripheral action.

Circulating levels of cholecystokinin and gastrin-releasing peptide in rainbow trout fed different diets.

Cholecystokinin (CCK) and gastrin-releasing peptide (GRP) are gastrointestinal peptides thought to be important regulators of intake and digestion of food in vertebrates. In this study, pre- and postprandial plasma levels of CCK and GRP were measured in rainbow trout (Oncorhynchus mykiss) by the establishment of homologous radioimmunoassays, and the hormonal levels assessed in relation to dietary lipid:protein ratio and food intake. Fish were acclimated to either a high protein/low lipid diet (HP/LL diet; 14.1% lipids) or a normal protein/high lipid diet (NP/HL diet; 31.4% lipids). On three consecutive sampling days, radio-dense lead-glass beads were included in the diets for assessment of feed intake. Fish were terminally sampled for blood and stomach contents prior to feeding at time 0, and at 0.3, 1, 2, 4, 6, and 24 h after feeding. There was a postprandial elevation of plasma CCK levels, which was most evident after 4 and 6 h. Fish fed the NP/HL diet had higher plasma CCK levels compared with those fed the HP/LL diet. Plasma CCK levels were not affected by the amount of food ingested. GRP levels in plasma were not influenced by sampling time, diet, or feed intake. The results indicate that the endocrine release of gastrointestinal CCK is increased during feeding and may be further influenced by the dietary lipid:protein ratio in rainbow trout. Plasma GRP levels, on the other hand, appear not to be influenced by feeding or diet composition.