Glycerol is not a physiologic signal in the control of food intake in rats

Integration of metabolism and intake regulation: a review focusing on periparturient animals

There has been great interest in dry matter intake regulation in lactating dairy cattle to enhance performance and improve animal health and welfare. Predicting voluntary dry matter intake (VDMI) is complex and influenced by numerous factors relating to the diet, management, housing, environment and the animal. The objective of this review is to identify and discuss important metabolic factors involved in the regulation of VDMI and their integration with metabolism. We have described the adaptations of intake and metabolism and discussed mechanisms of intake regulation. Furthermore we have reviewed selected metabolic signals involved in intake regulation. A substantial dip in VDMI is initiated in late pregnancy and continues into early lactation. This dip has traditionally been interpreted as caused by physical constraints, but this role is most likely overemphasized. The dip in intake coincides with changes in reproductive status, fat mass, and metabolic changes in support of lactation, and we have described metabolic signals that may play an equally important role in intake regulation. These signals include nutrients, metabolites, reproductive hormones, stress hormones, leptin, insulin, gut peptides, cytokines, and neuropeptides such as neuropeptide Y, galanin, and corticotrophin-releasing factor. The involvement of these signals in the periparturient dip in intake is discussed, and evidence supporting the integration of the regulation of intake and metabolism is presented. Still, much research is needed to clarify the complex regulation of VDMI in lactating dairy cows, particularly in the periparturient animal.

Food intake in the domestic fowl: effect of intrahepatic lipid and amino acid infusions

Five experiments were conducted to determine the effect of hepatic and jugular infusions of amino acid and lipid solutions on short-term food intake of cockerels. Neither jugular nor hepatic infusions of amino acids had any effect on food intake of Single-Comb White Leghorn (SCWL) cockerels. Lipid infusions had no effect on SCWL cockerels when administered intrajugularly but decreased food intake significantly when infused intrahepatically. In contrast, no effect on food intake was observed when Rock-Cornish (RC) cockerels were infused intrahepatically with lipid. Thus, it appears that the liver of the SCWL cockerel is not responsive to plasma amino acid concentrations but is sensitive to circulating lipid levels. This lipid sensitive mechanism may be important in the peripheral control of food intake in a chicken such as the SCWL which has been bred for egg production but may be desensitized in fowl such as the RC broiler which has been selected for large body size and feed consumption.

Different metabolites might reduce food intake by the mitochondrial generation of reducing equivalents

To test the possibility that hypophagic effects of injected metabolites are linked to a particular oxidative step in their metabolism, cumulative food intake in rats after subcutaneous injection of various metabolites or their immediate oxidation products was investigated. Glycerol was compared with dihydroxyacetone, L-malate with oxaloacetate, and L-lactate with pyruvate. Subcutaneous injection of 4.5 mmol/kg metabolic body weight (BW0.75) of glycerol or malate reduced food intake significantly, whereas the same doses of dihydroxyacetone or oxaloacetate did not. Lactate or pyruvate (4.5 mmol/kg BW0.75, each) both reduced food intake significantly. However, even higher doses (7.7 mmol/kg BW0.75) of lactate or pyruvate failed to affect food intake when rats were fed a high fat (HF) diet, which is known to decrease the activity of the enzyme pyruvate dehydrogenase. In an additional experiment, subcutaneous injection of lactate or pyruvate (7.7 mmol/kg BW0.75, each) increased plasma levels of lactate and pyruvate 2 h after the injection in HF-rats more than in rats fed the usual high-carbohydrate diet. The data are consistent with the hypothesis that the increased generation of reducing equivalents in the mitochondria, brought about by the oxidation of injected glycerol, malate, lactate, or pyruvate, reduces food intake in rats.

Patterns of hyperphagia in the Zucker obese rat: a role for fat cell size and number?

The hypothesis that adipocyte size and number influence feeding behavior, via as yet unidentified signals to the CNS, is reviewed. The proposal is made that, due to several metabolic alterations which favor lipid deposition, the genetically obese Zucker rat (fafa) may be an appropriate model in which to study feeding-adipose tissue relationships. Data from several studies are presented demonstrating that the developing male Zucker fatty rat displays hyperphagia during the growth period which reaches a peak, or "break point," and then declines such that intake of fatty and lean rats becomes comparable at approximately 20 weeks of age. Beyond week 20, cycles of hyperphagia of several weeks' duration can be detected in fatty rats. The above feeding changes are related to data showing that on a laboratory chow-type diet, adipocytes approach maximal size at 15-16 weeks in the fatty rat, while accelerated proliferation of adipocytes takes place following week 20. During growth, responding for food in an operant task by fatty rats varies in accord with the pattern of hyperphagia. Further studies in the fatty rat show that the duration and magnitude of developmental hyperphagia can be altered by manipulating the caloric density and macronutrient content of the diet, with fat containing diets leading to the earliest break point of developmental hyperphagia. Some theoretical problems with the notion of adipose tissue feedback control of feeding behavior are discussed.