Edmund T.S. Li

Meal composition influences subsequent food selection in the young rat

Food selection one-half hour after the ingestion (2 g) of either a protein containing or a protein-free (carbohydrate) diet was studied in young rats. Following a 12 hr fast the rats were allowed 15 minutes to consume the meal (premeal). Thirty minutes later, they had access to two isocaloric diets that differed only in protein and carbohydrate content. During the first hour of ad lib feeding, protein intake and protein concentration selected were lower in the 45% casein prefed group compared to the carbohydrate group. A further reduction in protein intake and protein concentration occurred when the protein content of the premeal was increased to 70% from 45% casein. Rats prefed with the 70% casein diet significantly reduced their daily total food intake (12 hr) compared to either the 45% casein or carbohydrate prefed group. It is concluded that both quantity and composition of food selected is influenced by the composition of the preceding meal.

Brain mechanisms and the quantitative and qualitative aspects of food intake

The brain receives a large number of signals from the ingestion of food. They provide the brain with information on both the adequacy of energy ingested and the macronutrient composition of the food. From this information brain feeding control systems are able to respond and direct the animal to make appropriate food choices so that both the quantitative (energy) needs as well as the qualitative (nutrient) needs are met. An understanding of brain mechanisms regulating feeding will only emerge if their dual purposes are recognized.

Different diurnal rhythms of protein and non-protein energy intake by rats☆

Abstract Rats allowed to self-select dietary constitutents control their intake of protein and energy (calories) independently. Although absolute protein consumption per meal remains almost constant, meal size increases progressively from the mid-light period through the dark period by selective increases in the non-protein content. Hence the composition of selected meals follows a daily rhythm with protein concentration decreasing from light to dark meals. The consistency of these feeding rhythms indicates separate short-term regulation of protein and energy intakes.

Effects of repeated administration of serotonergic agonists on diet selection and body weight in rats

Food intake, diet selection and body weight gain were examined in three separate experiments in which rats received saline or one of three serotonergic agonists, dexfenfluramine, RU 24969 and fluoxetine. In all experiments, food was available only in the dark period during which time rats were given simultaneous access to two isoenergetic diets which differed in their protein and carbohydrate content. After habituation to this feeding paradigm and intraperitoneal injections, rats were assigned to control or drug group. Saline or a serotonergic agonist was given to the same rat once daily, 15 min prior to feeding, for six consecutive days. All three agonists (1.5 mg/kg for dexfenfluramine and RU 24969; 3 mg/kg for fluoxetine) caused immediate (first two h of feeding) hypophagia which was accounted for by the selective suppression in intake of the high-carbohydrate-low-protein diet. This selective shift in diet choice was sustained upon repeated exposure. Although the effects of these agonists on daily (12-h) feeding was less pronounced, appetite suppression was due entirely to reduced intake of the high-carbohydrate-low-protein diet. Of the three agonists tested, partial tolerance was observed only after dexfenfluramine. Nevertheless, all three agonists caused comparable declines in weight gain. These results suggest that repeated administration of serotonergic agonists has sustained impacts on food intake, diet choice and weight gain.

Food intake and selection pattern of rats treated with dexfenfluramine, fluoxetine and RU 24969

The effects of two indirect (dexfenfluramine and fluoxetine) and one direct (RU 24969) serotonergic agonists on diet selection over a 12-hr period were examined. Rats were habituated to eat, during the dark period, from two isoenergetic diets that differed in carbohydrate and protein content. Drugs were injected intraperitoneally at 1845 hr, 15 min prior to food access. The drugs exerted their effects mainly during the first hour of feeding (1900-2000 hr). At this time, a selective suppression in intake of the high carbohydrate-low protein diet was the most prominent characteristic of all three serotonergic agonists. This macronutrient specific effect was particularly strong at low dosages (dexfenfluramine, fluoxetine and RU 24969: 0.5, 2.0 and 1.0 mg/kg, respectively). With time, as the effect of drugs wore off, diet selection pattern became more variable. The fact that both indirect 5-HT agonists and a direct selective 5-HT receptor agonist share a specific behavioral effect provides additional support for the role of serotonin in the control of macronutrient specific appetites.

Food intake and selection after peripheral tryptophan

Two studies investigated the effects of peripheral (IP) administration of the dietary indispensible amino acid tryptophan, on food intake and macronutrient selection in rats adapted to a 12 hr nocturnal feeding period and a choice of 10% and 60% casein diets. In a dose-response study (35, 55, 75, 95, 115 mg/kg), the threshold dose of 75 mg/kg produced a significant reduction in total food intake (3.6 to 2.3 g, p less than 0.05) during the first hour of feeding. The reduction in carbohydrate intake (2.1 vs. 1.2 g, p less than 0.05) was greater than that for protein intake (1.6 vs. 1.1 g, p less than 0.05). Twelve hr total food intake was also decreased (20.9 to 19.5 g, p less than 0.05) and this was attributable to decreased carbohydrate intake (13.2 to 11.8 g, p less than 0.05). In a second study designed to determine if tryptophans effects were mediated by the central nervous system, brain tryptophan uptake was blocked by co-injecting valine with tryptophan. The significant reduction in first hour total food intake by tryptophan was not prevented by co-injection of an equal quantity of valine (3.5 to 1.8 g, p less than 0.05). Again the suppression of carbohydrate intake (2.0 to 0.9 g p less than 0.05) was greater than that for protein intake (1.5 to 0.9 g, p less than 0.05). This dose of valine significantly reduced brain tryptophan uptake by 16% (21.3 to 17.8 micrograms/g, p less than 0.05) and when administered alone did not affect first hour total food intake (3.1 vs 3.2 g).(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary Fat Concentration Influences the Effects of trans-10, cis-12 Conjugated Linoleic Acid on Temporal Patterns of Energy Intake and Hypothalamic Expression of Appetite-Controlling Genes in Mice

This study tested the hypothesis that the effect of trans-10, cis-12 conjugated linoleic acid (t10, c12 CLA) on energy intake (EI) and body weight (BW)/composition is confounded by dietary fat concentration and involves hypothalamic appetite-controlling mechanisms. ICR mice received low-fat (LF; 5 g/100 g) or high-fat (HF; 30 g/100 g) diets, with or without 0.5 g/100 g t10, c12 CLA (>98% pure) for 27 d. By d 13, BW and cumulative EI of the mice fed CLA supplemented LF diet (LF/CLA) were 6.6 and 23.6% lower, respectively, than the LF mice. In the subsequent 14 d, their EI rebounded and did not differ from the LF group. BW and EI did not differ between the HF and CLA supplemented HF (HF/CLA) groups. Hypothalamic pro-opiomelanocortin (POMC) mRNA expression was elevated (P = 0.031) on d 13 but suppressed (P < 0.001) on d 27 due to CLA treatment. CLA also suppressed AMP-activated protein kinase alpha2 expression. Mice in Expt. 2 received the LF diet, the LF/CLA, or were pair-fed the LF diet to the EI of the CLA group (LF/PF). LF/CLA and LF/PF mice did not differ in the hypothalamic POMC:neuropeptide Y expression ratio on d 13, but it was significantly lower in the LF/PF group on d 27. We conclude that the habitual dietary fat concentration influences the magnitude of weight loss induced by dietary t10, c12 CLA. The effect is in part independent of EI. Hypothalamic neuropeptides and nutrient sensing mechanisms may play a role.

Dietary carbohydrate and the nervous system

Abstract The purpose of this review is to draw attention to the many effects of dietary carbohydrate on the nervous system. In addition to being dependent on glucose as its main energy source, the nervous system and many of its functions are influenced by the electrophysiologic and metabolic consequences of carbohydrate ingestion. The subject is discussed under the following topics; brain energy metabolism, brain links to glucose availability in the gut and liver, carbohydrates and brain neurochemistry, dietary sugars and brain neurochemistry, and carbohydrates and the sympathetic nervous system.

Effect of 5-HT agonists on rats fed single diets with varying proportions of carbohydrate and protein

Low doses of 5-HT agonists have been shown to selectively suppress carbohydrate intake in rats given dietary choices. To investigate further the relationship between dietary macronutrient composition and 5-HT-induced anorexia, the present study examined the effects of three 5-HT agonists on rats fed single isocaloric diets containing varying proportions of carbohydrate (CHO) and protein (PRO). Rats were habituated to eat one of the three diets (73.5% CHO - 10% PRO, 58.5% CHO - 25% PRO or 43.5% CHO - 40% PRO) during the dark period (1900–0700 h). Saline or 5-HT agonists (fluoxetine, RU 24969 and dexfenfluramine) were administered intraperitoneally at 1845 hours, 15 min prior to food access. At the doses used, food intake was significantly affected only during the first hour of eating. All 5-HT agonists caused dose-dependent decreases in food intake (P < 0.01). The magnitude of decrease, however, was significantly influenced by diet composition. Reduction in intake was greatest in rats fed the 73.5% CHO - 10% PRO diet. Thus, rats chronically fed a diet high in carbohydrate content were more sensitive to the anorectic effect of 5-HT agonists than rats fed diets containing moderate to low levels of carbohydrate.

Amino Acids in the Regulation of Food Intake and Selection

Until approximately ten years ago the prevalent view of the role of amino acids in food intake regulation was that they acted primarily as inhibitors when consumed in excess or in imbalanced proportions (1). There is now, however, evidence to show that proteins and amino acids function in physiological amounts, as consumed in normal diets, to influence food intake and possibly food choice (2, 3).