Hypophagia following dietary obesity

Effects of consuming a high carbohydrate diet after eight weeks of exposure to a ketogenic diet

BACKGROUND

Ketogenic diets have been utilized for weight loss and improvement in metabolic parameters. The present experiments examined the effects of returning to a chow diet after prolonged ingestion of a ketogenic diet.

METHODS

Rats were maintained on chow (CH) or a ketogenic diet (KD) for 8 weeks, after which the KD rats were given access to chow only (KD:CH) for 8 additional weeks. Caloric intake, body weight, and plasma leptin, insulin and ghrelin were measured before and after the dietary switch.

RESULTS

After 8 weeks of consuming a ketogenic diet, KD rats had increased adiposity and plasma leptin levels, and reduced insulin, as compared to CH controls. One week after the diet switch, fat pad weight and leptin levels remained elevated, and were normalized to CH controls within 8 weeks of the dietary switch. Switching from KD to chow induced a transient hypophagia, such that KD:CH rats consumed significantly fewer calories during the first week after the dietary switch, as compared to calories consumed by CH rats. This hypophagia was despite significantly increased plasma ghrelin in KD:CH rats. Finally, KD:CH rats developed hyperphagia over time, and during weeks 6-8 after the diet switch consumed significantly more calories per day than did CH-fed controls and gained more weight than CH-fed controls.

CONCLUSION

Collectively, these data demonstrate that returning to a carbohydrate-based diet after a period of consuming a ketogenic diet has post-diet effects on caloric intake, body weight gain, and insulin levels.

Effect of a lipid-enriched diet on body composition and some regulatory hormones of food intake in growing rats

OBJECTIVE

The aim of the present study was to investigate the effects of a lipid-enriched diet on body composition and on main regulatory hormones of food intake (insulin, adiponectin, leptin, ghrelin).

METHOD

Two groups of 16 rats, 35 days old, weighing 80+/-6 g, were constituted. One group (S) was given a standard diet during 10 weeks and served as control. The second group (L) was given a lipidic-enriched diet (containing: G: 41.5, L: 38.5, P: 20% calorie). Food and water were given "ad libitum".

RESULTS

Total food intake, body weight, skeletal area and lean body mass of rats eating lipid-enriched diet were lowered (6694+/-178 vs. 8160+/-184 kcal, P=0.01; 431+/-38 vs. 468+/-25 g, P=0.003; 72.19+/-0.96 vs. 76.07+/-1.31 cm2, P=0.03; 369+/-18 vs. 409+/-23 g, P=0.0006), fat mass difference was not statistically significant (82.5+/-17 vs. 80+/-17 g, P=0.7). Blood ghrelin, adiponectin levels were lowered (1517+/-224 vs. 1915+/-579 pg/ml, P=0.03; 10+/-3 vs. 19+/-3 microg/ml, P=0.003) whereas insulin and leptin were unchanged (1.8+/-1.5 vs. 2.6+/-1.4 ng/ml, P=0.1; 16+/-11 vs. 13+/-10 ng/ml, P=0.4).

CONCLUSION

A period of high fat diet in growing rats leads to a hypophagia, resulting in a lower lean body mass development. Some regulatory hormones of food intake did not change, while others significantly decreased, notably ghrelin being possible causal factor of the observed hypophagia linked to high fat diet.

Transient hypophagia in rats switched from high-fat diets with different fatty-acid pattern to a high-carbohydrate diet

The present study investigates the mechanisms underlying the transient hypophagia occurring when rats adapted to high-fat, carbohydrate-free diets are switched to high-carbohydrate, low-fat diets. The hypophagia after the high-fat, carbohydrate-free to high-carbohydrate, low-fat diet shift seems to depend on the amount of carbohydrate in the diet, since an attenuation of hypophagia was observed when high-fat, carbohydrate-free-adapted rats were switched to a medium-carbohydrate, medium-fat diet. A role of glucose intolerance in the hypophagia is supported by the attenuation of carbohydrate anorexia in rats adapted to a high-fat diet containing n -3 polyunsaturated fatty acids from fish oil (60% of fat as fish oil), which has been shown to improve glucose tolerance in rats. Furthermore, the increased plasma glucose concentration in the high-fat, carbohydrate-free diet to high-carbohydrate, low-fat shifted rats despite the suppression in food intake also suggests an involvement of glucose intolerance in the hypophagia. The failure of the inhibitor of hepatic-fatty-acid oxidation mercaptoacetate (400 micromol/kg, i.p.) to counteract carbohydrate anorexia in the HF-adapted rats argues against an involvement of fatty-acids oxidation in the inhibition of eating after high-fat, carbohydrate-free to high-carbohydrate, low-fat diet shift. This is also supported by the failure to demonstrate a relationship between plasma beta-hydroxybutyrate and the severity of the hypophagia. A role of leptin in the hypophagia seems unlikely, since plasma leptin after diet shift was unchanged. Ingestion of the high-carbohydrate, low-fat diet also produced an aversion towards this diet in high-fat, carbohydrate-free-adapted rats. It is concluded that the transient hypophagia induced by switching rats from a high-fat to a high-carbohydrate diet is not related to fatty acid oxidation but to transiently impaired carbohydrate utilization.

Dissociation of intake adjustments to sustained and day-to-day changes in food caloric density

Rats were subjected to day-to-day changes in food caloric density superimposed on a sustained (average) change above or below the basal value. This was done by giving them a different diet every day, so that caloric density gradually increased from a basal value. By repeating this procedure the caloric density varied in a sinusoidal pattern above or below the basal value. Short-term compensatory changes in food intake followed a reversed and delayed sinusoidal pattern (compared to that of caloric density), and achieved only an incomplete compensation of caloric intake. On the other hand, long-term changes in the average food intake led to a complete compensation of the average caloric intake for the high-calorie diets. The low-calorie diets produced a marked body weight loss, although these rats maintained the partly compensatory responses to the day-to-day changes in caloric density; that is, food intake decreased when the caloric density of the diet increased to the basal value, overriding any effect of body weight loss. It is concluded that short-term and long-term controls of food intake are relatively independent of each other and that the short-term control determines the daily caloric intake, with a delay that probably indicates learning processes, while the long-term control is determining the average caloric intake over a period of several days, accounting for a more or less complete compensation of caloric density changes.

Diet composition alters the acceptance of fat by rats

The acceptance of dietary fat by rats is influenced by changes in fat digestion and metabolism. In these experiments, rats were fed diets that differed in fat, carbohydrate and fiber content, and the acceptance of fat was measured. Rats fed a high fat (HF) diet ate more corn oil in 30-min or 6-h tests than did rats fed an isocaloric high carbohydrate (HC) diet. This effect was seen after the diets were switched and rats retested. Differences in dietary fiber between the isocaloric HF and HC diets did not account for this effect because rats fed HF diets, either high or low in fiber content, drank more oil than rats fed the HC diet. Rats fed the HF diet with added carbohydrate drank less oil than rats fed the HF diet, and the same amount of oil as rats fed the HC diet. Compared with rats fed the HC diet, rats fed the HF diet drank more oil in a two-bottle preference test with sucrose or when mixed with sucrose in a single-bottle test. Rats offered a variety of fats, sugars or other test foods, ate more nutritive liquid fats and some solid fats, but did not eat more sugar or other items if they were fed the HF diet rather than the HC diet. These studies taken together strongly suggest that rats fed a high fat diet show a greater acceptance of fat compared with rats fed a high carbohydrate diet.

Does dietary hyperphagia contradict the lipostatic theory?

It has frequently been suggested that body weight or fat somehow exerts an inhibitory influence on food intake in a way that acts to maintain a stable body weight or fat. The principal evidence supporting this idea is that animals that have been induced to overeat and become overweight by various means, eat less than control rats when they are permitted to eat freely. If the degree of suppression of appetite by overweight is as large as several experiments suggest, then dietary hyperphagia should be self-limiting. Any overeating induced by dietary treatments should disappear after animals become moderately overweight. Animals fed some kinds of hyperhagia-promoting diets do show this pattern. However, animals fed other kinds of diets do not show this pattern, and with most diets, dietary hyperphagia continues for extended periods. This implies that either 1) overweight does not suppress appetite as much as suggested by various authorities, 2) dietary manipulations can override normal regulatory mechanisms, or 3) certain diets induce irreversible changes in body fat that are not evident from changes in body weight.

Dietary hyperphagia and obesity: what causes them?

Diets that cause animals to overeat and become obese have been used in many investigations of obesity. Most of this research, however, has concentrated on the consequences rather than the causes of overeating. Furthermore, in most studies, several nutritional variables were manipulated simultaneously, making cause and effect relationship impossible to disentangle. Consequently, progress has been slow. Diets could alter energy intake by virtue of their effects on oral-sensory, gastrointestinal or postabsorptive effects. Palatability is the most popular oralsensory hypothesis but the empirical basis for this hypothesis is particularly weak. A substantial body of evidence is consistent with the possibility that the osmotic effects of diets in the gastrointestinal tract and metabolic postabsorptive factors may play a major role in dietary hyperphagia and obesity. Suggestions for future research directions are offered.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.