Adaptive changes in enzyme activity and metabolic pathways in adipose tissue from meal-fed rats

A number of metabolic factors and the activity of a number of enzymes were determined in meal-fed (animals fed a single daily 2 hr meal) and nibbling (ad libitum-fed) rats. The dependency of the observed adaptive changes on the ingestion of carbohydrate was studied by feeding diets high in carbohydrate or fat. Glucose-6-phosphate dehydrogenase and NADP-malic dehydrogenase were more active in adipose tissue from high carbohydrate meal-fed rats than in tissue from ad libitum-fed rats. The activity in adipose tissue of isocitric dehydrogenase, 6-phosphogluconate dehydrogenase, and NAD-malic dehydrogenase did not increase significantly in response to meal-feeding the high carbohydrate diet. No increase in lipogenesis or enzyme activity could be demonstrated in adipose tissue from rats meal-fed a high fat diet. Lipase activity of adipose tissue was increased by high carbohydrate meal-feeding and decreased by feeding a high fat diet. The in vitro uptake of palmitate-1-(14)C by adipose tissue was depressed by a high fat diet and enhanced in rats meal-fed a high carbohydrate diet. Diaphragm or slices of liver from high fat-fed rats oxidized palmitate-1-(14)C more rapidly than did tissue from ad libitum-fed animals. Evidence is presented for the quantitative importance of citrate as a source of extramitochondrial acetyl CoA in adipose tissue of meal-eating and ad libitum-fed rats. The relationship of extramitochondrially formed citrate to the NAD-malic dehydrogenase-malic enzyme system in adipose tissue is discussed.     

Effect of exercise and calorie restriction on biomarkers of aging in mice

Unlike calorie restriction, exercise fails to extend maximum life span, but the mechanisms that explain this disparate effect are unknown. We used a 24-wk protocol of treadmill running, weight matching, and pair feeding to compare the effects of exercise and calorie restriction on biomarkers related to aging. This study consisted of young controls, an ad libitum-fed sedentary group, two groups that were weight matched by exercise or 9% calorie restriction, and two groups that were weight matched by 9% calorie restriction + exercise or 18% calorie restriction. After 24 wk, ad libitum-fed sedentary mice were the heaviest and fattest. When weight-matched groups were compared, mice that exercised were leaner than calorie-restricted mice. Ad libitum-fed exercise mice tended to have lower serum IGF-1 than fully-fed controls, but no difference in fasting insulin. Mice that underwent 9% calorie restriction or 9% calorie restriction + exercise, had lower insulin levels; the lowest concentrations of serum insulin and IGF-1 were observed in 18% calorie-restricted mice. Exercise resulted in elevated levels of tissue heat shock proteins, but did not accelerate the accumulation of oxidative damage. Thus, failure of exercise to slow aging in previous studies is not likely the result of increased accrual of oxidative damage and may instead be due to an inability to fully mimic the hormonal and/or metabolic response to calorie restriction.     

Differential effects of high-fat and high-carbohydrate content isoenergetic meals on plasma active ghrelin concentrations in lean and obese women.

AIM: To study the effect of two different isoenergetic meals, one rich in carbohydrates and one rich in fat, on plasma active ghrelin levels in lean or obese subjects. METHODS: Eight obese and eight lean women, strictly matched for age, were fed two isoenergetic meals of different composition, one rich in fat and one rich in carbohydrates (CHO), on separate days. Plasma active ghrelin levels were measured just before and at 1, 2 and 3 hours after meal consumption. RESULTS: Overall, plasma active ghrelin levels were significantly lower in the obese compared to the lean women (71.7 +/- 29.7 vs. 222.2 +/- 127.2 pmol/liter respectively, p < 0.0001). Furthermore, ghrelin levels decreased significantly by 30 % from baseline values in the lean subjects in the first hour after the CHO-rich meal (mean difference +/- SD): -66.2 +/- 49.0 pmol/liter (p = 0.03), returning to near-baseline levels by 2 hours, while no significant change was observed in the obese subjects. After the fat-rich meal, active ghrelin levels did not change significantly in either group (p > 0.05). CONCLUSIONS: A fat-rich meal does not suppress plasma active ghrelin levels in either lean or obese women. Moreover, in obese, unlike lean women, a high carbohydrate meal also fails to suppress plasma ghrelin levels, which are already quite low. This suggests that ghrelin-induced satiety mechanisms may be compromised in these subjects.     

A transient surge of ghrelin secretion before feeding is modified by different feeding regimens in sheep

Ghrelin is a recently identified orexigenic hormone secreted by the stomach and has been implicated in meal-time hunger. Several experiments demonstrate a transient surge in ghrelin secretion shortly before a scheduled meal, suggesting from the involvement of cephalic mechanisms. If ghrelin secretion is stimulated by hunger in sheep, plasma levels of ghrelin should be modified by different feeding regimens that affect hunger drive. To test this hypothesis, we investigated changes in plasma ghrelin concentrations in fed Suffolk rams ad libitum and in rams either twice or four times daily. Plasma ghrelin levels increased (P<0.05) abruptly just before every feeding period in sheep fed twice and four times daily and then fell shortly after feeding. Peak levels of the pre-prandial ghrelin surge were higher (P<0.01) in animals fed twice daily than in animals fed four times daily, leading to greater (P<0.05) areas under response curves over 12h. In contrast, the plasma ghrelin levels remained relatively low and constant in sheep fed ad libitum, with no evidence of surges in plasma ghrelin levels. These results confirm that the transient surge in plasma ghrelin levels occurs just before feeding and demonstrate that this can be modified by the feeding regimen in sheep.     

Obese Adolescents Show Impaired Meal Responses of the Appetite‐Regulating Hormones Ghrelin and PYY

Ghrelin and peptide YY (PYY) stimulate hunger and satiety, respectively. The physiology of these hormones during normal meal intake remains unclear. This study was designed to compare the responses of these two hormones to meal intake between lean and obese Hispanic adolescents. A total of 10 obese and 7 lean Hispanic youth, aged 11–14 years, consumed two mixed meals, one small and one large, during which plasma measurements of active and total ghrelin and total PYY were obtained. Obese subjects tended to consume more calories during the small meal than lean subjects, although this did not reach statistical significance. Intake of the small meal significantly suppressed active ghrelin and stimulated PYY levels in the lean subjects, and these changes were further accentuated by the large meals. In obese subjects, the suppression of active ghrelin and stimulation of PYY by caloric intake were blunted. Interestingly, a paradoxical stimulation of active ghrelin levels was noted during the small meals in both lean and obese subjects. This stimulation was not seen during the larger meals in lean subjects, but remained present in the obese subjects. Thus, meal‐related changes in active ghrelin and PYY are blunted in obese as compared to lean Hispanic subjects. This blunting could contribute to the development or worsening of obesity.     

Ghrelin mediates anticipation to a palatable meal in rats

Food anticipatory activity (FAA) is displayed in rats when access to food is restricted to a specific time frame of their circadian phase, a behavior thought to reflect both hunger and the motivation to eat. Rats also display FAA in a feeding schedule with ad libitum access to normal chow, but limited availability of a palatable meal, which is thought to involve mainly motivational aspects. The orexigenic hormone ghrelin has been implicated in FAA in rodents with restricted access to chow. Because ghrelin plays an important role not only in the control of food intake, but also in reward, we sought to determine the role of ghrelin in anticipation to a palatable meal. Plasma ghrelin levels of non-restricted rats that anticipated chocolate correlated positively with FAA and were increased compared with chow-fed control rats. Furthermore, centrally injected ghrelin increased, whereas an antagonist of the ghrelin receptor decreased, the anticipation to chocolate. Therefore, we hypothesize that central ghrelin signaling is able to mediate the motivational drive to eat.     

Eating meals before wheel-running exercise attenuate high fat diet-driven obesity in mice under two meals per day schedule

Mice that exercise after meals gain less body weight and visceral fat compared to those that exercised before meals under a one meal/exercise time per day schedule. Humans generally eat two or three meals per day, and rarely have only one meal. To extend our previous observations, we examined here whether a “two meals, two exercise sessions per day” schedule was optimal in terms of maintaining a healthy body weight. In this experiment, “morning” refers to the beginning of the active phase (the “morning” for nocturnal animals). We found that 2-h feeding before 2-h exercise in the morning and evening (F-Ex/F-Ex) resulted in greater attenuation of high fat diet (HFD)-induced weight gain compared to other combinations of feeding and exercise under two daily meals and two daily exercise periods. There were no significant differences in total food intake and total wheel counts, but feeding before exercise in the morning groups (F-Ex/F-Ex and F-Ex/Ex-F) increased the morning wheel counts. These results suggest that habitual exercise after feeding in the morning and evening is more effective for preventing HFD-induced weight gain. We also determined whether there were any correlations between food intake, wheel rotation, visceral fat volume and skeletal muscle volumes. We found positive associations between gastrocnemius muscle volumes and morning wheel counts, as well as negative associations between morning food intake volumes/body weight and morning wheel counts. These results suggest that morning exercise-induced increase of muscle volume may refer to anti-obesity. Evening exercise is negatively associated with fat volume increases, suggesting that this practice may counteract fat deposition. Our multifactorial analysis revealed that morning food intake helps to increase exercise, and that evening exercise reduced fat volumes. Thus, exercise in the morning or evening is important for preventing the onset of obesity.     

Effect of Chronic Caloric Restriction on the Synchronization of Various Physiological Measures in Old Female Fischer 344 Rats

-A variety of physiological and behavioral parameters which relate to metabolism were continuously monitored in 18 month old female Fischer 344 rats which were maintained on either ad libitum or reduced calorie diets. Caloric restriction (CR) stimulated average motor activity per day, the duration of each feeding episode, food consumed per episode, and water consumed per gram lean body mass (LBM). However, CR limited total food consumption, feeding time, number of feeding episodes per day, total eating and drinking time, and the daily ratio of food consumed to water consumed, CR also decreased average body temperature per day, O₂ consumption, CO₂, production, and respiratory quotient. A variety of parameters concerning water consumption were not affected. CR rats ate their food immediately when food was presented during the light span, while ad libitum fed animals ate numerous small meals throughout the entire dark span. An anticipatory response to restricted feeding was also noted. Total motor activity, metabolism, and body temperature increased just prior to scheduled feeding and reached maximum values shortly after feeding, suggesting that these parameters were highly synchronized to feeding. Females and males were found to respond to caloric restriction in a similar fashion. Dramatic changes in respiratory quotient and body temperature suggest rapid shifts between metabolic pathways (glycolysis to giuconeogenesis) to obtain optimal efficiency. Lower body temperature and metabolism may provide protection against DNA damage, thereby increasing the survival potential of restricted rats. These responses may provide insight into the mechanisms by which caloric restriction acts to extend life span.     

Effects of fat, protein, and carbohydrate and protein load on appetite, plasma cholecystokinin, peptide YY, and ghrelin, and energy intake in lean and obese men

While protein is regarded as the most satiating macronutrient, many studies have employed test meals that had very high and unsustainable protein contents. Furthermore, the comparative responses between lean and obese subjects and the relationships between energy intake suppression and gut hormone release remain unclear. We evaluated the acute effects of meals with modest variations in 1) fat, protein, and carbohydrate content and 2) protein load on gastrointestinal hormones, appetite, and subsequent energy intake in lean and obese subjects. Sixteen lean and sixteen obese men were studied on four occasions. Following a standardized breakfast, they received for lunch: 1) high-fat (HF), 2) high-protein (HP), 3) high-carbohydrate/low-protein (HC/LP), or 4) adequate-protein (AP) isocaloric test meals. Hunger, fullness, and gut hormones were measured throughout, and at t = 180 min energy intake at a buffet meal was quantified. In lean subjects, hunger was less and fullness greater following HF, HP, and AP compared with HC/LP meals, and energy intake was less following HF and HP compared with HC meals (P < 0.05). In the obese subjects, hunger was less following HP compared with HF, HC/LP, and AP meals, and energy intake was less following HP and AP compared with HF and HC meals (P < 0.05). There were no major differences in hormone responses to the meals among subject groups, but the CCK and ghrelin responses to HP and AP were sustained in both groups. In conclusion, HP meals suppress energy intake in lean and obese subjects, an effect potentially mediated by CCK and ghrelin, while obese individuals appear to be less sensitive to the satiating effects of fat.     

A preexercise alpha-lactalbumin-enriched whey protein meal preserves lipid oxidation and decreases adiposity in rats

The composition of the preexercise food intake is known to affect substrate utilization during exercise and thus can affect long-term changes in body weight and composition. These parameters were measured in male rats exercised 2 h daily over 5 wk, either in the fasting state or 1 h after they ingested a meal enriched with glucose (Glc), whole milk protein (WMP), or alpha-lactalbumin-enriched whey protein (CPalphaL). Compared with fasting, the Glc meal increased glucose oxidation and decreased lipid oxidation during and after exercise. In contrast, the WMP and CPalphaL meals preserved lipid oxidation and increased protein oxidation, the CPalphaL meal increasing protein oxidation more than the WMP meal. At the end of the study, body weight was larger in the WMP-, Glc-, and CPalphaL-fed rats than in the fasted ones. This resulted from an increased fat mass in the WMP and Glc rats and to an increased lean body mass, particularly muscles, in the CPalphaL rats. We conclude that the potential of the CPalphaL meal to preserve lipid oxidation and to rapidly deliver amino acids for use during exercise improved the efficiency of exercise training to decrease adiposity.     

Individual rainbow trout can learn and anticipate multiple daily feeding times

The present study was conducted to investigate whether individual rainbow trout Oncorhynchus mykiss can simultaneously anticipate two daily meals, and to evaluate the influence of time-restricted feeding schedules and light-dark (L:D) cycles on the timing of self-feeding activity. Individual rainbow trout were capable of anticipating not only a single meal but also two daily meals scheduled at different times; the duration of anticipation was short and precise. This seems to be the first convincing evidence to show that a single fish can simultaneously anticipate two daily meals. Change in the onset and cessation of light did not affect the duration of food-anticipatory activity, although it led to shift the phases of the light-related self-feeding activity. Moreover, under constant light and ad lib feeding conditions, only one band of rhythmic activity was observed in individual rainbow trout that had previously developed food-anticipatory activity to each of the two daily meals.     

The GOAT-ghrelin system is not essential for hypoglycemia prevention during prolonged calorie restriction

Objective

Ghrelin acylation by ghrelin O-acyltransferase (GOAT) has recently been reported to be essential for the prevention of hypoglycemia during prolonged negative energy balance. Using a unique set of four different genetic loss-of-function models for the GOAT/ghrelin/growth hormone secretagogue receptor (GHSR) system, we thoroughly tested the hypothesis that lack-of-ghrelin activation or signaling would lead to hypoglycemia during caloric deprivation.

Methodology

Male and female knockout (KO) mice for GOAT, ghrelin, GHSR, or both ghrelin and GHSR (dKO) were subjected to prolonged calorie restriction (40% of ad libitum chow intake). Body weight, fat mass, and glucose levels were recorded daily and compared to wildtype (WT) controls. Forty-eight hour blood glucose profiles were generated for each individual mouse when 2% or less body fat mass was reached. Blood samples were obtained for analysis of circulating levels of acyl- and desacyl-ghrelin, IGF-1, and insulin.

Principal Findings

Chronic calorie restriction progressively decreased body weight and body fat mass in all mice regardless of genotype. When fat mass was depleted to 2% or less of body weight for 2 consecutive days, random hypoglycemic events occurred in some mice across all genotypes. There was no increase in the incidence of hypoglycemia in any of the four loss-of-function models for ghrelin signaling including GOAT KO mice. Furthermore, no differences in insulin or IGF-1 levels were observed between genotypes.

Conclusion

The endogenous GOAT-ghrelin-GHSR system is not essential for the maintenance of euglycemia during prolonged calorie restriction.     

Ethane production rate in vivo is reduced with dietary restriction

Dietary restriction without malnutrition prolongs life and has a beneficial effect on age-related diseases and metabolic derangements. To test the effect of food restriction on ethane production rate, ethane exhalation was measured in rats with partial food restriction. Ethane production rate in room air in rats fed 60% of food consumed by ad libitum-fed animals for 2 wk was significantly reduced (3.50 +/- 0.25 vs. 5.21 +/- 0.34 pmol.min-1.100 g body wt-1, P less than 0.01). In 100% oxygen, ethane production in food-restricted rats was not different from that of ad libitum-fed rats (21.81 +/- 1.25 vs. 19.57 +/- 1.89 pmol.min-1.100 g-1). Fifteen hours of fasting compared with ad libitum feeding reduced ethane production modestly in room air (4.37 +/- 0.45 vs. 5.21 +/- 0.34 pmol.min-1.100 g-1) and more significantly in 100% oxygen (12.37 +/- 0.78 vs. 19.57 +/- 1.89 pmol.min-1.100 g-1). Thus, in 100% oxygen, 15 h of fasting, compared with ad libitum feeding, resulted in an approximately 40% decrease in ethane production rate. It is concluded that short-term food restriction significantly reduces ethane exhalation rate in rats when measured in room air.     

Glucose metabolism in isolated adipocytes from ad Libitum- and restricted-fed lean and obese Zucker rats at two different ages

Chronic food restriction in Sprague-Dawley rats has been shown to alter adipose glucose metabolism. In the present study, lean and obese male Zucker rats were food restricted from 5 weeks until either 10 or 26 weeks of age and adipocyte glucose metabolism was measured. Adipocytes from restricted-fed lean and obese Zucker rats converted more glucose to CO2 and fatty acids than those from their ad libitum-fed counterparts in both the absence and the presence of increasing doses of insulin at 10 weeks of age. At the highest insulin dose, adipocytes from restricted-fed obese rats converted significantly more glucose to CO2 and fatty acids than did those from restricted-fed lean rats. Basal glyceride-glycerol values were similar in all groups at this age. At the 0.4 and 2.0 ng/ml insulin levels, glyceride-glycerol production was highest in restricted-fed lean rats; restricted-fed obese and ad libitum-fed lean rats had similar values; and ad libitum-fed obese rats had the lowest. At the 20 ng/ml dose, glyceride-glycerol values of restricted-fed rats were higher than those of ad libitum-fed rats. Basal and insulin-stimulated values were compared within each group. Most basal versus insulin-stimulated values were significantly different for the two lean groups. For ad libitum-fed obese rats, only 0 versus 20 ng/ml insulin values were significant. Restricted-fed obese rats had significant increases in 0 versus both 2 and 20 ng/ml insulin values. Restricted-fed obese rats had significantly lower serum insulin levels relative to ad libitum-fed obese rats at 10 weeks of age. Adipocytes from all rats at 26 weeks of age had similar basal rates of conversion of glucose metabolism to all three metabolites. In the presence of insulin, adipocytes from restricted-fed lean rats metabolized significantly more glucose to CO2 and glyceride-glycerol than adipocytes prepared from the three other groups. Fatty acid production was similar in all groups at each insulin level. Only restricted-fed lean rats showed consistent significant responses to insulin stimulation for the three metabolites. Whether these results are due to age, length of food restriction, or serum insulin levels remains to be determined.     

Maintenance of a normal meal-induced decrease in plasma ghrelin levels in children with Prader-Willi syndrome.

Ghrelin is a 28-amino acid peptide recently identified in the stomach as the endogenous ligand for the growth hormone secretagogue receptor (GHS-R1a). Ghrelin is a potent stimulator of GH secretion. It was recently shown that circulating ghrelin levels in humans rise shortly before and fall shortly after every meal, and that ghrelin administration increases voluntary food intake. The hypothesis that ghrelin hypersecretion might contribute to genetic obesity has never been investigated. In this context, Prader-Willi syndrome is the most common form of human syndromic obesity. As ghrelin affects appetite as well as GH secretion and both are abnormal in PWS, it has been surmised that these alterations might be due to ghrelin dysregulation. The aim of the study was to investigate whether ghrelin is suppressed by the meals differently in PWS children than in PWS adults. Overnight circulating fasting ghrelin levels and ghrelin levels 120 min after breakfast were assayed in 7 PWS children (10.2 +/- 1.7 yr), 7 subjects with morbid obesity (10.3 +/- 1.3 yr), and 5 normal controls (8.4 +/- 1.4 yr). Because of the data spread, no statistical difference was observed in fasting ghrelin levels between PWS and control children (p = NS); anyway, fasting ghrelin levels were significantly lower in obese children than in the other groups (p < 0.05 vs. control and PWS children). Ghrelin levels were slightly suppressed by the meal in control subjects (mean fasting ghrelin: 160.2 +/- 82 pg/ml; after the meal, 141.2 +/- 57 pg/ml, p = NS); the meal failed to suppress ghrelin levels in obese children (mean fasting ghrelin: 126.4 +/- 8.5 pg/ml; after the meal, 119.1 +/- 8.3 pg/ml, p = NS). Interestingly, the meal markedly suppressed ghrelin levels in PWS children (mean fasting ghrelin: 229.5 +/- 70.4 pg/ml; after the meal, 155.8 +/- 34.2 pg/ml, p < 0.01). In conclusion, since a lack of decrease in circulating ghrelin induced by the meal was previously reported in PWS adults, the finding of a meal-induced decrease in ghrelin levels in our population of young PWS would imply that the regulation of the ghrelin system involved in the orexigenic effects of the peptide is operative during childhood, although it progressively deteriorates and is absent in adulthood when hyperphagia and obesity progressively worsen.     

Food anticipatory activity behavior of mice across a wide range of circadian and non-circadian intervals

When rodents are fed in a limited amount during the daytime, they rapidly redistribute some of their nocturnal activity to the time preceding the delivery of food. In rats, anticipation of a daily meal has been interpreted as a circadian rhythm controlled by a food-entrained oscillator (FEO) with circadian limits to entrainment. Lesion experiments place this FEO outside of the light-entrainable circadian pacemaker in the suprachiasmatic nucleus. Mice also anticipate a fixed daily meal, but circadian limits to entrainment and anticipation of more than 2 daily meals, have not been assessed. We used a video-based behavior recognition system to quantify food anticipatory activity in mice receiving 2, 3, or 6 daily meals at intervals of 12, 8, or 4-hours (h). Individual mice were able to anticipate as many as 4 of 6 daily meals, and anticipation persisted during meal omission tests. On the 6 meal schedule, pre-prandial activity and body temperature were poorly correlated, suggesting independent regulation. Mice showed a limited ability to anticipate an 18 h feeding schedule. Finally, mice showed concurrent circadian and sub-hourly anticipation when provided with 6 small meals, at 30 minute intervals, at a fixed time of day. These results indicate that mice can anticipate feeding opportunities at a fixed time of day across a wide range of intervals not previously associated with anticipatory behavior in studies of rats. The methods described here can be exploited to determine the extent to which timing of different intervals in mice relies on common or distinct neural and molecular mechanisms.     

Circadian Mechanisms of Food Anticipatory Rhythms in Rats Fed Once or Twice Daily: Clock Gene and Endocrine Correlates

Circadian clocks in many brain regions and peripheral tissues are entrained by the daily rhythm of food intake. Clocks in one or more of these locations generate a daily rhythm of locomotor activity that anticipates a regular mealtime. Rats and mice can also anticipate two daily meals. Whether this involves 1 or 2 circadian clocks is unknown. To gain insight into how the circadian system adjusts to 2 daily mealtimes, male rats in a 12∶12 light-dark cycle were fed a 2 h meal either 4 h after lights-on or 4 h after lights-off, or a 1 h meal at both times. After 30 days, brain, blood, adrenal and stomach tissue were collected at 6 time points. Multiple clock genes from adrenals and stomachs were assayed by RT-PCR. Blood was assayed for corticosterone and ghrelin. Bmal1 expression was quantified in 14 brain regions by in situ hybridization. Clock gene rhythms in adrenal and stomach from day-fed rats oscillated in antiphase with the rhythms in night-fed rats, and at an intermediate phase in rats fed twice daily. Corticosterone and ghrelin in 1-meal rats peaked at or prior to the expected mealtime. In 2-meal rats, corticosterone peaked only prior the nighttime meal, while ghrelin peaked prior to the daytime meal and then remained elevated. The olfactory bulb, nucleus accumbens, dorsal striatum, cerebellum and arcuate nucleus exhibited significant daily rhythms of Bmal1 in the night-fed groups that were approximately in antiphase in the day-fed groups, and at intermediate levels (arrhythmic) in rats anticipating 2 daily meals. The dissociations between anticipatory activity and the peripheral clocks and hormones in rats anticipating 2 daily meals argue against a role for these signals in the timing of behavioral rhythms. The absence of rhythmicity at the tissue level in brain regions from rats anticipating 2 daily meals support behavioral evidence that circadian clock cells in these tissues may reorganize into two populations coupled to different meals.     

Ghrelin, macronutrient intake and dietary preferences in long-evans rats

Ghrelin is a recently discovered peptide that is primarily produced by the stomach. As a ligand of the growth hormone (GH) secretagogue (GHS) receptor, it stimulates GH secretion but it also stimulates feeding and has adipogenic effects in rodents. Although its circulating levels are modulated by fasting and refeeding, its relationship with diet composition is not known. In the present paper, we measured plasma ghrelin as well as two important hormones (leptin and insulin) in Long-Evans rats placed in two different feeding situations, e.g., either with imposed diets or with food choice. In the first case, the rats were fed unbalanced diets (either high-carbohydrate (HC) or high-fat (HF) diets) for 14 weeks, whereas in the second situation, they had the choice between these 2 diets for 2 weeks and were selected for their fat or carbohydrate preference. The intake of the HF diet for 14 weeks was associated with lower levels of ghrelin (-30% vs control diet; P < 0.01). These levels increased when the percentage of carbohydrate in the diet increased (+26 to +42% vs control diet; P < 0.01 or less). Ghrelin was inversely correlated with plasma leptin (r = -0.55; P < 0.003) and blood glucose (r = -0.58; P < 0.001) as well as with body weight (r = -0.63; P < 0.0001) and body fat content estimated by the sampling of specific fat pads (r = -0.62; P < 0.0001). In the food choice experiment, fat-preferring rats had plasma ghrelin levels lower than the carbohydrate-preferring rats (-33%; P < 0.0002). Ghrelin secretion was therefore very sensitive to the diet composition. Its down-regulation by fat ingestion might serve as a counterregulatory mechanism to limit the development of dietary-induced adiposity. Ghrelin may signal when a high calorie diet is ingested.     

Ghrelin response to protein and carbohydrate meals in relation to food intake and glycerol levels in obese subjects

Obese subjects have lower basal and an attenuated decrease of postprandial plasma ghrelin following carbohydrate-rich meals, while the response to protein is unknown. Therefore, plasma ghrelin levels were examined after ingestion of satiating amounts of a protein- or carbohydrate-rich meal in relation to food and energy intake and hunger/satiety ratings in 30 obese subjects followed 240 min later by ad lib sandwiches. Food intake and hunger/satiety ratings were identical while energy intake was significantly greater after bread (861 +/- 62.7 vs. 441 +/- 50.4 kcal, p < 0.001). Second meal food and energy intake were not different. Ghrelin decreased after bread, but increased by 50 pg/ml (p < 0.001) after meat. The corresponding increase of insulin was 55 vs. 9 microU/ml (p < 0.001). Glycerol levels decreased significantly less after the protein meal compared to carbohydrates. After protein glycerol was significantly correlated to the rise of ghrelin but not insulin. These data demonstrate that, in obese subjects, protein has no different satiating effect than carbohydrate despite divergent ghrelin levels. Energy intake corresponds to energy density of the respective food items. Ghrelin response to both meals is qualitatively similar but quantitatively attenuated compared to normal weight subjects. The relationship between ghrelin and glycerol would support recent observations of a possible role of ghrelin in fat metabolism.     

The modifying effects of fish oil on fasting ghrelin mRNA expression in weaned rats

Ghrelin expression and secretion seem to be influenced by the fat content of the diet. However, data on the probable adverse effect of high fat diet (HFD) with different dietary fats and saturation level of fatty acids is inconclusive. This study aimed at investigating the effects of HFDs on fasting total and acyl-ghrelin plasma levels, gastric fundus and duodenum ghrelin mRNA expressions. Weaned Wistar rats (n=50) were randomly divided to five groups of HFDs with fish oil (HF-F), olive oil (HF-O), soy oil (HF-S), butter (HF-B) and the controls. After 8weeks, blood samples were collected. While the animals were fasting for 24h, their blood and tissue samples were obtained. Plasma parameters of total and acyl ghrelin and ghrelin mRNA expression level in stomach and duodenum were measured. The HF-B fed group had lower fasting plasma acyl ghrelin level than the control, HF-F and HF-O groups (P<0.05); furthermore, the HF-F group had significantly higher acyl ghrelin level than the HF-S one (P<0.05). After feeding, all the groups, except for the HF-B one, had a significantly lower plasma acyl ghrelin levels (P<0.05), compared with the fasting state. Ghrelin mRNA expression levels in the gastric fundus and duodenum were significantly lower in the HF-B as compared to the control group. Furthermore, the HF-F group had significantly higher mRNA level in the duodenum, in comparison with the HF-B and HF-S groups. As HF-F and HF-O diets had the highest stimulatory effect on fasting ghrelin expression and plasma level, consumption of these dietary oils can play an important role in ghrelin regulation, which might affect feeding behavior and energy intake.