QTLs influencing carbohydrate and fat choice in a LOU/CxFischer 344 F2 rat population.

OBJECTIVE: Individual differences in macronutrient selection, particularly fat and carbohydrate, and associated body weight gain are partly inherited as polygenic traits, but the potential genetic pathways are unknown. To give an overview of the Quantitative Trait Loci (QTLs) and candidate gene pathways influencing these differences in rat was aimed in this study. DESIGN AND METHODS: To that end, F2 rats obtained from the crossbreeding between LOU/C and Fischer 344 rat strains to diet self-selection during 3 weeks were submitted. A genome scan was conducted with microsatellite markers covering evenly the whole genome. Genotypes and phenotypes were analyzed separately in male and female F2 rats by multiple interval mapping. Then, lists of candidate genes were treated by the Ingenuity Pathway software to propose gene pathways involved in our phenotypes. RESULTS: Among numerous others, a QTL on chromosome 12 that influences body weight gain, and fat and carbohydrate choices in the LOU/C x Fischer 344 F2 rat population was found. This locus contains notably the acyl-co-A dehydrogenase gene. CONCLUSION: A strong genetic determinism and complex pathways involving numerous candidate genes and processes, notably in accordance with the metabolic theory of feeding behavior control were found.

Lactose malabsorption and colonic fermentations alter host metabolism in rats.

Lactose malabsorption is associated with rapid production of high levels of osmotic compounds, such as organic acids and SCFA in the colon, suspected to contribute to the onset of lactose intolerance. Adult rats are lactase deficient and the present study was conducted to evaluate in vivo the metabolic consequences of acute lactose ingestion, including host-microbiota interactions. Rats received diets of 25% sucrose (S25 control group) or 25% lactose (L25 experimental group). SCFA and lactic acid were quantified in intestinal contents and portal blood. Expression of SCFA transporter genes was quantified in the colonic mucosa. Carbohydrate oxidation (Cox) and lipid oxidation (Lox) were computed by indirect calorimetry. Measurements were performed over a maximum of 13 h. Time, diet and time × diet variables had significant effects on SCFA concentration in the caecum (P<0·001, P=0·004 and P=0·007, respectively) and the portal blood (P<0·001, P=0·04 and P<0·001, respectively). Concomitantly, expression of sodium monocarboxylate significantly increased in the colonic mucosa of the L25 group (P=0·003 at t = 6 h and P<0·05 at t = 8 h). During 5 h after the meal, the L25 group's changes in metabolic parameters (Cox, Lox) were significantly lower than those of the S25 group (P=0·02). However, after 5 h, L25 Cox became greater than S25 (P=0·004). Thus, enhanced production and absorption of SCFA support the metabolic changes observed in calorimetry. These results underline the consequences of acute lactose malabsorption and measured compensations occurring in the host's metabolism, presumably through the microbiota fermentations and microbiota-host interactions.

Acute stress modifies food choice in Wistar male and female rats.

The present study investigates the effects of acute stress (15 min of swimming/day for three consecutive days) applied at the onset of the dark phase, just before the usual feeding time, on energy intake and more specifically on macronutrient selection, in male and female Wistar rats. The influence of stress regarding corticosterone and insulin kinetics was also examined. In the two experiments (1: food ad lib and 2: two feeding periods/day), three consecutive days of stress reduced daily body weight gain for both sexes. In the first experiment, the reduction in energy intake only occurred during the first 3h after stress. In males the 3h decrease in energy intake affected the three macronutrients, while in females, only the fat intake was decreased. In the second experiment, the stress only affected intake during the first feeding period. Protein, fat and CHO intakes were reduced in males, while in females only the protein and fat intakes were decreased. Unlike males, an increase in fat ingestion was observed in females; this occurred 6h after stress in experiment 1 and during the second feeding period 5h after stress in experiment 2. Stress raised plasma corticosterone levels in both sexes, while plasma insulin levels were decreased. These results demonstrate that the response to stress differed in males and females regarding macronutrient selection. Moreover, stress induced not only a quantitative effect on energy intake but also a qualitative one.

C57BL/6J and A/J mice fed a high-fat diet delineate components of metabolic syndrome.

OBJECTIVE: The aim of this study was to assess the suitability of A/J and C57BL/6J mice of both sexes as models of some components of the human metabolic syndrome (MetS) under nutritional conditions more comparable with the actual worldwide diet responsible for the increased incidence of the MetS. RESEARCH METHODS: We fed large cohorts (n = 515) of two strains of mice, A/J and the C57BL/6J, and of both sexes a high-fat diet (HFD; 60% fat) that, in contrast with most previous reports using saturated fats, was enriched in mono- and polyunsaturated fatty acids, thus more closely mimicking most Western diets, or a control diet (10% fat), for 20 weeks. RESULTS: In sharp contrast to previous reports, weight gain and hyperleptinemia were similar in both strains and sexes. Hyperinsulinemia, glucose tolerance, insulin resistance, and hypercholesterolemia were observed, although with important differences between strains and sexes. A/J males displayed severely impaired glucose tolerance and insulin resistance. However, in contrast with C57BL6/J mice, which displayed overt type 2 diabetes, A/J mice of both sexes remained normoglycemic. DISCUSSION: With important differences in magnitude and time course, the phenotypic and metabolic characteristics of both strains and both sexes on this HFD demonstrate that these models are very useful for identifying the mechanisms underlying progression or resistance to subsequent type 2 diabetes.

Comparison of energy balance in two inbred strains of rats: Fischer F344 prone to obesity and Lou rats resistant to obesity.

This study aimed to compare energy balance, metabolic profiles and body composition between two inbred strains of rats (F344 and Lou) submitted to a self-selecting macronutrient. During the 3 weeks of experiment, the two strains did not differ significantly for their total energy intake; males: F344 = 5875.4 +/- 171.4 kJ, Lou = 5619.2 +/- 349.4 kJ; and females: F344 = 4058.8 +/- 118.7 kJ, Lou = 3864.4+/-166.4 kJ. However, F344 rats showed a higher weight gain, and percentage of total fat tissue, together with a lower percentage of carcass weight than Lou rats regardless of sex. The percentages of each macronutrient revealed a lower preference of protein for F344 males and the opposite in females for CHO. The thermogenic activity measured in interscapular brown adipose tissue was lower in Fischer than in Lou while the reverse was observed for leptinemia and insulinemia. These results indicate that the mechanism responsible for the regulation of body composition observed in Lou rats takes place very early in life and attest the interest in this strain for studying the features of resistance to obesity.

In vivo leptin infusion impairs insulin and leptin signalling in liver and hypothalamus.

Leptin resistance contributes to the pathogenesis of common obesity related metabolic diseases, including insulin resistance. However, the relationship between leptin and insulin resistance is not clearly established. Here, we show that induced hyperleptinemia by leptin infusion alters insulin signalling in rat liver. Leptin infusion clearly reduced the insulin or leptin dependent IRS-1/IRS-2 association to p85 regulatory subunit of PI 3-kinase. Leptin infusion also abolished STAT-3 phosphorylation in response to insulin or leptin and similar results were obtained for MAP-kinase phosphorylation. Hypothalamic leptin resistance was also induced by leptin infusion since leptin was unable to induce STAT-3 phosphorylation. These results provide evidence that induced hyperleptinemia can contribute to the onset of insulin resistance at least at the hepatic level.

Cryptosporidium infection impairs growth and muscular protein synthesis in suckling rats.

This study aimed to explore the metabolic consequences of cryptosporidiosis in an acute experimental model both at the peak of infection and after parasite clearance. Four-day-old suckling rats were infected with 10(6) oocysts of Cryptosporidium parvum. At the peak of infection (day 8 PI), C. parvum resulted in a dramatic reduction both in nutrient intake (-50%) and body weight (16.3+/-5.2 vs 27.3+/-1.0 g, P<0.01) with a decrease in both lean body mass and adipose tissue. Muscular fractional and absolute synthesis rate were reduced (-15 and -55%, respectively). After parasite clearance (day 17 PI), body weight remained reduced in formerly infected animals (37.8+/-8.0 vs 47.8+/-4.2 g, P<0.01) whereas nutrient intake normalized and fractional synthesis rate slightly increased (+22%) compared to controls. Overall, our results show that the impact and consequences of cryptosporidiosis are far greater than generally appreciated, leading to major malnutrition in suckling rats.

Acute third ventricular administration of leptin decreases protein and fat in self-selecting rats.

The peripheral administration of leptin reduces food intake (FI) body weight gain (BWG) and modifies food choice. The aim of this study was to examine the effect of acute cerebral injections of leptin on food selection in rats. Male rats were first adapted to the food choice paradigm (protein, carbohydrate, fat) for 3 weeks. They were then implanted with a cannula in the third ventricle. Leptin (leptin group=L) or saline (control group=C) injections were performed at either the beginning or the end of the night at 4-day intervals. FI was recorded continuously, 3 days before, during and then after injections. Rats were sacrificed 86 h after the second injection. After both injections, BWG and FI were reduced. The reduction in FI concerned only nocturnal intake, whatever the timing of the injection. When the injection was given at the beginning of the night, the reductions after a 1-h latency period were -45% and -27.5% during the first and second days, respectively. Following the second injection, the same effects were observed immediately (-16% and -41%, respectively). Only the fat and protein intakes were significantly reduced. This lower FI was due to a reduction in meal size and duration. The reduction resulted in a lower BWG and total white adipose tissue mass. At the time of sacrifice, 6 h after food deprivation, leptinemia and insulinemia were reduced in leptin-treated rats. Glycemia values were identical. It was thus demonstrated that central leptin was a satiation factor rather than a satiety factor.

Comparison of fat storage between Fischer 344 and obesity-resistant Lou/C rats fed different diets.

OBJECTIVE: We aimed to characterize further the Lou/C (LOU) and Fischer 344 (F344) rat strains for nutritional traits to validate their use as contrasting strains for molecular genetic studies. RESEARCH METHODS AND PROCEDURES: Five batches of LOU and F344 rats were used to measure caloric intake, weight gain, and body composition when fed a chow diet, a self-selection diet (together with the study of preferences for macronutrients), hypercaloric diets, and a chow diet in a cold environment. RESULTS: Despite a higher caloric intake when fed a chow diet, LOU rats showed a lower weight gain, final body weight, and percentage of fat tissue, together with a higher percentage of carcass weight, than F344 rats. When fed a self-selection diet, LOU males ingested less protein and more fat than F344 males, and the reverse was observed for females. In this condition, feed efficiency was reduced in LOU but increased in F344 rats compared with the chow diet. Diet-induced obesity was observed in F344 rats but not in LOU rats fed hypercaloric diets. In a cold environment, both LOU and F344 rats displayed an increased percentage of brown adipose tissue compared with control groups, together with a higher caloric intake. DISCUSSION: The study shows robust nutritional differences between the LOU rat, a lean strain with a low feed efficiency and resistant to diet-induced obesity, and the contrasting F344 rat strain. It also shows the interest in these strains for studying the genetic components of resistance to obesity.

Intraperitoneal leptin modifies macronutrient choice in self-selecting rats.

This study aimed to evaluate the consequences on food intake and body weight (BW) of leptin administration in rats receiving a choice between the three macronutrients. Two studies were performed: during the first, rats received an acute intraperitoneal (IP) leptin administration (1 mg/kg) twice (at 8 and 14 weeks of age), at the beginning of the nocturnal cycle, while during the second, they received a chronic leptin infusion (osmotic minipump, 7 days). The total 24-h food intake after acute leptin injections was reduced by 14% and 17%, respectively. Body weight gain (BWG) after leptin injections was about half that seen on control days. Chronic leptin infusion reduced total intake, affecting mainly protein (P). Fat intake increased slightly since day 2 and became significant on the fourth day. After the leptin infusion, carbohydrate (CHO) eaters (>35% carbohydrate/total energy) significantly reduced the carbohydrate proportion in their total energy intake. There was no difference concerning macronutrient selection by fat eaters (Hfat). Leptin infusion reduced the number of mixed meals on the first day. In addition, the thermogenesis of brown adipose tissue (BAT) was higher in leptin than in control (C) rats. Consequently, leptin injections reduced food intake and BWG and increased thermogenesis, thus acting on the two terms of the energy balance. Moreover, leptin has different effects on macronutrient preferences, dependent upon age (tests 1 and 2) and the type (acute or chronic) of injection. High leptinemia level related to age or to minipump infusion lead to leptin resistance as found in old or obese subjects. It could explain our results.

Total subdiaphragmatic vagotomy does not suppress high protein diet-induced food intake depression in rats.

This study was undertaken to determine whether the subdiaphragmatic vagus nerve is involved in the depression of food intake induced by the ingestion of a high protein diet (P50) in rats. After total subdiaphragmatic vagotomy (Vago group) or sham surgery (Sham group), rats consumed the control diet for a 2-wk recovery period and then both groups consumed the high protein diet for 16 d. Daily food intake, meal pattern analysis and behavioral satiety sequence were measured. Total subdiaphragmatic vagotomy did not modify the daily intake of the control diet or suppress the dramatic depression in food intake produced by acute transition to a high protein diet. However, the daily intake of a high protein diet was slightly reduced under acute conditions or even after adaptation (P < 0.005). Analysis of meal parameters and the behavioral satiety sequence after adaptation indicated no major metabolic distress. In conclusion, these results suggest that the subdiaphragmatic vagus nerve does not constitute an obligatory pathway for the transfer of information to the brain, resulting in a depression of high protein diet intake. In contrast, a defect in this visceral regulating system could reinforce the metabolic-associated food intake depression signal.

A high-protein diet enhances satiety without conditioned taste aversion in the rat.

In order to determine the respective roles of conditioned food aversion, satiety and palatability, we studied behavioral responses to a 50% total milk protein diet, compared with those to a normal protein diet containing 14% total milk protein. Different paradigms were employed, including meal pattern analysis, two-choice testing, flavor testing, a behavioral satiety sequence (BSS) and taste reactivity. Our experiments showed that only behavioral and food intake parameters were disturbed during the first day when an animal ate the high-protein (P50) diet, and that most parameters returned to baseline values as soon as the second day of P50. Rats adapted to P50 did not acquire a conditioned taste aversion (CTA) but exhibited satiety, and a normal BSS. The initial reduction in high-protein diet intake appeared to result from the lower palatability of the food combined with the satiety effect of the high-protein diet and the delay required for metabolic adaptation to the higher protein level.

Wistar rats allowed to self-select macronutrients from weaning to maturity choose a high-protein, high-lipid diet.

The aim of this work was to study the evolution of rat food choice in relation to their age and metabolic parameters. Eighty Wistar rats were studied from birth to 13 weeks of age. At weaning, six litters were fed on a macronutrient self-selecting diet and four on a standard diet. In self-selecting males, protein intake was maximal at Week 7 of age and then plateaued (Week 13), whereas in females, protein consumption peaked at Week 7 and then steadily decreased. Females showed a strong and early preference for fat, which increased continuously with age. Males and females ingested their total energy intake during the dark period (respectively, 79% and 70%). Simple meals (composed of one item) were mainly ingested during the light phase, while mixed meals (at least two items) were ingested during the night. In males, most mixed meals began with carbohydrate bouts and finished with proteins, while in females no particular choice was observed at the beginning of meals, but most of them ended with protein bouts. Body weights of either male and female self-selecting or control fed rats were not significantly different at the end of the experiment. Differences between dietary groups in body fat mass were not observed with the exception of higher subcutaneous fat found in self-selecting rats. Moreover, insulinemia was lower in both male and female self-selecting rats. The high-protein, high-fat diet chosen by the self-selecting rats could be linked to a prevention of the age-related insulin resistance.