Effect of dietary carbohydrates on food efficiency and body composition in adult male rats during normal growth and during recovery from food restriction

High-fructose diet leads to visceral adiposity and hypothalamic leptin resistance in male rats--do glucocorticoids play a role?

Fructose overconsumption has been involved in the genesis and progression of the metabolic syndrome. Hypothalamus and adipose tissue, major organs for control of food intake and energy metabolism, play crucial roles in metabolic homeostasis. We hypothesized that glucocorticoid signaling mediates the effects of a fructose-enriched diet on visceral adiposity by acting on neuropeptide Y (NPY) in the hypothalamus and altering adipogenic transcription factors in the visceral adipose tissue. We analyzed the effects of 9-week consumption of 60% fructose solution on dyslipidemia, insulin and leptin sensitivity, and adipose tissue histology in male Wistar rats. Glucocorticoid signaling was assessed in both hypothalamus and visceral adipose tissue, while the levels of peroxisome-proliferator-activated receptor γ (PPARγ), sterol regulatory element-binding protein-1 (SREBP-1) and lipin-1, together with the levels of their target genes expression, were analyzed in the visceral adipose tissue. The results showed that long-term consumption of highly concentrated liquid fructose led to the development of visceral adiposity, elevated triglycerides and hypothalamic leptin resistance accompanied by stimulated glucocorticoid signaling and NPY mRNA elevation. Results from adipose tissue implied that fructose consumption shifted the balance between glucocorticoid receptor and adipogenic transcriptional factors (PPARγ, SREBP-1 and lipin-1) in favor of adipogenesis judged by distinctly separated populations of small adipocytes observed in this tissue. In summary, we propose that high-fructose-diet-induced alterations of glucocorticoid signaling in both hypothalamus and adipose tissue result in enhanced adipogenesis, possibly serving as an adaptation to energy excess in order to limit deposition of fat in nonadipose tissues.

When does sucrose increase appetite and adiposity?

Two methods of sucrose feeding have been employed in studies with rodents. In the nutritional method, part or all of the starch in a diet is replaced with sucrose. In the solution method, animals maintained on a nutritionally complete diet are given a sucrose solution to drink. The solution method is generally a more effective and reliable method of producing obesity except for weanling rodents. These two methods yield different results with regard to interactions with the fat and protein content of the diet, efficiency of weight gain, disaccharide effects and effects of meal feeding. It is suggested that for the nutritional method, sucrose alters food intake and adiposity via its effects on fat oxidation. For the solution method, the critical factor may be presenting a wet source of calories rather than sucrose per se. Differences in the way sucrose is fed do not account for all divergent results. Different investigators conducting similar experiments have often obtained different results. For these and other reasons, animal studies do not support the idea that sucrose intake causes obesity in humans.

Carbohydrate-induced hyperphagia and obesity in the rat: effects of saccharide type, form, and taste

Adult female rats were fed, in addition to chow and water, a carbohydrate source that differed in type (glucose, sucrose, or polysaccharide), form (32% solution, powder, or gel), or taste (very sweet, minimally sweet, or bitter). A control group was fed only chow and water during the 40-day experiment. The groups fed the glucose solution, sucrose solution, or one of three polysaccharide solutions (Polycose, maltose-dextrin 10, maltose-dextrin 42) all overrate and gained more body weight and fat than did the control group. The carbohydrate solution groups did not differ in their total caloric intake, weight gain, percent body fat, or basal insulin level. The polysaccharide groups, however, consumed more carbohydrate than did the sugar groups. The groups fed glucose, sucrose, or Polycose in powder form consumed less carbohydrate and total calories, gained less weight and fat, and had lower insulin levels than did the groups fed the saccharides in solution form. The powder groups did not reliably differ from the control group on these measures. Rats fed Polycose in solution form or in a solid gel form (32% Polycose + 1% agar) were similar in their carbohydrate intake, total caloric intake, weight gain, and percent body fat. Rats fed Polycose solutions that were minimally sweet (32% Polycose), sweet (0.2% saccharin + 32% Polycose), or bitter [0.05% sucrose octa acetate (SOA) + 32% Polycose] did not differ in their Polycose intake, total caloric intake, weight gain, or percent body fat. The results demonstrate that saccharide form is more important than saccharide type or taste in promoting hyperphagia and obesity in rats. The Polycose gel findings further indicate that it is the water of hydration, not liquidity that is responsible for the hyperphagia-inducing effect of carbohydrate solutions.