Differential effects of high-fat diets varying in fatty acid composition on the efficiency of lean and fat tissue deposition during weight recovery after low food intake

The energetics of body weight recovery after low food intake was examined in the rat during refeeding for 2 weeks with isocaloric amounts of high-fat (HF) diets providing 50% of energy as either lard, coconut oil, olive oil, safflower oil, menhaden fish oil, or a mixture of all these fat types. The results indicate that for both body fat and protein, the efficiency of deposition was dependent on the dietary fat type. The most striking differences were found (1) between diets rich in n-3 and n-6 polyunsaturated fatty acids (PUFA), with the diet high in fish oil resulting in a greater body fat deposition and lower protein gain than the diet high in safflower oil; and (2) between diets rich in long-chain (LCT) and medium-chain triglycerides (MCT), with the diet high in lard resulting in a greater gain in both body fat and protein than the diet high in coconut oil. Furthermore, the diet high in olive oil (a monounsaturated fat) and the mixed-fat diet (containing all fat types) were found to be similar to the fish oil diet in that the efficiency of fat deposition was greater (and that of protein gain lower) than with the diet high in safflower oil. Neither the efficiency of fat gain nor that of protein gain were found to correlate with fasting plasma insulin, the insulin to glucose ratio, or plasma lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Paraxanthine (metabolite of caffeine) mimics caffeine's interaction with sympathetic control of thermogenesis

The notion that paraxanthine (the major dimethylated by-product of caffeine) may be a biologically active metabolite that could mediate some of the effects of caffeine was tested in relation to the well-established property of caffeine as a thermogenic stimulant. From studies measuring the in vitro respiration rates of rat brown adipose tissue in the basal state and in response to ephedrine (an enhancer of norepinephrine release from sympathetic nerve endings), it is shown that paraxanthine has the same potency as its parent compound, caffeine, in interacting with the adrenergic system to potentiate thermogenesis. These data provide the first direct demonstration of a physiological effect of the main metabolite of caffeine and raise the possibility that paraxanthine may contribute importantly to the ability of caffeine to potentiate the thermogenic effects of well-known stimuli of the sympathetic nervous system such as cold exposure, moderate exercise, and sympathomimetic drugs.

Suppression of Ca(2+)-dependent heat production in mouse skeletal muscle by high fish oil consumption

The energy dissipation associated with calcium homeostasis amounts to more than 20% of muscle energy expenditure (EE) at rest and can be quantified from microcalorimetric measurements of heat production in response to chemical modulators of sarcoplasmic reticulum (SR) Ca2+ release. Using this approach, Ca(2+)-dependent heat production in both red- and white-fiber muscles from mice fed a high-fat (HF) diet rich in fish oil was found to be significantly lower than in other groups fed HF diets rich in saturated fat (hydrogenated coconut oil) or n-6 polyunsaturated fats corn oil) and in a group fed a low-fat diet. These findings reveal a potentially specific effect of fish oil on muscle-cell energy metabolism via interference with sarcoplasmic calcium homeostasis, and raise the possibility that modification of the energy cost for intracellular calcium homoeostasis may be a cellular mechanism by which diet could modulate skeletal muscle thermogenesis and whole-body EE.

Adaptive role of energy expenditure in modulating body fat and protein deposition during catch-up growth after early undernutrition

After 10 or 30 d of growth arrest due to undernutrition during the early weaning period, rehabilitated rats fed ad libitum showed the phenomenon of catch-up growth, ie, gains in body weight and body protein were greater by 50% and 25%, respectively, (P < 0.001) than those of controls with similar starting body weight [ie, weight-matched (WM) controls]. These increases, however, were entirely dependent on the higher food intake because they failed to occur when food intakes were maintained at similar amounts to those for WM controls. In contrast, independent of whether the rehabilitated groups were spontaneously hyperphagic relative to WM controls or made normophagic by pair-feeding to WM controls, the gross energetic efficiency was increased (P < 0.01) and body fat gain was more elevated (2-2.5 fold, P < 0.001) during the first 2 wk of refeeding compared with WM controls--differences that were uninfluenced by the duration of growth arrest. Taken together, these studies suggest that the often reported impressive gains in body fat during recovery from malnutrition may result not only from unbalanced diets or excess dietary intake, but also from a transitory enhancement in the efficiency of food utilization and a shift in energy partitioning in favor of an acceleration for the replenishment of fat stores.

Ephedrine, xanthines and prostaglandin-inhibitors: actions and interactions in the stimulation of thermogenesis

The pivotal role of the sympathoadrenal system in the defense of le milieu interieur has, in the last 15 years, been extended to include the fat stores-a notion that forms the basis of current strategies for thermogenic stimulation in obesity therapy. The search for effective and safe sympathetic stimulants has been directed at two main levels: (i) the development of novel beta-agonists selective for thermogenesis, and (ii) the evaluation of drugs already in clinical use for other purposes (e.g. ephedrine) which could conceivably increase the release of catecholamines to levels that enhance thermogenesis without significant cardiovascular effects. A re-direction of these strategies seem inevitable because at therapeutic doses, the thermogenic effects of these sympathomimetics seem to be considerably dampened by negative feedback mechanisms that operate both extracellularly (e.g. via adenosine & prostaglandins) as well as inside the cells (via cAMP phosphodiesterases). Such a contention is supported by studies both in man and in animals showing that methylxanthines and aspirin, drugs known to be capable of interfering with these modulators, potentiate the thermogenic effects of ephedrine. Future research aimed at clarifying the types and subtypes of these negative modulators of sympathomimetic-induced thermogenesis and their targeting by more selective antagonists would no doubt be pivotal in providing the safe drug combination with the necessary thermogenic properties to assist the management of obesity.

Ephedrine, caffeine and aspirin: safety and efficacy for treatment of human obesity

The safety and efficacy of a mixture of ephedrine (75-150mg), caffeine (150mg) and aspirin (330mg), in divided premeal doses, were investigated in 24 obese humans (mean BMI 37.0) in a randomized double blind placebo-controlled trial. Energy intake was not restricted. Overall weight loss over 8 weeks was 2.2kg for ECA vs. 0.7 kg for placebo (p < 0.05). 8 of 13 placebo subjects returned 5 months later and received ECA in an unblinded crossover. After 8 weeks, mean weight loss with ECA was 3.2 kg vs 1.3 kg for placebo (p = 0.036). 6 subjects continued on ECA for 7 to 26 months. After 5 months on ECA, average weight loss in 5 of these was 5.2 kg compared to 0.03 kg gained during 5 months between studies with no intervention (p = 0.03). The sixth subject lost 66 kg over 13 months by self-imposed caloric restriction. In all studies, no significant changes in heart rate, blood pressure, blood glucose, insulin, and cholesterol levels, and no differences in the frequency of side effects were found. ECA in these doses is thus well tolerated in otherwise healthy obese subjects, and supports modest, sustained weight loss even without prescribed caloric restriction, and may be more effective in conjunction with restriction of energy intake.

24 hour energy expenditure several months after weight loss in the underfed rat: evidence for a chronic increase in whole-body metabolic efficiency

The existence and quantitative contribution of an adaptive increase in whole-body metabolic efficiency in response to low caloric intake was examined after prolonged underfeeding in the rat. 24h energy expenditures (EE) and body composition were measured in chronically underfed rats which had been maintaining a stable body weight for several months after an initial period of substantial weight loss. Comparisons were made with ad libitum fed controls. The mean coefficient of variation for daily EE, assessed over three consecutive days, was 3% (range 1.7-4.0%) in the control group and 1.6% (range 0.7-2.1%) in the chronically underfed group, and hence reflect steady state conditions. Body composition was determined by chemical analysis, and as a proportion of body weight (BW), the fat content in the underfed group was reduced by 40% whereas lean body mass was significantly higher by 8%. Independently of the criteria utilized for metabolically active mass, 24h EE was lower in the underfed group than in controls, by 17%, 14% and 8% as a function of BW0.66, BW0.75 and lean body mass respectively (P < 0.01). Underfeeding was also associated with a substantial reduction in the level of external work, but in ad libitum fed animals similar reductions in external work (induced by limiting locomotion) failed to alter 24h EE significantly. Taken together, these studies provide evidence for an increased metabolic efficiency in response to low calorie intake well beyond the period of dynamic weight loss.

Influence of dietary composition on energy expenditure during recovery of body weight in the rat: implications for catch-up growth and obesity relapse

The influence of dietary composition on whole-body energetics was examined during the first 2 weeks of isocaloric refeeding after low food intake in a rat model. The high energetic efficiency and energy partitioning toward fat accretion characteristic of this refeeding period were unaltered by (1) dietary fat levels varying between 6% and 30% of energy intake; (2) protein levels between 15% and 40%; (3) carbohydrate types (glucose v fructose v sucrose v starch v unrefined carbohydrate); and (4) diets containing 30% fat but differing in fatty acid composition (long-chain triglycerides [LCT] v medium-chain triglycerides [MCT] v oleic v linoleic v alpha-linolenic metabolites eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA] omega-3 fatty acids). Changes were only observed for extreme diets, ie, those deficient in protein or very high in fat. Low-protein diet was the only condition in which the high metabolic efficiency characteristic of the refeeding period was partially suppressed, and this occurred despite a lack of concomitant reduction in body fat deposition. On the contrary, with high-fat diets (> 30% of dietary energy consumption) the elevated efficiency was further increased, an effect that was only partially accounted for by the lower energy cost of body fat gain from high-fat diets. These studies indicate that during body weight recovery, the mechanisms underlying the adaptive increase in metabolic efficiency favoring the replenishment of body fat stores override any effect of food type on thermogenesis, and suggest some convergence in the controlling neural pathway. The implications of these findings vis-a-vis nutritional rehabilitation (catch-up growth) and obesity relapse are discussed.

Potentiation of the thermogenic antiobesity effects of ephedrine by dietary methylxanthines: adenosine antagonism or phosphodiesterase inhibition?

Current concepts about the mechanisms underlying the therapeutic effects of dietary methylxanthines (caffeine, theophylline, and theobromine) favor their actions as antagonists of adenosine receptors, and attribute their other possible modes of action, namely those associated with translocation of intracellular calcium, inhibition of phosphodiesterase enzyme (PDE) activity, or the release of catecholamines, to high (near-toxic) doses. From studies measuring the respiration rate of brown adipose tissue (BAT), evidence is provided here that at concentrations compatible with therapeutic doses, the ability of methylxanthines (25 to 50 mumol/L) to potentiate the thermogenic effect of the sympathomimetic drug, ephedrine (0.25 mumol/L), particularly under conditions of caloric restriction, involves a minor contribution of adenosine antagonism, but could mainly be explained by the inhibition of PDE activity. In view of current interest in the pharmacological stimulation of metabolic rate to assist the management of obesity with low-calorie regimens, the targeting of PDE activity is therefore a rational approach in the search for drugs that could potentiate sympathomimetic stimulation of metabolic rate.

Dietary supplementation with fish oil enhances in vivo synthesis of tumor necrosis factor

Studies reported here investigate the influence of dietary fat types on cytokine production in response to endotoxin (LPS) challenge. Tumor necrosis factor (TNF) serum levels were markedly higher (by 10-fold) in mice fed chronically a diet rich in fish oil rather than either a diet rich in corn or coconut oil or a low fat diet. This in vivo hyper-responsiveness in LPS-induced TNF production following fish oil consumption concorded with similar exaggerated in vitro TNF release from macrophages exposed to LPS. These data suggest that high consumption of fish oils, by virtue of their high content of omega-3 polyunsaturated fatty acids, can lead to an exaggerated production of mediators of inflammation with potentially adverse consequences on the outcome and severity of infectious diseases.

Fish oil decreases natural resistance of mice to infection with Salmonella typhimurium

Mortality rate in mice fed fish oil for 4 weeks was remarkably higher after a very low peroral (PO) challenge with Salmonella typhimurium, as compared with those fed diets rich in either corn oil or hydrogenated coconut oil, or a low fat (chow) diet. None of the surviving mice fed the fish oil diet showed bacterial counts in their spleens, unlike 45.4% to 66.6% of surviving mice fed high fat or low fat diets. The spleens of mice fed fish oil presented the highest number of bacteria 7 days after intraperitoneal infection with the same bacterial strain. Thus, the current studies demonstrate that a diet rich in fish oil decreases host resistance to infection.

Peripheral mechanisms of thermogenesis induced by ephedrine and caffeine in brown adipose tissue

The peripheral mechanisms by which ephedrine and caffeine influence thermogenesis were investigated in innervated rat interscapular brown adipose tissue (IBAT) by assessing its rate of oxygen consumption (MO2) in vitro. Dose-response measurements with tissues from intact or sympathectomized (6-OHDA) animals indicate that the thermogenic effects of low concentrations of ephedrine and also of caffeine are entirely dependent upon the presence of intact sympathetic nerve endings, and thus depend on presynaptic mechanisms. Direct postsynaptic stimulation of thermogenesis is only apparent at much higher concentrations, namely greater than 1 microM for ephedrine and greater than 2mM for caffeine. At subminimal concentrations that neither ephedrine nor caffeine influenced basal tissue respiration, they induced a 4-5-fold increase in basal MO2 when administered in combination, a synergistic response prevented by pre-treatment of the rat with 6-OHDA. Synergistic increases in IBAT respiration were also obtained when subminimal concentration of ephedrine was added to 3-propylxanthine (a specific inhibitor of phosphodiesterase), to 8-phenyltheophylline (a potent adenosine receptor antagonist) or to adenosine deaminase (for enzymatic inactivation of endogenous adenosine). Conversely, the marked synergism in thermogenic response with ephedrine + caffeine was reduced in the presence of 2-chloroadenosine (an adenosine analogue). In tissues from fasted rats, the ephedrine + caffeine synergism in thermogenic response, although attenuated, was nevertheless present. These studies therefore demonstrate that ephedrine, at doses comparable with therapeutic use, stimulates thermogenesis in BAT via sympathetically released NA. In addition, a synergistic interaction between caffeine and ephedrine on BAT thermogenesis is explained by ephedrine's enhancement of sympathetic neuronal release of NA, together with caffeine's dual ability to antagonize adenosine and to inhibit cellular phosphodiesterase activity.

Effects of monosodium glutamate and gold thioglucose on dietary regulation of sympathetic nervous system activity in rodents

Neonatal administration of monosodium glutamate (MSG) disrupts hypothalamic regulation of a number of neuroendocrine systems. Studies described in this report using techniques of norepinephrine (NE) turnover examined sympathetic nervous system (SNS) activity in heart and interscapular brown adipose tissue (IBAT) of animals given MSG as neonates. Although in every experiment overall rates of NE turnover were lower in MSG-treated mice and rats, the differences were due exclusively to diminished tissue NE content, especially in IBAT. Fractional rates of NE turnover did not differ between groups. In contrast to animals with lesions in the ventromedial hypothalamus produced by gold thioglucose (AuTG) or electric current, MSG-treated mice and rats varied SNS activity in heart and IBAT in accord with changes in nutrient intake. Thus, SNS activity, both at baseline and in response to dietary manipulation, is probably not affected by neonatal MSG administration.

Adaptation to low calorie intake in obese mice: contribution of a metabolic component to diminished energy expenditures during and after weight loss

Adaptive changes in energy expenditure (EE) in response to low calorie intake were assessed quantitatively in obese mice using techniques that measure EE chronically over weeks (by the comparative carcass method), and also over 24 h (by indirect calorimetry). Grossly obese mice were slimmed (by restricting food to 50 per cent of normal intake) until their body weight reached that of the lean. They were subsequently either maintained in the post-obese state by continued food restriction at 25 per cent below the obese controls, or refed at the same calorie intake as controls. Total EE (assessed over 3 week periods) fell by 30 per cent during weight loss (obese controls 1356 kJ vs obese-slimmed 966 kJ, P less than 0.001), by 25 per cent during maintenance of the post-obese weight (obese control 1443 kJ vs post-obese 1062 kJ, P less than 0.001), and remained diminished by 17 per cent during weight regain upon refeeding (obese control 1443 kJ vs slimmed/refed 1176 kJ, P less than 0.001). Similar reductions in daily oxygen consumption were also obtained by indirect calorimetry, and as a function of lean body mass, 24 h VO2 were lower than controls by 17 per cent, 9 per cent and 10 per cent respectively (P less than 0.01 at least), and remained so under conditions of locomotory restraint. It is calculated that in these obese mice, a metabolic component (unassociated with changes in lean body mass and activity) comprised nearly two-thirds of the adaptive fall in EE during weight loss, and one-half of the change in EE during both phases of post-obese weight maintenance and during weight regain. These studies therefore support the notion that in response to low calorie intake, adaptive changes in the efficiency of cellular energy utilization are of considerable quantitative importance in the resistance to weight loss and in the ease with which the obese condition is rapidly regained after slimming.

Role of corticosterone in adaptive changes in energy expenditure during refeeding after low calorie intake

We examined the importance of corticosterone in elevated efficiency of energy utilization during refeeding after low food consumption. Energy balance studies during refeeding (over periods of 14 or 16 days) were conducted in rats previously food restricted for 16 days at 50% of normal food intake. Comparisons made with nonrestricted weight-matched controls after validation studies indicated that 2-wk-younger weight-matched controls had similar maintenance energy requirements and similar efficiency of energy utilization above maintenance (i.e., net efficiency) to nonrestricted age-matched controls. Results indicate that relative to controls refeeding after low food consumption was associated with enhanced energy conservation underlain by a 16-18% reduction (P less than 0.001) in total energy expenditure over a 14-day period. This metabolic adaptation for energy conservation resulted in a threefold increase (P less than 0.001) in body fat accretion but no difference in body protein deposition. Bilateral adrenalectomy (ADX) 2 days before refeeding reduced differences in energy expenditure between refed group and controls from 18 to 8% (P less than 0.01) and attenuated body fat gain from a three- to twofold increase (P less than 0.001) above control group. Effects of ADX were prevented by daily corticosterone replacement. Data suggest that after a period of low calorie intake an adaptive neurohormonal switching mechanism facilitates replenishment of fat stores during refeeding. This metabolic reorientation (characterized by an adaptive fall in energy expenditure) has both an adrenal as well as a nonadrenal component, because it is partially reversed by prior bilateral ADX, an effect attributed to removal of corticosterone-induced inhibition of thermogenesis.

Adaptive changes in energy expenditure during refeeding following low-calorie intake: evidence for a specific metabolic component favoring fat storage

Studies in this report investigate the existence and quantitative contribution of a metabolic component to an apparently elevated efficiency of energy utilization during refeeding following low-calorie consumption. Energy balance during a 25-d refeeding period was assessed in rats that had been food restricted (at 50% of normal intake) for either 30 or 10 d. Relative to weight-matched controls (with comparable lean tissue mass and similar food intake), refeeding following both periods of low food consumption (30 or 10 d) was associated with a 10% reduction in energy expenditure (p less than 0.001). Analysis of body composition revealed that virtually all the energy saved as a result of this metabolic (as opposed to tissue mass) adaptation was deposited as fat rather than protein. After the energy cost for depositing the extra fat is accounted for, the metabolic component represents a net 15% lower energy expenditure when normal food intake is resumed. These adaptive changes in energy expenditure may thus constitute an important mechanism for the rapid replenishment of energy stores in preparation for recurring food shortages and may also underlie the ease with which the obese condition is rapidly reachieved after weight loss.

Reduced adipsin expression in murine obesity: effect of age and treatment with the sympathomimetic-thermogenic drug mixture ephedrine and caffeine

Adipsin gene expression is greatly diminished in certain forms of genetic and acquired obesity. In the present study we evaluate the time course for the development of adipsin deficiency in obesity and its regulation by the sympathomimetic-thermogenic drug mixture ephedrine and caffeine. Previously, it was unknown whether adipsin deficiency occurred before or after the development of massive obesity. In the first series of experiments in which mice were treated with monosodium glutamate (MSG) for the first week of life, we demonstrate that adipsin deficiency occurs early in the development of MSG-induced obesity as evidenced by decreased circulating adipsin concentrations by 1 week of age and deficient adipsin mRNA levels in white adipose tissue (WAT) by 2 weeks. In db/db mice, diminished circulating adipsin was noted at 2 weeks of age. In both models, decreased adipsin gene expression precedes the development of marked obesity. Little is known about the factors which regulate adipsin gene expression in obesity. Common to the ob/ob, db/db and MSG models is diminished thermogenesis and sympathetic nervous system activity. In a second series of experiments we sought to determine whether adipsin deficiency in obesity could be corrected by treatment with ephedrine and caffeine (E+C), a sympathomimetic-thermogenic mixture previously shown to increase thermogenesis and reverse obesity in some models. In the present study, E+C treatment of MSG obese mice reversed obesity and markedly increased serum adipsin and adipsin mRNA levels in WAT and brown adipose tissue (BAT). In ob/ob mice, however, E+C treatment produced a negligible increase in adipsin mRNA levels in WAT and BAT as well as serum adipsin concentrations and this correlated with only a very small decrease in obesity. Thus, the ability of E+C to increase adipsin gene expression correlated with its ability to reverse obesity in these two models. Finally, the effect of E+C on adipsin gene expression may not be exerted directly on the fat cell since treatment of cultured 3T3-F442A adipocytes and isolated rat adipocytes in primary culture produced no effect on adipsin mRNA or secreted protein despite a lipolytic effect as measured by increased glycerol release. In summary, decreased adipsin gene expression occurs early in the development of MSG and db/db obesity and is markedly increased in the MSG model by the sympathomimetic-thermogenic drug mixture, E+C, which also reverses obesity. Elucidation of the factors responsible for these effects may enhance our understanding of fat cell gene regulation and obesity.

Energy expenditure and diet-induced thermogenesis in presence and absence of hyperphagia induced by insulin

The influence of experimental hyperinsulinemia on energy intake, energy expenditure, and body composition was investigated in rats treated chronically with high doses of insulin. Energy balance studies, each of 2-wk duration, were conducted with two different long-acting insulins (Protamine and Monotard), administered in the morning (IM), the late afternoon (IA), or both (IMA) and in animals of three different ages, namely in 4-, 8-, and 12-wk-old rats. The results indicate that the level of hyperphagia induced by insulin was markedly influenced by the type of long-acting insulin (P less than 0.001; Protamine greater than Monotard), by age (P less than 0.001; 12 greater than 8 greater than 4 wk), as well as by the timing of insulin administration (P less than 0.002, IMA greater than IM or IA). Body protein deposition was unaltered, but body fat and energy expenditure increased in parallel to the level of hyperphagia. Regression analysis shows a strong linear correlation (r = 0.963) between the change in energy expenditure and the change in energy intake in response to insulin and indicates that approximately 50% of the excess calories consumed was dissipated as heat. In the absence of hyperphagia, however, insulin administration had no effect on energy expenditure nor on energy partitioning. Similarly, the influence of altered meal pattern, induced by administering insulin at different times of the day, was also found to have no impact on energy expenditure. The current investigations therefore refute the notion that high doses of insulin via hyperinsulinemia and/or altered meal pattern have an inhibitory influence on whole body thermogenesis. In contrast, our data demonstrate that the adaptive phenomenon that tends to minimize the accumulation of excess caloric intake, i.e., diet-induced thermogenesis, persists in the hyperinsulinemic state.

Normal caffeine consumption: influence on thermogenesis and daily energy expenditure in lean and postobese human volunteers

Single-dose oral administration of 100 mg caffeine increased the resting metabolic rate of both lean and postobese human volunteers by 3-4% (p less than 0.02) over 150 min and improved the defective diet-induced thermogenesis observed in the postobese subjects. Measurements of energy expenditure (EE) in a room respirometer indicate that repeated caffeine administration (100 mg) at 2-h intervals over a 12-h day period increased the EE of both subject groups by 8-11% (p less than 0.01) during that period but had no influence on the subsequent 12-h night EE. The net effect was a significant increase (p less than 0.02) in daily EE of 150 kcal in the lean volunteers and 79 kcal in the postobese subjects. Caffeine at commonly consumed doses can have a significant influence on energy balance and may promote thermogenesis in the treatment of obesity.

A low-budget and easy-to-operate room respirometer for measuring daily energy expenditure in man

Accurate assessment of energy requirements of humans under various physiological, pathological, and environmental conditions is hampered by the high cost of techniques for measuring daily energy expenditure. A room respirometer novel in its novel in its low cost of construction and simplicity of operation is described, together with an appreciation of errors and assumptions. It is suitable for measurements of daily energy expenditure in humans with an accuracy of +/- 1.5% and can be reproduced without difficulty in both developed and developing countries.

Sympathetic nervous system responses to cold exposure and diet in rat skeletal muscle

Norepinephrine (NE) turnover measured in individual skeletal muscles of the rat assessed sympathetic responses to cold (4 degrees C) and diet. Acute cold exposure increased NE turnover slightly (15-50%) in all muscles examined, including gastrocnemius, tibialis anterior, tensor fascia lata, and soleus, in contrast, cold-accelerated NE turnover in heart by one- to twofold and in interscapular brown adipose tissue (IBAT) by more than threefold. Similar results in skeletal muscle and heart were obtained after 1 wk of cold exposure. Examination of dietary effects on NE turnover, comparing rats fed a sucrose-supplemented diet for 3 days with those fasted for 2 days, indicated that sympathetic activity in skeletal muscle was unresponsive to dietary intake. In heart and IBAT, on the other hand, NE turnover in sucrose-fed rats was twice that obtained in fasted animals. Taken together, the lesser effect of cold and the undetectable response to diet indicate that sympathetic outflow to skeletal muscle is regulated differently than that to heart or IBAT and imply that skeletal muscle is not a major site for sympathetically mediated thermogenic responses to cold exposure or to dietary alteration, such as fasting and sucrose feeding.

Prevention of genetic fa/fa obesity with an ephedrine-methylxanthines thermogenic mixture

A thermogenic mixture containing ephedrine and methylxanthines was administered to 8-wk-old genetically obese fa/fa rats (O-E/Mx group) for a period of 15 wk. Their energy balance and the final body composition were compared with an untreated ad libitum-fed (O-AL) group, as well as to other fa/fa obese animals that were either pair fed to lean controls (O-PF group), or that were food restricted to such an extent they maintained a similar body weight to that of lean animals (O-WF group). Energy intake was elevated above lean or O-PF levels by approximately 27 and 10% in the O-AL and O-E/Mx groups, respectively, but lower by 18% in the O-WF group. Energy expenditure, compared with the lean values, was 10% higher in both the O-AL and O-E/Mx groups, but reduced by 13 and 30% in the O-PF and O-WF groups, respectively. The gain in body energy and the efficiency of energy deposition remained elevated above the lean values by 2.3- to 3.5-fold in the O-AL, O-PF, and O-WF groups but were reduced to lean levels in the O-E/Mx groups. These studies indicate that, unlike food restriction, the ephedrine-methylxanthine mixture prevents or arrests the development of the obesity in the fa/fa mutant by normalizing their energetic efficiency to that of the lean.