Long-Evans and Sprague-Dawley rats exhibit divergent responses to refeeding after caloric restriction

Mature male Sprague-Dawley (SD) and Long-Evans (LE) rats were instrumented with telemetry transmitters for measurement of heart rate (HR) and housed in room calorimeters for assessment of food intake and oxygen consumption (Vo(2)) at standard laboratory temperatures (23 degrees C) to examine physiological responses to caloric restriction (CR; 60% of baseline ad libitum calories for 2 wk) and refeeding. Ad libitum controls had stable food intake (84-88 kcal/day) and gained weight at rates of 3-4 g/day. Groups from both strains assigned to CR exhibited similar patterns of weight loss and reductions in Vo(2) and HR. Upon refeeding, SD rats exhibited a mild, transient hyperphagic response (1 day) accompanied by sustained suppression of Vo(2) and HR that remained evident 8 days after refeeding. In contrast, LE rats exhibited sustained daily hyperphagia that persisted 8 days after refeeding and was accompanied by a complete restoration of HR and Vo(2). The lower HR and Vo(2) observed during refeeding in SD rats were not due to reduced locomotor activity. The results reveal a strain-dependent divergent response to recovery from CR. We conclude that during recovery from CR, homeostatic stimulation of appetite or suppression of energy expenditure may occur selectively to restore body weight.     

Homeostatic responses to caloric restriction: influence of background metabolic rate.

The biological responses to caloric restriction (CR) are generally examined in rats with elevated metabolic rates due to being housed at ambient temperatures (T(a)) below the zone of thermoneutrality. We determined the physiological and behavioral responses to 2 wk of 30-40% CR in male FBNF1 rats housed in cool (T(a) = 12 degrees C) or thermoneutral (TMN; T(a) = 30 degrees C) conditions. Rats were instrumented with telemetry devices and housed continuously in home-cage calorimeters for the entire experiment. At baseline, rats housed in cool T(a) had reduced rate of weight gain; thus a mild CR (5%) group at thermoneutrality for weight maintenance was also studied. Rats housed in cool T(a) exhibited elevated caloric intake (cool = 77 +/- 1; TMN = 54 +/- 2 kcal), oxygen consumption (Vo(2); cool = 9.9 +/- 0.1; TMN = 5.5 +/- 0.1 ml/min), mean arterial pressure (cool = 103 +/- 1; TMN = 80 +/- 2 mmHg), and heart rate (cool = 374 +/- 3; TMN = 275 +/- 4 beats/min). Cool-CR rats exhibited greater CR-induced weight loss (cool = -62 +/- 3; TMN = -42 +/- 3 g) and reductions in Vo(2) (cool = -2.6 +/- 0.1; TMN = -1.5 +/- 0.1 ml/min) but similar CR-induced reductions in heart rate (cool = -59 +/- 1; TMN= -51 +/- 7 beats/min). CR had no effect on arterial blood pressure or locomotor activity in either group. Unexpectedly, weight maintenance produced significant reductions in Vo(2) and heart rate. At thermoneutrality, a single day of refeeding effectively abolished CR-induced reductions in Vo(2) and heart rate. The results reveal that rats with low or high baseline metabolic rate exhibit comparable compensatory reductions in Vo(2) and heart rate and suggest that T(a) can be used to modulate the metabolic background on which the more prolonged effects of CR can be studied.     

Sexually dimorphic responses to neonatal basal forebrain lesions in mice: II. Cortical morphology

Previous studies in the mouse have shown that neonatal lesions to the cholinergic basal forebrain (nBM) areas result in transient cholinergic depletion of neocortex and precipitate altered cortical morphogenesis. Lesion-induced morphological alterations in cortex persist into adulthood and are accompanied by behavioral changes, including spatial memory deficits. The current study investigated whether neonatal nBM lesions affect male and female mice differently in adulthood. Quantitative morphometry of cortical layer width was employed to assess alterations in cytoarchitecture in neonatally nBM-lesioned and littermate control mice of both sexes following behavioral testing. Our results showed significant decreases in cortical layer IV and V widths across somato/motor cortex in neonatally nBM lesioned mice of both sexes. Sexually dimorphic responses were observed in cortical layer II/III and total cortical width, limited to the area containing the “barrel cortex” representation of the whisker hairs. In lesioned females, layer II/III and total cortical width were decreased relative to female controls, and in lesioned males, layer II/III was increased relative to controls, whereas total cortical width was unchanged. In male but not female mice we observed significant correlations between decreased widths in layer IV and V and impaired performance on a spatial memory task. The current data further support a role of developing cholinergic cortical afferents in the modulation of cortical morphogenesis and cortical circuits involved in cognitive behaviors. In addition, our observations provide further evidence for sexually dimorphic development and function in cognitive centers of the rodent brain. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 595–606, 1998     

Sexually dimorphic responses to neonatal basal forebrain lesions in mice: I. Behavior and neurochemistry

The nucleus basalis magnocellularis (nBM) provides the primary source of cholinergic input to the cortex. Neonatal lesions of the nBM produce transient reductions in cholinergic markers, persistent abnormalities in cortical morphology, and spatial navigation impairments in adult mice. The present study examined sex differences in the effects of an electrolytic nBM lesion on postnatal day 1 (PND 1) in mice on behavior and neurochemistry in adulthood. Mice were lesioned on PND 1 and tested at 8 weeks of age on a battery of behavioral tests including passive avoidance, cued and spatial tasks in the Morris water maze, simple and delayed nonmatch to sample versions of an odor discrimination task, and locomotor activity measurements. Following behavioral testing, mice were sacrificed for either morphological assessment or neurochemical analysis of a cholinergic marker or catecholamines. There were no lesion or sex differences in acquisition or retention of passive avoidance, performance of the odor discrimination tasks, or activity levels. Control mice showed a robust sex difference in performance of the spatial water maze task. The lesion produced a slight cued but more dramatic spatial navigation deficit in the water maze which affected only the male mice. Neurochemical analyses revealed no lesion-induced changes in either choline acetyltransferase activity or levels of norepinephrine or serotonin at the time of testing. The subsequent report shows a sex difference in lesion-induced changes in cortical morphology which suggests that sexually dimorphic cholinergic influences on cortical development are responsible for the behavioral deficits seen in this study. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 582–594, 1998     

Transcriptomics applied to obesity and caloric restriction

Caloric restriction still remains the most efficient way to promote weight loss. Deciphering the molecular basis of adaptation to energy restriction is critical for the tailoring of new therapeutic strategies. This review focuses on the recent input of gene profiling on adipose tissue in obesity pathogenesis and on the new insights on adaptations occurring during very low caloric diet (VLCD) in humans. Hypocaloric diets improve a wide range of metabolic parameters including lipolytic efficiency, insulin sensitivity, and inflammatory profile. In the subcutaneous white adipose tissue (scWAT) the VLCD induced a decrease in the mRNA levels for the antilipolytic alpha2-adrenergic receptor associated with changes in catecholamine-induced adipocyte lipolytic capacity. The improvement in insulin sensitivity was not associated with a change in subcutaneous adipose tissue adiponectin gene expression or in its plasma level, suggesting that adiponectin is not involved in the regulation of VLCD-induced improvement of insulin sensitivity and that there is a small contribution of subcutaneous adipose tissue to plasma adiponectin levels. Pangenomic microarray studies in human scWAT revealed that a panel of inflammatory markers and acute phase reactants were over expressed in obese compared to lean subjects. Caloric restriction improved the inflammatory profile of obese subjects through a decrease of pro-inflammatory factors and an increase of anti-inflammatory molecules. These genes were mostly expressed in the stroma vascular fraction of the adipose tissue. Specific cell-type isolation and immunohistochemistry demonstrated that monocyte/macrophage lineage cells were responsible for the expression of both mRNA and protein inflammatory markers. The acute phase proteins serum amyloid A was highly expressed in mature adipocytes from obese subjects. Caloric restriction decreased both serum amyloid mRNA and circulating levels. Obesity now clearly appears as chronic low-grade inflammation state. Modulation of the inflammatory pathways may represent new therapeutic targets for the treatment of obesity-related complications.     

Cardiovascular responses to caloric restriction and thermoneutrality in C57BL/6J mice

We utilized variations in caloric availability and ambient temperature (T(a)) to examine interrelationships between energy expenditure and cardiovascular function in mice. Male C57BL/6J mice (n = 6) were implanted with telemetry devices and housed in metabolic chambers for measurement of mean arterial pressure (MAP), heart rate (HR), O(2) consumption (VO(2)), and locomotor activity. Fasting (T(a) = 23 degrees C), initiated at the onset of the dark phase, resulted in large and transient depressions in MAP, HR, VO(2), and locomotor activity that occurred during hours 6-17, which suggests torporlike episodes. Food restriction (14 days, 60% of baseline intake) at T(a) = 23 degrees C resulted in progressive reductions in MAP and HR across days that were coupled with an increasing occurrence of episodic torporlike reductions in HR (<300 beats/min) and VO(2) (<1.0 ml/min). Exposure to thermoneutrality (T(a) = 30 degrees C, n = 6) reduced baseline light-period MAP (-14 +/- 2 mmHg) and HR (-184 +/- 12 beats/min). Caloric restriction at thermoneutrality produced further reductions in MAP and HR, but indications of torporlike episodes were absent. The results reveal that mice exhibit robust cardiovascular responses to both acute and chronic negative energy balance. Furthermore, we conclude that T(a) is a very important consideration when assessing cardiovascular function in mice.     

Autologous fat transplants influence compensatory white adipose tissue mass increases after lipectomy

Direct tests of the hypothesized total body fat regulatory system have been accomplished by partial surgical lipectomy. This usually results in the restoration of the lipid deficit through compensatory increases in nonexcised white adipose tissue (WAT) masses of ground squirrels, laboratory rats, and mice, as well as Siberian and Syrian hamsters. We challenged this hypothesized total body fat regulatory system by testing the response of Siberian hamsters to 1) lipid deficits [lipectomy; primarily bilateral epididymal WAT (EWAT) removal], 2) lipid surfeits (addition of donor EWAT with no lipectomy), 3) no net change in lipid [EWAT or inguinal WAT (IWAT) lipectomy with the excised fat replaced to a new location (autologous)], 4) lipectomy with the same pad (EWAT lipectomy only) added from a sibling (nonautologous), and 5) sham surgeries for each treatment. Food intake generally was not affected. Body mass was not affected across all treatments. Grafts approximately 3 mo later had normal appearance both macro- and microscopically and were revascularized. The normal lipectomy-induced compensatory increases in nonexcised WAT masses surprisingly were exaggerated with autologous EWAT transplants, but not for autologous IWAT or nonautologous EWAT transplants. There was no compensatory decrease in native WAT masses with nonautologous EWAT additions. Collectively, only lipectomy triggered reparation of the lipid deficit, but the other manipulations did not, suggesting a system biased toward rectifying decreases in lipid or an inability of the hypothesized total body fat regulatory system to recognize WAT transplants.     

Resveratrol does not increase body fat loss induced by energy restriction

Resveratrol (RSV) is known to have an antiobesogenic effect because it mimics energy restriction. However, hardly any evidence exists concerning the combined effects of RSV and energy restriction on body fat reduction. So, the aim of the present study was to determine whether RSV increases body fat reduction induced by energy restriction. Male Wistar rats were fed a high-fat, high-sucrose diet for 6 weeks to obtain a diet-induced obesity model. Then they were submitted to a mild energy restriction (25 %) without or with RSV supplementation (30 mg/kg body weight/day) for 2 weeks. Final body weight, subcutaneous and intra-abdominal white adipose tissues weights, Adipose Index, and serum triacylglycerol, cholesterol, glucose, and insulin were assessed. Lipoprotein lipase (LPL), fatty acid synthase (FAS), and acetyl coenzyme A carboxylase (ACC) activities, as well as their genetic expressions, were measured in white adipose tissue. Final body weight, white adipose tissue weights, Adipose Index, and serum triacylglycerol, cholesterol, and insulin were reduced in both groups, but no differences were found among them. FAS, ACC, and LPL activities and expressions were also similar in both groups. These results suggest a lack of any adjuvant effect of RSV on energy restriction for obesity treatment purposes.     

T cell potentiation in normal and autoimmune-prone mice after extended exposure to low doses of ionizing radiation and/or caloric restriction

In order to better understand the apparent physiologic up-regulation in response to low levels of potentially lethal insults, murine T lymphocytes were analysed for functional and phenotypic alterations after exposure to 0.005 Gy/day, 0.01 Gy/day and 0.04 Gy/day in groups of ad-libitum-fed and calorie-restricted mice. These studies were conducted in two strains of mice: the long-lived and immunologically normal C57Bl/6 +/+ and the congenic short-lived immunologically depressed C57Bl/6 lpr/lpr. Whole-body exposure to 0.01 Gy/day and 0.04 Gy/day for an extended period of 20 days was associated with an increase in splenic proliferative response and with shifts in the proportions of T cell subpopulations in the thymus and spleen of both strains. Caloric restriction independently altered functional activity and T cell subpopulations in the same direction as low dose rates of ionizing radiation. Although the dose-response augmentation in proliferative activity was similar in the two strains, observed alterations in thymic and splenic T cell subpopulations were clearly different, suggesting that different mechanisms were responsible for immune enhancement in each strain.     

Sex-related differences in energy balance in response to caloric restriction

Sex-related differences in energy balance were studied in young Wistar rats fed standard chow pellets either ad libitum or in restricted amounts (60% of ad libitum intake) for 100 days. Caloric intake, indirect calorimetry, organ and adipose tissue weights, energy efficiency, liver mitochondrial respiration rate, and brown adipose tissue (BAT) uncoupling protein-1 (UCP1) content were measured. Ad libitum-fed females showed greater oxygen consumption (Vo(2)) and carbon dioxide production (Vco(2)) and lower energy efficiency than males. Caloric restriction induced a chronic drop of Vo(2) and Vco(2) in females but not in males over the period studied. Restricted females showed a better conservation of metabolic active organ mass and a greater decrease in adipose depots than restricted males. Moreover, changes of BAT size and UCP1 content suggest that BAT may be the main cause responsible for sex differences in the response of energy balance to caloric restriction. In conclusion, our results indicate that females under caloric restriction conditions deactivate facultative thermogenesis to a greater degree than males. This ability may have obvious advantages for female survival and therefore the survival of the species when food is limiting.     

Sirt3 mediates reduction of oxidative damage and prevention of age-related hearing loss under caloric restriction

Caloric restriction (CR) extends the life span and health span of a variety of species and slows the progression of age-related hearing loss (AHL), a common age-related disorder associated with oxidative stress. Here, we report that CR reduces oxidative DNA damage in multiple tissues and prevents AHL in wild-type mice but fails to modify these phenotypes in mice lacking the mitochondrial deacetylase Sirt3, a member of the sirtuin family. In response to CR, Sirt3 directly deacetylates and activates mitochondrial isocitrate dehydrogenase 2 (Idh2), leading to increased NADPH levels and an increased ratio of reduced-to-oxidized glutathione in mitochondria. In cultured cells, overexpression of Sirt3 and/or Idh2 increases NADPH levels and protects from oxidative stress-induced cell death. Therefore, our findings identify Sirt3 as an essential player in enhancing the mitochondrial glutathione antioxidant defense system during CR and suggest that Sirt3-dependent mitochondrial adaptations may be a central mechanism of aging retardation in mammals.     

C-reactive protein and coronary artery disease: influence of obesity,caloric restriction and weight loss

C reactive protein (CRP) values in blood are a good indicator of the likelihood of acute coronary and cerebral events in both healthy subjects and patients with coronary artery disease. This indicates that atherosclerotic lesions rich in inflammatory cells and cytokines are more likely to produce acute events either through vasospasm and/or thrombosis and also can be readily detected through elevations in CRP when measured using a high sensitivity assay (hsCRP). However the arterial wall is only one potential source of cytokines which induce CRP production. Fat cells also produce cytokines, in particular IL-6 which induces the synthesis of CRP by the liver. Obesity, especially abdominal obesity, is associated with elevations of hsCRP. This may be of pathogenic significance as CRP stimulates the uptake of LDL by macrophages, induces complement activation which may cause cellular damage in the artery, and enhances monocyte production of tissue factor, thus enhancing the risk of thrombosis. Caloric restriction and weight loss lowers IL-6 and CRP levels and may beneficially suppress an immune response. Whether particular dietary macronutrients or micronutrients alter IL-6 or CRP is unknown but this issue is clearly becoming more important.     

Sexually dimorphic micturition in rats: relationship of perineal muscle activity to voiding pattern

In the present study we examined the possibility that striated muscle activity may underlie sexually dimorphic micturition in rats. Micturition dynamics, the gross anatomy of the external urethral sphincter, and the participation of the striated perineal muscles in micturition were compared in urethane-anesthetized adult male and female rats. Bladder contraction characteristics, particularly the magnitude of bladder high-frequency pressure waves during voiding, differed between sexes. Dissections indicated that the sphincter was more extensive and thicker in males than in females. Electromyography showed that in both sexes the sphincter discharged in bursts that correlated with the rising phase of high-frequency bladder pressure oscillations. Regional differences in discharge pattern were seen in the sphincters of males, with the proximal part of the sphincter showing components activated during bladder filling. Bulbospongiosus, ischiocavernosus, and cremaster muscles also were activated during bladder contraction in males. In both sexes transection of the motor branch of the lumbosacral plexus eliminated the bladder high-frequency oscillations and reduced voided volume. Neurectomy did not affect bladder pressure but reduced voiding efficiency by 45% in males. In females the bladder pressure was dramatically decreased, but voiding efficiency only decreased by 24%. Our findings suggest that, in rats, striated perineal muscles contribute to the sexually dimorphic micturition. Activity of the dimorphic perineal muscles may regulate genital and urinary urethra expulsive functions, helping to expel seminal plug and fluids through the long urethra in the male.     

Sexually dimorphic development of mouse primordial germ cells: switching from oogenesis to spermatogenesis

During embryogenesis, primordial germ cells (PGCs) have the potential to enter either spermatogenesis or oogenesis. In a female genital ridge, or in a non-gonadal environment, PGCs develop as meiotic oocytes. However, male gonadal somatic cells inhibit PGCs from entering meiosis and direct them to a spermatogenic fate. We have examined the ability of PGCs from male and female embryos to respond to the masculinising environment of the male genital ridge, defining a temporal window during which PGCs retain a bipotential fate. To help understand how PGCs respond to the male gonadal environment, we have identified molecular differences between male PGCs that are committed to spermatogenesis and bipotential female PGCs. Our results suggest that one way in which PGCs respond to this masculinising environment is to synthesise prostaglandin D(2). We show that this signalling molecule can partially masculinise female embryonic gonads in culture, probably by inducing female supporting cells to differentiate into Sertoli cells. In the developing testis, prostaglandin D(2) may act as a paracrine factor to induce Sertoli cell differentiation. Thus part of the response of PGCs to the male gonadal environment is to generate a masculinising feedback loop to ensure male differentiation of the surrounding gonadal somatic cells.     

Cognitive performances are selectively enhanced during chronic caloric restriction or resveratrol supplementation in a primate

Effects of an 18-month treatment with a moderate, chronic caloric restriction (CR) or an oral supplementation with resveratrol (RSV), a potential CR mimetic, on cognitive and motor performances were studied in non-human primates, grey mouse lemurs (Microcebus murinus).Thirty-three adult male mouse lemurs were assigned to three different groups: a control (CTL) group fed ad libitum, a CR group fed 70% of the CTL caloric intake, and an RSV group (RSV supplementation of 200 mg.kg(-1).day(-1)) fed ad libitum. Three different cognitive tests, two motor tests, one emotional test and an analysis of cortisol level were performed in each group.Compared to CTL animals, CR or RSV animals did not show any change in motor performances evaluated by rotarod and jump tests, but an increase in spontaneous locomotor activity was observed in both groups. Working memory was improved by both treatments in the spontaneous alternation task. Despite a trend for CR group, only RSV supplementation increased spatial memory performances in the circular platform task. Finally, none of these treatments induced additional stress to the animals as reflected by similar results in the open field test and cortisol analyses compared to CTL animals.The present data provided the earliest evidence for a beneficial effect of CR or RSV supplementation on specific cognitive functions in a primate. Taken together, these results suggest that RSV could be a good candidate to mimic long-term CR effects and support the growing evidences that nutritional interventions can have beneficial effects on brain functions even in adults.     

DNA replication stress-induced loss of reproductive capacity in S. cerevisiae and its inhibition by caloric restriction

In many organisms, attenuation of growth signaling by caloric restriction or mutational inactivation of growth signaling pathways extends lifespan and protects against cancer and other age-related diseases. The focus of many efforts to understand these effects has been on the induction of oxidative stress defenses that inhibit cellular senescence and cell death. Here we show that in the model organism S. cerevisiae, growth signaling induces entry of cells in stationary phase into S phase in parallel with loss of reproductive capacity, which is enhanced by elevated concentrations of glucose. Overexpression of RNR1 encoding a ribonucleotide reductase subunit required for the synthesis of deoxynucleotide triphosphates and DNA replication suppresses the accelerated loss of reproductive capacity of cells cultured in high glucose. The reduced reproductive capacity of these cells is also suppressed by excess threonine, which buffers dNTP pools when ribonucleotide reductase activity is limiting. Caloric restriction or inactivation of the AKT homolog Sch9p inhibits senescence and death in stationary phase cells caused by the DNA replication inhibitor hydroxyurea or by inactivation of the DNA replication and repair proteins Sgs1p or Rad27p. Inhibition of DNA replication stress represents a novel mechanism by which caloric restriction promotes longevity in S. cerevisiae. A similar mechanism may promote longevity and inhibit cancer and other age-related diseases in humans.     

Sexually dimorphic gall structures correspond to differential phytohormone contents in male and female wasp larvae

Sexually dimorphic galls are rare among gall‐inducing insects and the reason for their occurrence is unknown. The pteromalid wasp Trichilogaster acaciaelongifoliae, which induces galls on Acacia longifolia, is one such species. In the present study, the anatomical and physiological attributes of male and female galls of T. acaciaelongifoliae are examined and compared. Histological preparations are used to characterize anatomical differences between male and female gall chambers. Bioassays, high‐performance liquid chromatography‐mass spectrometry and an enzyme immunoassay are used to measure concentrations of auxin and cytokinin in normal buds, galled tissues, and larvae of both sexes. Female chambers are found to be 3.3‐fold larger, and are associated with 1.5‐fold more storage tissue and 3.5‐fold more vascular tissues than male chambers. Tissues from female chambers induce stronger cytokinin‐like bioactivity than tissues from male chambers. Female larvae have considerably higher concentrations of cytokinin free bases, ribosides, glucosides and monophosphates than male larvae; higher auxin‐like bioactivity than in normal or galled plant tissues; and almost twice the concentration of auxin than male larvae. Both male and female larvae contain much higher auxin concentrations than either galled or normal plant tissues. These findings suggest that differing levels of phytohormones are involved in the development of sexual dimorphism of gall structures in this species.     

Sexually dimorphic,androgen sensitive,enkephalinergic afferents to a lumbar motor nucleus of rats

In male rats, methionine-enkephalin immunoreactivity (enkephalin-ir) has been observed in the dorsal lateral nucleus (DLN), a longitudinal pool of motoneurons in the lumbar spinal cord. Within the DLN a mediodorsal crescent of intense enkephalin-ir staining surrounds the motoneurons innervating the ischiocavernosus muscle of the penis, which suggests a function of the enkephalinergic afferents in male copulatory activities. The present study attempted to determine the roles of gender and adult exposure to androgen in shaping the striking subnuclear distribution of enkephalin-ir. Transverse sections through L5–6 were obtained from mature male and female rats that were gonadally intact, gonadectomized, or gonadectomized and treated with testosterone, as well as from male rats genetically deficient in androgen receptors (Tfm). The sections were incubated with primary antiserum raised against methionine enkephalin and bound antibodies were visualized using the avidin-biotin peroxidase technique. A microphotometer was used to compare the staining density in laminae I-II of the dorsal horn, ventral grey matter, and the DLN. In all groups the DLN stained more darkly than the ventral grey, demonstrating the presence of enkephalin-ir in the DLN regardless of gender or exposure to androgen. However, the mediodorsal crescent of dense staining in the DLN was obvious only in gonadally intact males, while the entire DLN stained darkly in both sexes of gonadectomized rats treated with androgen. Therefore, the preferential distribution of enkephalin-ir in the mediodorsal crescent of the DLN is sexually dimorphic though the overall content of enkephalin-ir within the DLN responds to androgen.     

SirT1 regulates energy metabolism and response to caloric restriction in mice

The yeast sir2 gene and its orthologues in Drosophila and C. elegans have well-established roles in lifespan determination and response to caloric restriction. We have studied mice carrying two null alleles for SirT1, the mammalian orthologue of sir2, and found that these animals inefficiently utilize ingested food. These mice are hypermetabolic, contain inefficient liver mitochondria, and have elevated rates of lipid oxidation. When challenged with a 40% reduction in caloric intake, normal mice maintained their metabolic rate and increased their physical activity while the metabolic rate of SirT1-null mice dropped and their activity did not increase. Moreover, CR did not extend lifespan of SirT1-null mice. Thus, SirT1 is an important regulator of energy metabolism and, like its orthologues from simpler eukaryotes, the SirT1 protein appears to be required for a normal response to caloric restriction.