The effects of prolonged caloric restriction on protein kinetics in lean subjects has not been previously investigated. PURPOSE: To test the hypotheses that 21 days of caloric restriction (CR) in lean subjects would a) result in significant losses of lean mass despite a suppression in leucine turnover and oxidation, and b) negatively impact exercise performance. METHODS: Nine young, normal weight men (23+/-5 y, 78.6+/-5.7 kg, VO2peak: 45.2+/-7.3 ml(.)kg(-1)(.)min(-1),mean+/-SD) were underfed by 40% of the calories required to maintain body weight (BW) for 21 days and lost 3.8+/-0.3 kg BW and 2.0+/-0.4 kg lean mass. Protein intake was kept at 1.2 g(.)kg(-1)(.)day(-1). Leucine kinetics were measured using KIC reciprocal pool model in the post-absorptive state during rest and 50 minutes of exercise (EX) at 50% of VO2peak. Body composition, basal metabolic rate (BMR) and exercise performance were measured throughout the intervention. RESULTS: At rest, leucine flux (~131 micromol(.)kg(-1)(.)hr(-1)) and oxidation (Rox; ~19 micromol(.)kg(-1)(.)hr(-1)) did not differ pre- and post- CR. During EX, leucine flux (129+/-6 vs. 121+/-6) and Rox (54+/-6 vs. 46+/-8)were lower following CR than pre-CR. Nitrogen balance was negative throughout the intervention (~3.0gN(.)d(-1)) and BMR declined from 1898+/-262 kcal(.)d(-1) to 1670+/-203. Aerobic performance (VO2peak, endurance cycling) was not impacted by CR, but arm flexion endurance decreased by 20%. CONCLUSIONS: Three weeks of caloric restriction reduced leucine flux and oxidation during exercise in normal weight young men. However, despite negative nitrogen balance and loss of lean mass, whole body exercise performance was well maintained in response to CR.

Calorie restriction (CR) has been shown to improve peripheral insulin resistance and type 2 diabetes in animal models. However, the exact mechanism of CR on GLUT4 expression and translocation in insulin-sensitive tissues has not been well elucidated. In the present study, we examine the effect of CR on the expression of glucose transporter 4 (GLUT4), GLUT4 translocation, and glucose transport activity in adipose tissue from Otsuka Long-Evans Tokushima Fatty (OLETF) rat and control (LETO) rats. CR (70% of satiated group) ameliorated hyperglycemia and improved impaired glucose tolerance (IGT) in OLETF rats. In skeletal muscle, the expression levels of GLUT4 and GLUT1 were not significantly different between LETO and OLETF rats, and were not affected by CR. By contrast, the expression level of GLUT4 was markedly decreased in the adipose tissue of OLETF rats, but was dramatically increased by CR. The GLUT4 recruitment stimulated by insulin was also improved in OLETF rat adipocytes by CR. The insulin-stimulated 2-deoxyglucose (2DG) uptake was significantly increased in adipocytes from the CR OLETF rats, as compared with the satiated OLETF rats. Taken together, these results suggest that CR improves whole body glucose disposal and insulin resistance in OLETF rats, and that these effects may associate with the increased adipocyte-specific GLUT4 expression.

Oxidative damage to cellular macromolecules has been postulated to be a major contributor to the ageing of diverse organisms. Oxidative damage can be limited by maintaining high anti-oxidant defenses and by clearing/repairing damage efficiently. Protein turnover is one of the main routes by which functional proteins are maintained and damaged proteins are removed. Protein turnover rates decline with age, which might contribute to the accumulation of damaged proteins in ageing cells. Interestingly, protein turnover rates are maintained at high levels in caloric restricted animals. Whether changes in protein turnover are a cause or a consequence of ageing is not clear, and this question has not been a focal point of modern ageing research. Here we survey work on protein turnover and ageing and suggest that powerful genetic models such as the nematode Caenorhabditis elegans are well suited for a thorough investigation of this long-standing question.

Many reports have been published on the effects of lifelong dietary restriction (DR) on a variety of parameters such as life span, carcinogenesis, immunosenescence, memory function, and oxidative stress. There is, however, limited available information on the effect of late onset DR that might have potential application to intervene in human aging. We have investigated the effect of DR initiated late in life on protein and protein degradation. Two months of DR in 23.5-month-old mice significantly reduced heat-labile altered proteins in the liver, kidney, and brain. DR reversed the age-associated increase in the half-life of proteins, suggesting that the dwelling time of the proteins is reduced in DR animals. In accordance with this observation, the activity of proteasome, which is suggested to be responsible for degradation of altered proteins, was found increased in the liver of rats 30 months of age subjected to 3.5 months of DR. Thus, DR can increase turnover of proteins, thereby possibly attenuating potentially harmful consequences by altered proteins. Likewise, DR in old rats reduced carbonylated proteins in liver mitochondria, although the effect was not observed in cytosolic proteins. Fasting induced apoA-IV synthesis in the liver of young mice for efficient mobilization of stored tissue fats, while it occurred only marginally in the old. DR for 2 months from 23 months of age partially restored inducibility of this protein, suggesting the beneficial effect of DR. Taking all these findings together, it is conceivable that DR conducted in old age can be beneficial not only to retard age-related functional decline but also to restore functional activity in young rodents. Interestingly, recent evidence that involves DNA array gene _expression analysis supports the findings on the age-related decrease in protein turnover and its reversion by late-onset DR.

Differential 'fuel usage' has been proposed as a mechanism for life-span extension by caloric restriction (CR). Here, we report the effects of CR, initiated after weaning, on metabolic enzyme gene _expression 0, 1.5, 5, and 12 h after feeding of 24-month-old mice. Plasma glucose and insulin were reduced by approximately 20 and 80%. Therefore, apparent insulin sensitivity, as judged by the glucose to insulin ratio, increased 3.3-fold in CR mice. Phosphoenolpyruvate carboxykinase mRNA and activity were transiently reduced 1.5 h after feeding, but were 20-100% higher in CR mice at other times. Glucose-6-phosphatase mRNA was induced in CR mice and repressed in control mice before, and for 5 h following feeding. Feeding transiently induced glucokinase mRNA fourfold in control mice, but only slightly in CR mice. Pyruvate kinase and pyruvate dehydrogenase activities were reduced approximately 50% in CR mice at most times. Feeding induced glutaminase mRNA, and carbamyl phosphate synthetase I and glutamine synthase activity (and mRNA). They were each approximately twofold or higher in CR mice. These results indicate that in mice, CR maintains higher rates of gluconeogenesis and protein catabolism, even in the hours after feeding. The data are consistent with the idea that CR continuously promotes the turnover and replacement of extrahepatic proteins.

Our studies show that dietary caloric restriction (CR) alters the _expression of key metabolic enzymes in a manner consistent with an increased rate of extrahepatic protein turnover and renewal during aging. Of the key hepatic gluconeogenic enzyme genes affected by CR, glucose 6-phosphatase mRNA increased 1.7- and 2.3-fold in young and old CR mice. Phosphoenolpyruvate carboxykinase mRNA increased 2-fold in young mice, and its mRNA and activity increased 2.5- and 1.7-fold in old mice. These changes indicate that CR enhances the enzymatic capacity for gluconeogenesis. The carbon required for gluconeogenesis appears to be generated from peripheral protein turnover. Muscle glutamine synthetase mRNA increased 1.3- and 2.1-fold in young and old CR mice, suggesting increased disposal of nitrogen and carbon derived from protein catabolism for energy. mRNA for the key liver nitrogen disposal enzymes glutaminase, carbamyl phosphate synthase I, and tyrosine aminotransferase were increased by 2.4-, 1.8-, and 1.8-fold in CR mice. Consistent with increased hepatic nitrogen disposal, hepatic glutamine synthetase mRNA and activity were each decreased about 40% in CR mice. Together, these and our other published data suggest that CR enhances and maintains protein turnover, and thus protein renewal, into old age. These effects are likely to resist the well-documented decline in whole body protein renewal with age. Enhanced renewal may reduce the level of damaged and toxic proteins that accumulate during aging, contributing to the extension of life span by CR.

The age-associated increase in oxidative damage in ad libitum-fed mice is attenuated in mice fed calorically restricted (CR) diets. The objective of this study was to determine if this effect results from a slowing of age-related accumulation of oxidative damage, or from a reversible decrease of oxidative damage by caloric restriction. To address these possibilities, crossover studies were conducted in C57BL/6 mice aged 15 to 22 months that had been maintained, after 4 months of age, on ad libitum (AL) or a 60% of AL caloric regimen. One half of the mice in these groups were switched to the opposite regimen of caloric intake for periods up to 6 weeks, and protein oxidative damage (measured as carbonyl concentration and loss of sulfhydryl content) was measured in homogenates of brain and heart. In AL-fed mice, the protein carbonyl content increased with age, whereas the sulfhydryl content decreased. Old mice maintained continuously under CR had reduced levels of protein oxidative damage when compared with the old mice fed AL. The effects of chronic CR on the carbonyl content of the whole brain and the sulfhydryl content of the heart were fully reversible within 3-6 weeks following reinstatement of AL feeding. The effect of chronic CR on the sulfhydryl content of the brain cortex was only partially reversible. The introduction of CR for 6 weeks in the old mice resulted in a reduction of protein oxidative damage (as indicated by whole brain carbonyl content and cortex sulfhydryl), although this effect was not equivalent to that of CR from 4 months of age. The introduction of CR did not affect the sulfhydryl content of the heart. Overall, the current findings indicate that changes in the level of caloric intake may reversibly affect the concentration of oxidized proteins and sufhydryl content. In addition, chronic restriction of caloric intake also retards the age-associated accumulation of oxidative damage. The magnitude of the reversible and chronic effects appears to be dependent upon the tissue examined and the nature of the oxidative alteration.

Rates of protein synthesis (measured in vivo) and growth of the small intestine were studied as a function of age in ad libitum fed (control) and chronic dietary-restricted rats. At weaning, the fractional rates of synthesis in the mucosal and muscularis externa and serosal layers of the small intestine of control animals were similarly high (90-100% per day). Although these rates subsequently declined with age in the muscularis externa and serosa, they remained constant in the mucosa. Restricted feeding (50% reduced intake), when imposed from weaning onwards, significantly extends the maximum life span of rodents. However, the change in nutritional status slows the accumulation of protein, RNA, and DNA in both layers of the small intestine. Although underfeeding did not prevent the age-related fall in muscularis externa and serosal protein synthesis, significantly higher rates (both fractional and per ribosome) were found when compared age for age with controls. Mucosal fractional synthetic rates were similarly increased by the reduced food intake. These changes in protein turnover in the small intestine are consistent with the higher rates of whole body turnover previously observed in chronically underfed rats.

Changes in whole body growth, nucleic acids, and protein turnover have been studied in conjunction with ageing and chronic dietary restriction. Normal developmental changes between weaning and senescence included progressive decreases in the fractional rates of growth, protein synthesis, and protein breakdown; the decline in the synthetic rate correlating with decreases in the ribosomal capacity. Dietary intervention was imposed at weaning and involved pair feeding to 50% of the ad libitum food intake. Although this regime slowed whole body growth by retarding the developmental decline in protein turnover, growth was extended into the second and third years of life. The dietary-induced increase in longevity resulting from a retardation of the ageing process(es) appears therefore to be associated with an enhanced turnover of proteins during the major portion of the life span of dietary restricted rats. These observations are strange as up to 50% of basal metabolic rate may be due to the energy requirements for protein synthesis. So an increased protein synthesisi in caloric restricted animals becomes difficult to reconcile with the sharply decreased energy availability in these seme animals, althouh on a lean body mass basis, these may be no decrease. (from Weindruch & Walford 1988).