***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: This study was design ed to evaluate the effects of enriching an essential amino acid ( EAA ) mixture with leucine on muscle protein metabolism in elderly and young individuals . Four ( 2 elderly and 2 young ) groups were studied before and after ingestion of 6.7 g of EAAs . EAAs were based on the composition of whey protein [ 26 % leucine ( 26 % Leu ) ] or were enriched in leucine [ 41 % leucine ( 41 % Leu ) ] . A primed , continuous infusion of L-[ring-2H5]phenylalanine was used together with vastus lateralis muscle biopsies and leg arteriovenous blood sample s for the determinations of fractional synthetic rate ( FSR ) and balance of muscle protein . FSR increased following amino acid ingestion in both the 26 % ( basal : 0.048 + /- 0.005%/h ; post-EAA : 0.063 + /- 0.007%/h ) and the 41 % ( basal : 0.036 + /- 0.004%/h ; post-EAA : 0.051 + /- 0.007%/h ) Leu young groups ( P < 0.05 ) . In contrast , in the elderly , FSR did not increase following ingestion of 26 % Leu EAA ( basal : 0.044 + /- 0.003%/h ; post-EAA : 0.049 + /- 0.006%/h ; P > 0.05 ) but did increase following ingestion of 41 % Leu EAA ( basal : 0.038 + /- 0.007%/h ; post-EAA : 0.056 + /- 0.008%/h ; P < 0.05 ) . Similar to the FSR responses , the mean response of muscle phenylalanine net balance , a reflection of muscle protein balance , was improved ( P < 0.05 ) in all groups , with the exception of the 26 % Leu elderly group . We conclude that increasing the proportion of leucine in a mixture of EAA can reverse an attenuated response of muscle protein synthesis in elderly but does not result in further stimulation of muscle protein synthesis in young subjects It is recognized that both whey protein ( WY ) and essential amino acids ( EAA ) are stimuli for muscle protein anabolism . The aim of the present study was to determine if the effects of WY ingestion on muscle protein accrual in elderly persons are due solely to its constituent EAA content . Fifteen elderly persons were r and omly assigned to ingest a bolus of either 15 g of WY , 6.72 g of EAA , or 7.57 g of nonessential amino acids ( NEAA ) . We used the leg arteriovenous model to measure the leg phenylalanine balance , which is an index of muscle protein accrual . Phenylalanine balance ( nmol x min(-1 ) kg lean leg mass(-1 ) ) during the 3.5 hours after the bolus ingestion improved in the WY ( -216 + /- 14 vs -105 + /- 19 ; P < .05 ) but not in the EAA ( -203 + /- 21 vs -172 + /- 38 ; P > .05 ) or NEAA groups ( -203 + /- 19 vs -204 + /- 21 ; P > .05 ) . The insulin response ( uIU x mL(-1 ) 210 min(-1 ) ) during the same period was lower in both the NEAA ( 48 + /- 40 ) and EAA ( 213 + /- 127 ) when compared to the WY ( 1073 + /- 229 ; P < .05 ) . In conclusion , WY ingestion improves skeletal muscle protein accrual through mechanisms that are beyond those attributed to its EAA content . This finding may have practical implication s for the formulation of nutritional supplements to enhance muscle anabolism in older individuals Decreases in whole-body lean mass can cause sarcopenia , a disease frequently found in the elderly . This condition is frequently associated with frailty and disability in aging as well as the onset and progression of several geriatric syndromes . Sarcopenia therefore must be managed with multidimensional approaches that include physical training , nutritional support , and metabolic and anabolic treatment . The purpose of our study was to assess the effect of an orally administered special mixture of amino acids ( AAs ) in elderly subjects with reduced lean body mass and sarcopenia . A r and omized , open-label , crossover study was conducted in 41 elderly subjects ( age range : 66 - 84 years ) with sarcopenia , assigned to 2 distinct treatments ( AAs and placebo ) . All subjects had normal body weight ( body mass index within 19 - 23 ) . The AA treatment consisted of 70.6 kcal/day ( 1 kcal = 4.2 kJ ) of 8 g of essential AA snacks , given at 10:00 am and 5:00 pm . Lean mass was measured with dual-energy x-ray absorptiometry in leg , arm , and trunk tissues . Significant increases in whole-body lean mass in all areas were seen after 6 months and more consistently after 18 months of oral nutritional supplementation with AAs .  Fasting blood glucose , serum insulin , and homeostatic model assessment of insulin resistance ( an index of insulin resistance ) significantly decreased during AA treatment . Furthermore , a significant reduction in serum tumor necrosis factor-alpha ( TNF-alpha ) and a significant increase in both insulin-like growth factor-1 ( IGF-1 ) serum concentrations and in the IGF-1/TNF-alpha ratio were also found . No significant adverse effects were observed during AA treatment . These preliminary data indicate that nutritional supplements with the oral AA mixture significantly increased whole-body lean mass in elderly subjects with sarcopenia . The improvement in the amount of whole-body lean mass could be linked to increased insulin sensitivity and anabolic conditions related to IGF-1 availability CONTEXT Inadequate dietary protein intake has been implicated in sarcopenia . OBJECTIVE AND DESIGN The objectives of this study were to determine whether : 1 ) chronic essential amino acid ( EAA ) supplementation improves postabsorptive muscle protein fractional synthesis rate ( FSR ) , lean body mass ( LBM ) , and one-repetition maximum muscle strength , and and rogen receptor and IGF-I muscle protein expression ; and 2 ) the acute anabolic response to EAA ingestion is preserved after a 3-month supplementation period . Using a r and omized , double-blinded , placebo-controlled design , older women ( 68 + /- 2 yr ) were assigned to receive either placebo ( n = 7 ) , or 15 g EAA/d [ supplemented treatment group ( SUP ) ] ( n = 7 ) for 3 months . Metabolic outcomes were assessed in association with stable isotope studies conducted at 0 and 3 months . SETTING The study was performed at The University of Texas Medical Branch General Clinical Research Center . RESULTS Ingestion of 7.5 g EAA acutely stimulated FSR in both groups at 0 months ( P < 0.05 ) . Basal FSR at 3 months was increased in SUP only . The magnitude of the acute response to EAA was unaltered after 3 months in SUP . LBM increased in SUP only ( P < 0.05 ) . One-repetition maximum strength remained unchanged in both groups . Basal IGF-I protein expression increased in SUP after 3 months ( P = 0.05 ) , with no changes in and rogen receptor or total and phosphorylated Akt , mammalian target of rapamycin , S6 kinase , and 4E-binding protein . CONCLUSIONS EAA improved LBM and basal muscle protein synthesis in older individuals . The acute anabolic response to EAA supplementation is maintained over time and can improve LBM , possibly off setting the debilitating effects of sarcopenia BACKGROUND It has been reported that the blunted muscle protein synthetic response to food intake in the elderly can be normalized by increasing the leucine content of a meal . OBJECTIVE The objective was to assess the effect of 3 mo of leucine supplementation on muscle mass and strength in healthy elderly men . DESIGN Thirty healthy elderly men with a mean ( + /-SEM ) age of 71 + /- 4 y and body mass index ( BMI ; in kg/m(2 ) ) of 26.1 + /- 0.5 were r and omly assigned to either a placebo-supplemented ( n = 15 ) or leucine-supplemented ( n = 15 ) group . Leucine or placebo ( 2.5 g ) was administered with each main meal during a 3-mo intervention period . Whole-body insulin sensitivity , muscle strength ( one-repetition maximum ) , muscle mass ( measured by computed tomography and dual-energy X-ray absorptiometry ) , myosin heavy chain isoform distribution , and plasma amino acid and lipid profiles were assessed before , during , and /or after the intervention period . RESULTS No changes in skeletal muscle mass or strength were observed over time in either the leucine- or placebo-supplemented group . No improvements in indexes of whole-body insulin sensitivity ( oral glucose insulin sensitivity index and the homeostasis model assessment of insulin resistance ) , blood glycated hemoglobin content , or the plasma lipid profile were observed . CONCLUSION Long-term leucine supplementation ( 7.5 g/d ) does not augment skeletal muscle mass or strength and does not improve glycemic control or the blood lipid profile in healthy elderly men . This trial was registered at clinical trials.gov as NCT00807508 BACKGROUND The progressive loss of skeletal muscle mass with aging is attributed to a disruption in the regulation of skeletal muscle protein turnover . OBJECTIVE We investigated the effects on whole-body protein balance and mixed-muscle protein synthesis rates of the ingestion of carbohydrate with or without protein and free leucine after simulated activities of daily living . DESIGN Eight elderly ( 75 + /- 1 y ) and 8 young ( 20 + /- 1 y ) lean men were r and omly assigned to 2 crossover experiments in which they consumed either carbohydrate ( CHO ) or carbohydrate plus protein and free leucine ( CHO+Pro+Leu ) after performing 30 min of st and ardized activities of daily living . Primed , continuous infusions with L-[ring-13C6]phenylalanine and L-[ring-2H2]tyrosine were applied , and blood and muscle sample s were collected to assess whole-body protein turnover and the protein fractional synthetic rate in the vastus lateralis muscle over a 6-h period . RESULTS Whole-body phenylalanine and tyrosine flux were significantly higher in the young than in the elderly men ( P < 0.01 ) . Protein balance was negative in the CHO experiment but positive in the CHO+Pro+Leu experiment in both groups . Mixed-muscle protein synthesis rates were significantly greater in the CHO+Pro+Leu than in the CHO experiment in both the young ( 0.082 + /- 0.005%/h and 0.060 + /- 0.005%/h , respectively ; P < 0.01 ) and the elderly ( 0.072 + /- 0.006%/h and 0.043 + /- 0.003%/h , respectively ; P < 0.01 ) subjects , with no significant differences between groups . CONCLUSIONS Co-ingestion of protein and leucine with carbohydrate after activities of daily living improves whole-body protein balance , and the increase in muscle protein synthesis rates is not significantly different between lean young and elderly men BACKGROUND We previously showed that human muscle protein synthesis ( MPS ) increased during infusion of amino acids ( AAs ) and peaked at ≈120 min before returning to baseline rates , despite elevated plasma AA concentrations . OBJECTIVE We tested whether a protein meal elicited a similar response and whether signaling responses that regulate messenger RNA translation matched MPS changes . DESIGN Eight postabsorptive healthy men ( ≈21 y of age ) were studied during 8.5 h of primed continuous infusion of [1,2-¹³C₂]leucine with intermittent quadriceps biopsies for determination of MPS and anabolic signaling . After 2.5 h , subjects consumed 48 g whey protein . RESULTS At 45 - 90 min after oral protein bolus , mean ( ± SEM ) myofibrillar protein synthesis increased from 0.03 ± 0.003 % to 0.10 ± 0.01%/h ; thereafter , myofibrillar protein synthesis returned to baseline rates even though plasma essential AA ( EAA ) concentrations remained elevated ( + 130 % at 120 min , + 80 % at 180 min ) . The activity of protein kinase B ( PKB ) and phosphorylation of eukaryotic initiation factor 4 G preceded the rise of MPS and increases in phosphorylation of ribosomal protein kinase S6 ( S6K1 ) , and 4E-binding protein 1 ( 4EBP1 ) was superimposable with MPS responses until 90 min . However , although MPS decreased thereafter , all signals , with the exception of PKB activity ( which mirrored insulin responses ) , remained elevated , which echoed the slowly declining 
    Output:
    
These findings suggest that leucine supplementation is useful to address the age-related decline in muscle mass in elderly individuals , as it increases the muscle protein fractional synthetic rate