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Aging does not impair the anabolic response to a protein-rich meal^1,2,3

¹ From the Division of Rehabilitation Sciences (TBS) and the Departments of Surgery (SES, TLC, DLC, and RRW) and Physical Therapy and Internal Medicine (DPJ), The University of Texas Medical Branch, Galveston, TX

² Supported by the National Cattlemen's Beef Association (NCBA) Checkoff Program (to DPJ); by the National Institutes of Health/National Institute on Aging Claude D Pepper Older Americans Independence Center at the University of Texas Medical Branch (grant #P30 AG024832); and by grant no. MO1 RR-00073 from the National Institutes of Health.

³ Reprints not available. Address correspondence to D Paddon-Jones, Department of Physical Therapy, The University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555-1144. E-mail: djpaddon{at}utmb.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Background: Sarcopenia is a debilitating condition afflicting the elderly that may be facilitated by insufficient or ineffectual intake of dietary protein. We previously showed that free-form essential amino acids acutely stimulate muscle protein synthesis in both the young and the elderly. However, the ability of an actual protein-rich food to stimulate anabolism in the young and the elderly has not been explored.

Objective: We aimed to characterize changes in plasma amino acid concentrations and to quantify muscle protein synthesis in healthy young (41 ± 8 y old; n = 10) and elderly (70 ± 5 y old; n = 10) persons after ingestion of a 113-g (4-oz) serving of lean beef.

Design: Venous blood samples and vastus lateralis muscle biopsy samples were obtained during a primed (2.0 µmol/kg) constant infusion (0.08 µmol·kg^–1·min^–1) of L-[ring-¹³C₆] phenylalanine. Plasma amino acid concentrations were measured and a mixed-muscle fractional synthesis rate (FSR) was calculated during the premeal period and for 5 h after beef ingestion.

Results: Mixed-muscle FSR increased by 51% in both the elderly (mean ± SE measurements: 0.072 ± 0.004%/h and 0.108 ± 0.006%/h before and after the meal, respectively) and the young (0.074 ± 0.005%/h and 0.113 ± 0.005%/h before and after the meal, respectively) after beef ingestion (P < 0.001). Plasma amino acid concentrations peaked at 100 min after beef ingestion in both age groups but were substantially higher in the elderly (2185 ± 134 nmol/mL compared with 1403 ± 96 nmol/mL; P < 0.001).

Conclusion: Despite differences in the concentration of amino acids in the plasma precursor pool, aging does not impair the ability to acutely synthesize muscle protein after ingestion of a common protein-rich food.

Key Words: Nutrition • stable isotopes • sarcopenia • diet • beef • amino acids

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Sarcopenia is a debilitating consequence of aging characterized by a host of negative outcomes, including loss of muscle mass and functional capacity (1-5), increased risk of falls, and increased susceptibility to illness (6). There is no single cause of sarcopenia. However, inappropriate or insufficient nutritional intake, termed "anorexia of aging," may be a key contributor (7, 8). Specifically, insufficient or ineffectual protein intake in the elderly may facilitate the loss of muscle by blunting muscle anabolism and thus promoting net muscle protein catabolism (9, 10). In some persons, this insidious process may begin during middle age (40-50 y old). However, the overt expression of muscle loss and physical frailty is more pronounced in persons >65 y old (1, 2).

Many attempts have been made to combat inadequate nutritional or protein intake and attenuate the loss of skeletal muscle by the provision of protein-containing supplements. Unfortunately, these trials have largely been unsuccessful (11-13), in part because the elderly tend to concomitantly reduce their energy intake to accommodate the supplement (14). Furthermore, although some forms of supplementation [eg, whey protein and free essential amino acids (EAAs)] were shown to acutely stimulate net protein synthesis in both elderly and young persons (15, 16), a number of age-related differences may influence the subsequent incorporation or utilization of amino acids (AAs) ingested as part of an intact protein source. These potential differences include the rate of protein digestion and AA absorption, splanchnic uptake and clearance, and whole-body AA utilization and turnover (16, 17). The extent to which these factors influence precursor availability is of great importance, given that a rapid increase in extracellular AA concentrations is a major stimulus for muscle protein synthesis (18). Furthermore, if there is indeed an age-related impairment in anabolism after protein ingestion that may contribute to sarcopenia, it is imperative that this phenomenon be investigated in a realistic meal-like context.

Beef is a common source of dietary protein and is considered to be of high biological value because it contains the full complement of EAAs in a proportion similar to that found in human skeletal muscle (19). A standard serving of 113.4 g (4 oz) lean beef provides 10 g EAAs and 30 g AAs in total. Although the direct protein-synthetic effects of ingestion of an intact protein such as beef were not previously examined, it was proposed that, in older adults who consume the equivalent of 10-15 g EAAs in the form of animal protein (eg, meat, eggs, dairy, and fish), a strong anabolic response could potentially be elicited from the resultant increase in plasma EAA concentration (20).

The purpose of this study was to characterize changes in plasma AA concentrations and to quantify muscle protein synthesis in healthy young and elderly persons after ingestion of a 113.4-g (4 oz) serving of 90% lean beef.

	SUBJECTS AND METHODS

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Subjects and experimental design
Twenty healthy volunteers [10 elderly ( ± SD age: 70 ± 5 y old), 10 young (41 ± 8 y old)] participated in this study (Table 1). Elderly subjects were recruited through The Sealy Center on Aging Volunteer Registry at The University of Texas Medical Branch, and young subjects were recruited through newspaper advertisements and flyers. A battery of medical tests, including medical history, blood count, plasma electrolytes, blood glucose concentration, and liver and renal function tests, assessed subject eligibility. Exclusion criteria included recent injury, the presence of a metabolically unstable medical condition, low hematocrit or hemoglobin concentration, vascular disease, hypertension, or cardiac abnormality. All subjects were physically active and independent but not athletically trained.

The experimental design is depicted in Figure 1. Subjects were instructed to maintain their normal diet and to avoid strenuous activity for 72 h before admission. Subjects underwent full-body dual-energy X-ray absorptiometry (QDR-4500A; Holologic, Waltham, MA) before admission for measurement of lean muscle mass (LMM). Subjects were studied in the General Clinical Research Center after an overnight fast.

View larger version (6K): [in this window] [in a new window]   FIGURE 1.. The infusion protocol. After background blood samples were obtained, L-[ring-13C6]phenylalanine was infused for 10 h.

Venous blood samples were obtained at 3 time points during the fasting premeal period and every 20 min after beef ingestion. A 5-mm Bergstrom biopsy needle was used to take for biopsy 3 samples of muscle, 50-100 mg each, from the lateral portion of the vastus lateralis, 10-15 cm above the knee, while the patient was under local anesthesia (2% lidocaine), as previously described (21).

The 113.4-g lean ground-beef patties were prepared and supplied by Texas Tech University. Patties were delivered to the University of Texas Medical Branch already cooked, individually vacuum-sealed, and frozen. At mealtime, a single beef patty was gently warmed in a microwave oven and provided to each subject without condiments. All volunteers were able to consume the meal within 5 min. The consistency of the AA profile of the lean ground beef was verified by HPLC analysis (Table 2).

Muscle tissue was weighed, and the protein was precipitated with 800 µL of 10% perchloroacetic acid. Intracellular phenylalanine concentration was measured by adding an internal standard (2 µL/mg wet wt) containing 3 µmol/L L-[ring-¹³C₉ phenylalanine. Supernatant fluid (1.5 mL) was collected after tissue homogenization and centrifugation (2400 x g, 4 °C, 10 min) and processed in the same manner as the supernatant fluid from the blood samples. Intracellular phenylalanine enrichment and concentrations were measured with tert-butyldimethylsilyl derivative (15, 25). The remaining muscle pellet was washed and dried, and the proteins were hydrolyzed in 6N HCl at 110 °C for 24 h. The protein-bound L-[ring-¹³C₆] phenylalanine enrichment was measured with the use of gas chromatography-mass spectrometry with electron impact ionization (22).

Calculations
The fractional synthesis rate (FSR) of mixed muscle protein was calculated by measuring the direct incorporation of L-[ring-¹³C₆] phenylalanine into protein by using the precursor-product model (26). The following equation was used:

(1)

where E_p1 and E_p2 are the enrichments of bound L-[ring-¹³C₆] phenylalanine in 2 sequential biopsies, t is the time interval between 2 biopsies, and E_m is the mean L-[ring-¹³C₆] phenylalanine enrichment in the muscle intracellular pool.

An assumed prerequisite for the determination of muscle intracellular phenylalanine enrichment is an isotopic steady state. Although this assumption is easily met during the premeal period, it is complicated by the ingestion of beef (a source of nonlabeled phenylalanine), which transiently reduces the tracer-to-tracee ratio (ie, decreased plasma L-[ring-¹³C₆] phenylalanine enrichment), which in turn results in an underestimation of FSR. When free AAs or rapidly digested proteins are ingested, an isotopic steady state may be maintained by adding a small quantity of tracer to the supplement (20, 27). However, the addition of L-[ring-¹³C₆] phenylalanine to the beef was clearly inappropriate; thus, a correction factor (CF) was used to account for the decrease in the precursor enrichment during the postmeal period (a nonsteady state), as shown in the following equation:

(2)

where Ev_(AUC) is the actual venous enrichment area under the curve between sequential biopsies (ie, biopsy 2 and 3) (Figure 1), and Ev_(m2,m3) is the average venous enrichment at each biopsy time point. The CF reduces the potential for erroneous underestimation of the postmeal FSR; it is based on the assumption that the depression in plasma phenylalanine enrichment reflects the depression in the muscle intracellular phenylalanine enrichment.

Statistical analysis
Data are presented as means ± SEMs. Changes in AA concentration during the premeal and postmeal periods were analyzed with the use of repeated-measures analysis of variance with one between-subject factor (age) and one repeated-measures factor (time). Two-tailed unpaired t tests for equal variance were used to determined differences in physical characteristics. Changes in FSR were analyzed with the use of repeated-measures analysis of variance with 2 between-subject factor (age and sex) and one repeated-measures factor (time). Statistical analyses were conducted with SPSS software (version 9; SPSS Inc, Chicago, IL). Statistical significance for all analyses was set at P < 0.05.

	RESULTS

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Physical characteristics
Subject characteristics are presented in Table 1. All volunteers were healthy, free-living persons with no discernable or reported disability or disease. Nevertheless, body weight, body mass index (in kg/m²), and LMM were significantly (P < 0.03) lower in the elderly than in the young, which is consistent with the involuntary, yet progressive loss of muscle mass with age (1-5).

Muscle protein synthesis
Our primary outcome measure, mixed muscle FSR, did not differ significantly between the 2 groups during the premeal period. After beef ingestion, FSR (corrected for nonsteady state conditions) in both the elderly and the young increased by 51% (Figure 2).

View larger version (9K): [in this window] [in a new window]   FIGURE 2.. Mean (±SEM) corrected mixed-muscle fractional synthesis rate before and after ingestion of 113 g of 90% lean beef in elderly (n = 10) and young (n = 10) persons. Changes in fractional synthesis rate were analyzed with the use of repeated-measures ANOVA with 2 between-subject factors (age and sex) and 1 repeated-measures factor (time). *Significant main effect for time after beef ingestion (P < 0.001).

Examination of interactions showed no time x age (P = 0.8), time x sex (P = 0.8), or time x age x sex (P = 0.6) differences. Values were 0.070 ± 0.003%/h (before the meal) and 0.106 ± 0.005%/h (after the meal) in the women and 0.076 ± 0.005%/h (before the meal) and 0.115 ± 0.005%%/h (after the meal) in the men.

Plasma amino acid concentrations
Premeal plasma concentrations of EAAs did not differ significantly between the 2 age groups (Figure 3). After beef ingestion, total plasma concentrations of EAAs increased in both age groups, peaking at 100 min after the meal. Both the rate of increase and the peak concentration were greater in the elderly than in the young. A similar pattern was evident for the branched-chain AAs (data not shown) and leucine (Figure 3).

View larger version (14K): [in this window] [in a new window]   FIGURE 3.. Mean (±SEM) total plasma concentrations of essential amino acids (EEAs) and leucine before and after ingestion of 113 g of 90% lean beef in elderly (n = 10) and young (n = 10) persons. Changes in amino acid concentration were analyzed with the use of repeated-measures ANOVA with one between-subject factor (age) and one repeated-measures factor (time). *Significant increase in concentration after beef ingestion in both age groups; significant difference between groups in concentration after beef ingestion. Time x age interaction, P = 0.012.

Figure 4

View larger version (9K): [in this window] [in a new window]   FIGURE 4.. Mean (±SEM) plasma insulin concentrations before and after ingestion of 113 g of 90% lean beef in elderly (n = 10) and young (n = 10) persons. Changes were analyzed with the use of repeated-measures ANOVA with one between-subject factor (age) and one repeated-measures factor (time). *Significant increase from the premeal values in both age groups (P < 0.005). Time x age interaction, P = 0.7.

	DISCUSSION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

The ability of a common, protein-rich food item, such as lean beef, to stimulate muscle protein synthesis is good news for elderly persons. Previous studies have found a blunted responsiveness to the anabolic effect of small doses of AAs (19, 29) but a normal response to a larger dose (29). The current data suggest that a normal serving of beef provides enough AAs to overcome any deficiency in responsiveness.

Ingestion of lean beef significantly increased all plasma concentrations of EAAs (Figure 3), including leucine (Table 2), which has been targeted as a key factor in translation initiation and the regulation of muscle protein synthesis (30, 31). However, compared with the peak plasma concentrations of the more rapidly digested and absorbed free AAs or whey protein supplements (27, 28, 32), peak plasma concentrations of AAs in both age groups occurred more slowly (eg, 100 min) after beef ingestion. This was a consistent, albeit somewhat faster time than the 120-150 min reported by Uhe et al (33) after a 230-g lean beef meal.

The most pronounced difference between age groups in this study was the significantly (P < 0.001) greater increase in plasma concentrations of AAs after beef ingestion in the elderly than in the young. This finding is counterintuitive if considered in the context of a potentially slower rate of gastric emptying (17) and greater first-pass splanchnic uptake (16) in the elderly. However, the difference may be explained, in part, by the smaller body size (lean and total mass) of the elderly than of the young subjects (Table 1). Specifically, it makes sense that a smaller person would experience a greater increase in plasma EAA concentration than would a larger person after ingestion of the same absolute amount of protein. A corollary of this result was that the greater relative precursor availability in the elderly subjects did not translate to a greater increase in muscle protein synthesis (18). As a function of the smaller LMM in the elderly subjects, the relative protein intake for the elderly was 27% greater than that for the young (0.65 ± 0.04 compared with 0.51 ± 0.04 g protein/LMM; P = 0.031). This suggests either 1) that a 113-g serving of lean beef (10 g EAAs) provides a maximal anabolic stimulus, or 2) that a greater increase in precursor availability (ie, a proportionally greater EAA intake) is required to counteract a lower efficiency of protein synthesis in the elderly.

Although we have no direct evidence to address this first suggestion, we previously showed an increase of >60% in FSR (51% from beef) in the young and the elderly after ingestion of 15 g free-form EAAs (15, 16). Furthermore, we established that the elderly have less of a reduced anabolic response to 7 g free-form EAAs than do the young (29) Thus, it is unlikely that a 113-g serving of beef (10 g EAAs) elicits a maximal anabolic response. In contrast, the findings of the present study are consistent with the suggestion that the decreased ability of the elderly to incorporate intracellular EAAs into protein (20) is due to their less-efficient anabolic response after ingestion of a protein-rich food. As stated, these data are consistent with the assertion that the elderly experience a deficit in the efficiency of protein synthesis compared with the young. However, we also suggest that a slightly greater relative protein intake (g/kg LMM) and concomitantly greater relative increase in precursor availability may be sufficient to overcome any such deficit, which would result in an absence of age-related difference in mixed-muscle FSR. In practical terms, this possibility means that, if a person continues to ingest a similar absolute quantity of high-quality protein as he or she ages, there should be no overt impairment in the person's protein-synthetic response.

Promoting muscle anabolism by using an actual protein-rich food has several distinguishing advantages over protein or AA supplementation. Many protein-rich foods, such as beef, poultry, fish, or dairy products, are readily accessible, relatively inexpensive, and palatable, whereas EAAs frequently are not. Furthermore, supplements often act as a meal replacement, especially in the elderly, which results in no additional protein, caloric, or essential nutrient intake (14). Targeted AA supplementation may indeed be necessary in the case of advanced sarcopenia, cachexia, trauma, or another condition that results in accelerated protein catabolism. However, for most of the population, including older adults, the most practical means of promoting or increasing skeletal muscle protein anabolism is to include a protein of high biological value during each meal. Our data suggest that lean beef is an effective and practical source of dietary protein for both the elderly and the young. In conclusion, despite potential differences in the efficiency of utilization, aging does not impair the ability to acutely synthesize muscle protein after ingestion of a common protein-rich food.

	ACKNOWLEDGMENTS

 
We thank Asle Aarsland, Melissa Bailey, Stephanie Blasé, Dan Creson, Christopher Danesi, Arny Ferrando, Guy Jones, Ola Ronsen, and Melinda Sheffield-Moore for their invaluable assistance in data collection and analysis.

The author's responsibilities were as follows—DPJ, DLC, and RRW: the original experimental design; TBS, SES, DLC, and TLC: data acquisition and data analysis; TBS: performed statistical analysis and drafted the manuscript under the supervision of DPJ, DLC, and RRW; all authors: interpretation of the results. DPJ and RRW have received compensation for speaking engagements and consultation with the National Cattlemen's Beef Association.. The other author had no personal or financial conflict of interest.

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TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 

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