Sports Med DOI 10.1007/s40279-014-0269-4

SYSTEMATIC REVIEW

Effects of Protein Supplementation in Older Adults Undergoing Resistance Training: A Systematic Review and Meta-Analysis De´bora Finger • Fernanda Reistenbach Goltz • Daniel Umpierre • Elisabeth Meyer • Luis Henrique Telles Rosa • Cla´udia Dornelles Schneider

Ó Springer International Publishing Switzerland 2014

Electronic supplementary material The online version of this article (doi:10.1007/s40279-014-0269-4) contains supplementary material, which is available to authorized users.

ized controlled trials—RCTs), sample mean age (60 years and over) and intervention (a resistance training program for a period of 6 weeks or longer combined with protein or amino acids supplementation). Two independent reviewers performed the study selection and data extraction. Continuous data on fat-free mass, muscle mass and muscle strength were pooled using a random-effects model. Results Of the 540 articles reviewed, 29 eligible articles underwent full-text evaluation. Nine RCTs (462 subjects) met the inclusion criteria and were included in the study. The mean age of the participants ranged from 61 to 79 years old. Protein supplementation protocols varied widely throughout the studies. Three studies used quantities related to the body mass of the participants and the other six trials provided supplements in daily amounts, independently of subjects’ body masses. Overall, protein supplementation in combination with resistance training was associated with gains in fat-free mass, resulting in a standardized mean difference (SMD) of 0.23 [95 % confidence interval (CI), 0.05–0.42]. However, protein supplementation was not associated with changes in muscle

D. Finger Universidade Federal de Cieˆncias da Sau´de de Porto Alegre, Rua Sarmento Leite, 245, Porto Alegre, Rio Grande do Sul, CEP 90050-170, Brazil

E. Meyer Instituto de Cardiologia do Rio Grande do Sul/Fundac¸a˜o Universita´ria de Cardiologia, Av. Princesa Isabel, 370, Porto Alegre, Rio Grande do Sul, CEP 99620-000, Brazil

F. R. Goltz
L. H. T. Rosa Postgraduate Program of Rehabilitation Sciences, Universidade Federal de Cieˆncias da Sau´de de Porto Alegre, Rua Sarmento Leite, 245, Porto Alegre, Rio Grande do Sul, CEP 90050-170, Brazil

C. D. Schneider (&) Department of Nutrition, Universidade Federal de Cieˆncias da Sau´de de Porto Alegre, Rua Sarmento Leite, 245, sala 611, Porto Alegre, Rio Grande do Sul, CEP 90050-170, Brazil e-mail: [email protected]; [email protected]

Abstract Background Older individuals present reductions in muscle mass and physical function, as well as a blunted muscle protein synthesis response to amino acid administration and physical activity. Although resistance training is an effective intervention to slow down muscle impairments in the elderly, there is no consensus whether a combination with protein supplementation could offer additional benefits to an older population. Objective We aimed to systematically summarize and quantify whether protein supplementation could optimize the effects of resistance training on muscle mass and strength in an aged population. Design A structured literature search was conducted on MEDLINE (PubMed), Cochrane, EMBASE and LILACS databases. The search had no period or language restrictions. Inclusion criteria comprised study design (random-

D. Umpierre Exercise Pathophysiology Research Laboratory, Postgraduate Program in Cardiovascular Sciences, Universidade Federal do Rio Grande do Sul, Rua Ramiro Barcelos, 2350, Porto Alegre, Rio Grande do Sul, CEP 90035-903, Brazil

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mass (0.14, 95 % CI -0.05 to 0.32) or muscle strength (0.13, 95 % CI -0.06 to 0.32). Limitations Studies among the very elderly population are scarce. The variation regarding the supplementation protocol, namely the different protein sources, amounts and timing of ingestion, also made it harder to compare the results. The general quality of the studies was low, reflecting increased risk of bias in some studies. Despite these limitations, this systematic review provides a general overview of the role of protein supplementation with no other added macronutrients to augment muscle mass and strength during resistance training in older adults. Conclusion Combining protein supplementation with resistance training is effective for eliciting gains in fat-free mass among older adults, but does not seem to increase muscle mass or strength.

Key Points Protein supplementation in older people is associated with increases in fat-free mass when compared with control groups undergoing resistance training only. Protein supplementation in older people is not associated with increases in muscle mass and muscle strength in comparison with the control groups undergoing resistance training only.

1 Introduction The aging process leads to a number of physiological changes that are accompanied by gradual declines in physical performance and cognitive function [1, 2]. These changes are often associated with a progressive loss of skeletal muscle mass and strength and a concomitant decrease in fat-free mass (FFM), which is accelerated after the age of 60 years and leads to frailty, disability and functional impairment. Such a process, known as sarcopenia, results in a reduction of the abilities to perform activities of daily living, increasing dependence on others [3, 4]. Various interventions have been proposed on how to effectively improve the rate of protein synthesis and slow the rate of protein degradation in order to elicit skeletal muscle hypertrophy and improve strength in the elderly [1]. The effects of resistance training programs on muscle mass and strength in older men have been well established throughout the past 2 decades [5]. However, the role of protein supplementation in concert with resistance exercise in the elderly remains inconclusive. The majority of studies

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in humans suggest that protein or essential amino acids ingestion associated with resistance training can enhance skeletal muscle hypertrophy [4–8]. Nonetheless, whereas some studies report greater gains in muscle fiber size, FFM and/or muscle strength with the combination of dietary protein supplementation and resistance exercise, others failed to confirm the proposed surplus benefits [4]. Especially in older populations, inadequate food intake and protein-poor diets seem to contribute to sarcopenia and overall reduction in FFM. The recommended dietary allowance (RDA) of a nutrient should provide the amount sufficient for most subjects; however, the RDA for protein of 0.8 g/kg/day is clearly insufficient in aged individuals for whom a protein allowance of 1.25–1.5 g/kg/day is more advisable [7, 8]. Moreover, elderly people may not eat sufficient protein due to reduced appetite or the occurrence of chronic diseases (e.g., diabetes, low-grade inflammation) which further increase their dietary protein requirements [9]. Recent work seems to suggest that the elderly show a blunted muscle protein synthesis response to amino acid administration and physical activity when compared with young subjects. Thus, a higher level of protein intake is likely required in elderly individuals to elicit significant effects on muscle protein synthesis [8]. Although sensitivity analyses [4] have indicated that protein supplementation may augment the adaptive response of skeletal muscle to resistance training in older adults, well designed randomized controlled trials (RCTs) also present conflicting results [10, 11]. Therefore, there is still uncertainty as to whether protein supplementation should be considered an effective aid in combination with resistance training in older individuals. To our knowledge, the previous meta-analysis available in the literature did not focus on the effect of the supplementation exclusively composed of protein or amino acids during resistance training, i.e., without the presence of other macronutrients. Accordingly, this study consists of a systematic review with meta-analysis of RCTs aiming to assess the association between protein supplementation and resistance training in variables related to muscle mass and muscle strength in the aged population.

2 Methods 2.1 Search Strategy and Study Selection A search for articles up to and including January 2014 was carried out using the following electronic databases: MEDLINE (accessed via PubMed), Cochrane Central Register of Controlled Trials, EMBASE and LILACS. The search had no period nor language restriction.

Protein Supplementation in Older Adults Undergoing Resistance Training

The initial search comprised terms such as ‘‘elderly,’’ ‘‘protein,’’ and ‘‘resistance training,’’ and related entry terms associated with a high-sensitivity strategy search. Most of the eligible studies were found on PubMed database. The complete and extensive search strategy used for the PubMed database is shown in Electronic Supplementary Material Appendix S1. The titles and abstracts of retrieved articles were individually appraised by two reviewers (D.F. and F.R.G.) to assess whether they were eligible for the present study. Reviewers were not blinded to authors, institutions or manuscript journals. Abstracts that did not provide enough information regarding the inclusion and exclusion criteria were retrieved for full-text evaluation. Corresponding authors of potentially eligible articles were contacted if there were missing data. After reviewing the studies and applying the inclusion criteria independently, the reviewers held a consensus meeting to compare their results and to decide which articles should be included in the study. Disagreements were solved by consensus or, if necessary, by a third reviewer (C.D.S.). This systematic review and meta-analysis is reported in accordance with the recommendations and criteria outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement [12]. 2.2 Eligibility Criteria We included only RCTs that lasted at least 6 weeks and had one subject group receiving protein supplementation or modified diet with increased protein content and being compared with a group without changes in protein intake. The control groups could either be submitted to resistance training alone (without supplementation) or to exercise combined with non-protein placebo supplementation. Furthermore, eligible trials were required to have subjects with a mean age C60 years. The 2007 American College of Sports Medicine (ACSM)/American Heart Association (AHA) joint recommendations for physical activity in older adults suggest that ‘‘old age’’ usually applies to individuals aged C65 years. However, for some individuals, sarcopenia may occur prior to this age. On the basis of this fact and in order to find a greater number of studies, we decided to accept studies with a participant mean age of C60 years [13]. Papers were reviewed if they met the following criteria: (1) a control group existed; (2) supplementation was composed of protein or amino acids; (3) subjects were involved in a resistance training program; and (4) muscle mass, FFM, or muscle strength were evaluated. Exclusion criteria encompassed any study in which (1) the intervention aimed to treat a specific disease or medical condition; (2) protein supplementation was provided in

combination with other supplements known to stimulate muscle hypertrophy (e.g., creatine); (3) supplementation contained other added macronutrients than protein (carbohydrate or lipid); (4) no information regarding the subjects’ mean age was provided (5) studies were duplicate publications or sub-studies of included trials. 2.3 Data Extraction The two reviewers separately and independently evaluated full-text articles and conducted data extraction. Pertinent information regarding the details of population characteristics (number, age, sex, body composition), dropout and compliance to treatment percentage, study methods/design, exercise training (duration, frequency, intensity, type, repetitions per sessions), protein supplementation (amount, type, timing) and outcomes was collected using a standardized predefined form. When provided, details on the number of patients excluded and their compliance with treatment were also recorded. Resistance exercise intensities were computed as percentages of the 1-repetition maximum (1-RM). Within the body composition term, muscle mass and FFM were considered. The latter term was sometimes referred to as lean body mass, depending on how it was described in the original article. Therefore, both terms are used in this review, although a considerable difference exists between their literal meanings. Shortly after extraction, the authors crosschecked the data to confirm their accuracy. Any discrepancies were discussed in order to find a consensus decision. 2.4 Assessment of Risk of Bias Risk of bias was evaluated according to the PRISMA recommendation [12]. Study quality assessment included adequate sequence generation for subjects’ randomization, allocation concealment, blinding of outcomes assessors and participants, use of intention-to-treat analysis, and description of losses and exclusions. Studies without clear descriptions of an adequate sequence generation or how the allocation list was concealed were considered not to have fulfilled these criteria. Two unblinded reviewers (D.F. and F.R.G.) independently performed quality assessments, and disagreements were solved by consensus or by a third reviewer (C.D.S.). The complete assessment of risk of bias is shown in Electronic Supplementary Material Appendix S2. 2.5 Statistical Analysis Absolute changes in muscle mass, FFM and muscle strength were extracted as differences between arithmetic

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means before and after interventions. In each study, the effect size for the intervention was calculated by the difference between the means of the post-test and pretest at the end of the resistance training program. The metaanalysis effect size of studies which reported muscle mass is expressed in standardized mean difference (SMD), since distinct units (cm, cm2, and kg) were used in the analysis. In the meta-analysis of FFM and muscle strength, pooled effect sizes are also reported as the SMD between groups. Data from intention-to-treat analyses were entered whenever available in the included RCTs. Heterogeneity refers to the existence of variation between studies for each main effect being evaluated. Statistical heterogeneity of the treatment effect among studies was assessed using Cochran Q test, a threshold p value of 0.1 was considered statistically significant, and for the inconsistency I2 test, values greater than 75 % were considered indicative of high heterogeneity. This procedure quantifies the proportion of variability in the results that is due to a function of heterogeneity, rather than by chance. With this method, I2 ranges from 0 to 100 %, where 0 % reflects low heterogeneity and 100 % indicates substantial heterogeneity [4]. The analyses of pooled data were conducted with a random-effects model to account for measurement variability among the included studies. For each outcome, a forest plot was generated to illustrate the study-specific effect sizes and their respective 95 % confidence intervals (CIs). All analyses were conducted using Stata software, version 11.0 (Stata Inc., College Station, Texas, USA).

A flow diagram of search and selection is shown in Fig. 1. The diagram follows the recommendations of the PRISMA statement [12]. The characteristics of all the studies included [10, 11, 14–20] are summarized in Table 1 and described below. Trials were published from 1995 to 2013, and the majority were small studies with sample sizes ranging from twelve, in the smallest study, to 87 subjects in the largest study. The mean age of the 462 participants ranged from 61.2 to 79.2 years old. Dropout rates were less than 28 %, with an average value of 14 %, and no major adverse effects were reported. Among the nine studies, one [18] included only sarcopenic individuals. Another study was limited to pre-frail and frail older people [20], and another one was focused on mobility-limited older adults [16]. Regarding the sex of the participants, five studies included men and women, three studies evaluated only men, and one study had its sample composed exclusively of women. Three of the included studies were carried out in the USA, one in Canada, one in Japan, one in Australia and three in the Netherlands. Five studies reported data on adherence to the exercise and supplementation protocol. The overall compliance ranged from 63 to 98 %. 3.2 Quality (Risk of Bias) and Publication Bias Assessment Among the included studies, 33.3 % presented adequate sequence generation (three of nine), 33.3 % reported allocation concealment (three of nine), 77.7 % had blinded assessment of outcomes (seven of nine) and 55.5 % blinded assessment of participants (five of nine), 100 %

3 Results 3.1 Description of Studies The search strategy identified a total of 540 articles. Of these, 511 clearly did not fulfill the eligibility criteria and were excluded on the basis of titles and abstracts. Full-text copies of 29 studies were obtained for further examination. Of these, nine RCTs met the inclusion criteria and provided data on 462 subjects. If studies were based on the same research subjects and contained similar information, the most recent publication was included in the review. Eleven studies were excluded because they were duplicate publications. One study did not have a control group. In the other four articles, the mean age of subjects was below 60 years and one study did not specify the mean age. Two others had the addition of some kind of carbohydrate in the supplement and were excluded as well. Lastly, one was a sub-study of an article previously included in the analysis [14].

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Fig. 1 Flow chart diagram of the study selection

C: 72 (13)

I: 72 (13)

C: 79 (31)

I: 78 (31)

C-M: 70 (14)

C-W: 69 (12)

I-M: 70 (15)

I-W: 72 (12)

C: 60.7 (44)

I: 61.7 (43)

M

M/W

M/W

M

Exercise number

Training volume

3 d/w 9 12 w

2 d/w 9 24 w

3 d/w 9 24 w

3 d/w 9 72 w

2 d/w 9 12 w

3 d/w 9 12 w

3 d/w 9 24 w

3 d/w 9 12 w

3 d/w 9 12 w

L

U?L

U?L

U?L

U?L

U?L

U?L

U?L

U?L

80

75

80

85

NA

80

80

70

80

2

6

6

4 sets 9 8–15 reps

3(U)–4(L) sets 9 8–15 reps

3(U)–4(L) sets 9 8–15 reps

2–3 sets 9 8–20 reps

1 set 9 up to 8 reps

6a 6b

3 sets 9 8 reps or fatigue

2–3 sets 9 10–12 reps

3 sets 9 10 reps

3 sets 9 8–12 reps or fatigue

8

5

9

4

Casein hydrolysate

Milk protein concentrate

Milk protein concentrate

Fortified milk

Essential amino acid

Egg, milk, dairy (through diet)

Whey protein

Whey protein and egg albumin

Milk-based beverages

20 g daily

30 g daily

15 g daily

13.2 g daily

6 g daily

0.3 g/kg

40 g daily

0.3 g/kg

0.8 g/kg

Extra protein amount

B?A

NRE

NRE

NRE

NRE

NRE

A

B?A

NRE

Timing

Type of protein

RT intensity %1-RM

Exercise frequency and study length

RT type

Protein and placebo supplementation details

Training details

TD

ED

ED

ED

ED

ED

ED

TD

ED

Suppl day

Flavored water

Flavored beverage with carbohydrate

Beverage with only lactose and calcium

Exercise only

Exercise only

Low-protein diet

Maltodextrin

Flavored beverage with carbohydrate

Low-protein diet

Placebo

b

a

Refers to the main exercise program; however, there were additional exercises that were alternated throughout the study

Refers to the number of exercises comprised in a seated-chair training program; however, there were other training variations (i.e., balance and gait training)

A after exercise, B ? A before and after exercise, C control subjects, d day(s), ED every day, I intervention subjects, I1/2 intervention subjects groups 1 and 2, L lower-body exercises, M men, NA not available, NRE not related to exercise, reps repetitions, RM repetition maximum, RT resistance training, Suppl supplementation, TD training days, U upper-body exercises, w week(s), W women

Verdijk et al. [11]

Tieland et al. [20]

Leenders et al. [10]

Kukuljan et al. [19]

W

Kim et al. [18]

C: 79 (36)

M/W

I: 61 (18)

M/W

M

C: 62 (18) I: 79.5 (34)

C: 77.3 (38)

I: 78 (42)

C: 64.6 (10)

I2: 66.5 (10)

I1: 63.3 (9)

M/W

Sex

Iglay et al. [17]

Chale´ et al. [16]

Candow et al. [15]

I: 64 (6)

Campbell et al. [14]

C: 66 (6)

Mean age in years (no. of subjects)

References

Table 1 Details of the included studies

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described losses to follow-up and exclusions (nine of nine), and 22.2 % used the intention-to-treat principle for statistical analyses (two of nine). 3.3 Resistance Training Characteristics As shown in Table 1, trials lasted between 12 and 72 weeks (mean duration = 22 weeks) and the mean exercise frequency was 3 sessions per week. Mean exercise intensity was 78 % of 1-RM (minimum–maximum 70–85 %). The number of total sets per session ranged from 8 to 27, and repetitions in each individual set varied from 8 to 20. The number of different exercises prescribed to the participants ranged from two to nine. Eight of the studies included lower- and upper-body exercises [10, 14– 20], and only one study [11] limited the training protocol to lower-body exercises. 3.4 Protein Supplementation Characteristics The protocol for protein supplementation varied widely across studies. Regarding the amount of protein, three studies [14, 15, 17] used quantities related to the body mass of the participants, with amounts of extra protein ranging from 0.3 to 0.8 g/kg/day (mean = 0.46 g/kg/day). The other six trials [10, 11, 16, 18–20] provided supplements in daily amounts, independent of subjects’ body masses, and the mean amount of protein supplementation provided was 20.7 g/day (ranging from 6 to 40 g/day). This included supplements and extra protein provided in habitual diet. The protein source of the supplementation was different in each study, consisting of milk-based beverages, a combination of whey protein and egg albumin, essential amino acids, fortified milk, milk protein concentrate, casein hydrolysate, and egg, milk and dairy through diet. Seven of the studies [12, 14, 16–20] provided supplementation every day throughout the study, and in other two studies [11, 15], extra protein was provided only on training days. Regarding the control groups, in two trials [18, 19], the control group was submitted to exercise only without placebo. In the other two studies [14, 17], the control group received a low-protein diet. Four studies [10, 15, 16, 20] used flavored beverages with carbohydrate as placebo, and one study [11] provided flavored water as a placebo. 3.5 Association of Protein Supplementation with Muscle Mass, FFM and Muscle Strength Meta-analyses of studies were performed in order to generate effect sizes for each variable: muscle mass, FFM, and muscle strength. Each outcome measure was independently assessed, and is presented sequentially. As shown in Fig. 2,

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protein supplementation was associated with increases in FFM (SMD = 0.23; 95 % CI 0.0–0.42, I2 = 0.0 %, p value for heterogeneity = 0.997), when compared with the control groups undergoing resistance training only. However, the intervention was not associated with changes in muscle mass (SMD = 0.14; 95 % CI -0.05 to 0.32, I2 = 0.0 %, p value for heterogeneity = 0.780) and muscle strength (SMD = 0.13; 95 % CI -0.06 to 0.32, I2 = 0.0 %, p value for heterogeneity = 0.810) in comparison with the control groups (Figs. 3, 4).

4 Discussion 4.1 Summary of Findings The present systematic review provides evidence from nine RCTs involving 462 individuals and represents an updated synthesis of interventions consisting of protein supplementation combined with resistance training in the elderly. Overall, there was no association between protein supplementation and changes in muscle strength and muscle mass in older subjects undergoing resistance training programs lasting an average of 22 weeks. However, we did observe an association of this combined intervention with increased FFM in the elderly. The age-related loss of muscle protein may be attributed to an imbalance between muscle protein synthesis and breakdown rates, resulting in a negative muscle protein balance and, over time, a decline in skeletal muscle mass [21]. Therefore, nutritional interventions are important strategies to blunt muscle loss. In this context, a systematic review [5] with 17 studies and 1,287 patients from 65 to 85 years of age has shown that dietary supplementations were effective in the treatment of sarcopenia in older individuals. Importantly, some of the untreated groups analyzed by Malafarina et al. [5] actually reduced muscle mass during the follow-ups, which suggests that dietary adjustments could ultimately prevent or reduce the extent of sarcopenia in the elderly. Since resistance exercise and consumption of protein or amino acids are both stimulators of muscle protein synthesis, it is important to assess whether dietary and exercise interventions would lead to additive effects on structure and function of skeletal muscles. To our knowledge, only one previous meta-analysis has addressed the efficacy of protein supplementation combined with resistance training in the elderly [4]. In a subanalysis of older subgroups, Cermak et al. [4] found that protein supplementation combined with resistance training was associated with increased FFM and 1-RM leg press strength, but not with changes in muscles mass, when

Protein Supplementation in Older Adults Undergoing Resistance Training

Fig. 2 Forest plot of the changes in fat-free mass of subjects in the individual studies included in the meta-analysis of resistance training combined with protein supplementation vs. control group. Results are shown as pooled mean differences with 95 % CIs (SMD = 0.23; 95 % CI 0.05–0.42, I2 = 0.0 %, p value for heterogeneity = 0.997).

For each study, the horizontal lines join the lower and upper limits of the 95 % CI of this effect. The area of the shaded squares reflects the relative weight of the study in the meta-analysis. The diamond represents the pooled mean difference. CI confidence interval, SMD standardized mean difference

compared with groups performing resistance training with placebo supplementation. Although our results on FFM and muscle mass are in agreement with those found by Cermak et al. [4], there are important methodological differences that may partially explain distinct results in muscle strength. First, since our data are expressed as SMD, this statistic metric might be less sensitive with regard to detecting changes in means in proportion of the respective standard deviations in each individual study. Second, the type of exercise assessment was not the same in trials analyzed by both reviews, as we used knee extension muscle strength in the analysis with ten observations, while Cermak et al. [4] used leg press muscle strength from three observations. Third, the number of older groups analyzed is quite different. The previous meta-analysis consisted of six trials, but we updated the analyses to include ten observations (derived from nine RCTs). Fourth, we aimed to analyze the potential effects of exclusive protein or amino acids supplementation without the presence of other macronutrients, such as carbohydrates [21]. Finally, the age cutoff of 60 years old in our study may reflect the findings in individuals at more advanced ages.

It has been well accepted that muscle hypertrophy is achieved from cumulative periods of positive protein balance, which occurs following resistance exercise when protein is consumed [7]. In this context, an important nutritionally regulated signaling component affecting messenger ribonucleic acid (mRNA) translation is the mammalian target of rapamycin complex 1 (mTORC1), which integrates anabolic signals for muscle protein synthesis in human skeletal muscle. In addition, mTORC1 is also the central component of the insulin-signaling cascade that regulates protein synthesis and mRNA translation. Human studies have demonstrated that leucine consumption during exercise stimulates the mTORC1 pathway, increases muscle protein synthesis, and decreases whole-body proteolysis [21, 22]. Interestingly, Cuthbertson et al. [23] have demonstrated that protein concentrations of mTORC1, and its downstream target p70S6K, differ between healthy young and older individuals, which may contribute to the reduced capacity of the muscle protein synthetic machinery to ‘‘sense’’ a nutrient signal in senescent muscle [23]. On the other hand, some mechanisms possibly responsible for an anabolic resistance to protein and/or amino acid

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Fig. 3 Forest plot of the changes in muscle mass of subjects in the individual studies included in the meta-analysis of resistance training combined with protein supplementation vs. control group. Results are shown as pooled SMDs with 95 % CIs (SMD = 0.14; 95 % CI -0.05 to 0.32, I2 = 0.0 %, p value for heterogeneity = 0.780). For each

study, the horizontal lines join the lower and upper limits of the 95 % CI of this effect. The area of the shaded squares reflects the relative weight of the study in the meta-analysis. The diamond represents the pooled SMD. CI confidence interval, SMD standardized mean difference

administration in the elderly remain to be elucidated. It is unclear whether the blunted muscle protein synthetic response to food intake is accompanied by an attenuated postprandial decline in muscle protein breakdown in the elderly [24]. In addition, a higher level of protein intake is likely required in the elderly in order to overcome a low responsiveness to protein intake and to elicit a muscle protein synthesis level comparable to that of younger individuals [9]. The main issue might be the still imprecise protein needs of older people who perform resistance training [7]. In a recent randomized trial, D’Souza et al. [25] observed increased p70S6K phosphorylation in a dose-dependent manner with ingestion of up to 40 g of whey protein, showing that, in contrast to young men, older subjects may require more than the 20 g of highquality protein in order to maximize the muscle anabolic response during the postexercise recovery [25]. While dietary data are limited, there is reasonable evidence that protein intake at rates higher than 0.8 g/kg/day is beneficial for the majority of elderly individuals. It is, therefore, safe to conclude that the optimal protein intake is greater than the RDA and, thus, 0.8 g/kg/day should be considered as the minimum intake for all elderly [9].

It is interesting to note that evidence is still limited on the choice of the best source of protein to be supplemented. Some of the studies suggest that protein sources containing a relatively high proportion of essential amino acids, such as proteins found in meat and dairy products, would be more effective than vegetable proteins [7, 9]. Nonetheless, in a study by Haub et al. [6], 21 men were counseled to self-select a basal meat-free diet along with 0.6 g/kg/day from either texturized vegetable protein (soy) or beef. The soy and beef groups experienced comparable increases in maximal strength among the muscle groups exercised. This led the authors to conclude that when older men consume adequate amounts of total protein, resistance traininginduced muscle hypertrophy is not influenced by whether the predominant protein source is from soy or beef sources. Campbell and Leidy [7] have suggested that protein quantity is a more substantial determinant of body composition and muscle hypertrophy responses to resistance training in older people than the predominant source of protein. In the present review, when trials were examined individually, only two studies reported a significant benefit of protein supplementation on the gain of muscle mass

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Protein Supplementation in Older Adults Undergoing Resistance Training

Fig. 4 Forest plot of the changes in muscle strength of subjects in the individual studies included in the meta-analysis of resistance training combined with protein supplementation vs. control group. Results are shown as pooled mean differences with 95 % CIs (SMD = 0.13; p value for 95 % CI -0.06 to 0.32, I2 = 0.0 %,

heterogeneity = 0.810). For each study, the horizontal lines join the lower and upper limits of the 95 % CI of this effect. The area of the shaded squares reflects the relative weight of the study in the metaanalysis. The diamond represents the pooled SMD. CI confidence interval, SMD standardized mean difference

compared with placebo. Kim et al. [18] demonstrated a significant increase of leg muscle mass in the group treated with exercise and supplementation compared with the group without treatment (p = 0.007). Another recent trial, Tieland et al. [20], also showed positive effects for the cointervention of exercise plus supplementation. Regardless, once the data were pooled, it became clear that protein supplementation is associated with increases in FFM when compared with the control groups undergoing resistance training only. Although the increases in FFM were not reflected by increases in muscle mass, we point out that longer periods of intervention as well as higher doses of protein supplementation appropriate to older individuals’ needs could indicate how the benefits in FFM may be further extended to muscle mass and strength. Additionally, reduction in fat mass and a consequent increase in body percentage of FFM represent a marked advantage promoted by the combined intervention for older populations. There are other important points that should be considered in order to maximize the gains in muscle mass and strength. First of all, the characteristics and details of the training sessions, such as training volume, total work and intensity, as well as the period between sessions, should be carefully planned and controlled. These training

characteristics were poorly investigated and should receive more attention, since they will definitely influence the outcomes of each study. Second, not only is the adequacy of protein intake necessary, but it is also extremely important to provide enough energy supply for building muscle mass. Thus, the supplement strategy should take this into consideration. Third, regarding the timing of the protein ingestion, there is a physiological basis for why the consumption of protein close to the performance of resistance exercise would have a stimulatory effect on muscle protein synthesis and lean mass building. Increasing the amino acid concentration in the blood in conjunction with performance of resistance exercise would ‘‘take advantage’’ of mechanisms at the cellular level that are activated following resistance exercise [26]. Unfortunately, only two of the seven studies included in our meta-analysis considered the timing of protein ingestion related to the exercise session. 4.2 Limitations This review, as well as other similar ones, is limited by the relative lack of data specific to very old adults. Most of the controlled trials focus on older adults with an average age

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of 55–70 years, while studies among the very elderly population are scarce. Subjects at a more advanced age and, more specifically, frail elderly could demonstrate an even greater effect of protein supplementation on FFM during a period of resistance exercise training, since these subpopulations generally consume insufficient amounts of dietary protein [4]. Most of the studies reviewed in our analysis refer to sarcopenia and frailty in their introduction and discussion; however, only one trial included sarcopenic patients [18], and another one was limited to pre-frail and frail older people [20]. Also, in our systematic review of published literature, we extracted information that was sometimes dynamic throughout the studies, such as progressive exercise durations and/or intensities, and despite trying for maximum uniformity in the characteristics of the papers included in this meta-analysis, it was very difficult to compare data. The variation regarding the supplementation protocol, namely the different protein sources, amounts and timing of ingestion, also made it harder to compare the results. None of the studies, for instance, specified the formula used to calculate muscle mass. The relatively small number of papers falling within our inclusion criteria was another main limitation of this review, since various studies were omitted in an attempt to make the study treatment groups as homogenous as possible. Finally, the general quality of the studies was low, reflecting increased risk of bias in some studies. This may have contributed to the heterogeneity of our analyses. Despite these limitations, this systematic review with meta-analysis provides a general overview of the research literature addressing the role of protein supplementation with no other added macronutrients to augment muscle mass and strength during resistance exercise training in older adults.

5 Conclusion This systematic review provides evidence that combining protein supplementation with resistance training is effective for eliciting gains in FFM in older adults; however, it does not seem to increase muscle mass or strength gains. Even in the absence of a parallel gain in strength, the increase in lean mass is a great advantage in elderly individuals. Evidence for the predicted value of the supplementation with protein or amino acids, or even a modified diet with increased protein content, is still scarce in older people. Although the overall consensus is that resistance training and protein supplementation could have additive effects to prevent or offset sarcopenia, there is a need for future studies with larger samples, as well as testing of

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types of protein sources, timing for intakes, and daily doses for the supplementation. Acknowledgments This study was partially funded by Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio Grande do Sul (FAPERGS Brazil), protocol number 1760-2551/12-3, and Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (PNPD 2818/2011, CAPES-Brazil). Sponsors had no role in the study design; collection, analysis and interpretation of data; the writing of the report; and the decision to submit the paper for publication. Authors had full access to the analyzed data in the study and take responsibility for the integrity of the data and the accuracy of data analysis. No conflicts of interest are declared by the authors. The manuscript does not contain clinical studies or patient data.

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