The relation between protein
intake and muscle strength in frail
elderly: an observational
cross-sectional study
Melanie Hazeleger and Sabine Bezemer
Student numbers: 500731700 and
500751640
Voeding en Diëtetiek
Hogeschool van Amsterdam
Project number: 2020205
School counselor: Michael Tieland
Examiner: Marielle Engberink
2
Abstract
Background: Frailty is considered highly prevalent in older age. Frailty causes a high risk of
falls, disability, hospitalization, and mortality. This eventually might be the turning-point for older patients to shift from independent living to assisted living. Poor nutritional status might be an important reversible risk factor for frailty. Given nutritional status as risk factor and weakness in frailty, there might be a relation between protein intake and muscle strength. Data of the relation between protein intake and muscle strength among frail patients is scarce and show conflicting results. We aimed to assess the following research question: What is the relation between protein intake and muscle strength in frail elderly?
Method: Primary measures were protein intake and muscle strength (handgrip strength and leg
press). A correlation test was used to determine the correlation between protein intake and muscle strength. A multiple regression analysis was conducted to determine whether the relation between muscle strength and protein intake differ between men and women. The confounding factors were age, gender, lean body mass (DXA), energy intake and physical activity (counts/min).
Results: A total of 62 subjects were included in this study (Mean ± SD; Age: 78 ± 7.6 years;
hand grip strength: 26.7 ± 9.6 kg; leg press strength: 120.2 ± 32.1 kg; protein intake: 77 ± 22.1 g). The correlation analysis showed that there was a significant, weak positive correlation between protein intake and hand grip strength (r = .288; p = .025) and between protein intake and leg press strength (r = .303; p = .018). After adjusting for confounders, protein intake was not significantly associated with hand grip strength (p = .213) and leg press strength (p = .169). Separating men and women gave no different outcome in association between protein intake and muscle strength (p = .101).
Conclusion: There is no association between protein intake and muscle strength in frail elderly
above 65 years. More research is needed to determine the factors that provide an increase in muscle strength.
3
Contents
Introduction 4 Method 6 Results 8 Discussion 12 Conclusion 14 Recommendations 14 References 16 Appendix 19Appendix 1 Spss output correlation analysis 19
Appendix 2 Regression analysis of effect modification by gender 19
Appendix 3 SPSS output confounders body strength 20
Hand grip strength 20
Leg press 21
Appendix 4 SPSS output multiple regression analyses protein intake and muscle strength 23
Hand grip strength 23
4
Introduction
The number of elderly above the age of 65 is growing strongly, due to the increasing life expectancy in the last century (1). The elderly formed 19.2% of the Dutch population in 2019. Which is expected to be increased by 4% in 2030 (2). With aging, the number of elderly with comorbidities or functional limitations is also increasing. Elderly with complex health problems can be referred to as "frail" (3). The following criteria can be present when diagnosed with frailty: unintentional weight loss, weakness, exhaustion, slowness, and low physical activity level. Frailty is considered highly prevalent in older age (4). The prevalence of frailty among elderly varies from 4.9% to 27.3% and varies from 34.6% to 50.9% in pre-frailty (5). Frailty causes a high risk of falls, disability, hospitalization, and mortality (4). Weakness is associated with a risk of longer hospitalization, faster re-admission and a higher mortality rate after discharge (6,7). Weakness will make it harder to execute daily activities (8). This eventually might be the turning-point for older patients to shift from independent living to assisted living (6).
Weakness is determined by measuring the hand grip strength (4). Figure 1 shows that there is a natural loss of muscle strength in elderly (9,10). The process of losing muscle strength is
accelerated during hospitalization (6). The rate of muscle strength loss depends on multiple factors. Poor nutritional status might be an important reversible risk factor for frailty. Studies have suggested that adequate protein intake may reduce the risk of frailty (11,12). Given nutritional status as a risk factor for frailty and weakness as a criteria for frailty, there might be a relation between protein intake and muscle strength.
Figure 1Handgrip strength in the general population dependent on age, with a change point at the age of 50 years (10).
Data of the relation between protein intake and muscle strength are limited and show conflicting results. Where one study among adults aged 29 to 85 showed a positive relation between protein intake and muscle strength, others showed little or no relation (13–16). The Guideline ‘Malnutrition in the Geriatric Patient’ has also found contradictory outcomes with low evidential value (17). Data of the relation between protein intake and muscle strength among frail patients is scarce. The risk profiles of the loss of muscle strength need further specification (6).
5 Therefore, we aim to assess the following question: What is the relation between protein intake and muscle strength in frail elderly?
With the following sub-questions:
• Is there a difference in relation between protein intake and muscle strength in men and women?
• What is the relation between protein intake and the hand grip strength? • What is the relation between protein intake and the leg press strength?
6
Method
Study design
Baseline measurements of the subjects were used to show the relationship between protein intake and muscle strength in frail elderly. In this way, the intervention had no influence on the subjects and the results. This study is designed as an observational cross-sectional study. Primary measures were protein intake and muscle strength (handgrip strength and leg press). In addition, data about age, gender, lean body mass (DXA), energy intake and physical activity (counts/min) were collected. At the time of data collection, measurements could not be taken in the hospitals due to the situation surrounding COVID-19. Therefore, an existing data set was used in this study. The used data set is from the ProMuscle study (18).
Subjects
Recruitment of the patients took place between December 2009 and September 2010. Patients were recruited by using an existing database, flyers and local information meetings. In this study, 686 elderly people were invited to participate. Potentially eligible patients of 65 years and older were screened for (pre)frailty. A total of 233 elderly were screened for prefrailty and frailty. The screening was performed using the Fried criteria. Patients were defined as frail if 3 or more criteria were present and as prefrail when 1 or 2 criteria were present (4,18). Subjects were excluded when they were diagnosed with cancer, type 2 diabetes (7 mmol/L), renal insufficiency (eGFR <60 mL/min/1.73 m2), chronic obstructive pulmonary disease, or muscle disease or who were unable to perform the exercise regimen. Patients with silent ischemia were also excluded, which was tested with a resting electrocardiogram. A total of 171 people were excluded. This gave a total of 62 subjects that were included in this study, also shown in figure 2. If patients were willing to participate in the study, they had to sign their written informed consent (18).
Figure 2 Participant flow chart
Dietary intake
Protein and energy intake were measured using a 3 days record of the subject’s food intake. The recorded days were randomly assigned so that all days of the week, including weekend days, were equally represented. The subjects got oral and written instructions from a trained dietitian about recording type of foods and estimating portion sizes in household measures. Firstly, the subjects recorded their own intake using a food diary. During a second visit from the dietitian, the dietitian checked the food records for completeness, obtained additional
7 information about unclear items or amounts, and used examples of household measures to improve the estimation of portion sizes. After that, energy and macronutrient intakes were calculated using a food calculation system (BAS nutrition software 2004, Arnhem, The
Netherlands) in which the Dutch food composition database 2006 was included. The mean of these days was calculated for energy and protein intake (18).
Muscle strength
Muscle strength was measured using a hand grip meter and leg-press machine.
A hydraulic hand dynamometer (Jamar, Jackson, MI) was used to measure the muscle strength in the upper body. The hand grip strength was measured 3 times consecutively on the left and right side at one moment at the start of the study. Subjects needed to sit in an upward position and the measured arm in a 90-degree angle. We used the mean of the 3 attempts on the dominant hand (18).
A leg-press machine (Technogym, Rotterdam, the Netherlands) was used to measure the maximum lower body strength. The maximum leg press strength was assessed by using the 1 repetition maximum (1-RM) strength tests. During a first familiarization session, the proper lifting technique was practiced. So that the participants perform the measurements responsibly and methodically. After this session, the maximum strength was estimated. The 1-RM strength was determined in a second session. The maximum strength was expressed in kilograms (18).
Physical activity and body composition
Physical activity was assessed by using the ActiGraph. This was expressed in counts/min. Subject wore this device 24 hours a day for at least 5 days in a row. The mean was calculated over these days and used for the statistical analyses (18,19).
Lean body mass was measured by using the dual-energy x-ray absorptiometry [DXA] in fasted state (Lunar Prodigy Advance; GE Health Care, Madison, WI). This was expressed in kilograms (18).
Statistical analyses
We defined 1 primary outcome: the regression coefficient between the protein intake and muscle strength. The analyses were split in upper body strength (hand grip strength) and lower body strength (leg press strength). At which the protein intake was the independent variable and upper body and lower body strength were the dependent variable. To prevent mistakes in the data entry, the data was directly imported from Excel into SPSS. IBM SPSS version 25 was used for the analyses. Descriptive statistics were used to describe characteristics of the study participants and were presented as means and standard deviation.
A correlation test was used to determine the correlation between protein intake and muscle strength. The Pearson correlation coefficient was tested using a two-sided test to determine whether it is significant, based on the critical values of r. Outcomes were presented in two scatter plots with a regression line.
A multiple regression analysis was conducted to determine whether the relation between muscle strength and protein intake differs between men and women. An interaction term was made by multiplying a dummy variable of gender and protein intake. The significance of the interaction term would determine if effect modification by gender was found.
A multivariable linear regression was used to correct for confounding factors. Confounding factors were checked by the 10% marge rule or biological rule. The possible confounding factors were age, gender, lean body mass, energy intake and physical activity. For the
regression coefficient we used the standardized Beta coefficient. The regression coefficient was tested for significance, using a t-test. The ANOVA-test was performed to see if the regression model is significant by means of an f-test. Statistical significance was preset at a = 0.05 (20).
8
Results
Demographic characteristics
Descriptive characteristics and dietary intake of the study populations are presented in table 1. A total of 62 subjects were included for the analysis, of which 66% were women.
Participants' ages ranged from 66 to 96 years, with a mean age of 78 years (± 7.6). Subjects had a mean lean body mass of 46.3 kg (± 9.4). Handgrip strength had an average of 26.7 kg (± 9.6). This was 120.2 kg (± 32.1) in leg press strength. The mean counts per minute was 139.7 (± 92.4). Protein intake had a mean of 77 grams (± 22.1) of protein varying from 42 to 185. Daily energy intake was 8188 kJ (± 2304.7).
Correlation between protein intake and muscle strength
The correlation analysis showed that there was a significant, weak positive correlation between protein intake and hand grip strength (r = .288; p = .025; N = 60). For protein intake and leg press strength it showed a significant, weak positive correlation too (r = .303; p = .018; N = 61) (see also table 2 and appendix 1). The scatter plots in figure 3 and 4 show the linear correlation between protein intake and muscle strength in the analysed group.
Table 1 Baseline characteristics
Variable N Mean St. dev.
Gender Women 41 (66%)
Men 21 (34%)
Age (years) 62 78 7.6
Body composition Lean body mass (kg) 56 46.3 9.4
Handgrip strength (kg) 61 26.7 9.6
Leg press strength (kg) 62 120.2 32.1
Physical activity Counts/minute 57 139.7 92.4
Dietary intake Energy intake (kJ) 61 8188.0 2304.7
9 Table 2 Correlation between protein intake and muscle strength in frail elderly
Hand grip strength Leg press strength
Pearson Correlation .288 .303
Significance (2-tailed) .025 .018
N 60 61
Association between protein intake and hand grip strength
A multiple regression analysis showed no significant difference in the relationship between protein intake and muscle strength between men and women (p = .101) (Appendix 2).
Therefore, gender is not an effect modification and men and women were combined in the rest of the analysis.
Lean body mass, physical activity, gender and energy intake gave a >10% difference on the hand grip strength, so these were confounding factors. Age had only a 3.2% difference, but was considered a confounding factor on biological grounds. The analysis can be found in appendix 3. After adjusting for these confounders, protein intake was not significantly associated with hand grip strength (β = .261; t (43) = 1.263; p = .213) (Shown in table 3 and appendix 4).
Association between protein intake and leg press strength
Gender, age, lean body mass and energy intake gave a >10% difference on the leg press strength, so these were confounding factors. Counts per minute only gave a 0.4% difference, but was considered a confounding factor on biological grounds (Appendix 3). After adjusting for these confounders, a multiple regression analysis showed that protein intake is not significantly associated with leg press strength (β = -.300; t (44) = -1.399; p = .169) (Shown in table 4 and appendix 4).
10 Figure 3 Scatter plot of protein intake and hand grip strength in frail elderly
11 Table 3 Relation between protein intake and hand grip strength in frail elderly adjusted for
confoundersa Model
Standardized
Beta Sig. 95% CIb
Lower bound Upper bound
1 Protein intake (g) .288 .025 .016 .234
2 Protein intake (g) .261 .213 -.065 .282
Age (Years) -.117 .299 -.447 .141
Gender .681 .000 6.564 20.569
Lean body mass (kg) .130 .437 -.220 .499
Physical activity (Counts/minute)
-.018 .884 -.027 .023
Energy intake (kJ) -.194 .330 -.003 .001
Model 1: crude model
Model 2: corrected for confounders: age, lean body mass, counts/minute and energy intake a: Dependent variable: hand grip strength
b: Confidence interval for Beta
Table 4 Relation between protein intake and leg press strength in frail elderly adjusted for
confoundersa
Model
Standardized
Beta Sig. 95% CIb
Lower bound Upper bound
1 Protein intake (g) .303 .018 .080 .807
2 Protein intake (g) -.300 .169 -1.056 .191
Age (Years) -.388 .002 -2.781 -.700
Gender .295 .111 -4.844 45.482
Lean body mass (kg) .285 .180 -.238 2.334
Physical activity (Counts/minute)
-.011 .931 -.093 .086
Energy intake (kJ) .457 .030 .001 .013
Model 1: crude model
Model 2: corrected for confounders: age, gender, lean body mass and energy intake a: Dependent variable: leg press strength
12
Discussion
The aim of this study was to research the relation between protein intake and muscle strength in frail elderly above the age of 65. Results of this study show that there was a significant, weak positive correlation between protein intake and muscle strength. After adjustment for
confounders, the regression analysis showed a not significant association between protein intake and muscle strength in frail elderly.
Looking at other studies, we noticed there are limited studies who exclusively examine the relation between protein intake and muscle strength in frail elderly.
However, there was a study where they compared muscle strength between two groups with low and high protein intake. There was no association found between protein intake and hand grip strength in community dwelling elderly (21).
In another study among frail elderly, in which the intervention group received a drink with 30 grams of protein, there were also no significant differences found in handgrip strength compared to the control group who got a placebo. This effect was measurable up to 18 months (22). Moreover, this has also been tested in sarcopenic elderly. The results showed that an increase in protein intake led to an improvement in handgrip strength, however this effect was not found significant (23).
Lastly, a systematic review of the relation between protein intake and muscle mass, muscle strength and physical performance in middle aged to older adults showed conflicting outcomes (24). This review showed that overall protein does not have a significant effect on improvement in handgrip strength or leg press strength. This is consistent with the results of our study. Multiple studies have examined the relation between protein intake and muscle strength in combination with exercise training. Looking at the effect of protein intake on muscle strength in combination with training, it shows a positive effect on muscle strength. A meta-analysis showed that higher protein intake and exercise training had significantly greater effects on frailty indices, including handgrip strength and leg press strength, compared to exercise training only (25). More studies examined a combined intervention of physical activity and protein intake. An RCT among 352 frail elderly who received a 12-week intervention of exercise, protein
supplementation, memory training, and medication review showd a significant positive effect on hand grip strength. This effect was maintained up to at least 18 months (26). When a
combination of a twice-weekly exercise training with branched-chain amino acids (BCAAs) was performed this effect was also found in leg press strength (27).
Other research shows that supplementation of whey protein, essential amino acids and vitamin D in combination with exercise has a positive effect in handgrip strength in sarcopenic elderly (28). This is remarkable, because another study that looked at the effect of protein and vitamin D, without exercise in sarcopenic elderly does not show a significant difference (29). It indicates that the combination of exercise and protein supplementation might have the greatest effect on muscle strength. The outcome of our study differs and can be explained by the fact that a combination of exercise training gives an improvement in muscle mass and therefore likely an improvement in muscle strength. However, further research will be needed to demonstrate whether this also applies to frail elderly.
An important strength of this study is that we used an objective measure of muscle strength in the upper body which is highly reliable. According to the fabricant, the accuracy of the hand grip dynamometer is 5%. Study suggests this is even 3%. The test-retest reliability of the hand grip strength is high to very high, where it is highest when the mean of the three trails was used. The inter-tester reliability is very high (30–32).
13 The 1-RM of the leg press was used to measure the upper body strength. Although
dynamometry is considered the golden standard for measuring muscle strength, 1-RM testing on training-specific equipment is more commonly applied. The 1-RM test of the leg press is proven to be a reliable method to measure the lower body strength. The leg press triggers more muscle groups than the leg extension, giving a better representation of the whole limb extension strength rather than the knee extensors alone (33,34).
Validity of a food diary depends on the length of time and the quality of the instructions. In our study this was guaranteed by keeping track of the intake for three days which were randomly assigned, so all days of the week were equally present. The instructions were given by a
certified dietitian. The reliability is guaranteed at the second appointment with the dietitian, when the dietitian enquires about the food diary and complementing the food diary (35,36).
The confounders were measured in reliable ways, as explained below. Physical activity was measured using an ActiGraph, which is a reliable tool for independent living adults (37). The mean of several days was used to overcome possible differences with frail elderly. Although the DXA scan has flaws in measuring lean body mass, it is a commonly used and valid instrument (38,39). Energy intake was measured in the same way as protein intake, so the validity and reliability are in accordance with protein intake.
The data of lean body mass, hand grip strength and physical activity were not fully measured by all participants. Consequently, the subgroups we could use for the statistical analyses got smaller, but the number of subjects for the final conclusion stayed the same.
The multiple regression analysis in our study shows that there was no significant association between protein intake and muscle strength in frail elderly after adjustment for confounding factors. This can be explained by other factors that may have an influence on frailty. In our study we have corrected for age, gender, physical activity and energy intake. Nevertheless, there are more factors that can influence muscle strength in frail elderly. Factors like education level, marital status, family per capita annual income, constipation, urinary retention, urinary incontinence, serum prealbumin and hemoglobin are also correlated with frailty (40). Considering the fact that not all confounders were measured, we cannot draw causal links. Therefore, we focused on statistical coherence.
14
Conclusion
This study found that there is no significant association between protein intake and hand grip strength in frail elderly. This also applies for protein intake and leg press strength. There is no difference between men and women in this matter. Therefore, there is no association between protein intake and muscle strength in frail elderly above 65 years. More research is needed to determine the factors that provide an increase in muscle strength in frail elderly, maybe in collaboration with protein intake.
Recommendations
Recommendations for the target group
Frail elderly can not increase their muscle strength by solely consuming more protein.
Increasing or maintaining muscle strength is of interest for this group, given the consequences that were discussed in the introduction. Since only increasing the protein intake is not enough to increase the muscle strength for frail elderly, it becomes more complex to increase muscle strength by themselves. Therefore, we recommend this group of elderly to visit a dietitian for advice on nutritional intake as soon as they are diagnosed with frailty. In addition, we
recommend visiting a physiotherapist as well to train the muscles, given the vulnerability of the elderly. The biggest gains in increasing the strength of frail elderly will be generated when the dietitian and the physiotherapist work together.
Recommendations/applications for the initiator
The aim of ProIntens is to achieve an increase in protein intake and improve physical
functioning during admission, hospitalization and after discharge. This will be achieved through research into a more intensive nutrition therapy in combination with a physical activity
intervention.
In our study the subjects were defined as frail by the Fried criteria. This target group is different from the target group of ProIntens, which are malnourished hospitalized elderly. However, there are some similarities between the two groups. One of the Fried criteria is weight loss, which is also a criteria of malnutrition. It is possible that some of the subjects in our study are
malnourished too, but we must remain careful with this.
For ProIntens we can conclude from our study that there is no association found between protein intake and muscle strength in frail elderly. Therefore, we can state that our study points out, there is no scientific evidence for giving patients more protein with the purpose to increase muscle strength. ProIntens must look further than only protein intake for the increase of muscle strength. Further research is needed to demonstrate whether this applies for malnourished hospitalized elderly as well.
Recommendations/applications for professional practice
Dietitians can prescribe protein supplementation to frail elderly. We conclude for dietitians, that an increase in protein will not consequently increase muscle strength in frail elderly. Therefore, we cannot give advice about the amount of protein, but we suggest dietitians to hold on to recommendations for protein by frailty that are already stated.
Additionally, it will be necessary to look further than only protein consumption to increase muscle strength in frail elderly. Other studies show that the combination of protein
15 an opportunity for dietitians to collaborate with physiotherapists more often in order to focus on nutrition and exercise training at the same time.
Recommendations for further research
There is limited data that examines the relation between protein and muscle strength in frail elderly. No association is found by our study of 62 frail elderly people, but more research is needed. Our study was observational, therefore we focused on statistical coherence and we could not draw causal links. For further research we recommend a randomized control trial that examines the effect of the combination of protein intake and exercise training on muscle strength.
To examine this among frail elderly we advise to split the study population into three groups. A group that only gets extra proteins, a group that receives extra proteins and gets exercise training twice a week and a control group. The protein supplementation could most easily be ingested through a drink. The measuring moments for muscle strength are after 1, 3, 6 and 12 months. Muscle strength can be measured with the handgrip dynamometer and leg press. At the end of the study, a conclusion can be drawn about the effect of proteins and exercise training on muscle strength. This method allows to also examine the maintenance of muscle strength over time. This was not possible in our study since we only examined baseline measurements. By adding a control group, it can be determined whether the loss of muscle strength in frail elderly can be slowed down, maintained, or improved by the combination of higher protein intake and exercise training twice a week.
We recommend further research among malnourished hospitalized elderly, because the
malnourishment, inflammation and underlying disease can also have consequences for muscle strength. This was not researched in our study. We suggest a randomized control trial that examines protein intake and muscle strength in malnourished hospitalized elderly. There will be two groups. A group that gets a protein intake of 1.5 g/kg body weight with help from a dietitian, and a control group with a protein intake of 1.0 g/kg body weight. Protein intake and muscle strength will be measured after 3, 6 and 12 months. In this study, inflammation can be
measured as one of the confounding factors. At the end of the study there can be determined if the muscle strength has increased, maintained or decreased at a slower rate over time as a result of a higher protein intake in malnourished hospitalized elderly.
16
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Appendix
Appendix 1 Spss output correlation analysis
Appendix 2 Regression analysis of effect modification by gender
Regression analysis effect modification by genderaUnstandardized B Coefficients Std. Error Standardized Coefficients Beta t Sig. (Constant) 26.904 4.326 6.219 .000 Protein intake .112 .049 .259 2.285 .026 Women -3.492 6.510 -.174 -.536 .594 ProWomenb -.135 .081 -.529 -1.667 .101
a. Dependent variable: Hand grip strength dominant hand b. ProWomen: interaction term (protein intake x women)
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Appendix 3 SPSS output confounders body strength
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Appendix 4 SPSS output multiple regression analyses protein
intake and muscle strength
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