Abstract
[Purpose]
Evidence on the effects of low-grade dehydration on muscle function in non-athletic middle-aged adults is scarce and inconclusive. The objective was to assess the effect of small variations in hydration status due to 12-hour overnight fasting on right-leg muscle function in middle-aged adults.
[Methods]
Single-group before-and-after study in volunteers aged 35-65 years. Participants were assessed at two time points: after 12 hours of absolute fasting and after breakfast and drinking 1.5 L of water. Body composition and hydration status were assessed by bioelectrical impedance analysis (BIA) and muscle function by an isokinetic test.
[Results]
Fifty-nine volunteers (47 years, 81.4% women) were recruited. Under fasting conditions, total body water, intracellular water (ICW), and extracellular water were positively correlated with knee flexion and extension strength, work, and power in men and women. The ICW to fat-free mass ratio and phase angle were positively correlated with flexion and extension strength and work in women but not men. Flexion strength and work improved significantly after water intake in women but not men. No changes in muscle function were observed in individuals younger than 45 years. In those aged 45 years or older, significant differences were observed for flexion strength (58.7 vs 61.8 N, p = 0.004), work (269.0 vs 284.0 J, p = 0.003) and power (39.5 vs 41.3 W, p = 0.018).
[Conclusion]
Hydration status appears to have a role in muscle function, especially with increasing age. More studies, however, are required to better understand the influence of hydration on muscle function and the underlying mechanisms.
Keywords: dehydration, muscle function, muscle strength, bioimpedance analysis, isokinetic test
INTRODUCTION
Water has numerous essential functions in the body. It provides structure to cells; acts as a reaction medium, a solvent, and a reagent in body metabolism; transports nutrients and waste; and regulates body temperature [1]. It flows from intracellular to extracellular compartments throughout the body by osmotic pressure. Accordingly, any increase in extracellular osmolarity will cause intracellular dehydration, which can have deleterious effects on protein structure and function [2]. Starting in adulthood, there is a progressive decline in body water content, which is explained mainly by the relative decrease in fat free mass (FFM) but also by low-grade chronic dehydration, especially at advanced ages [3]. Older individuals are hence at an increased risk of dehydration and consequently adverse health outcomes [4]. Chronic dehydration is a relatively common clinical condition in aged population, but it is often asymptomatic and under-diagnosed. No universally accepted definition of chronic dehydration exists, being elevated blood urea, urea nitrogen to creatinine ratio > 20, serum osmolarity ≥ 295 mOsm/kg, weight loss ≥ 1% due to fluid loss or a dehydrated state lasting more than 72 hours the most used diagnostic criteria [5]. The lack of a clear diagnostic criteria claims for an international consensus on the definition of chronic dehydration [5].
Muscle, the heaviest organ system in the body, contains approximately 76% water, making it the main reservoir of this essential nutrient. The relationship between hydration status and muscle function has been widely studied in young adult athletes [6,7], with findings showing a link between dehydration and decreased physical performance [8]. Dehydration causes changes in cardiovascular, thermoregulatory, metabolic and central nervous function and provokes physical performance impairments, especially during prolonged exercise in the heat [9]. Few studies, however, have analyzed hydration status and muscle function in non-athletes. In addition, the effects of hyperosmotic stress and low-grade chronic dehydration on muscle function are poorly understood in middle-aged and older adults. Yamada and colleagues reported a reduction in intracellular water (ICW) in the muscle of older individuals [10], and they also found that an increase in the extracellular water (ECW)/ICW ratio in older adults is an independent predictor of poor muscle strength and decreased gait speed [11,12]. Our group has reported that cellular hydration, measured by the ICW/FFM ratio, decreases with age and that cellular dehydration is associated with lower muscle strength, poorer functional capacity, and an increased risk of frailty [13]. As individuals age, there is typically a reduction in muscle mass, an expansion of the muscle’s extracellular space, and a tendency for myocyte contraction and fat infiltration. All these changes contribute to poor muscle quality, poor muscle function, and sarcopenia [14]. Other authors have reported that inadequate dietary water intake in older individuals can contribute to sarcopenia, considered to be a dehydration-related complication [15]. Although there is evidence suggesting that low-grade dehydration affects muscle function in adults, it is still scarce and inconclusive [14]. Further research is thus recommended. The aim of this study was to assess the effect of small variations in hydration status due to 12-hour absolute fasting on right-leg muscle function in middle-aged adults.
METHODS
Design and population
We conducted a single-group before-and-after study in volunteers aged 35-65 years. The volunteers were assessed after 12 hours of absolute overnight fasting (no solids or liquids) and again after breakfast and drinking 1.5 L of water. In the single group before-and-after design, each individual acts as his or her own control (paired data). Participants were recruited among workers at Consorci Sanitari del Maresme (CSdM) and relatives and friends of these workers. Exclusion criteria were an acute illness, active cancer, a muscular disorder (muscular dystrophy or post-stroke paresis), a neurodegenerative disease (e.g., Parkinson disease), or bilateral hip or knee prostheses. Volunteers were invited to attend an initial visit to check selection criteria, receive information about the study, and provide signed informed consent. The main outcome measure for the sample size calculation was quadriceps extension strength. Sample sizes were calculated for both men and women to allow the identification of sex-specific effects. To detect a difference equal to or greater than 3 newton in strength between dehydration (before) and normohydration (after), with an assumed standard deviation of 5.5-6 in paired data, we determined a required sample size of 27-30 individuals, accepting an alpha risk of 0.05 and a beta risk of 0.2 in a two-sided test. A loss to follow-up rate of 0% was considered. The study protocol was approved by the local ethics committee (CEIm CSdM 86/20).
Procedures and data collection
Participants attended the baseline visit between 8 am and 10 am after 12 hours of absolute fasting for the pre-intervention evaluation. Under these conditions, they were considered to have low-grade dehydration. After the baseline evaluation, they were allowed to have a breakfast of their choice and instructed to drink 1.5 litres of mineral water over the following 2 hours. The post-intervention evaluation was conducted between 12 noon and 2 pm (4 hours after the participants had started to drink the water and 2 hours after they had finished). Pre- and post-intervention evaluations included bioelectrical impedance analysis (BIA) to assess body composition and hydration status and an isokinetic test to assess muscle function. Figure 1 presents a schematic diagram of the study design.
Figure 1. Schematic diagram of the study design.
BIA is a quick, non-invasive, reproducible test that has been validated and is widely accepted as a highly accurate method for determining body composition [17]. It involves applying a very small alternating electric current at different frequencies and measuring the impedance of biological tissues to the passage of this current. It is used to estimate fat mass, FFM, and muscle mass (in kg and as % of total body weight); total body water (TBW) and ECW and ICW for the whole body, each extremity, and the trunk (in litres and % of TBW); and phase angle (PhA). Prior to the BIA, the study participants had not performed intense exercise in 24 hours, had not consumed alcohol in 8 hours, had not consumed solids or liquids for at least 2 hours, and had emptied their bladder. The BIA was performed using the InBody s10 device (Biospace, Seul), a multifrequency device that uses frequencies of 0, 1, 5, 10, 50, 100, and 200 kHz. The ICW/FFM ratio was used as an indicator of cell hydration [13].
The isokinetic test is the most appropriate test for objectively assessing muscle function in terms of strength, work, and power. The system comprises three elements: a goniometer to measure range of motion, a tachymeter to measure the speed of movement, and a dynamometer to measure force at a given moment. Isokinetic machines allow muscle exercises to be performed at a speed that remains constant throughout the range of joint motion. The resistance generated by the machine is consistently proportional to the strength exerted by the patient. Accordingly, the engaged muscles invariably generate maximal tension and maximal fibre activation. The test was used to measure strength in newtons (N), work in joules (J), and power in watts (W) for right knee flexion and extension. The isokinetic test (System 4 Pro device from PRIM) was used to assess flexion and extension of the knee. A note was also made of the participants’ age, sex, main comorbidities, number of chronic medication, and level of physical activity according to the International Physical Activity Questionnaire (IPAQ).
Statistical analysis
The main characteristics of the study sample were described using means and standard deviations for numerical variables and percentages for categorical variables. Correlation coefficients (r) and linear regression coefficients (beta) were used to analyze the relationship between BIA indicators of hydration and body composition and muscle function parameters. Analyses were performed separately for men and women and for flexion and extension. The participants were also split into two age groups: those younger than 45 years and those aged 45 years or older and a stratified analysis was performed according to age groups. The effect of hydration indicators on muscle function was adjusted for age, number of medications, and level of physical activity using multiple linear regression models. The Wilcoxon test and the generalized linear model were used to analyze the effects of hydration/water intake on body composition and muscle function (pre-post paired data). Statistical significance was established at a p value < 0.05.
RESULTS
Description of study sample
Fifty-nine participants (48 women, 81.4%) with a mean (SD) age of 47.1 [10] years were recruited; 89.8% were never smokers. The main comorbidities were hypertension (15.3%), arthritis (11.9%), dyslipidemia (11.9%), and depression (10.2%). Mean BMI was 25.0 (4.4) and the mean number of medications was 0.57 (0.9).
Baseline relationship between BIA hydration indicators and muscle function
Under fasting conditions, TBW, ICW, and ECW were positively correlated with knee flexion and extension strength, work, and power in men and women. In women but not in men, whole body ICW/FFM ratio was positively correlated with knee flexion and extension strength and work, right leg ICW/FFM ratio was positively correlated with knee flexion work and knee extension strength and work, and PhA was positively correlated with knee flexion strength and work and with knee extension strength, work and power(Table 1). The results of the multivariate linear regression analyses adjusting the effects of BIA hydration indicators on thigh muscle function by age, number of medications, and physical activity are presented in Table 2. The results show an independent effect of TBW and ICW (in litres) on knee flexion and extension muscle strength, work, and power in women, and on knee flexion and extension muscle strength in men. They also show an independent and positive effect for whole body and right leg ICW (% of TBW) and the ICW/FFM ratio on knee extension muscle strength in women but not men.
Table 1.
Correlations between bio-impedance analysis indicators of hydration status and isokinetic parameters of right leg muscle function of the right leg under fasting conditions (pre-evaluation)
| r (p) | Right Knee Flexion |
|||||
|---|---|---|---|---|---|---|
| Women (n=48) |
Men (n=11) |
|||||
| Strength (N) | Work (J) | Power (W) | Strength (N) | Work (J) | Power (W) | |
| Whole body: | ||||||
| ● TBW (L) | 0.51 (0.001) | 0.51 (0.001) | 0.47 (0.001) | 0.77 (0.006) | 0.73 (0.011) | 0.74 (0.009) |
| ● ICW (L) | 0.52 (0.001) | 0.53 (0.001) | 0.48 (0.001) | 0.77 (0.005) | 0.69 (0.019) | 0.74 (0.008) |
| ● ECW (L) | 0.47 (0.001) | 0.47 (0.001) | 0.45 (0.001) | 0.75 (0.007) | 0.76 (0.006) | 0.74 (0.010) |
| ● ECW/ICW | -0.29 (0.04) | -0.30 (0.035) | -0.22 (0.133) | 0.005 (0.99) | -0.03 (0.94) | 0.00 (1.000) |
| ● ICW/FFM (L/kg) | 0.31 (0.034) | 0.33 (0.022) | 0.24 (0.097) | 0.04 (0.915) | 0.09 (0.790) | 0.04 (0.915) |
| ● PhA | 0.32 (0.025) | 0.37 (0.009) | 0.26 (0.075) | 0.28 (0.406) | 0.32 (0.344) | 0.04 (0.904) |
| Right leg: | ||||||
| ● ECW/ICW | -0.27 (0.07) | -0.31 (0.034) | -0.19 (0.184) | -0.13 (0.70) | -0.01 (0.97) | -0.15 (0.66) |
| ● ICW/FFM (L/kg) | 0.27 (0.067) | 0.31 (0.032) | 0.20 (0.181) | -0.07 (0.83) | -0.08 (0.81) | -0.05 (0.87) |
|
Right Knee Extension
|
||||||
|
Women (n=48)
|
Men (n=11)
|
|||||
| Strength (N) | Work (J) | Power (W) | Strength (N) | Work (J) | Power (W) | |
| Whole body: | ||||||
| ● TBW (L) | 0.64 (0.001) | 0.53 (0.001) | 0.47 (0.001) | 0.69 (0.018) | 0.70 (0.017) | 0.71 (0.014) |
| ● ICW (L) | 0.66 (0.001) | 0.55 (0.001) | 0.49 (0.001) | 0.66 (0.026) | 0.65 (0.029) | 0.71 (0.015) |
| ● ECW (L) | 0.59 (0.001) | 0.48 (0.001) | 0.44 (0.002) | 0.72 (0.013) | 0.74 (0.010) | 0.71 (0.015) |
| ● ECW/ICW | -0.35 (0.01) | -0.34 (0.02) | -0.24 (0.19) | 0.05 (0.873) | -0.14 (0.97) | 0.14 (0.689) |
| ● ICW/FFM (L/kg) | 0.35 (0.014) | 0.35 (0.016) | 0.24 (0.103) | 0.02 (0.958) | 0.06 (0.85) | -0.09 (0.79) |
| ● PhA | 0.43 (0.002) | 0.43 (0.002) | 0.30 (0.036) | 0.28 (0.406) | 0.32 (0.344) | 0.04 (0.904) |
| Right leg: | ||||||
| ● ECW/ICW | -0.36 (0.01) | -0.37 (0.01) | -0.24 (0.09) | 0.07 (0.842) | 0.09 (0.80) | 0.04 (0.915) |
| ● ICW/FFM (L/kg) | 0.38 (0.008) | 0.39 (0.006) | 0.26(0.074) | -0.07 (0.83) | -0.08 (0.81) | -0.05 (0.87) |
Abbreviations: ECW, extracellular water; FFM, fat-free mass; ICW, intracellular water; PhA, phase angle; TBW, total body water.
Table 2.
Effect of bio impedance analysis indicators of hydration status on right thigh muscle function under fasting conditions with adjustment for age, number of medications, and physical activity (multivariate linear regression analysis)
| beta (p) | Right Knee Flexion |
|||||
|---|---|---|---|---|---|---|
| Women (n=48) |
Men (n=11) |
|||||
| Strength (N) | Work (J) | Power (W) | Strength (N) | Work (J) | Power (W) | |
| Whole body: | ||||||
| ● TBW (L) | 2,21 (<0.001) | 12.29 (<0.001) | 1.58 (<0.001) | 3.19 (0.026) | 14.90 (0.036) | 2.18 (0.070) |
| ● ICW (L) | 3.53 (<0.001) | 19.74 (<0.001) | 2.50 (<0.001) | 5.01 (0.028) | 23.56 (0.036) | 3.41 (0.076) |
| ● TBW (%) | 38.6 (0.223) | 67.5 (0.720) | 24.3 (0.325) | -644.4 (0.264) | -1988.3 (0.493) | -401.9 (0.382) |
| ● ICW (%) | 608.7 (0.096) | 3760.3 (0.080) | 285.8 (0.320) | -144.4 (0.960) | 3655.1 (0.791) | -439.6 (0.844) |
| ● ECW/ICW | -231.4 (0.097) | -1425.4 (0.083) | -108.2 (0.324) | 42.36 (0.970) | -1495.7 (0.782) | 161.4 (0.853) |
| ● ICW/FFM (L/kg) | 1055.7 (0.074) | 6806.2 (0.050) | 552.1 (0.235) | -15.4 (0.997) | 8521.6 (0.699) | -531.9 (0.882) |
| ● PhA | 7.01 (0.086) | 47.47 (0.047) | 3.58 (0.264) | 13.27 (0.588) | 122.03 (0.278) | 6.24 (0.746) |
| Right leg: | ||||||
| ● ECW/ICW | -172.6 (0.110) | -1211.7 (0.056) | -81.5 (0.337) | -204.4 (0.826) | -1247.2 (0.781) | -39.9 (0.956) |
| ● ICW/FFM (L/kg) | 727.3 (0.110) | 5146.6 (0.054) | 348.5 (0.330) | 955.3 (0.803) | 5341.2 (0.772) | 201.5 (0.946) |
|
Right Knee Extension
|
||||||
|
Women (n=48)
|
Men (n=11)
|
|||||
| Strength (N) | Work (J) | Power (W) | Strength (N) | Work (J) | Power (W) | |
| Whole body: | ||||||
| ● TBW (L) | 3.82 (<0.001) | 16.17 8<0.001) | 2.22 (<0.001) | 4.22 (0.020) | 18.666 (0.065) | 2.71 (0.053) |
| ● ICW (L) | 6.13 (<0.001) | 25.9 (<0.001) | 3.49 (<0.001) | 6.63 (0.022) | 29.56 (0.065) | 4.22 (0.060) |
| ● TBW (%) | -31.4 (0.539) | -82.4 (0.455) | -8.02 (0.826) | -637.5 (0.408) | -1417.8 (0.725) | -416.0 (0.453) |
| ● ICW (%) | 1168.9 (0.045) | 4647.0 (0.097) | 311.0 (0.464) | -216.2 (0.954) | 5779.4 (0.759) | -981.3 (0.711) |
| ● ECW/ICW | -442.1 (0.047) | -1744.8 (0.103) | -115.7 (0.475) | 64.3 (0.965) | -2348.5 (0.749) | 369.5 (0.721) |
| ● ICW/FFM (L/kg) | 1873.7 (0.047) | 7654.1 (0.092) | 481.7 (0.485) | 352.4 (0.953) | 14082.8 (0.637) | -1182.2 (0.781) |
| ● PhA | 15.04 (0.020) | 64.06 (0.039) | 4.68 (0.322) | 22.10 (0.481) | 173.10 (0.256) | 5.25 (0.819) |
| Right leg: | ||||||
| ● ECW/ICW | -360.1 (0.036) | -1634.4 (0.047) | -103.3 (0.410) | 42.52 (0.972) | -636.1 (0.917) | 261.9 (0.761) |
| ● ICW/FFM (L/kg) | 1605 (0.026) | 7286.6 (0.035) | 496.5 (0.346) | -73.9 (0.988) | 2762.9 (0.913) | -1041.4 (0.769) |
Abbreviations: ECW, extracellular water; FFM, fat-free mass; ICW, intracellular water; PhA, phase angle; TBW, total body water.
Pre-post changes in BIA hydration and thigh muscle function indicators
Pre-post comparisons are presented in Table 3, which shows that TBW, ICW, ECW, and PhA increased significantly after hydration in women. Only ECW increased significantly in men. The only significant improvements observed for women in the isokinetic test were for knee flexion strength and work. No changes were observed in men. On stratifying by age, no changes in muscle function were observed after water intake in individuals younger than 45 years. In individuals aged 45 or older, significant differences were observed for knee flexion strength (58.7 vs 61.8 N, p = 0.004), work (269.0 vs 284.0 J, p = 0.003), and power (39.5 vs 41.3, p = 0.018). Increases in hydration status indicators were not correlated with improvements in muscle function in men or women.
Table 3.
Comparison of bioimpedance analysis hydration indicators and isokinetic parameters of muscle function between fasting (PRE) and normohydration (POST) by age and sex groups
| Hydration indicators |
||||||
|---|---|---|---|---|---|---|
| Women (n=48) |
Men (n=11) |
|||||
| Fasting (PRE) | Normohydration (POST) | p | Fasting (PRE) | Normohydration (POST) | p | |
| Whole body: | 0.001 | |||||
| ● TBW (L) | 33.10 (3.80) | 33.39 (3.73) | 47.31 (7.51) | 47.60 (7.47) | 0.130 | |
| ● ICW (L) | 20.53 (2.39) | 20.69 (2.33) | 0.003 | 29.66 (4.75) | 29.77 (4.73) | 0.506 |
| ● ECW (L) | 12.57 (1.43) | 12.70 (1.43) | <0.001 | 17.64 (2.77) | 17.83 (2.75) | 0.015 |
| ● ECW/ICW | 0.612 (0.02) | 0.614 (0.01) | 0.084 | 0.595 (0.01) | 0.599 (0.01) | 0.075 |
| ● ICW/FFM (L/Kg) | 0.483 (0.004) | 0.482 (0.003) | 0.070 | 0.487 (0.003) | 0.486 (0.003) | 0.091 |
| ● PhA | 5.47 (0.51) | 5.53 (0.52) | 0.006 | 6.46 (0.54) | 6.40 (0.52) | 0.347 |
| Right leg: | ||||||
| ECW/ICW | 0.614 (0.02) | 0.615 (0.02) | 0.104 | 0.592 (0.02) | 0.598 (0.02) | 0.012 |
| ICW/FFM (L/Kg) | 0.483 (0.005) | 0.482 (0.004) | 0.147 | 0.488 (0.004) | 0.486 (0.004) | 0.003 |
|
Isokinetic parameters of muscle function
|
||||||
|
Women (n=48)
|
Men (n=11)
|
|||||
| Fasting (PRE) | Normohydration (POST) | p | Fasting (PRE) | Normohydration (POST) | p | |
| Right knee flexion: | ||||||
| ● Strength (N) | 63.28 (23.6) | 65.00 (21.5) | 0.027 | 95.71 (31.3) | 95.29 (26.20) | 1.00 |
| ● Work (J) | 296.86 (113.1) | 308.21 (98.6) | <0.001 | 432.67 (140.7) | 439.16 (116.1) | 0.594 |
| ● Power (W) | 43.51 (17.8) | 44.22 (15.7) | 0.168 | 67.50 (23.5) | 65.99 (19.7) | 0.050 |
| Right knee extension: | ||||||
| ● Strength (N) | 121.81 (36.6) | 121.96 (34.9) | 0.901 | 171.66 (40.1) | 172.22 (38.8) | 0.859 |
| ● Work (J) | 507.29 (152.7) | 522.49 (146.1) | 0.077 | 694.04 (185.9) | 704.96 (169.2) | 0.248 |
| ● Power (W) | 77.91 (24.4) | 77.25 (22.4) | 0.369 | 112.26 (28.1) | 107.02 (25.8) | 0.878 |
|
Isokinetic parameters of muscle function
|
||||||
|
<45 years (n=27)
|
≥45 years (n=32)
|
|||||
| Fasting (PRE) | Normohydration (POST) | p | Fasting (PRE) | Normohydration (POST) | p | |
| Right knee flexion: | ||||||
| ● Strength (N) | 68.8 (23.5) | 68.8 (20.7) | 0.829 | 58.7 (23.1) | 61.8 (22.0) | 0.004 |
| ● Work (J) | 329.9 (105.0) | 336.9 (100.1) | 0.084 | 269.0 (113.6) | 284.0 (92.0) | 0.003 |
| ● Power (W) | 48.2 (16.7) | 47.7 (15.1) | 0.666 | 39.5 (18.0) | 41.3 (15.9) | 0.018 |
| Right knee extension: | ||||||
| ● Strength (N) | 128.4 (33.2) | 126.8 (32.1) | 0.322 | 116.3 (35.4) | 117.9 (37.0) | 0.483 |
| ● Work (J) | 553.1 (143.0) | 573.5 (146.2) | 0.068 | 468.6 (152.0) | 479.5 (133.6) | 0.525 |
| ● Power (W) | 84.4 (22.7) | 83.8 (20.2) | 0.792 | 72.4 (24.8) | 71.7 (23.0) | 0.400 |
DISCUSSION
The main study results indicate that a) TBW, ICW, and ECW were positively correlated with muscle strength, work, and power in both men and women in fasting conditions in the study population, b) the ICW/FFM ratio and PhA were positively correlated and the ECW/ICW ratio negatively correlated with muscle strength and work in women but not men, c) knee flexion strength and work improved after water intake in women but not men, and d) muscle function was only affected by water intake following 12 hours of fasting in individuals aged 45 years or older.
TBW, ICW, and ECW (expressed in litres) were clearly related to muscle function in both men and women. These BIA parameters should be considered as indicators of muscle mass rather than hydration, as the greater the mass, the greater the water amount (in absolute terms). The correlations observed between muscle strength and TBW, ICW, and ECW corroborate previous reports of a link between muscle mass and strength [18]. However, the baseline analysis also showed that the ICW/FFM ratio, an indicator of cell hydration and muscle quality [13], and PhA, an indicator of hydration status and membrane cell integrity [19], were correlated with muscle strength and work in women, suggesting that cell hydration might influence muscle function. The mechanisms by which dehydration could affect the functioning of muscles remain to be fully elucidated. Some authors have indicated that dehydration might contribute to elevated glycogenolysis during exercise and poor glycogen resynthesis during recovery [20]. This could have long-term effects on glucose availability in the case of prolonged dehydration. Other authors have reported that the intake of hydrogen-rich water (regular water with hydrogen gas added) improves muscle function, reduces lactate response, and alleviates muscle soreness [21]. Studies of patients with chronic obstructive pulmonary disease (COPD), in turn, have shown that a decreased impedance ratio, an indicator of body cell mass and muscle quality [19], was associated with a lower forced expiratory volume in 1 second and lower vital capacity, respiratory muscle strength, and hand grip strength [22]. Other studies have shown an increased ECW/ICW ratio in sarcopenic patients with COPD [23] and an inverse association between this ratio and peak VO2 [24]. These findings all suggest that hydration status may play a role in muscle function, but further studies are needed to corroborate this hypothesis in individuals of different ages with low-grade dehydration and to gain a detailed understanding of the mechanisms involved.
BIA detected changes in hydration status from baseline (after 12 hours of overnight fasting) to 2 hours after an “ad libitum” breakfast and intake of 1.5 L water. TBW, ECW, ICW, and PhA all changed significantly in women. The only change observed in men was for ECW, probably because of the very small sample size. These findings indicate that 12-hour overnight fasting might be a good model for low-grade dehydration. Nonetheless, the effects of intermittent low-grade dehydration may differ from those caused by chronic or persistent dehydration. There are 2 possible explanations for the greater increase observed in ECW than in ICW 2 hours after water intake. First, a period of 2-4 hours may not be enough time for the water to be absorbed, distributed throughout the body, and penetrated into de cells. Second, 1.5 L of water consumed over a period of 2 hours might be excessive for a well-hydrated middle-aged individual (even after 12 hours of fasting), who, without enough time to eliminate the excess water in urine, could experience a relative increase in ECW.
The observed effect of hydration on muscle function after 12 hours of overnight fasting in healthy middle-aged individuals is difficult to interpret because of the relatively small effect sizes (2.7% increase in right knee flexion strength and 3.8% increase in right knee flexion work), observed only in women. The lack of a significant change in men may be due to the small sample size and the limited statistical power in this group. In addition, no effects were observed for extension strength or work, and there were no correlations between improved hydration and improved muscle function. The experimental 12-hour overnight fasting model may cause insufficient dehydration to detect an evident, relevant effect on muscle function in well-hydrated middle-aged individuals. Although BIA is capable of detecting changes in body hydration parameters between fasting and non-fasting conditions, 12 hours of fasting did not appear to be sufficient to produce clinically relevant dehydration in the study population. The lack of effects observed in men may be due to underpowering and perhaps the higher muscle mass and water content in this population. Moreover, mild dehydration may have affected knee flexion but not extension because extension requires engagement of a much stronger and more powerful muscle group. Our findings suggest that larger and more powerful muscle groups have more functional reserves for dealing with stressors such as dehydration. Muscle-specific differences may be due to different proportions of type I and type II fibres within each muscle [25]. Overall, our results do not allow for definitive conclusions, but a potential relationship between low-grade dehydration and muscle dysfunction cannot be ruled out and requires further investigation, especially in more vulnerable older populations at risk of dehydration. Notably, dehydration only affected muscle function in individuals aged 45 years or older. Older adults may therefore experience not only a reduction in body water content and an increased risk of dehydration, but also more evident effects from fasting and water restriction. This may be because muscle function is affected when body (and muscle) water content falls below a certain threshold, which would be easier to cross in older individuals.
The main limitations of this study are: a) the lack of a control group, which does not allow to ensure that the observed changes are due to the (re-) hydration intervention itself (or to other non-controlled factors), b) the small sample size and consequent lack of statistical power (particularly for men) and c) the overnight fasting model used, as 12 hours does not seem to be enough to produce a relevant dehydration in middle-aged adults to clinically affect muscle function. Moreover, although BIA was able to detect small changes in body water composition, it is not the gold standard for assessing hydration status. In addition, BIA hydration parameters are indirectly estimated from closed-formulas and are interrelated, making it difficult to clearly distinguish between muscle mass and muscle hydration.
In conclusion, this study provides some evidence on the relationship between hydration status and muscle function in middle-aged individuals. It shows that BIA indicators of muscle hydration are correlated with muscle strength, work, and power in fasting conditions and that water intake after 12 hours of overnight fasting improves knee flexion strength and work in women. The effect of water intake on muscle function appears to be age-dependent, as it was only observed in individuals aged 45 years or older. Our findings suggest that hydration status plays a role in muscle strength, work, and power, especially with increasing age (and risk of dehydration). Nonetheless, more studies are required to better understand the influence of hydration status on muscle function and the mechanisms involved.
Footnotes
AUTHOR CONTRIBUTIONS
Conceptualization, MSP and IL; methodology, MSP; formal analysis, EP; investigation, JM, MC, PF; writing—original draft preparation, MSP; writing—review and editing, EPle; funding acquisition, MSP. All the authors have read and agreed to the final version of this manuscript.
INSTITUTIONAL REVIEW BOARD STATEMENT
The study was conducted in accordance with the principles of the Declaration of Helsinki and approved by the ethics committee at Consorci Sanitari del Maresme (codes CEIm CSdM 65/19 and CEIm CSdM 07/23).
INFORMED CONSENT STATEMENT
Informed consent was obtained from all individuals involved in the study.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are not openly available due to confidentiality considerations, but are available from the corresponding author upon reasonable request and approval from the ethics committee.
ACKNOWLEDGEMENTS
This research was funded by the Spanish Ministry of Health-Instituto de Salud Carlos III (ISCIII), grant number PI19/00500, PIR22/00327, and co-financed by the European Union (FEDER funds).
The authors declare no conflict of interest. This study is part of the doctoral thesis of Isabel Lorenzo at the Universitat de Vic-Universitat Central de Catalunya (UVIC).
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