Resting metabolic rate is increased after a series of whole body vibration in young men

Resting metabolic rate (RMR) is the largest component of total energy expenditure and increasing it can be of great importance in reducing excess body fatness. Whole body vibration (WBV) can affect energy expenditure during single session of WBV, but the effects of repeated WBV on resting metabolic rate have not been reported. The purpose of this study was to investigate whether a series of WBV would increase resting metabolism in young men. Thirty-two healthy men aged of 21–23 were recruited and randomly assigned to two 16-member groups: a group participating in the WBV intervention and a group without the intervention. The intervention lasted 2 weeks and WBV was performed 5 times a week. Diet, physical activity, body composition and resting metabolic rate were analyzed in the participants. In WBV group significantly increased resting oxygen uptake (p = 0.049) and consequently RMR (p = 0.035) after the intervention. Similar changes were not observed in the control group. This indicates that applied type of vibration in this study can be an addition to obesity therapy, in which, WBV can, among other beneficial metabolic effects, increase RMR and thus total energy expenditure.


Body height, body mass and body composition
The body composition of the men was examined twice before and after the intervention.Body composition was measured by Dual Energy X-ray Absorptiometry (DEXA) (Lunar Prodigy, GE, USA) according to the manufacturer's guidelines.Body mass (BM), fat-free mass (FFM), fat mass (FM) and percent body fat (%FAT) were determined.Body height was measured without shoes, in a standing position to the nearest 1 mm, with the head in the Frankfurt plane, using a stadiometer (Seca, Germany).In addition, body mass index (BMI) was calculated for each subject.

Diet and physical activity
Men completed 4-day food diaries, in which they recorded the weight or volume of each food consumed.The serving size was assessed subjectively by the participant on the basis of the Album of Product and Food Photography 19 .The caloric content of the diet and the proportion of carbohydrates, proteins and fats in the diet was then calculated by a qualified nutritionist, using the Diet 6.0 software (Food and Nutrition Institute, Warsaw, Poland).
Physical activity was assessed using the International Physical Activity Questionnaire (IPAQ, short Polish version 20 ), which the participants completed on the first day of the intervention.Before completing the questionnaire, they were instructed on its purpose and how to fill it out.The questionnaire was completed in the presence of the researcher, who instructed or answered any questions the respondents might have.

Resting metabolic rate
RMR was measured in fasting, always at the same time of day (morning), between 6:00 a.m. and 10:00 a.m.Prior to the first RMR measurement, participants were instructed on how to prepare for the measurement, i.e. avoid exercise for 3 days before the scheduled measurement, be properly hydrated, and not use any stimulants before the study (nicotine, caffeine).Participants were also recommended not to change their diet or physical activity during the intervention.RMR was measured in the supine position in an air-conditioned laboratory, at a constant temperature of 21 °C, after a prior rest of about 15 min in the supine position.Resting metabolic rate, was measured by indirect calorimetry using a Cortex MetaLyzer 3R ergospirometer (Germany), using the breath by breath method.The ergospirometer was calibrated each time according to the manufacturer's requirements (gas and volume calibration).Oxygen uptake (VO 2 ), carbon dioxide production (VCO 2 ), respiratory quotient (RQ), RMR, and substrate utilization (carbohydrate [CHO], fat [FAT], protein [PRO]) and energy expenditure (EE) from each substrate were measured during the measurement.Resting metabolism was expressed absolutely (kcal/day) and relatively to body mass (kcal/kg/day) and to body surface area (RMR/BSA) (kg/m 2 /day).RMR and other indices measured by calorimetry were determined from a 5-min measurement period during which steady state was observed in oxygen uptake.The steady state was considered to be fluctuations in oxygen uptake within ± 10%.The averages from steady state were used to calculate REE using the Weir 21 formula without using urinary urea nitrogen.All calculations were performed using dedicated metabolic rate measurement software provided by ergospirometer manufacturer (Cortex, Germany).

Statistical analysis
Sample size was estimated a priori using G*Power 3.1.9.7 (Dusseldorf, Germany).The following options were selected in the software: test family = f tests; statistical test = ANOVA repeated measures, within-between interaction; type of power analysis = compute required sample size-given α, power, and effect size.Input parameters into the software were as follows: effect size f = 0.4; α error probability = 0.01; power = 0.95; number of groups and measurements = 2; correlation among measures = 0.5; nonsphericity correction = 1.0.The required sample size was 16 subjects per group (total sample size = 32).Data distribution was checked using the Shapiro-Wilk test.Homogeneity of variance within the groups was tested via Levene's test Analysis of variance (ANOVA) with repeated measures or one-way ANOVA was used to analyze the data obtained.In case of significant changes (p < 0.05) in the analysis of variance, post-hoc analysis was performed using the Tukey test.Additionaly, in post hoc analysis (if significant), the effect size (Cohen's d) between baseline and WBV (placebo) treatment was calculated and interpreted as small (0.20), medium (0.50), or large (0.80) 22 .Data are presented as mean and standard deviation.The STATISTICA 13 package (StatSoft, Inc., Tulsa, OK, USA) was used for calculations.

Ethics declaration
The study was conducted in accordance with the Declaration of Helsinki and approved by the Bioethics Committee of Opole Medical School in Poland (no.KB/56/NOZ/2019).All participants were informed about the study protocol, voluntarily took part in the experiment and signed informed consent.

Results
No adverse effects were reported during the intervention.

Somatic build
There were no significant differences in body height, body mass or body composition between WBV and PL group.The parameters did not change significantly during the intervention either (Table 1).

Resting metabolic rate
There were no significant intergroup differences in the level of the parameters studied (Table 2).However, significant changes were observed between measurements.In the WBV group significantly increased resting oxygen uptake (p = 0.049) and consequently RMR (p = 0.035) after intervention.The significant increase in RMR in the WBV group was due to increased resting carbohydrate utilization i.e. 330.88 ± 98.1 g/day (baseline) vs. 407.13± 111.9 g/day (post intervention) (p = 0.024).Similar changes were not observed in the PL group (Table 2).

Discussion
To the best of our knowledge, this is the first study to evaluate the effects of a series of WBV on resting metabolism.A previous study 17 , which only examined the effect of a single WBV on RMR, showed that RMR was increased during WBV and the effect disappeared after WBV session.This study showed that there was a significant increase in RMR in young men after just two weeks of WBV intervention.
Resting metabolism is affected by age, physical activity, and lean body mass 3 .Long-term effects of physical activity result in increases in RMR due to increases in lean muscle mass 3 and lean body mass correlates highly with RMR 23 .Many other factors: anxiety, diurnal variation, the thermic effect of food, elevated post-exercise oxygen consumption, stimulants, and pharmaceuticals can also affect the resting metabolic rate 24 .In our study, both age, physical activity, and lean body mass were similar in both groups, so it is likely that they also had similar effects on metabolism in both groups.The participants in both groups also consumed similar amounts of carbohydrates, proteins and fats.The study's inclusion criteria also allowed for ineligibility of stimulant users and pharmaceuticals.The participants enrolled the study were rested (after an overnight rest) and avoided physical exertion for three days prior to RMR measurement.This allowed us to control most of the factors that could affect RMR.Haugen et al. 24 , also showed that repeated morning and evening measurements of RMR are stable and highly correlated and day-to-day measurements of RMR are not significantly different.This indicates that RMR measurement is highly repeatable.
In this study, by applying WBV in the prone position, it was possible, on the one hand, to deactivate postural muscles and, on the other hand, to apply additional local vibration, to stimulate directly the abdominal cavity.Local vibration, could affect the function of the gastrointestinal tract and thus, indirectly, the thermic effect of food (TEF) i.e. the energy cost of chewing, swallowing, digesting, absorbing and storing food.TEF is a significant part of energy expenditure and is influenced by, among other things, the caloric content of the diet, eating habits, physical activity and sympathetic nervous system activity 25 .It has been shown that sympathetic stimulation in the upper gastrointestinal tract may increase metabolic rate up to 15.6% 26 .Previous paper has shown that WBV can be an effective method in improving gastrointestinal function.Two weeks of low-intensity WBV was an effective therapy for reducing symptom severity in patients with chronic functional constipation 27 .For this reason, vibrating capsules have begun to be used in patients with constipation.Vibrating capsule may improve constipation by augmenting the physiological effects of waking and meals on bowel movements and circadian rhythm 28 .Vibrating capsule can promote defecation, as well as ameliorating symptoms and improving the quality of life in patients with functional constipation with sustained efficacy 29 .The applied local abdominal vibration www.nature.com/scientificreports/ was a specific abdominal vibro-massage.Abdominal massage can stimulate the parasympathetic activity and thus the gastrointestinal response 30 .It increases peristaltic movements and accelerates the passage of food through the gastrointestinal tract 31,32 .Our participants did not declare constipation or have a medical interview in this direction, nor did they report functional changes in the gastrointestinal tract after intervention, this applied local vibration, targeting the abdominal cavity, could significantly affect gastrointestinal tract function and improve its function, thus affecting TEF.
The study used WBV in combination with local vibration.Although local vibration may have affected TEF, the observed metabolic effect may also be due to the effects of WBV or the combination of local vibration and WBV.Increased metabolism during WBV may also be the result of changes in circulation because the peripheral vascular system is sensitive to vibration stimulation 33 .In a previous study 8 , an increase in mean blood flow was observed during WBV.Whole-body vibration also increases muscle activity by inducing a tonic vibration reflex in the muscles 34 .RMR is mostly dependent on the amount of metabolically active tissue in an individual (mainly muscle mass) 35 .Thus, both increasing muscle activity and improving tissue blood flow can also affect metabolic rate.

Conclusion
The results of our study indicate that after just 2 weeks of using WBV in combination with local vibration applied to the abdominal area, resting metabolism increased significantly.This indicates that this type of vibration can be an addition to obesity therapy, in which WBV can, among other beneficial metabolic effects, increase RMR and thus total energy expenditure.

Strength and limitation of the study
The strength of this study is the rigorous methodology and research methods used.Our intervention lasted 2 weeks, and after that time we observed an increase in RMR in men.Perhaps the results would have been different after a different intervention period.The reported effect applies only to vibration with the described characteristics and body position.Only young healthy men without overweight or obesity participated in this study.Further studies should also be conducted in women and those with obesity or other metabolic disorders to confirm the reported effect.

Table 1 .
Age, body height and mass and body composition of study participants before and after the intervention.BH body height, BM body mass, BMI body mass index, FFM fat free mass, WBV whole-body vibration, PL placebo.

Table 2 .
Effects of whole-body vibration series on resting metabolic rate in participants.VO 2 oxygen uptake, VCO 2 carbon dioxide production, RQ respiratory quotient, RNR resting metabolic rate, BM body mass, BSA body surface area, CHO carbohydrates, PRO proteins, EE energy expenditure, WBV whole-body vibration, PL placebo.