Introduction

The autonomic nervous system (ANS) regulates many vital bodily systems, including the cardiovascular, endocrine, and inflammatory systems. The parasympathetic (PNS) branch of the ANS is of particular importance as it represents the body’s integrative system to adaptively self-regulate and maintain homeostatic balance (1, 2). Vagally mediated heart rate variability (vmHRV), defined as the rapid beat-to-beat fluctuations in heart rate mediated by the vagus nerve, serves as a non-invasive index of PNS activity (1, 2). In a resting state, greater vmHRV is associated with better overall health, in addition to adaptive and flexible physiological responses to an ever-changing environment, with lower vmHRV being associated with disease and all-cause mortality (1–3).

Body mass index (BMI) remains an important indicator of physical health condition. Given that adaptive patterns of PNS activity are essential in maintaining good physical health (1–3), understanding how BMI impact PNS activity is of interest. Converging evidence suggests that greater BMI is associated with lower resting PNS activity as index by lower resting vmHRV, providing a psychophysiological pathway linking obesity with health and disease (4–6).

Given that obesity is associated with poorer health outcomes and that lower resting vmHRV is typically indicative of maladaptive physiological regulatory abilities (1–3), it is plausible that those with higher BMI may experience maladaptive patterns of PNS activity in response to physical stressors in comparison to those with normal BMI. However little research has been conducted to examine how BMI may influence PNS reactivity indicated by changes in vmHRV to changing environmental demands. One study found that in moderately obese subjects (BMI 26–40), vmHRV was lower in an orthostatic (standing) position compared to non-obese (BMI < 25) individuals (7). Similarly, another study found lower vmHRV to be associated with greater BMI while standing (8). However, studies have not addressed how BMI affects changes in vmHRV from rest to standing and back to rest. This is important, as the normal BMI and otherwise healthy individual should show a decrease in vmHRV and an increase in heart rate from rest to stand and a return to baseline levels when resting again. One study sought to examine BMI influences on vmHRV patters from rest to standing but results were not significant (9). Thus, the influence of BMI on PNS reactivity during orthostatic stress is not well understood given the conflicting aforementioned investigations. Moreover, research suggests that underweight individuals may experience poor health outcomes similar to obese individuals (10). However, research has yet to examine vmHRV reactivity differences between underweight and normal weight individuals.

Thus, the present investigation examines individual patterns of vmHRV as a function of low, moderate, and high BMI throughout baseline (seated rest), orthostatic (standing), and recovery (seated rest) positions. Overall, we sought to better understand the link between poor health outcomes and individuals who are over/underweight by directly assessing how BMI may influence PNS reactivity.

Materials and Methods

The study was approved by the Institutional Review Board of SRH University in Heidelberg, Germany (data collection site), and all participants provided written informed consent prior to their assessment. A total of 59 apparently healthy Caucasian students (44 females, mean age = 24.37 years, age range 19–65 years) were included in the trial. Subjects’ body measures (weight and height) were taken and BMI was obtained according to common calculation (kg/m2). Subjects reported (i) no medication intake (that may influence vmHRV) and (ii) no current suffering from chronic or acute disease.

Following informed consent, participants sat in a comfortable chair for 5-mintues for a resting (baseline) period, followed by a 5-minute orthostatic (standing) period, and finally another 5-minute resting (recovery) period. Continuous interbeat-intervals (IBI) was measured throughout the experiment at sampling frequency of 1000 Hz using a Polar RS800CX portable device, providing a temporal resolution of 1 ms for each R– R interval. The device’s transmitter consists of a stable polyamide case with electrodes attached to an elastic belt fixated to the chest of the subjects. Device-specific software (Polar ProTrainer 5) was used to transfer recordings to a personal computer. IBI data were imported and analyzed with “Kubios HRV - Heart Rate Variability Analysis Software” (Biosignal Analysis and Medical Imaging Group, University Kuopio, Finland, Version 2.0, (11s)) and analyzed. The square root of the mean squared difference of successive RR intervals (RMSSD, ms) was calculated and is considered to be a stable and valid (12) time-domain index of vmHRV. Autoregressive estimates were also calculated, yielding high frequency power HRV (HF-HRV, 0.15-0.4 Hz). In the present study RMSSD correlated highly with HF power throughout all phases of the experiment (each r > .88, p < .001), and as such we report vmHRV results using RMSSD — results were identical using HF-HRV (results not shown). Data on vmHRV measures was skewed and successfully log-transformed to meet assumptions of linear analyses. Individuals we stratified into three separate BMI groups: (i) low BMI (BMI < 20 | n = 14) ; (ii) moderate BMI (BMI = 20–25 | n = 33), (iii) high BMI (BMI > 25 | n = 12). StatsSoft Statistica 6.0 (StatSoft, Inc.,Tulsa, OK) was used to conduct a 2-way (1 between factor and 1 within factor) analysis of variance (ANOVA) to examine differences in vmHRV patterns between BMI groups. Experimental phases (baseline, standing, recovery) were used as the within factor and BMI groups (low, moderate, high) as the between factor. Because these are physiological data, within and withinbetween ANOVA interactions are reported using multivariate ANOVA tests (Wilks’ Lambda (λ) and associated degrees of freedom), and preplanned contrasts were used as a direct test of differences in vmHRV throughout the experiment (13,14). All tests were two-tailed with significance set at alpha of 0.05.

Results

Adjusted means and standard deviations (for age and gender) of vmHRV in each position split by BMI group are displayed in Table 1. In this sample, individuals’ BMI ranged from 17.16 -32.35 kg/m2.

Table 1
figure 1

Adjusted Means and Standard Deviations of vmHRV by BMI Group throughout Experimental Positions

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Controlling for age and gender, repeated measure ANOVA results showed no significant main effect of BMI group (F(2, 55) = .166, r = .077, p = .848), a significant main effect of position (λ = .720 F(2,53) = 10.28, r = .529, p < .001), and a strong trending interaction between the two (λ = .841, F(4,106) = 3.00, r = .319, p = .058) (Figure 1). Preplanned contrasted showed a significant quadratic trend overtime in each BMI group (each p < .001), such that participants’ vmHRV decreased from baseline while standing, and increased when sitting again. However in the low BMI group, vmHRV during recovery was significantly lower than vmHRV during baseline (F(1,54) = 5.57, r = .306, p = .022), indicating that this group’s vmHRV did not fully recover. Individuals in the moderate and high BMI groups did not show significant linear decreases from baseline to recovery, suggesting good vmHRV recovery from an orthostatic position.

Figure 1
figure 2

BMI Group by Position Interaction

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The high BMI group showed a significantly greater decrease in vmHRV from baseline to standing in comparison to the moderate BMI group (F(1,54) = 4.746, r = .284, p = .034). Although there were no other significant contrasts between groups, a trend showed that the moderate BMI group had slightly higher vmHRV during the orthostatic position in comparison to the high BMI group (F(1,54) = 3.29, r = .240, p = .075).

Discussion

The present study sought to explore differences in vmHRV reactivity throughout a simple orthostatic stress test (baseline-rest, standing, recovery-rest) based on BMI profiles. To our knowledge, few studies have investigated the impact BMI may have on vagal activity when making simple movements, showing that those with greater BMI had lower vmHRV during an orthostatic position (7,8). Our data replicate these previous findings, as the moderate BMI group showed slightly higher vmHRV in comparison to the high BMI group while standing. In contrast to previous research (9) and adding to the literature, those in the high BMI group showed a steeper decrease in vagal activity from baseline to standing, suggesting that those with high BMI have lower PNS activity when altering body positions. Thus, the present investigation, coupled with previous research, suggests that individuals with high BMI have lower PNS at rest (4–6) which is considered maladaptive (1–6), in addition greater withdraw of the PNS while standing, and possibly other physical activities (15, 16). We theorize that these maladaptive physiological responses may have a negative impact on overall health in these individuals.

Results also showed lower vmHRV during recovery compared to baseline in low BMI group, suggesting that these individuals’ vmHRV did not fully recover from the orthostatic position as it did in individuals with moderate and high BMI groups. This suggests prolonged vmHRV reactivity in response to the orthostatic position, which can be considered maladaptive and harmful for health (3). Future research is needed to understand potential physiological mechanisms and time course underlying this phenomenon in those with low BMI.

Both age and gender can influence vmHRV (17, 18) and thus, were included as covariates in the current investigation. Importantly, the age-range for the current sample was between 19 and 65 years, with 46 individuals falling between 19 and 25, eight individuals between 25 and 30, three individuals between 30 and 40, and 2 individuals over 40. Therefore, it is important to note that interpretations and implications of the current study are applicable primarily to younger adults.

One limitation of the current study is that we did not control for body composition (i.e., fat vs. lean mass), and future investigations should include this variable as a covariate as the interpretation of BMI can be misleading depending on body composition (19).

Overall, the present investigation suggests that both low and high BMI can lead to maladaptive patterns in vmHRV reactivity under different conditions. In line with previous research (7,8), those with high BMI show a greater decrease in vmHRV in response to standing and possibly other physical activities, whereas those with low BMI show prolonged decreased vmHRV following standing, as they did not recovery back to baseline within the 5-minute rest period. Future research needs to replicate the present findings extending the experimental paradigm to other physical activities.

Ethical Standards: The study was approved by the Institutional Review Board of SRH University in Heidelberg, Germany (data collection site), and all participants provided written informed consent prior to their assessment.

Conflict of Interest: The authors did not receive funding for the work presented. The authors declare no conflict of interests, all authors are listed, and all have contributed substantially to the manuscript.