Reliability of low-power cycling efficiency in energy expenditure phenotyping of inactive men and women

Interindividual variability of human thermoregulation: Toward personalized ergonomics of the indoor thermal environment

OBJECTIVE

The study was undertaken to show the magnitude of interindividual differences in energy expenditure (i.e., heat production) under normal living conditions with the aim of providing physiological evidence to support the advancement of a personalized thermal conditioning approach.

METHODS

Three sets of experimental protocols with six participants were conducted at neutral and mild cold temperatures. Energy expenditure, local skin temperatures, and core body temperature were measured continuously, while cognitive performance and thermal sensation were surveyed intermittently. The protocols were designed to study the effects of several normal day activities, low-level physical activity and eating a meal, on metabolic and physiological parameters.

RESULTS

Large interindividual differences among the subjects were demonstrated using non-normalized data by design. The resting metabolic rate difference was 58%, the percentage change in energy expenditure during standing compared to sitting was up to 31%, and the difference in mechanical work efficiency between the least and the most efficient individual was 39.1%. Energy expenditure increase due to the meal effect was 11.2% to 23.3% at neutral and 9.9% to 33.9% at mild cold temperatures across individuals.

CONCLUSIONS

Large interindividual differences in metabolic rate under typical everyday living and office activities suggest facilitating personalized thermal conditioning instead of providing uniform temperature. Therefore, it is necessary to find noninvasive markers that can be easily measured and used as surrogates for human heat production to individualize the climate control of buildings.

Dynamics of Fat Oxidation from Sitting at Rest to Light Exercise in Inactive Young Humans

Societal erosion of daily life low-level physical activity has had a great influence on the obesity epidemic. Given that low fat oxidation is also a risk factor for obesity, we investigated, in a repeated measures design, the dynamics of fat oxidation from a resting state to a light-intensity leg cycling exercise (0–50 watts) in inactive, healthy young adults. Using indirect calorimetry, energy expenditure and the respiratory quotient (RQ) were assessed in a sitting posture at rest and during a cycling exercise in 35 subjects (20 women). The rate of perceived exhaustion (RPE) was assessed using the Borg Scale. During graded leg cycling, the mean RPE did not exceed values corresponding to the exercise being perceived as ‘light’. However, analysis of individual data at 50 watts revealed two distinct subgroups among the subjects: those having RPE values corresponding to the exercise being perceived as ‘very light to light’ and showing no increase in RQ relative to resting levels, as opposed to an increase in RQ in those who perceived the exercise as being ‘somewhat hard to hard’ (p < 0.001). Our study in inactive individuals showing that high fat oxidation was maintained during ‘light-perceived’ physical activity reinforced the potential importance of light physical activity in the prevention of obesity.

A Comparison of Methodological Approaches to Measuring Cycling Mechanical Efficiency

BACKGROUND

Much is known about theoretical bases of different mechanical efficiency indices and effects of physiological and biomechanical factors to them. However, there are only a few studies available about practical bases and interactions between these efficiency indices, which were the aims of the present study.

METHODS

Fourteen physically active men (n = 12) and women (n = 2) participated in this study. From the incremental test, six different mechanical efficiency indices were calculated for cycling work: gross (GE) and net (NE) efficiencies, two work efficiencies (WE), and economy (T) at 150 W, and in addition delta efficiency (DE) using 3-5 observation points.

RESULTS

It was found that the efficiency indices can be divided into three groups by Spearman's rank correlation: GE, T, and NE in group I; DE and extrapolated WE in group II; and measured WE in group III. Furthermore, group II appeared to have poor reliability due to its dependence on a work-expended energy regression line, which accuracy is poorly measured by confidence interval.

CONCLUSION

As efficiency indices fall naturally into three classes that do not interact with each other, it means that they measure fundamentally different aspects of mechanical efficiency. Based on problems and imprecisions with other efficiency indices, GE, or group I, seems to be the best indicator for mechanical efficiency because of its consistency and unambiguity. Based on this methodological analysis, the baseline subtractions in efficiency indices are not encouraged.

Mechanical Efficiency at Different Exercise Intensities Among Adolescent Boys With Different Body Fat Levels

This study investigated the mechanical efficiency (ME) and associated factors in obese, overweight, and normal-weight adolescent boys during incremental cycle exercise test to exhaustion. Forty-five sedentary adolescent boys (13–14 years old) were separated in three groups according to the percentage of fat mass as follows: 15 normal-weight (NW) (body fat: 16.0 ± 1.9%), 15 overweight (OW) (body fat: 24.0 ± 1.6%), and 15 obese (OB) (body fat: 31.0 ± 3.0%). All groups completed an incremental cycle exercise to exhaustion in which energy consumption (E, W), ME (%), lipid oxidation rate (LO, %), plasma epinephrine and norepinephrine concentrations were determined consecutively at rest and at three intensity levels corresponding to 50 and 75% of each participant’s maximal heart rate (50%HRmax and 75%HRmax) and peak oxygen consumption (V˙O_2peak). During the incremental cycle exercise test, plasma epinephrine, and norepinephrine responses as well as ME determined at 50%HRmax, 75%HRmax, and at VO_2peak stages were significantly lower in OB compared to NW and OW individuals (ps < 0.01). Multiple linear regressions showed that body weight (ß = -0.64, p < 0.001), energy consumption (ß = -0.24, p < 0.05) and lipid oxidation (ß = 0.69, p < 0.01) were significant predictors of ME at 50%HRmax. However, at 75%HRmax and V˙O_2peak, significant predictors of ME were epinephrine (ß = 0.34, ß = 0.49, respectively, ps = 0.01), norepinephrine (ß = 0.26, ß = 0.60, respectively, ps < 0.05) and power output (ß = 0.62, ß = 0.71, respectively, ps < 0.01). These findings suggest that excess in body weight exerts a negative effect on ME at a low intensity by increasing energy consumption for obese and overweight adolescent boys, while at higher intensities (75%HRmax and VO_2peak) the lower ME could be better explained by the lower power output and catecholamine responses that were attenuated among obese and overweight adolescent boys.

Superior Intrinsic Mitochondrial Respiration in Women Than in Men

Sexual dimorphism is apparent in humans, however, to date no studies have investigated mitochondrial function focusing on intrinsic mitochondrial respiration (i.e., mitochondrial respiration for a given amount of mitochondrial protein) and mitochondrial oxygen affinity (p50_mito) in relation to biological sex in human. A skeletal muscle biopsy was donated by nine active women, and ten men matched for maximal oxygen consumption (VO_2max) and by nine endurance trained men. Intrinsic mitochondrial respiration, assessed in isolated mitochondria, was higher in women compared to men when activating complex I (CI_P) and complex I+II (CI+II_P) (p < 0.05), and was similar to trained men (CI_P, p = 0.053; CI+II_P, p = 0.066). Proton leak and p50_mito were higher in women compared to men independent of VO_2max. In conclusion, significant novel differences in mitochondrial oxidative function, intrinsic mitochondrial respiration and p50_mito exist between women and men. These findings may represent an adaptation in the oxygen cascade in women to optimize muscle oxygen uptake to compensate for a lower oxygen delivery during exercise.

Cycling Efficiency During Incremental Cycle Ergometry After 24 Hours of Overfeeding or Fasting

OBJECTIVE

The objective of this study was to determine whether net cycling efficiency (NET) is altered by 24-hour fasting or overfeeding and whether it correlates with dietary-related energy expenditure (EE) and future weight change.

METHODS

In a crossover design, healthy subjects fasted or were overfed for 24 hours while in a whole-room calorimeter using five diets with doubled energy needs: standard, high-carbohydrate (75%), high-fat (60%), high-protein (30%), and low-protein (3%) diets. Graded cycling exercise at low power outputs (10-25-50 W) was performed the day before and after each dietary intervention.

RESULTS

NET did not change following any dietary intervention (all P > 0.05 vs. 0). Individual changes in NET did not correlate with EE responses to dietary interventions. However, the change in NET after low-protein overfeeding was inversely correlated with baseline body fat (r = -0.60, P = 0.01); that is, NET increased in lean but decreased in overweight subjects (Δ = 0.010 ± 0.010 vs. -0.013 ± 0.009, P = 0.0003). Increased NET following the low-protein diet was associated with weight gain after 6 months (r = 0.60, P = 0.05).

CONCLUSIONS

Despite no substantial effect of acute overfeeding or fasting on NET, the change in NET following low-protein overfeeding depends on adiposity and may influence weight change, suggesting that increased efficiency in a setting of protein scarcity is an adaptive response that may ultimately lead to weight gain.

Posture Allocation Revisited: Breaking the Sedentary Threshold of Energy Expenditure for Obesity Management

There is increasing recognition that low-intensity physical activities of daily life play an important role in achieving energy balance and that their societal erosion through substitution with sedentary (mostly sitting) behaviors, whether occupational or for leisure, impact importantly on the obesity epidemic. This has generated considerable interest for better monitoring, characterizing, and promoting countermeasures to sedentariness through a plethora of low-level physical activities (e.g., active workstations, standing desks, sitting breaks), amid the contention that altering posture allocation (lying, sitting, standing) can modify energy expenditure to impact upon body weight regulation and health. In addressing this contention, this paper first revisits the past and more recent literature on postural energetics, with particular emphasis on potential determinants of the large inter-individual variability in the energy cost of standing and the impact of posture on fat oxidation. It subsequently analyses the available data pertaining to various strategies by which posture allocations, coupled with light physical activity, may increase energy expenditure beyond the sedentary threshold, and their relevance as potential targets for obesity management.

Reliability of low-power cycling efficiency in energy expenditure phenotyping of inactive men and women

Standardized approaches to assess human energy expenditure (EE) are well defined at rest and at moderate to high-intensity exercise, but not at light intensity physical activities energetically comparable with those of daily life (i.e., 1.5-4 times the resting EE, i.e., 1.5-4 METs). Our aim was to validate a graded exercise test for assessing the energy cost of low-intensity dynamic work in physically inactive humans, that is, those who habitually do not meet the guidelines for moderate-to-vigorous aerobic physical activity levels. In healthy and inactive young men and women (n = 55; aged 18-32 years), EE was assessed in the overnight-fasted state by indirect calorimetry at rest and during graded cycling between 5 and 50W for 5 min at each power output on a bicycle ergometer. Repeatability was investigated on three separate days, and the effect of cadence was investigated in the range of 40-90 rpm. Within the low power range of cycling, all subjects perceived the exercise test as "light" on the Borg scale, the preferred cadence being 60 rpm. A strong linearity of the EE-power relationship was observed between 10 and 50 W for each individual (r > 0.98), and the calculation of delta efficiency (DE) from the regression slope indicated that DE was similar in men and women (~29%). DE showed modest inter-individual variability with a coefficient of variation (CV) of 11%, and a low intra-individual variability with a CV of ~ 5%. No habituation or learning effect was observed in DE across days. In conclusion, the assessment of the efficiency of low power cycling by linear regression - and conducted within the range of EE observed for low-intensity movements of everyday life (1.5-4 METs) - extends the capacity for metabolic phenotyping in the inactive population.

Issues in Continuous 24-h Core Body Temperature Monitoring in Humans Using an Ingestible Capsule Telemetric Sensor

BACKGROUND

There is increasing interest in the use of pill-sized ingestible capsule telemetric sensors for assessing core body temperature (Tc) as a potential indicator of variability in metabolic efficiency and thrifty metabolic traits. The aim of this study was to investigate the feasibility and accuracy of measuring Tc using the CorTemp^® system.

METHODS

Tc was measured over an average of 20 h in 27 human subjects, with measurements of energy expenditure made in the overnight fasted state at rest, during standardized low-intensity physical activity and after a 600 kcal mixed meal. Validation of accuracy of the capsule sensors was made ex vivo against mercury and electronic thermometers across the physiological range (35-40°C) in morning and afternoon of 2 or 3 consecutive days. Comparisons between capsule sensors and thermometers were made using Bland-Altman analysis. Systematic bias, error, and temperature drift over time were assessed.

RESULTS

The circadian Tc profile classically reported in free-living humans was confirmed. Significant increases in Tc (+0.2°C) were found in response to low-power cycling at 40-50 W (~3-4 METs), but no changes in Tc were detectable during low-level isometric leg press exercise (<2 METs) or during the peak postprandial thermogenesis induced by the 600 kcal meal. Issues of particular interest include fast "turbo" gut transit with expulsion time of <15 h after capsule ingestion in one out of every five subjects and sudden erratic readings in teletransmission of Tc. Furthermore, ex vivo validation revealed a substantial mean bias (exceeding ±0.5°C) between the Tc capsule readings and mercury or electronic thermometers in half of the capsules. When examined over 2 or 3 days, the initial bias (small or large) drifted in excess of ±0.5°C in one out of every four capsules.

CONCLUSION

Since Tc is regulated within a very narrow range in the healthy homeotherm's body (within 1°C), physiological investigations of Tc require great accuracy and precision (better than 0.1°C). Although ingestible capsule methodology appears of great interest for non-invasively monitoring the transit gut temperature, new technology requires a reduction in the inherent error of measurement and elimination of temperature drift and warrants more interlaboratory investigation on the above factors.