Mild intermittent hypoxia exposure induces metabolic and molecular adaptations in men with obesity.

OBJECTIVE: Recent studies suggest that hypoxia exposure may improve glucose homeostasis, but well-controlled human studies are lacking. We hypothesized that mild intermittent hypoxia (MIH) exposure decreases tissue oxygen partial pressure (pO2) and induces metabolic improvements in people who are overweight/obese. METHODS: In a randomized, controlled, single-blind crossover study, 12 men who were overweight/obese were exposed to MIH (15 % O2, 3 × 2 h/day) or normoxia (21 % O2) for 7 consecutive days. Adipose tissue (AT) and skeletal muscle (SM) pO2, fasting/postprandial substrate metabolism, tissue-specific insulin sensitivity, SM oxidative capacity, and AT and SM gene/protein expression were determined. Furthermore, primary human myotubes and adipocytes were exposed to oxygen levels mimicking the hypoxic and normoxic AT and SM microenvironments. RESULTS: MIH decreased systemic oxygen saturation (92.0 ± 0.5 % vs 97.1 ± 0.3, p < 0.001, respectively), AT pO2 (21.0 ± 2.3 vs 36.5 ± 1.5 mmHg, p < 0.001, respectively), and SM pO2 (9.5 ± 2.2 vs 15.4 ± 2.4 mmHg, p = 0.002, respectively) compared to normoxia. In addition, MIH increased glycolytic metabolism compared to normoxia, reflected by enhanced fasting and postprandial carbohydrate oxidation (pAUC = 0.002) and elevated plasma lactate concentrations (pAUC = 0.005). Mechanistically, hypoxia exposure increased insulin-independent glucose uptake compared to standard laboratory conditions (~50 %, p < 0.001) and physiological normoxia (~25 %, p = 0.019) through AMP-activated protein kinase in primary human myotubes but not in primary human adipocytes. MIH upregulated inflammatory/metabolic pathways and downregulated extracellular matrix-related pathways in AT but did not alter systemic inflammatory markers and SM oxidative capacity. MIH exposure did not induce significant alterations in AT (p = 0.120), hepatic (p = 0.132) and SM (p = 0.722) insulin sensitivity. CONCLUSIONS: Our findings demonstrate for the first time that 7-day MIH reduces AT and SM pO2, evokes a shift toward glycolytic metabolism, and induces adaptations in AT and SM but does not induce alterations in tissue-specific insulin sensitivity in men who are overweight/obese. Future studies are needed to investigate further whether oxygen signaling is a promising target to mitigate metabolic complications in obesity. CLINICAL TRIAL REGISTRATION: This study is registered at the Netherlands Trial Register (NL7120/NTR7325).

Validity and reproducibility of VO2 max testing in a respiration chamber.

The aim of this study was to investigate whether VO2 max can be accurately measured in a respiration chamber. Thirty participants aged 23.4 ± 3.9 years with a wide range in VO2 max were included. Participants performed four incremental cycle ergometer tests (VO2 max) with a minimum of 5 days between tests. These tests consisted of one familiarization test with face mask, followed by two VO2 max tests in the respiration chamber and one test with face mask in randomized order. Oxygen consumption and CO2 production were measured continuously using Omnical (Maastricht University, the Netherlands) gas analysis system. The mean VO2 max was 3634 ± 766 ml, which resulted in mean VO2 max per lean body mass of 60.8 ± 8.0 ml/kg. Repeated respiration chamber tests showed a high concordance, and no significant differences were detected between tests (Lin's concordance correlation coefficient (Rc) = 0.99; ∆70 ± 302 ml/min; p = .38). There was high concordance between the mean VO2 max from both respiration chamber tests and the mean face mask tests, and no significant difference (Rc = 0.99; ∆41 ± 173 ml/min; p = .22) was observed. The Bland-Altman plots showed no proportional bias between different tests. In conclusion, the respiration chamber has been found to be a valid and reproducible method for measuring VO2 max. New research opportunities are possible in the respiration chamber, such as maximal exercise testing during 24-hour measurements.

Test-retest variability of VO2max using total-capture indirect calorimetry reveals linear relationship of VO2 and Power.

This study aimed to analyze the intra-individual variation in VO2max of human subjects using total-capture and free-flow indirect calorimetry. Twenty-seven men (27 ± 5 year; VO2max 49-79 mL•kg-1 •min-1 ) performed two maximal exertion tests (CPETs) on a cycle ergometer, separated by a 7 ± 2 day interval. VO2 and VCO2 were assessed using an indirect calorimeter (Omnical) with total capture of exhalation in a free-flow airstream. Thirteen subjects performed a third maximal exertion test using a breath-by-breath calorimeter (Oxycon Pro). On-site validation was deemed a requirement. For the Omnical, the mean within-subject CV for VO2max was 1.2 ± 0.9% (0.0%-4.4%) and for ergometer workload P max 1.3 ± 1.3% (0%-4.6%). VO2max values with the Oxycon Pro were significantly lower in comparison with Omnical (P < 0.001; t test) with mean 3570 vs 4061 and difference SD 361 mL•min-1 . Validation results for the Omnical with methanol combustion were -0.05 ± 0.70% (mean ± SD; n = 31) at the 225 mL•min-1 VO2 level and -0.23 ± 0.80% (n = 31) at the 150 mL•min-1 VCO2 level. Results using gas infusion were 0.04 ± 0.75% (n = 34) and -0.99 ± 1.05% (n = 24) over the respective 500-6000 mL•min-1 VO2 and VCO2 ranges. Validation results for the Oxycon Pro in breath-by-breath mode were - 2.2 ± 1.6% (n = 12) for VO2 and 5.7 ± 3.3% (n = 12) for VCO2 over the 1000-4000 mL•min-1 range. On a Visual analog scale, participants reported improved breathing using the free-flow indirect calorimetry (score 7.6 ± 1.2 vs 5.1 ± 2.7, P = 0.008). We conclude that total capturing free-flow indirect calorimetry is suitable for measuring VO2 even with the highest range. VO2max was linear with the incline in P max over the full range.

Determining the Accuracy and Reliability of Indirect Calorimeters Utilizing the Methanol Combustion Technique.

BACKGROUND: Several indirect calorimetry (IC) instruments are commercially available, but comparative validity and reliability data are lacking. Existing data are limited by inconsistencies in protocols, subject characteristics, or single-instrument validation comparisons. The aim of this study was to compare accuracy and reliability of metabolic carts using methanol combustion as the cross-laboratory criterion. METHODS: Eight 20-minute methanol burn trials were completed on 12 metabolic carts. Respiratory exchange ratio (RER) and percent O2 and CO2 recovery were calculated. RESULTS: For accuracy, 1 Omnical, Cosmed Quark CPET (Cosmed), and both Parvos (Parvo Medics trueOne 2400) measured all 3 variables within 2% of the true value; both DeltaTracs and the Vmax Encore System (Vmax) showed similar accuracy in measuring 1 or 2, but not all, variables. For reliability, 8 instruments were shown to be reliable, with the 2 Omnicals ranking best (coefficient of variation [CV] < 1.26%). Both Cosmeds, Parvos, DeltaTracs, 1 Jaeger Oxycon Pro (Oxycon), Max-II Metabolic Systems (Max-II), and Vmax were reliable for at least 1 variable (CV ≤ 3%). For multiple regression, humidity and amount of combusted methanol were significant predictors of RER (R2 = 0.33, P < .001). Temperature and amount of burned methanol were significant predictors of O2 recovery (R2 = 0.18, P < .001); only humidity was a predictor for CO2 recovery (R2 = 0.15, P < .001). CONCLUSIONS: Omnical, Parvo, Cosmed, and DeltaTrac had greater accuracy and reliability. The small number of instruments tested and expected differences in gas calibration variability limits the generalizability of conclusions. Finally, humidity and temperature could be modified in the laboratory to optimize IC conditions.

Overnight energy expenditure determined by whole-body indirect calorimetry does not differ during different sleep stages.

BACKGROUND: Sleep has been associated with the regulation of energy balance, yet the relation between sleep stages and energy expenditure remains unclear. OBJECTIVE: The objective was to investigate the relation between sleep stages and energy expenditure, with sleep stage and overnight energy expenditure patterns taken into account. DESIGN: Thirteen subjects aged (mean ± SD) 24.3 ± 2.5 y with a BMI (in kg/m(2)) of 23.6 ± 1.7 slept in a respiration chamber while sleep was polysomnographically recorded to determine wake after sleep onset (WASO), slow-wave sleep (SWS), and rapid eye movement (REM) sleep. Energy expenditure was calculated during each sleep stage for the whole night and separately for sleeping metabolic rate (SMR; ie, 3-h period during the night with the lowest mean energy expenditure) and non-SMR. RESULTS: Energy expenditure and sleep stages showed characteristic patterns during the night, independently of each other. Sleep stages exerted no effect on energy expenditure during the whole night, except for WASO compared with SWS (P < 0.05) and WASO compared with REM sleep (P < 0.05). During the SMR and non-SMR periods of the night, no overall effect of sleep stage on energy expenditure, except for WASO compared with SWS (P < 0.05) and WASO compared with REM sleep (P < 0.01) during the non-SMR period of the night, was found. Energy expenditure and activity counts during the night were positively correlated (r = 0.927, P < 0.001). CONCLUSIONS: Energy expenditure does not vary according to sleep stage overnight, except for higher energy expenditure during wake episodes than during SWS and REM sleep. Coincidence of the sleep stage pattern and the overnight energy expenditure pattern may have caused accidental relations in previous observations. This trial was registered at http://apps.who.int/trialsearch as NTR2926.

Eating what you like induces a stronger decrease of 'wanting' to eat.

Human eating behavior may be influenced non-homeostatically by the rewarding value of foods, i.e. 'liking' (pleasure/palatability) and 'wanting' (incentive motivation). The objectives of this study were to validate a computer test for assessment of rewarding value of food, and to assess how rewarding value of food is affected by eating a dessert-specific (chocolate mousse, CM) vs. dessert non-specific, neutral food item (cottage cheese, CC). Seventy-three subjects (47f/26m, age 27.8+/-10.0 y, BMI 24.1+/-3.3 kg/m(2)), studied in a randomized cross-over design, came to the university twice, fasted. A computer test was developed to determine rewarding value, i.e. 'liking' and 'wanting', for 72 items divided in six categories (bread, filling, drinks, dessert, sweets, stationery). 'Liking' was measured by indicating relative preference of paired items (within/between categories), 'wanting' by working to earn items to choose from. Subjects completed the computer test before and after consumption of CM/CC, matched for energy content (5.6 kJ/g) and daily energy requirements (10%). 'Liking' and 'wanting' scores of all fasted subjects on the two test-days showed 62-73% reproducibility. CM was liked more than CC (p<0.001). Consumption of CM decreased 'wanting' for bread, filling, drinks and dessert (p<0.03). Consumption of CC decreased 'wanting' for bread only (p<0.05). Contrary to CC, CM decreased relative 'liking' for the dessert category (p<0.001). In conclusion, the computer test for measurement of 'liking' and 'wanting' is sufficiently valid. Eating a highly liked food item induces a more distinct decrease in 'wanting' for food items in general and category-specific 'liking', than eating a sufficiently liked neutral food item.

Intra-individual variability and adaptation of overnight- and sleeping metabolic rate.

The largest component of daily energy expenditure is resting energy expenditure as reflected in overnight metabolic rate (OMR) and sleeping metabolic rate (SMR). Here, we determined the variation in OMR (24:00-6:00 h) and SMR values (3 h intervals) as affected by physical activity (PA) during the day and the night. Subjects were 32 females and 17 males, age 18-52 years. Energy expenditure (EE) was measured for 36 h in a whole room calorimeter (14 m3), starting in the evening, providing values before and after behavioral limitation. The mean intra-individual coefficient of variation was 1.8+/-1.4% for SMRmin (minimum EE), 2.8+/-2.0% for SMRact (minimum PA), 2.4+/-2.5% for SMRres (minimum residual EE, residual calculated from 24 h relationship between EE and PA) and 2.8+/-2.2% for OMR (n=49). Mean clock time for SMR ranged from 3:15 till 4:13 h. EE and PA increased in the hour before awakening. Surprisingly, OMR showed a significant 2.7% increase (P<0.05) during the second night of the 36 h measurement, but only for a second visit, and was related to increased physical activity during night period (R2=0.50, P<0.001). OMR measurements following unrestricted daily activity showed identical results for first and second (repeat) visits: 6.82+/-0.86 MJ/day and 6.79+/-0.93 MJ/day (n=49), respectively. It is advised to measure SMR based on minimum residual EE during nights following free-living conditions, and to exclude EE measures 1 h before awakening from SMR and OMR calculations to prevent influences of habitual wake-up time.

Heat production and body temperature during cooling and rewarming in overweight and lean men.

OBJECTIVE: To compare overweight and lean subjects with respect to thermogenesis and physiological insulation in response to mild cold and rewarming. RESEARCH METHODS AND PROCEDURES: Ten overweight men (mean BMI, 29.2 +/- 2.8 kg/m(2)) and 10 lean men (mean BMI, 21.1 +/- 2.0 kg/m(2)) were exposed to cold air for 1 hour, followed by 1 hour of rewarming. Body composition was determined by hydrodensitometry and deuterium dilution. Heat production and body temperatures were measured continuously by indirect calorimetry and thermistors, respectively. Muscle activity was recorded using electromyography. RESULTS: In both groups, heat production increased significantly during cooling (lean, p = 0.004; overweight, p = 0.006). The increase was larger in the lean group compared with the overweight group (p = 0.04). During rewarming, heat production returned to baseline in the overweight group and stayed higher compared with baseline in the lean group (p = 0.003). The difference in heat production between rewarming and baseline was larger in the lean (p = 0.01) than in the overweight subjects. Weighted body temperature of both groups decreased during cold exposure (lean, p = 0.002; overweight, p < 0.001) and did not return to baseline during rewarming. DISCUSSION: Overweight subjects showed a blunted mild cold-induced thermogenesis. The insulative cold response was not different among the groups. The energy-efficient response of the overweight subjects can have consequences for energy balance in the long term. The results support the concept of a dynamic heat regulation model instead of temperature regulation around a fixed set point.

Intra-individual variation of basal metabolic rate and the influence of daily habitual physical activity before testing.

The present study determined the intra-individual variation of BMR measurements, using a standard out-patient protocol, with the subjects transporting themselves to the laboratory for the BMR measurements after spending the night at home. The effect of a non-fasting state and variation in daily habitual physical activity the day before testing was evaluated. Eight male and eleven female subjects participated in three BMR measurements with 2-week intervals. Physical activity was estimated with a tri-axial accelerometer for movement registration, during the 3 d before each BMR measurement. There were no significant differences in estimated BMR (ANOVA repeated measures, P=0.88) and in physical activity (ANOVA repeated measures, P=0.21). Mean within-subject CV in BMR was found to be 3.3 (SD 2.1) %, ranging from 0.4 to 7.2 %. Differences between BMR measurements could not be explained by differences in physical activity the day before; however the mean within-subject CV in BMR changed from 5.7 to 5.2 % after correcting for within-machine variability and from 5.2 to 3.3 % after excluding five measurements because of non-compliance to the protocol including fasting. In conclusion, BMR values measured with a standard out-patient protocol are sufficiently reproducible for most practical purposes despite the within-subject variability in physical activity the day before the measurement. For this purpose, however, non-fasting subjects must be excluded and a regular function check of the ventilated-hood system is recommendable.