The influence of cardiorespiratory fitness level on the relationship between work rates at the aerobic threshold (AerT) and the point of maximal fat oxidation (Fatmax) in untrained adults.

Introduction: In this study, we investigated the impact of cardiorespiratory fitness (CRF), quantified as peak oxygen consumption (VO2peak), on the relationship between work rates (WR) at the aerobic threshold (AerT) and the point of maximal fat oxidation rate (Fatmax). Methods: A total of 761 untrained adults aged 41-68 completed a one-minute incremental exercise test on a cycle ergometer, using breath-by-breath gas analysis to determine VO2peak, AerT, and Fatmax. AerT was determined using automatic and visual detection methods, and Fatmax was determined using indirect calorimetry. Participants were categorized into CRF-groups: low (<25th percentile), medium (≥25th percentile and <75th percentile), and high (≥75th percentile). Results: Fatmax was found at 43 ± 7% WRpeak, 37% ± 6% WRpeak and 35% ± 7% WRpeak in the low, medium, and high CRF-groups, respectively. In contrast, AerT was located at significantly higher relative work rates: 51% ± 8% WRpeak, 47% ± 10% WRpeak, and 47% ± 11% WRpeak in the respective CRF-groups. There was a weak agreement between Fatmax and AerT [intraclass correlation coefficient (ICC) = .19, p < .001], and the ICC decreased from .35 to .12 to .13, while the mean bias ±95% limits of agreement increased from 8% ± 14% WRpeak to 8% ± 19% WRpeak to 12% ± 44% WRpeak from CRF-low to CRF-medium to CRF-high. The mean difference between Fatmax and AerT was significantly different among the CRF subgroups: 8% ± 7% WRpeak vs. 10% ± 10% WRpeak vs. 12% ± 11% WRpeak in low, medium, and high CRF-groups, respectively. Nonetheless, multiple regression analysis revealed only a weak positive correlation between the difference in relative work rates (% WRpeak) between Fatmax and AerT (dependent variable) and the predictor variables CRF and sex, both identified as significant (R = .19, p < .001). Conclusion: Our study confirms substantial differences in exercise intensities between Fatmax and AerT in untrained adults (10% ± 19% WRpeak, ranging from -14% to 53% WRpeak). Importantly, this difference remains relatively consistent across varying CRF levels, emphasizing the distinct nature of Fatmax and AerT, with CRF playing a limited role in influencing their relationship in our study's untrained adults.

Influence of the time of day in the effect of caffeine on maximal fat oxidation during exercise in women: a randomized, crossover, double-blind, and placebo-controlled study.

PURPOSE: Caffeine is a stimulant with well-recognized performance and metabolic benefits, however, there is a lack of studies investigating the time-of-day influence in the properties of caffeine to enhance fat oxidation in women. Thus, the aim of the present study was to evaluate the influence of the time of the day on the effect of caffeine on the maximal rate of fat oxidation during aerobic exercise in trained women. METHODS: Fourteen female athletes (25.5 ± 7.1 years) took part in a randomized, crossover, double-blind study. All participants undertook four different experimental trials combining the ingestion of 3 mg/kg caffeine and a placebo either in the morning (8.00-10.00 h) and in the evening (17.00-19.00 h) realizing an incremental test on a cycle ergometer with 3 min stages at workloads from 30 to 70% of maximal oxygen uptake (VO2max). Substrate oxidation rates were measured by indirect calorimetry. In each trial, the maximum rate of fat oxidation (MFO) and the intensity that elicited MFO (Fatmax) were measured. RESULTS: In comparison to placebo, MFO was significantly higher with caffeine both in the morning (0.24 ± 0.13 vs 0.30 ± 0.14 g/min; p < 0.001; ES = 0.79) and in the evening (0.21 ± 0.08 vs 0.28 ± 0.10 g/min; p = 0.002; ES = 0.72). No time-of-day effect on the capacity of caffeine to increase MFO was found (all p = 0.336) CONCLUSION: The intake of 3 mg/kg of caffeine increased the use of fat as a fuel during exercise independently of the time-of-day in trained women. TRIAL REGISTRATION: The study was registered in ClinicalTrials.gov with the following ID: NCT05880186 by 15 May 2023.

Training zones through muscle oxygen saturation during a graded exercise test in cyclists and triathletes.

Use of muscle oxygen saturation (SmO2) has been validated as a performance factor during incremental exercise with portable near-infrared stereoscopy (NIRS) technology. However, there is little knowledge about the use of SmO2 to identify training zones. The objective of this study was to evaluate the metabolic zones by SmO2: maximum lipid oxidation zone (Fatmax), ventilatory thresholds (VT1 and VT2) and maximum aerobic power (MAP) during a graded exercise test (GXT). Forty trained cyclists and triathletes performed a GXT. Output power (W), heart rate (HR), oxygen consumption (VO2), energy expenditure (kcal/min) and SmO2 were measured. Data were analysed using the ANOVA test, ROC curves and multiple linear regressions. Significance was established at p ≤ 0.05. SmO2 decreases were observed from baseline (LB) to Fatmax (Δ = -16% p < 0.05), Fatmax to VT1 (Δ = -16% p < 0.05) and VT1 to VT2 (Δ = -45% p < 0.01). Furthermore, SmO2 together with weight, HR and output power have the ability to predict VO2 and energy expenditure by 89% and 90%, respectively. We conclude that VO2 and energy expenditure values can be approximated using SmO2 together with other physiological parameters and SmO2 measurements can be a complementary parameter to discriminate aerobic workload and anaerobic workload in athletes.

No diurnal variation is present in maximal fat oxidation during exercise in young healthy women: A cross-over study.

Maximal fat oxidation during exercise (MFO) and the intensity that elicits MFO (Fatmax) seems to show a diurnal variation in men, which favours an increased performance in the afternoon than the morning. At present, it remains unknown whether the observed MFO and Fatmax diurnal variation in men is also present in women. Therefore, the current study examined the diurnal variations of MFO and Fatmax in women. Nineteen healthy women (age: 26.9 ± 8.7 years, maximum oxygen uptake: 39.8 ± 6.5 ml/kg/min) participated in the study. MFO and Fatmax were determined by a graded exercise test in cycloergometer using a cross-over design performed on two separate daytime schedules, one conducted in the morning (8am-11am) and one in the afternoon (5pm-8pm). Stoichiometric equations were used to calculate fat oxidation rates. There were no significant differences between MFO-morning and MFO-afternoon (0.24 ± 0.10 vs. 0.23 ± 0.07 g/min, respectively; P = 0.681). Similarly, there was no significant differences between Fatmax-morning and Fatmax-afternoon (41.1 ± 4.7 vs. 42.6 ± 5.5% of maximal oxygen uptake, respectively; P = 0.305). These results persisted after controlling for fat mass percentage (all P > 0.5). In summary, the main finding of the present study was that MFO and Fatmax were similar independent of the time-of-day when the exercise test is performed in healthy women. These results have important clinical implications since they suggest that, in contrast to what was found in men, MFO and Fatmax show similar rates during the course of the day in women.HighlightsMFO and Fatmax were similar during the afternoon and morning in young healthy women.Our results suggest that, in women, it does not matter when endurance exercise is performed in term of fat metabolism during exercise.

Fat Oxidation during Exercise in People with Spinal Cord Injury, and Protocols Used: A Systematic Review.

BACKGROUND: The aim of this study was to summarize evidence on energy metabolism through peak fat oxidation (PFO) and maximum fat oxidation (Fatmax), as well as to analyze the protocols used in people with spinal cord injury (SCI) and to examine the main factors related to fat oxidation ability (i.e., age, sex, level of physical activity, and level and degree of injury). METHODS: Studies to determine PFO and Fatmax using indirect calorimetry with an arm exercise protocol for SCI patients were included after a systematic search. Other endpoints included study design, sample size, control group, demographic data, level of injury, physical condition, protocol, outcomes measured, and statistical findings. RESULTS: Eight studies (n = 560) were included. The mean value of VO2peak was 1.86 L∙min-1 (range 0.75-2.60 L∙min-1) (lowest value in the tetraplegic subjects). The PFO ranged between 0.06 and 0.30 g∙min-1 (lowest rates: the non-trained subjects with cervical SCI; highest: the tetraplegic subjects). Two types of exercise protocol were found: arm cycle ergometer, and wheelchair propulsion with a computerized ergometer. Five studies used an incremental protocol (2-3 min/stage, different load increments); the rest performed tests of 20 min/stage at three intensities. CONCLUSION: There are few existing studies measuring fat oxidation in SCI, many of which used small and heterogeneous samples. PFO was lower in SCI subjects when compared with non-injured people performing lower-limb exercise; however, comparing upper-limb exercise, people with SCI showed higher values.

Resting and exercise metabolic characteristics in obese children with insulin resistance.

Purpose: This study aimed to explore the characteristics of resting energy expenditure (REE) and lipid metabolism during incremental load exercise in obese children and adolescents with insulin resistance (IR) to provide evidence for exercise intervention in obese children and adolescents with IR. Method: From July 2019 to August 2021, 195 obese children and adolescents aged 13-17 were recruited through a summer camp. The participants were divided into IR (n = 67) and no-IR (without insulin resistance, n = 128) groups and underwent morphology, blood indicators, body composition, and resting energy consumption gas metabolism tests. Thirty participants each were randomly selected from the IR and no-IR groups to carry out the incremental treadmill test. Results: Significant metabolic differences in resting and exercise duration were found between the IR and no-IR groups. In the resting state, the resting metabolic equivalents (4.33 ± 0.94 ml/min/kg vs. 3.91 ± 0.73 ml/min/kg, p = 0.001) and REE (2464.03 ± 462.29 kcal/d vs. 2143.88 ± 380.07 kcal/d, p < 0.001) in the IR group were significantly higher than in the no-IR group. During exercise, the absolute maximal fat oxidation (0.33 ± 0.07 g/min vs. 0.36 ± 0.09 g/min, p = 0.002) in the IR group was significantly lower than in the no-IR group; maximal fat oxidation intensity (130.9 ± 8.9 bpm vs. 139.9 ± 7.4 bpm, p = 0.040) was significantly lower in the IR group. Conclusion: Significant resting and exercise metabolic differences were found between obese IR and no-IR children and adolescents. Obese IR children and adolescents have higher REE and lower maximal fat oxidation intensity than obese no-IR children and adolescents.

Maximal Fat Oxidation during Incremental Upper and Lower Body Exercise in Healthy Young Males.

The aim of this study is to determine the magnitude of maximal fat oxidation (MFO) during incremental upper and lower body exercise. Thirteen non-specifically trained male participants (19.3 ± 0.5 y, 78.1 ± 9.1 kg body mass) volunteered for this repeated-measures study, which had received university ethics committee approval. Participants undertook two incremental arm crank (ACE) and cycle ergometry (CE) exercise tests to volitional exhaustion. The first test for each mode served as habituation. The second test was an individualised protocol, beginning at 40% of the peak power output (POpeak) achieved in the first test, with increases of 10% POpeak until volitional exhaustion. Expired gases were recorded at the end of each incremental stage, from which fat and carbohydrate oxidation rates were calculated. MFO was taken as the greatest fat oxidation value during incremental exercise and expressed relative to peak oxygen uptake (%V˙O2peak). MFO was lower during ACE (0.44 ± 0.24 g·min-1) than CE (0.77 ± 0.31 g·min-1; respectively, p < 0.01) and occurred at a lower exercise intensity (53 ± 21 vs. 67 ± 18%V˙O2peak; respectively, p < 0.01). Inter-participant variability for MFO was greatest during ACE. These results suggest that weight loss programs involving the upper body should occur at lower exercise intensities than for the lower body.

Associations between heart rate variability and maximal fat oxidation in two different cohorts of healthy sedentary adults.

BACKGROUND AND AIMS: Resting heart rate variability (HRV) and maximal fat oxidation (MFO) during exercise are both considered as a noninvasive biomarkers for early detection of cardiovascular risk factors. Thus, this study aimed to analyze the relationship between resting HRV parameters and MFO during exercise, and the intensity of exercise that elicit MFO (Fatmax) in healthy sedentary adults. METHODS AND RESULTS: A total of 103 healthy young adults (22.2 ± 2.3 years old, 67% female; from the ACTIBATE cohort) and 67 healthy middle-aged adults (53.1 ± 5.0 years old, 52% female; from the FIT-AGEING cohort) were included in this cross-sectional study. HRV was assessed using a Polar RS800CX heart rate monitor, while MFO and Fatmax were determined during a graded exercise treadmill test using indirect calorimetry. No significant associations were observed for healthy young adults (standardized ß coefficients ranged from -0.063 to 0.094, and all P ≥ 0.347) and for middle-aged adults (standardized ß coefficients ranged from -0.234 to 0.090, and all P ≥ 0.056). Nevertheless, only a weak association was observed between one HRV parameter in time-domain (the percentage of R-R intervals that shows a difference higher than 50 ms [pNN50]) and MFO in the cohort of middle-aged adults (ß coefficient = -0.279, and P = 0.033). CONCLUSION: The results of this study suggest that resting HRV parameters are not associated with MFO and Fatmax during exercise in two independent cohorts of healthy sedentary young and middle-aged adults, respectively.

Impact of Ageing on Female Metabolic Flexibility: A Cross-Sectional Pilot Study in over-60 Active Women.

BACKGROUND: Ageing affects metabolic flexibility, although physical status could influence this relationship. This cross-sectional study aims to describe and analyse the metabolic flexibility/inflexibility in a group of active older women, together with the impact of ageing and physical status on their oxidation rates and maximal fat oxidation (MFO). METHODS: Fifteen volunteers (69.00 ± 6.97 years)-from 24 women-completed an incremental cycling test until the second ventilatory threshold. Intensity increased 10 W each 3 min 15 s, starting at 30 W. Gas exchange, heart rate, rate of perceived effort, pain scale and muscle power were registered, together with lactate. VO2 and VCO2 were considered for fat and carbohydrate oxidation (FATox and CHOox; Frayn's equation) at intensities 60%, 80% and 100% from the peak power in the test (P100). Psychophysiological parameters were compared at MFO/FATmax and P100, together with the energy expenditure calculations around MFO (included FAT and CHO contributions), and the main correlation analyses, with and without P100 and VO2 as covariates. RESULTS: FATox was low at MFO (0.13; 95% CI [0.09-0.17] mg/min/kgFFM; 3.50; 95% CI [2.49-4.50] mg/min/kgFFM), with short oxidation-rate curves shifting down and leftward. CHOox and FATox were both low for reduced power with age (77.14 ± 18.58 W and 39.29 ± 9.17 W at P100 and MFO, respectively), all accompanied by a fall in energy expenditure (5.44 ± 2.58 kcal/min and 3.32 ± 1.55 kcal/min at P100 and MFO, respectively). Power appears as a determinant factor, given its strong and negative significant association with age (r = - 0.85, p < 0.005; R2 = 0.72) and moderate with MFO (r = - 0.54, p = 0.04; R2 = 0.29). In turn, energy expenditure shows a positive and moderate association with muscle power (r = 52, p = 0.04). CONCLUSIONS: Despite the drop in substrates oxidation with age, physical status (i.e. larger muscular power and energy expenditure) suggests a key role in the preservation of metabolic health with ageing in active women.

Dihydrocapsiate does not increase energy expenditure nor fat oxidation during aerobic exercise in men with overweight/obesity: a randomized, triple-blinded, placebo-controlled, crossover trial.

Background: Prior evidence suggests that capsinoids ingestion may increase resting energy expenditure (EE) and fat oxidation (FATox), yet whether they can modulate those parameters during exercise conditions remains poorly understood. We hypothesized that dihydrocapsiate (DHC) ingestion would increase EE and specifically FATox during an acute bout of aerobic exercise at FATmax intensity (the intensity that elicits maximal fat oxidation during exercise [MFO]) in men with overweight/obesity. Since FATmax and MFO during aerobic exercise appear to be indicators of metabolic flexibility, whether DHC has an impact on FATox in this type of population is of clinical interest. Methods: A total of 24 sedentary men (age = 40.2 ± 9.2 years-old; body mass index = 31.6 ± 4.5 kg/m2 [n = 11 overweight, n = 13 obese]) participated in this randomized, triple-blinded, placebo-controlled, crossover trial (registered under ClinicalTrials.gov Identifier no. NCT05156697). On the first day, participants underwent a submaximal exercise test on a cycle ergometer to determine their MFO and FATmax intensity during exercise. After 72 hours had elapsed, the participants returned on 2 further days (≥ 72 hours apart) and performed a 60 min steady-state exercise bout (i.e. cycling at their FATmax, constant intensity) after ingesting either 12 mg of DHC or placebo; these conditions were randomized. Respiratory gas exchange was monitored by indirect calorimetry. Serum marker concentrations (i.e. glucose, triglycerides, non-esterified fatty acids (NEFAs), skin temperature, thermal perception, heart rate, and perceived fatigue) were assessed. Results: There were no significant differences (P > 0.05) between DHC and placebo conditions in the EE and FATox during exercise. Similarly, no significant changes were observed in glucose, triglycerides, or NEFAs serum levels, neither in the skin temperature nor thermal perception across conditions. Heart rate and perceived fatigue did not differ between conditions. Conclusions: DHC supplementation does not affect energy metabolism during exercise in men with overweight/obesity.

A Systematic Review and Meta-Analysis on the Association and Differences between Aerobic Threshold and Point of Optimal Fat Oxidation.

Over the past two decades, scientists have attempted to evaluate whether the point of maximal fat oxidation (FATmax) and the aerobic threshold (AerT) are connected. The existence of such a relationship would allow a more tailored training approach for athletes while improving the efficacy of individualized exercise prescriptions when treating numerous health-related issues. However, studies have reported conflicting results, and this issue remains unresolved. This systematic review and meta-analysis aimed: (i) to examine the strength of the association between FATmax and AerT by using the effect size (ES) of correlation coefficient (r) and standardized mean difference (SMD); (ii) to identify potential moderators and their influence on ES variability. This study was registered with PROSPERO (CRD42021239351) and ClinicalTrials (NCT03789045). PubMed and Google Scholar were searched and fourteen articles, consisting of overall 35 ES for r and 26 ES for SMD were included. Obtained ESs were analyzed using a multilevel random-effects meta-analysis. Our results support the presence of a significant association between FATmax and AerT exercise intensities. In conclusion, due to the large ES variance caused by clinical and methodological differences among the studies, we recommend that future studies follow strict standardization of data collection and analysis of FATmax and AerT-related outcomes.

Beyond the Calorie Paradigm: Taking into Account in Practice the Balance of Fat and Carbohydrate Oxidation during Exercise?

Recent literature shows that exercise is not simply a way to generate a calorie deficit as an add-on to restrictive diets but exerts powerful additional biological effects via its impact on mitochondrial function, the release of chemical messengers induced by muscular activity, and its ability to reverse epigenetic alterations. This review aims to summarize the current literature dealing with the hypothesis that some of these effects of exercise unexplained by an energy deficit are related to the balance of substrates used as fuel by the exercising muscle. This balance of substrates can be measured with reliable techniques, which provide information about metabolic disturbances associated with sedentarity and obesity, as well as adaptations of fuel metabolism in trained individuals. The exercise intensity that elicits maximal oxidation of lipids, termed LIPOXmax, FATOXmax, or FATmax, provides a marker of the mitochondrial ability to oxidize fatty acids and predicts how much fat will be oxidized over 45-60 min of low- to moderate-intensity training performed at the corresponding intensity. LIPOXmax is a reproducible parameter that can be modified by many physiological and lifestyle influences (exercise, diet, gender, age, hormones such as catecholamines, and the growth hormone-Insulin-like growth factor I axis). Individuals told to select an exercise intensity to maintain for 45 min or more spontaneously select a level close to this intensity. There is increasing evidence that training targeted at this level is efficient for reducing fat mass, sparing muscle mass, increasing the ability to oxidize lipids during exercise, lowering blood pressure and low-grade inflammation, improving insulin secretion and insulin sensitivity, reducing blood glucose and HbA1c in type 2 diabetes, and decreasing the circulating cholesterol level. Training protocols based on this concept are easy to implement and accept in very sedentary patients and have shown an unexpected efficacy over the long term. They also represent a useful add-on to bariatric surgery in order to maintain and improve its weight-lowering effect. Additional studies are required to confirm and more precisely analyze the determinants of LIPOXmax and the long-term effects of training at this level on body composition, metabolism, and health.

Intermittent fasting & performance: The iFast clinical trial protocol.

There is increasing evidence from animal and human studies suggesting that fasting can play a role in disease prevention, weight control and longevity. However, few studies have compared exercise performances in individuals adhering to an intermittent fasting (IF) in comparison to individuals who are not. Given the rising popularity of IF we aim to investigate whether this type eating pattern will improve cardiovascular performance over a period of 12 weeks through VO2 max measurements in participants from a Lebanese community. Additionally, we will study the variation of different health parameters, physical performance and biomarkers potentially affected by IF. Participants will be recruited from a large university community and randomized into 4 arms. Baseline information will be collected from all participants, which includes biological, physical, nutritional, medical and psychological data. Two arms will follow a time-restricted fasting diet with and without physical exercise, one arm will exercise without fasting, and one will act as a control group. Throughout the study, measurements will be repeated, and data analysis will follow to evaluate results.

Uncertain association between maximal fat oxidation during exercise and cardiometabolic risk factors in healthy sedentary adults.

The present work examines the relationships between maximal fat oxidation during a graded exercise test (MFO), the intensity of exercise that elicits MFO (Fatmax), and traditional cardiometabolic risk factors in healthy, sedentary adults. A total of 119 (81 women) young, sedentary adults (22.1 ± 2.2 years old), and 71 (37 women) middle-aged, sedentary adults (53.4 ± 4.9 years old) participated in the current study. Systolic and diastolic blood pressures were determined following standard procedures. Plasma glucose, insulin, total cholesterol, high-density lipoprotein cholesterol and triglycerides were determined in a fasted state and the homeostatic model assessment of insulin resistance index and low-density lipoprotein cholesterol levels subsequently calculated. A sex and age group-specific cardiometabolic risk Z-score was also calculated for each subject based on waist circumference, systolic and diastolic blood pressure, plasma glucose, high-density lipoprotein cholesterol and triglycerides. MFO and Fatmax were determined using a walking graded exercise test using indirect calorimetry. No clear association was seen of MFO and Fatmax with any cardiometabolic risk factor (all P≥0.05), except for a weak, inverse association between Fatmax and the fatty liver index (P=0.027). Similarly, neither MFO nor Fatmax was apparently associated with the cardiometabolic risk Z-score (all P≥0.05). The current findings suggest an uncertain association of MFO and Fatmax during a graded exercise test with the cardiometabolic profile of healthy, sedentary adults.HighlightsThe study of the physiological mechanisms that trigger the onset of metabolic disorders has received considerable attention in recent years, with changes in MFO and Fatmax being highlighted as a potential key factor.This work shows that MFO and Fatmax during a graded exercise test are not associated with the cardiometabolic profile in sedentary, healthy adults.Further studies are needed to elucidate which other physiological disorders are related to cardiometabolic risk.

Inter-Day Reliability of Resting Metabolic Rate and Maximal Fat Oxidation during Exercise in Healthy Men Using the Ergostik Gas Analyzer.

The attainment of high inter-day reliability is crucial to determine changes in resting metabolic rate (RMR), respiratory exchange ratio (RER), maximal fat oxidation during exercise (MFO) and the intensity that elicits MFO (Fatmax) after an intervention. This study aimed to analyze the inter-day reliability of RMR, RER, MFO and Fatmax in healthy adults using the Ergostik gas analyzer. Fourteen healthy men (age: 24.4 ± 5.0 years, maximum oxygen uptake (VO2max): 47.5 ± 11.9 mL/kg/min) participated in a repeated-measures study. The study consisted of two identical experimental trials (Day 1 and Day 2) in which the participants underwent an indirect calorimetry assessment at resting and during an incremental exercise test. Stoichiometric equations were used to calculate energy expenditure and substrate oxidation rates. There were no significant differences when comparing RMR (1999.3 ± 273.9 vs. 1955.7 ± 362.6 kcal/day, p = 0.389), RER (0.87 ± 0.05 vs. 0.89 ± 0.05, p = 0.143), MFO (0.32 ± 0.20 vs. 0.31 ± 0.20 g/min, p = 0.776) and Fatmax (45.0 ± 8.6 vs. 46.4 ± 8.4% VO2max, p = 0.435) values in Day 1 vs. Day 2. The inter-day coefficient of variation for RMR, RER, MFO and Fatmax were 4.85 ± 5.48%, 3.22 ± 3.14%, 7.78 ± 5.51%, and 6.51 ± 8.04%, respectively. In summary, the current results show a good inter-day reliability when RMR, RER, MFO and Fatmax are determined in healthy men using the Ergostik gas analyzer.

Maximal fat oxidation rate in women with sedentary behaviour and at-risk body fat percentage

ABSTRACT The purpose of this study was to examine how the maximal fat oxidation rate is affected in two groups of women with different fat mass percentages. Thirty-four women with sedentary behavior (aged 18-25 years) were divided into 2 groups according to their body fat percentage. (<32% of body fat was considered as a threshold following American College Sport Medicine [ACSM] indications). Body fat percentages were assessed by anthropometry, then subjects performed an adapted graded exercise test on a treadmill to determine maximal oxygen consumption and fat oxidation rates during exercise. There were significant differences (p<0.05) in maximal cardiorespiratory capacity (34.4±4.4 vs 30.4±10.4 mL x kg-1 x min-1) and maximal fat oxidation rates (0.39±0.05 vs 0.31±0.06 g x min-1) between the lower risk group when compared to the higher risk group. There was no correlation between body fat percentage (BF%) and fat oxidation rates in both groups. In conclusion, cardiorespiratory capacity and substrate oxidation are different when two groups of women divided by ACSM recommendations are considered and these results could help clinicians and trainers to prevent weight gain and/or promote body fat and body weight loss.

RESUMEN El propósito de este estudio fue examinar el cambio de la tasa de oxidación de grasas en dos grupos de mujeres con diferentes porcentajes de masa grasa. Metodología: Treinta y cuatro mujeres con comportamiento sedentario (edades de 18 a 25 años) fueron divididas en dos grupos de acuerdo con su porcentaje de masa grasa (<32% fue considerado como umbral siguiendo las indicaciones del American College Sport Medicine [ACSM]). El porcentaje de masa grasa fue medido por antropometría y luego los sujetos realizaron una prueba adaptado en trotadora eléctrica para determinar el consumo máximo de oxígeno y las tasas de oxidación durante el ejercicio. Resultados: Existieron significativas (p<0,05) en la capacidad cardiorrespiratoria (34,4±4,4 vs 30,4±10,4 mL x kg-1 x min-1) y máxima tasa de oxidación de grasas (0,39±0,05 vs 0,31±0,06 g x min-1) entre el grupo de menor riesgo cuando se compara con el de mayor riesgo. No hubo correlación entre el porcentaje de masa grasa (BF%) vs oxidación máxima de grasas. Conclusión: la capacidad cardiorespiratoria y la oxidación de sustratos fue diferente cuando dos grupos de mujeres son divididos por las consideraciones de la ACSM.

Peak Fat Oxidation Rate Is Closely Associated With Plasma Free Fatty Acid Concentrations in Women; Similar to Men.

Introduction: In men, whole body peak fat oxidation (PFO) determined by a graded exercise test is closely tied to plasma free fatty acid (FFA) availability. Men and women exhibit divergent metabolic responses to fasting and exercise, and it remains unknown how the combined fasting and exercise affect substrate utilization in women. We aimed to investigate this, hypothesizing that increased plasma FFA concentrations in women caused by fasting and repeated exercise will increase PFO during exercise. Then, that PFO would be higher in women compared with men (data from a previous study). Methods: On two separate days, 11 young endurance-trained women were investigated, either after an overnight fast (Fast) or 3.5 h after a standardized meal (Fed). On each day, a validated graded exercise protocol (GXT), used to establish PFO by indirect calorimetry, was performed four times separated by 3.5 h of bed rest both in the fasted (Fast) or fed (Fed) state. Results: Peak fat oxidation increased in the fasted state from 11 ± 3 (after an overnight fast, Fast 1) to 16 ± 3 (mean ± SD) mg/min/kg lean body mass (LBM) (after ~22 h fast, Fast 4), and this was highly associated with plasma FFA concentrations, which increased from 404 ± 203 (Fast 1) to 865 ± 210 µmol/L (Fast 4). No increase in PFO was found during the fed condition with repeated exercise. Compared with trained men from a former identical study, we found no sex differences in relative PFO (mg/min/kg LBM) between men and women, in spite of significant differences in plasma FFA concentrations during exercise after fasting. Conclusion: Peak fat oxidation increased with fasting and repeated exercise in trained women, but the relative PFO was similar in young trained men and women, despite major differences in plasma lipid concentrations during graded exercise.

Determinants of Peak Fat Oxidation Rates During Cycling in Healthy Men and Women.

This study explored lifestyle and biological determinants of peak fat oxidation (PFO) during cycle ergometry, using duplicate measures to account for day-to-day variation. Seventy-three healthy adults (age range: 19-63 years; peak oxygen consumption [V˙O2peak]: 42.4 [10.1] ml·kg BM-1·min-1; n = 32 women]) completed trials 7-28 days apart that assessed resting metabolic rate, a resting venous blood sample, and PFO by indirect calorimetry during an incremental cycling test. Habitual physical activity (combined heart rate accelerometer) and dietary intake (weighed record) were assessed before the first trial. Body composition was assessed 2-7 days after the second identical trial by dual-energy X-ray absorptiometry scan. Multiple linear regressions were performed to identify determinants of PFO (mean of two cycle tests). A total variance of 79% in absolute PFO (g·min-1) was explained with positive coefficients for V˙O2peak (strongest predictor), FATmax (i.e the % of V˙O2peak that PFO occurred at), and resting fat oxidation rate (g·min-1), and negative coefficients for body fat mass (kg) and habitual physical activity level. When expressed relative to fat-free mass, 64% of variance in PFO was explained: positive coefficients for FATmax (strongest predictor), V˙O2peak, and resting fat oxidation rate, and negative coefficients for male sex and fat mass. This duplicate design revealed that biological and lifestyle factors explain a large proportion of variance in PFO during incremental cycling. After accounting for day-to-day variation in PFO, V˙O2peak and FATmax were strong and consistent predictors of PFO.

The training induced increase in whole-body peak fat oxidation rate may be attenuated with aging.

An attenuated ability to appropriately oxidize fat (metabolic inflexibility) has been associated with the development of obesity and type 2 diabetes. Previous studies have found that regular exercise training increases the body's ability to oxidize fat during exercise, but also shown that fat oxidation at the same relative and absolute exercise intensity is lower in old compared with young adults. Based on these studies we investigated the effect of training status on the whole-body peak fat oxidation rate (PFO) during exercise in young and middle-aged trained and untrained men. We hypothesized that aging was associated with decreased PFO, but regular exercise training could counteract this decline. 36 healthy non-overweight young and middle-aged men were recruited into a four groups: young (27 [24-30] yrs, (Mean [95% CI])) untrained (⩒O2peak: 47 [44-49] ml/min/kg), young (28 [26-30] yrs) trained (⩒O2peak: 64 [62-67] ml/min/kg), middle-aged (55 [53-57] yrs) untrained (⩒O2peak: 37 [32-42] ml/min/kg) and middle-aged (54 [51-57] yrs) trained (⩒O2peak: 55 [51-58] ml/min/kg). PFO was measured by indirect calorimetry while subjects performed a validated incremental exercise protocol on a cycle ergometer. Whole-body peak fat oxidation rate was higher in the young trained compared to young untrained subjects (0.70 [0.65-0.75] vs.0.45 [0.36-0.54] g/min, post-hoc: p < 0.001); however, this training effect was attenuated in middle-aged trained and untrained subjects (0.44 [0.38-0.50] vs. 0.41 [0.35-0.47] g/min, post-hoc: p = 0.83, respectively). In summary, these findings suggest that the training induced effects on whole-body fat oxidation found in young men may be attenuated in middle-aged men.

Fat Oxidation Kinetics Is Related to Muscle Deoxygenation Kinetics During Exercise.

PURPOSE: The present study aimed to determine whether whole-body fat oxidation and muscle deoxygenation kinetics parameters during exercise were related in individuals with different aerobic fitness levels. METHODS: Eleven cyclists [peak oxygen uptake ( V . O 2 ⁢ p ⁢ e ⁢ a ⁢ k ): 64.9 ± 3.9 mL⋅kg-1⋅min-1] and 11 active individuals ( V . O 2 ⁢ p ⁢ e ⁢ a ⁢ k : 49.1 ± 7.4 mL⋅kg-1⋅min-1) performed a maximal incremental cycling test to determine V . O 2 ⁢ p ⁢ e ⁢ a ⁢ k and a submaximal incremental cycling test to assess whole-body fat oxidation using indirect calorimetry and muscle deoxygenation kinetics of the vastus lateralis (VL) using near-infrared spectroscopy (NIRS). A sinusoidal (SIN) model was used to characterize fat oxidation kinetics and to determine the intensity (Fatmax) eliciting maximal fat oxidation (MFO). The muscle deoxygenation response was fitted with a double linear model. The slope of the first parts of the kinetics (a 1) and the breakpoint ([HHb]BP) were determined. RESULTS: MFO (p = 0.01) and absolute fat oxidation rates between 20 and 65% V . O 2 ⁢ p ⁢ e ⁢ a ⁢ k were higher in cyclists than in active participants (p < 0.05), while Fatmax occurred at a higher absolute exercise intensity (p = 0.01). a 1 was lower in cyclists (p = 0.02) and [HHb]BP occurred at a higher absolute intensity (p < 0.001) than in active individuals. V . O 2 ⁢ p ⁢ e ⁢ a ⁢ k was strongly correlated with MFO, Fatmax, and [HHb]BP (r = 0.65-0.88, p ≤ 0.001). MFO and Fatmax were both correlated with [HHb]BP (r = 0.66, p = 0.01 and r = 0.68, p < 0.001, respectively) and tended to be negatively correlated with a 1 (r = -0.41, p = 0.06 for both). CONCLUSION: This study showed that whole-body fat oxidation and muscle deoxygenation kinetics were both related to aerobic fitness and that a relationship between the two kinetics exists. Individuals with greater aerobic fitness may have a delayed reliance on glycolytic metabolism at higher exercise intensities because of a longer maintained balance between O2 delivery and consumption supporting higher fat oxidation rates.