Edited by: Davide Malatesta, Université de Lausanne, Switzerland
Reviewed by: Jean-Frédéric Brun, INSERM U1046 Physiologie et Médecine Expérimentale du Coeur et des Muscles, France; Todd Anthony Astorino, California State University San Marcos, United States
This article was submitted to Exercise Physiology, a section of the journal Frontiers in Physiology
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Using a short-duration step protocol and continuous indirect calorimetry, whole-body rates of fat and carbohydrate oxidation can be estimated across a range of exercise workloads, along with the individual maximal rate of fat oxidation (MFO) and the exercise intensity at which MFO occurs (Fatmax). These variables appear to have implications both in sport and health contexts. After discussion of the key determinants of MFO and Fatmax that must be considered during laboratory measurement, the present review sought to synthesize existing data in order to contextualize individually measured fat oxidation values. Data collected in homogenous cohorts on cycle ergometers after an overnight fast was synthesized to produce normative values in given subject populations. These normative values might be used to contextualize individual measurements and define research cohorts according their capacity for fat oxidation during exercise. Pertinent directions for future research were identified.
During prolonged exercise, carbohydrate and fat are the primary substrates oxidized to fuel energy metabolism (Romijn et al.,
Carbohydrate is the quantitatively most important metabolic substrate during prolonged exercise of moderate-to-high intensities (Romijn et al.,
In contrast, human fat reserves are effectively unlimited in the context of exercise, and so identifying the determinants of, and enhancing, fat oxidation during exercise is a pertinent training and research goal in endurance sport. Indeed, fat oxidation capacity has been correlated with performance in Ironman triathlons, which are ultra-endurance events (>8 h) in which carbohydrate availability is likely limiting (Frandsen et al.,
Perhaps the most fundamental determinant of whole-body fat oxidation rate is exercise intensity. The relationship between exercise intensity and fat oxidation is generally parabolic; with fat oxidation initially increasing with exercise intensity before declining at high work rates (Romijn et al.,
However, impaired mitochondrial fatty acid uptake might also contribute to the reduction in whole-body fat oxidation observed at high exercise intensities, given the observed reduction in mitochondrial uptake and oxidation of long-chain fatty acids with increasing exercise intensity (Sidossis et al.,
In order to comprehensively define the relationship between whole-body fat oxidation rate and exercise intensity, the “Fatmax” test was developed (Achten et al.,
Representative illustration of fat oxidation (g.min−1) against exercise intensity (W) during a graded, cycling Fatmax test, where MFO, maximal rate of fat oxidation (g.min−1) and Fatmax, the intensity at which MFO occurs (W).
The reliability of Fatmax assessments has been examined. The first reliability study of the Fatmax protocol described above reported a coefficient of variation (CV) of 9.6% for Fatmax in a cohort of overnight fasted moderately-trained males with 24-h pre-trial dietary repetition (Achten and Jeukendrup,
As described above, the validity of the original Fatmax protocol was examined against prolonged exercise bouts at intensities equivalent to those in the step test, with results from the step test demonstrated to be reflective of those over longer duration (Achten et al.,
In a health context, MFO has been significantly positively correlated with insulin sensitivity in a large cohort (
Therefore, Fatmax tests appear a practical monitoring tool in performance settings where the capacity to utilize fat as a metabolic substrate is of concern, and might also be useful in clinical exercise physiology as an indicator of metabolic health. The purpose of the present review is to extend previous summaries (Jeukendrup and Wallis,
In order to explore the determinants of MFO and Fatmax, a systematic literature search was performed to identify all studies using Fatmax protocols in adult populations. As such, “maximal fat oxidation,” “peak fat oxidation,” and “Fatmax” were searched in the PubMed and Web of Science databases (27/03/2018). Hand searches of reference lists and key journals were also conducted. Studies published in English and reporting directly measured MFO and/or Fatmax values in adult populations were included. This search approach yielded 53 studies for inclusion in the review.
Five studies were identified that directly compared MFO and/or Fatmax between subjects groups of different training status (Nordby et al.,
A moderating effect of training status on MFO is not surprising given the previously observed significantly higher whole-body fat oxidation rates in trained compared to untrained males exercising at the same absolute workload (van Loon et al.,
Seven studies were identified that compared males (
However, some studies making comparisons between-sexes have reported MFO relative to fat-free mass (FFM). When expressed in these terms (mg.kg FFM−1.min−1), two large cohort studies have reported greater MFO in females compared to males (Venables et al.,
The existing literature therefore suggests that whilst absolute MFO is generally greater in males compared to females, MFO relative to FFM is likely greater in non-obese females compared to non-obese males. There also appears a minor tendency toward greater Fatmax in females compared to males. Given sex-related differences in body mass and composition, MFO relative to FFM might be more descriptive when comparing between sexes. Whether these effects are observed in endurance-trained cohorts is unknown. Similarly, effects of the menstrual cycle on MFO and Fatmax have not been studied, but warrant consideration in the context of serial inter-individual measurement.
Only one study has directly examined the effect of acute feeding status on MFO and Fatmax (Achten and Jeukendrup,
From a chronic dietary perspective, a recent large study of 150 male and 155 female subjects used hierarchical regression to elucidate the influence of a 4-day dietary record on MFO, and reported absolute carbohydrate and fat intakes accounted for 3.2% of the variation, with carbohydrate and fat intakes contributing negatively and positively to MFO, respectively (Fletcher et al.,
In a cross-sectional study involving a homogenous cohort of male ultra-endurance runners, MFO (1.54 ± 0.18 vs. 0.67 ± 0.14 g.min−1) and Fatmax (70 ± 6 vs. 55 ± 8%VO2max) were significantly higher in those habitually consuming a ketogenic vs. high carbohydrate diet (Volek et al.,
It is also possible that protein intake exerts an effect on MFO. During 3-month consumption of a weight-maintenance diet, increasing protein intake by ~10 g.d−1 has been shown to significantly increase MFO by ~19% in a mixed-sex sample of previously weight-stable volunteers (Soenen et al.,
A further consideration is exercise modality. In general, studies comparing running and cycling at given exercise intensities have reported greater fat and reduced carbohydrate oxidation rates during running (Snyder et al.,
It has been demonstrated that the training status, sex, and acute and chronic nutritional status of the subject population or individual under study are clear determinants of MFO and Fatmax, with a possible effect of exercise modality. These determining factors must be considered when interpreting results between-studies and in serial intra-individual measurement.
Given the interest in measurement of MFO and Fatmax in research and non-research settings, it would be prudent to generate normative values from existing data in order to contextualize individually measured values and define the fat oxidation capacity of given research cohorts. However, in order to do this, the aforementioned determinants of MFO and Fatmax need to be considered. Accordingly, published MFO and Fatmax values were synthesized from studies with homogeneous cohorts performing assessments after an overnight fast on a cycle ergometer. These criteria were applied in order to generate sufficient data to produce meaningful normative values.
Studies were subsequently partitioned into five populations: endurance-trained, lean males (Achten et al.,
Normative percentile values for MFO (g.min−1) in different subject populations during assessments performed on a cycle ergometer after an overnight fast.
Endurance-trained, lean males | 201 | 0.53 ± 0.16 | 0.40 | 0.49 | 0.58 | 0.67 |
Recreationally-active, lean males | 105 | 0.46 ± 0.14 | 0.34 | 0.42 | 0.49 | 0.58 |
Recreationally-active, lean females | 68 | 0.35 ± 0.12 | 0.25 | 0.32 | 0.38 | 0.45 |
Overweight/obese males | 193 | 0.28 ± 0.14 | 0.16 | 0.24 | 0.31 | 0.39 |
Overweight/obese females | 144 | 0.16 ± 0.05 | 0.12 | 0.15 | 0.17 | 0.20 |
Normative percentile values for Fatmax (%VO2max) in different subject populations during assessments performed on a cycle ergometer after an overnight fast.
Endurance-trained, lean males | 201 | 56 ± 8 | 49 | 54 | 58 | 63 |
Recreationally-active, lean males | 67 | 51 ± 8 | 44 | 48 | 53 | 58 |
Recreationally-active, lean females | 38 | 50 ± 10 | 41 | 47 | 52 | 58 |
Overweight/obese males | 190 | 43 ± 18 | 28 | 38 | 47 | 57 |
Overweight/obese females | 27 | 61 ± 10 | 52 | 58 | 64 | 70 |
A trend toward greater MFO with increasing training status was observed (Table
Many determinants of MFO and Fatmax have been identified in the ~16 years since the original protocol was developed (Achten et al.,
Schematic illustration of the identified determinants of maximal fat oxidation during graded protocols (black) and key identified unknown factors (gray).
An unexplored parameter likely to alter MFO and Fatmax is environmental temperature. Environmental heat stress increases muscle glycogenolysis, hepatic glucose output, and whole-body carbohydrate oxidation rates, whilst reducing fat oxidation rates at given intensities (Febbraio et al.,
The effect of cold environments on substrate metabolism during prolonged exercise is less certain. Some investigations have reported augmented carbohydrate utilization in cold vs. temperate conditions (Galloway and Maughan,
Direct investigation of the impact of environmental temperature on laboratory measures of MFO and Fatmax, and the environmental thresholds at which they occur, is therefore warranted. This data would have strong applied relevance given the diverse environmental conditions in which endurance competitions take place (Racinais et al.,
Fourteen longitudinal studies have measured the effect of exercise training interventions on MFO and/or Fatmax (Venables and Jeukendrup,
Training-induced increases in MFO have been observed with interval (~10–80%) (Alkahtani et al.,
The most favorable training regimen for increasing MFO cannot presently be discerned. Training studies have generally utilized either prolonged moderate-intensity aerobic exercise (Mogensen et al.,
There is also a notable absence of data concerning the responsiveness of MFO and Fatmax to training in endurance-trained cohorts. Existing studies have generally been in overweight/obese populations (Venables and Jeukendrup,
Therefore, whilst it has been demonstrated that exercise training
A hypothesis linking MFO, Fatmax, and performance in prolonged exercise where carbohydrate availability is limiting (>2 h) has clear intuitive appeal. If an individual makes extensive use of fat oxidation to support metabolism during prolonged exercise at their competitive or operational intensity, this should reduce the requirement for endogenous carbohydrate oxidation, and therefore muscle glycogen depletion, which is linked to fatigue (Bergström et al.,
However, the importance of MFO and Fatmax for exercise performance has not yet been comprehensively studied, and such research is warranted. A recent study of 64 Ironman triathletes reported a significant, albeit modest, correlation between MFO and performance time in the 2016 Copenhagen Ironman (
An interesting avenue for future research might therefore be to determine if MFO and Fatmax are indicators of the degree of endogenous carbohydrate utilization and skeletal muscle glycogenolysis during prolonged exercise within a homogenous group of endurance-trained athletes, and consequently if such an effect has implications for endurance exercise performance. Such data would provide indication of the functional relevance of monitoring MFO and Fatmax in endurance-trained athletes, and could serve to build on existing models of endurance exercise performance (McLaughlin et al.,
This review has systematically identified several key determinants of MFO and Fatmax. These include training status, sex, acute nutritional status, and chronic nutritional status, with the possibility of an effect of exercise modality. Accordingly, normative percentile values for MFO and Fatmax in different subject populations are provided to contextualize individually measured values and define the fat oxidation capacity of given research cohorts. However, the effect of environmental conditions on MFO and Fatmax remain to be established, as does the most appropriate means of training MFO and Fatmax, particularly in endurance-trained cohorts. Furthermore, direct links between MFO, Fatmax, and rates of muscle glycogenolysis during prolonged exercise remain to be established, as do relationships between MFO, Fatmax, and exercise performance. This information might add to existing models of endurance exercise performance, and indicate how useful MFO and Fatmax monitoring might be in endurance sport.
EM performed data analysis. EM, DP, and AK wrote the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
EM is funded by an Education New Zealand scholarship (no role in preparation of the manuscript).