Abstract
Objective
To describe the fat‐oxidation rate in triathlon and different modalities of endurance cycling.
Methods
34 endurance athletes (15 male triathletes, 4 female triathletes, 11 road cyclists and 4 male mountain bikers) underwent a progressive cycloergometer test until exhaustion. Relative work intensity (VO2max), minimal lactate concentration (La−min), lactic threshold, individual lactic threshold (ILT), maximal fat‐oxidation rate (Fatmax, Fatmax zone) and minimal fat‐oxidation rate (Fatmin) were determined in each of the groups and were compared by means of one‐way analysis of variance.
Results
No significant differences were found for Fatmax, Fatmin or for the Fatmax zone expressed as fat oxidation rate (g/min). Intensities −20%, −10% and −5% Fatmax were significantly lower for mountain bikers with respect to road cyclists and female triathletes, expressed as % VO2max. Intensities 20%, 10% and 5% Fatmax were significantly lower for mountain bikers with respect to male triathletes and female triathletes, and for male triathletes in comparison with female triathletes, expressed as % VO2max. Lactic threshold and La−min did not show significant differences with respect to Fatmax. Lactic threshold was found at the same VO2max with respect to the higher part of the Fatmax zone, and La−min at the same VO2max with respect to the lower part of the Fatmax zone.
Conclusions
The VO2max of Fatmax and the Fatmax zone may explain the different endurance adaptations of the athletes according to their sporting discipline. Lactic threshold and La−min were found at different relative work intensities with respect to those of Fatmax even though they belonged to the Fatmax zone.
Fat and carbohydrates (CHO) are the main energetic substrates of aerobic metabolism that resynthesise the ATP in the skeletal muscle. The relative use of fat and CHO during exercise may vary in an important way and depends fundamentally on the maximal oxygen uptake VO2max.1 Moreover, it should be taken into account that, during exercise, women oxidise more intramuscular triglycerides than men,2 and use less muscle glycogen.3
In endurance specialities, having a large capacity to metabolise CHO is a determining factor for performance, but so is the ability to economise this substrate as an effect of a greater capacity to use fat.4 It has been observed that athletes have a lower fat‐oxidation rate at low and high intensities compared with moderate intensities.1,5 Also, there is a relative individual intensity at which the maximal fat‐oxidation rate is produced.
Until recently, the methods followed by some authors only measured the athletes fat‐oxidation rate in a reduced number of work intensities: two,5,6,7 three1,8,9 or four.10 Now there is a method to measure the fat‐oxidation rate for each of the relative work intensities, by means of indirect calorimetry and using stoichiometric equations.11 However, this method has only been applied in a few studies carried out with cyclists.11,12,13
Some authors have observed that cycling endurance performance at a concrete work intensity improves with a training focused on this intensity.14 After this principle (training specificity), other authors have observed that the fat‐oxidation rate increases specifically for the concrete intensity that has been trained.7 Thus, differences in the training of triathlon and cycling endurance modalities may produce different adaptations of the fat‐oxidation rate at each of the aerobic work intensities.
The aim of this study was to use a cross‐sectional design to describe the fat‐oxidation rate in different modalities of endurance cycling and triathlon (mountain biking and road cycling in men, and short distance men's and women's triathlon), where the pace of the race is developed at different relative work intensities.
Methods
Participants
A total of 34 athletes, mean (standard deviation (SD)) age 23.8 (4.7) years, and different sporting specialities participated in the study: short‐distance male triathletes (n = 11) and female triathletes (n = 4), road cyclists (n = 11) and male mountain bikers (n = 4). These participants were all the endurance athletes included in our study group with a high performance level. They were all active competitors at national and international performance levels. The mean level of competitive experience was 4.5 (1.7) years. The study was carried out during the competitive periods in the individual schedule of the 2004 season. The athletes were informed of the procedures they would have to undergo. All experimental procedures were approved by the ethics committee of the High Performance Centre of Sant Cugat.
Study protocol
The tests were conducted for 4 weeks and always during the afternoon. Before the trial, during the 2 days before the test, all the athletes had to eat a high‐CHO diet (70–80%) and perform endurance training at low intensities (40–50% VO2max). The laboratory test was conducted on a cycloergometer with electromagnetic brakes (Cardgirus; G&G Innovación, La Bastida, Alava, Spain). After a 10‐min warm‐up at 100 W, the test started with an initial stage of 130 W for the women and 200 W for the men. The workload was increased by 30 W every 4 min, until the athlete reached exhaustion. Athletes were familiarised with this kind of protocol.
Gas exchange variables
The ventilatory frequency, tidal volume, fraction of O2 exhaled, fraction of CO2 exhaled, ventilatory volume, rate of ventilatory exchange and oxygen consumption were measured in real time, breath by breath, throughout the test, by means of an integrated system of indirect calorimetry (Quark PFT; Cosmed, Rome, Italy). VO2max was determined as the mean value of the VO2 of the last 30 s of effort, when at least two of the criteria recommended by the British Association of Sport and Exercise Sciences15 were fulfilled.
Capillary blood lactate
During the last 15 s of each stage, as well as at the third minute after the end of the effort, the lactic acid concentration (La−max) was measured. This final recording was considered to be La−max.16 Lactate concentration was determined in a portable lactate analyser (Lactate Pro Arkray, Kyoto, Japan). For this analysis, blood samples were taken from the ear lobe. Once the cycloergometer test was finished, the evaluation of La−max with respect to the power developed was adjusted by a second‐order polynomial equation, which allowed the determination of the La−min and the individual lactic threshold (ILT) of each participant of the study. The La−min was determined as the minimum La− measured during the test. The ILT was calculated as the La−min plus the concentration of a fixed lactate value established at 1.5 mM.17 In addition, the lactic threshold was determined as the work intensity at which the La− started to increase progressively.18 The values of La−min, lactic threshold and ILT were expressed as La− (mM), VO2 (mL/kg/min), % VO2max and heart rate (beats/min).
Determination of the fat‐oxidation rate
The fat‐oxidation rate was calculated by means of the equation proposed by Frayn,19
Fat oxidation = (1.67 × VO2) − (1.67 × VCO2)
in which it is assumed that the excretion of nitrogen in urine is negligible, using as values for VO2 and VCO2 the average of the measures (litres/min) realised during the last 2 min of each of the stages.
For the determination of the fat‐oxidation rate, the point of maximal fat oxidation rate (Fatmax), expressed as an absolute value (g/min), was calculated for each of the participants of the study. With these data, a curve of the fat‐oxidation rate versus the relative work intensity (% VO2max) was constructed. The function was used to calculate the following relative variables: Fatmax (% VO2max at which the greatest fat‐oxidation rate was found), Fatmin (% VO2max at which the lowest fat oxidation rate was reached (R = 1.0)) and the % VO2max corresponding to 5%, 10% and 20% Fatmax. The Fatmax zone was defined as the range of relative work intensities between +10% and –10% Fatmax.11
Statistical analysis
Descriptive statistics were generated for all variables. To compare the physiological variables between the different groups of study, a one‐way analysis of variance test was carried out. The determination of the Pearson correlation coefficient was used to verify the existence of relationships between the different variables studied, taking the relationships into account when r ⩾ 0.8. The significance level was determined at p<0.05 for all statistical tests carried out.
Results
No significant differences were found for the different anthropometric variables studied between the different groups, except for the body mass, which was significantly greater in the male triathlete group than in the female triathlete group, and for the body mass index (BMI (weight (kg)/height2(m2)), which was higher in the male triathlete group than in the female triathlete and road cyclists groups (table 1).
Table 1 Comparison of the anthropometric variables of the groups studied (mean (SD)).
| Age (years) | Height (cm) | Body mass (kg) | BMI (kg/m2) | |
|---|---|---|---|---|
| TM | 26.4 (4.3) | 174.6 (6.8) | 73.0 (7.9)* | 23.6 (1.9) |
| TF | 20.5 (1.7) | 165.7 (6.5) | 60.1 (4.9) | 21.9 (0.1)† |
| C | 21.1 (4.5) | 176.7 (6.8) | 67.1 (5.6) | 21.5 (1.3)† |
| M | 23.0 (4.7) | 178.1 (5.2) | 71.3 (7.3) | 22.5 (1.9) |
| Total | 23.8 (4.7) | 176.3 (6.6) | 69.4 (7.6) | 22.6 (1.8) |
BMI, body mass index; C, road cyclists; M, mountain bikers; TF, female triathletes; TM, male triathletes.
*p<0.05 (significant differences with respect to group TF).
†p<0.05 (significant difference with respect to group TM).
With respect to the maximal cardiorespiratory and metabolic variables, the female triathlete group had a lower VO2max, expressed in absolute values, than the other study groups, and the male and female triathlete groups a lower maximum heart rate than the mountain biker group. No significant differences were observed regarding the maximal La−max (table 2).
Table 2 Comparison of the maximum cardiorespiratory and metabolic variables of the groups studied (mean (SD)).
| VO2max | HRmax (beats/min) | La−max (mM) | ||
|---|---|---|---|---|
| (ml/kg/min) | (l/min) | |||
| TM | 58.8 (4.4) | 4.2 (0.5)* | 180.4 (11.5)† | 8.3 (2.7) |
| TF | 58.4 (6.9) | 3.5 (0.2) | 177.0 (2.3)† | 7.4 (3.3) |
| C | 64.8 (5.5) | 4.3 (0.3)* | 190.2 (6.5) | 8.1 (1.8) |
| M | 65.6 (5.9) | 4.6 (0.2)* | 197.8 (9.5) | 10.4 (3.7) |
| Total | 61.5 (5.9) | 4.2 (0.5) | 185.2 (11.1) | 8.4 (2.6) |
C, road cyclists; HRmax, maximal heart rate; La−max, maximal lactate concentration; M, mountain bikers; TF, female triathletes; TM, male triathletes; VO2max, maximal oxygen uptake.
*p<0.001 (significant differences with respect to group TF).
†p<0.05 (significant differences with respect to group M).
No significant differences were found between the different groups of the study for the lactic threshold, expressed as La−, or as relative (ml/kg/min) or absolute (l/min) VO2, or as heart rate. La−min only showed significant differences between male triathletes and road cyclists when expressed as La− (1.9 (0.7) v 1.0 (0.2) mM, respectively).
The ILT was found at a significantly higher La− in the male triathlete group than in the other groups. When the ILT was expressed in absolute VO2, it was significantly lower for the female triathlete group than in the other groups, although the ILT presented no significant differences when it was expressed in relative VO2 values. Finally, when the ILT was expressed as HR, it was significantly higher for the mountain biker group than in the female triathlete group (table 3).
Table 3 Comparison of the cardiorespiratory and metabolic variables corresponding to the ILT of the groups studied (mean (SD)).
| La− (mM) | VO2 (ml/kg/min) | VO2 (l/min) | VO2max (%) | HR (beats/min) | |
|---|---|---|---|---|---|
| TM | 3.44 (0.73) | 45.7 (4.3) | 3.2 (0.4)† | 74.3 (8.6) | 154 (14) |
| TF | 2.77 (0.79)* | 43.2 (3.9) | 2.6 (0.0) | 70 (0.8) | 143 (8) |
| C | 2.53 (0.17)* | 51.3 (5.4) | 3.5 (0.3)† | 77.4 (4.4) | 162 (8) |
| M | 3.07 (0.80)* | 51.0 (5.1) | 3.6 (0.1)‡ | 77.8 (1.7) | 169 (7)§ |
| Total | 3.01 (0.71) | 47.9 (5.5) | 3.3 (0.4) | 75.2 (6.6) | 157 (13) |
C, road cyclists; HR, heart rate; La−, lactate concentration; M, mountain bikers; TF, female triathletes; TM, male triathletes; VO2, oxygen uptake; VO2max, maximal oxygen uptake.
*p<0.01 (significant differences with respect to group TM).
†p<0.001 (significant differences with respect to group TF).
‡p<0.01 (significant differences with respect to group TF).
§p<0.05 (significant differences with respect to group TF).
La−min, lactic threshold and ILT, expressed in terms of % VO2max, did not show significant differences between the different groups of this study.
The average Fatmax was 0.45 (0.12) g/min, and was found at a relative work intensity of 52.3 (7.0)% VO2max, whereas the Fatmin value was found at a relative work intensity of 87.8 (6.8)% VO2max. The Fatmax zone was found to be between 43.3 (6.6) and 62.3 (7.0)% VO2max, which corresponded to a range between −8.7 (4) and 10.5 (8.4)% of the mean Fatmax value. At intensities above the upper limit of the Fatmax zone (+ 10% Fatmax), the fat‐oxidation rate deceased markedly.
Neither significant differences for the Fatmax were found between the different groups of study, nor for the different relative intensities evaluated (5%, 10% and 20% Fatmax), and for the Fatmin, expressed all these terms as fat‐oxidation rate (g/min). No difference was found for the limits of the Fatmax zone, expressed as percentages of the difference with respect to the Fatmax. However, when the fat‐oxidation rate was expressed as % VO2max, significant differences were found for the different relative intensities evaluated, except for the Fatmin.
Relative work intensities −20%, −10% and −5% Fatmax were significantly lower for the mountain biker group than for the road cyclist and female triathlete groups when they were expressed as % VO2max. Relative work intensities 20%, 10% and 5% Fatmax were significantly lower for the mountain biker group than for the male and female triathlete groups, and for male triathletes in comparison with female triathletes when they were expressed as VO2max (fig 1, table 4).
Figure 1 Comparison of the relationship of fat‐oxidation rate (g/min) and the relative work intensity (% VO2max) of the different groups studied. C, road cyclists; M, mountain bikers; TF, female triathletes; TM, male triathletes.
Table 4 Comparison of the relative intensities (% VO2max) at which the variables of the fat‐oxidation rate are found in the groups of study (mean±SD).
| TM | TF | C | M | Total | |
|---|---|---|---|---|---|
| −20% Fatmax | 32.6±5.3 | 39.3±0.8* | 34.2±6.5* | 23.6±5.2 | 33.1±6.4 |
| −10% Fatmax | 42.6±5.3 | 49.3±0.8* | 44.2±6.5* | 33.6±5.2 | 43.1±6.4 |
| −5% Fatmax | 47.3±5.3 | 54.3±0.8* | 49.2±6.5* | 38.6±5.2 | 48.1±6.4 |
| Fatmax | 52.6±5.3 | 59.3±0.8‡ | 52.3±7.2 | 43.0±4.4† | 52.1±6.8 |
| +5% Fatmax | 57.6±5.3 | 64.3±0.8‡ | 57.3±7.2 | 48.0±4.4† | 57.1±6.8 |
| +10% Fatmax | 62.6±5.3 | 69.3±0.8‡ | 62.3±7.2 | 53.0±4.4† | 62.1±6.8 |
| +20% Fatmax | 72.6±5.3 | 79.3±0.8‡ | 72.3±7.2 | 63.0±4.4† | 72.1±6.8 |
| Fatmin | 86.4±5.4 | 93.8±3.0 | 87.3±8.6 | 86.4±7.8 | 87.6±6.9 |
C, road cyclists; Fatmin, minimal fat‐oxidation rate; Fatmax, maximal fat‐oxidation rate; M, mountain bikers; TF, female triathletes; TM, male triathletes.
*p<0.05 (significant differences with respect to group M).
†p<0.05 (significant differences with respect to groups TM and TF).
‡p<0.01 (significant differences with respect to group TM).
At the same time, the Fatmax did not correlate with the BMI and showed a very low correlation with the VO2max.
La‐min was found at a significantly lower relative work intensity than Fatmax (42.0 (16.9) v 52.1 (6.8)% VO2max, respectively). Lactic threshold was found at a significantly higher relative work intensity than Fatmax (64.6 (8.9) v 52.1 (6.8)% VO2max, respectively). No significant differences were found for the La−min with respect to relative intensities of the lower part of the Fatmax zone (between −20% and −5% Fatmax; 42 (16.9) v 33.1 (6.4) –48.1 (6.8)% VO2max, respectively), or for the lactic threshold with respect to relative intensities of the higher part of the Fatmax zone (10% Fatmax to be precise; fig 2).
Figure 2 Comparison of La−min and lactic threshold (LT) and Fatmax zones in terms of relative work intensity, of the total group studied (*no significant differences for La−min with respect to−20% and −5% Fatmax; †no significant differences for lactic threshold with respect to 10% Fatmax).
Discussion
The Fatmax of the sample studied presented a relative intensity of 52% VO2max, and the range of values varied between the 43% VO2max of the mountain biker group, the 52% and 53% VO2max of road cyclist and male triathlete, groups respectively, and the 59% VO2max of the female triathlete group. This range of Fatmax agreed with the data on cycloergometer efforts found in the literature (33–75% VO2max).1,5,6,7,8,9,10,11,12,13
When relating the data of this study to those of other authors, reference should be made to two important aspects. Firstly, few studies have measured the Fatmax of trained cyclists. To this effect, a Fatmax at a relative intensity of 65% VO2max has been observed in female cyclists9 and of 57% and 75% VO2max in highly trained cyclists of both sexes.8 The remaining studies have been conducted on less well trained populations such as sedentary men (36–55% VO2max),5,10 sedentary women (65% VO2max),7 students of physical education of both sexes (49% VO2max),6 and male amateur cyclists (33–65% VO2max).1,13 The second aspect to be remarked on is of a methodological nature: as the evaluation of the fat‐oxidation rate at all the submaximal work intensities has been rarely applied, few studies have been found that can be used as reference.11,12,13
To this effect, authors who have measured the Fatmax in amateur cyclists found Fatmax at 63–64% VO2max and at 0.6 g/min, and a Fatmax zone at 51–55% and 69–72% VO2max. Fat oxidation was null above 81–89% VO2max. These results are higher than those obtained with respect to the road cyclist group of this study (Fatmax, 52% VO2max; Fatmax zone, 43–62% VO2max), which may be because the performance level of the road cyclist group was heterogeneous. The Fatmin found in this study (88 (7)% VO2max) was similar to that observed in amateur cyclists (89 (3)% VO2max).11
The fact that no significant differences were found in the Fatmax zone, expressed in absolute values, between the different groups in this study may be due to the heterogeneity of the road cyclist and male triathlete groups. It has been observed in other studies that, in cyclists with a greater level of VO2max, the Fatmax expressed in absolute values is greater than that seen in cyclists with a lower VO2max.12 At the same time, the Fatmax did not correlate with either the BMI or the VO2max. The same results are reported by Achten and Jeukendrup12 for the BMI, although, in contrast with their findings, a significant, but low, correlation was found in this study for the VO2max (r = 0.636; p<0.01).
With respect to the Fatmax zone, significant differences were found when expressed in relative work intensity. In this sense, in the female triathlete group, the relative work intensities at which the Fatmax zone was found were greater than in the other male groups and, in many participants, significantly so. This fact might be attributable not so much to the type of training as to the differentiation due to the effects of the female hormones, the oestrogens. These hormones mean that, at a relative work intensity, the intramuscular triglycerides are used in a more important way in energy output,2 which is facilitated by greater reserves of intramuscular triglycerides in women.20
On the other hand, regarding the male groups, significant differences were found between mountain biker and male triathlete groups with respect to the high part of the Fatmax zone, and between mountain biker and road cyclist groups with respect to the low part. These differences may be attributable to the type of training of the different sporting modalities. It is well known that aerobic endurance training enhances fat‐oxidation capacity because of the increase in the mitochondrial enzyme content and in the enzymes responsible for β‐oxidation.3,21 The fact that the Fatmax zone of the mountain biker group was found at a significantly lower % VO2max than in the road cyclist and male triathlete groups might be attributable to the type of training carried out according to the competition requirements of each of the sporting modalities. It has been observed that the percentage of total competitive time developed below the first ventilatory threshold in high‐level road cyclists corresponds to 71.5–74.6%, compared with 18.5% in high‐level mountain bikers. Regarding the zone between the first and second ventilatory thresholds, the percentage of total competitive time developed in this zone is 22.7–25.2% in road cycling and 51.4% in mountain biking, and regarding the zone above the second ventilatory threshold, it is 2.7–3.3% in road cycling stages competitions,22,23 compared with 30.1% in high level 1‐day mountain‐bike competitions.24 Therefore, it is logical that the lower part of the Fatmax zone of the road cyclist group is significantly higher than that of the mountain biker group. Moreover, the pace of the race of the male triathlete group is found around the ILT,25 which is why it seems logical to consider that the higher part of the Fatmax zone is significantly higher than that of the mountain biker group.
It seems obvious that when the same relative work intensity is measured, after an endurance training period, the fat oxidation will increase,21 as this type of training has been observed to enhance not only the oxidation of intramuscular triglycerides in an absolute manner but also the relative work intensity at which fats are oxidised.7 Consequently, performance also improves.3
What is already known on this topic
Until recently, methods followed by some authors measured only the athletes' fat‐oxidation rate at a reduced number of work intensities.
Now, a method exists to measure the fat‐oxidation rate for all the range of aerobic work intensities, by means of indirect calorimetry and using stoichiometric equations.
This method, however, has only been applied in a few studies carried out with amateur cyclists.
What this study adds
This study used the indirect calorimetry method to calculate the fat‐oxidation rate on high performance‐level cyclists and triathletes.
The results show different adaptations of the endurance athletes' fat‐oxidation rate depending on their sporting discipline.
In any case, we should be cautious when interpreting the data of this study. It has to be remarked that the group sample sizes vary considerably and are not adequately powered. This is due to the difficulty in finding elite athletes of the different modalities.
Regular physical activity at low and moderate intensities enhances the mobilisation of intramuscular triglycerides7 and subcutaneous fat oxidation.1 This variable may also be used to orient training in medium‐endurance and long‐endurance specialities, as this type of training increases the muscular capability to oxidise fats and, in this way, to economise the use of glycogen, thus improving performance.4
Another important finding of this study is the relationship of Fatmax and Fatmax zone with respect to La−min and lactic threshold. La−min was found at a lower relative work intensity than Fatmax, although it coincided with that of the lower part of the Fatmax zone. This suggests that La−min may be more related to lactate clearance capability26 of blood than to fat‐oxidation rate. Lactic threshold was found at a higher relative intensity than Fatmax, coinciding with the higher part of the Fatmax zone. So, lactic threshold appears when the fat‐oxidation rate starts to decrease. One of the main causes of this may be an increase in the glycolytic flux, as it has been suggested to induce intramuscular changes that affect fat oxidation. A decreased muscle pH as a result of increased proton release during anaerobic glycolysis has been proposed by Starritt et al27 as a possible mechanism explaining decreased fat oxidation.
Traditionally, lactic threshold has been considered as one of the performance determinant factors in road cycling,28 and has been used to control endurance sports training.29 The findings of this study highlight the importance of the Fatmax and the Fatmax zone as variables to be taken into account when determining training rhythms.
In conclusion, the relative work intensity at which the Fatmax and the Fatmax zone are found may explain the different endurance adaptations of the athletes according to their sporting discipline. Furthermore, lactic threshold and La−min were found at different relative work intensities with respect to Fatmax, but they belong to the Fatmax zone. Longitudinal studies are required to evaluate the effect of endurance training on athlete fat oxidation.
Acknowledgements
We thank the participating athletes for their cooperation, and Maria Sarrado for her contribution to this study.
Abbreviations
BMI - body mass index
CHO - carbohydrate
Fatmax - maximal fat‐oxidation rate
Fatmin - minimal fat‐oxidation rate
ILT - individual lactic threshold
La−min - minimal lactate concentration
VO2max - maximal oxygen uptake
Footnotes
Funding: This work was supported by a grant (DOGC number 3885 16.05.2006) from the Direcció General de l'Esport, Generalitat de Catalunya.
Competing interests: None declared.
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