The purpose of this study was to assess substrate oxidation during a GXT to exhaustion while determining the point at which FATmin occurs in ATL and NATL. Our secondary aim was to determine if there was a correlation between VO2 at AnT and at the FATmin. Our results showed that FATmin was obtained at 87.60 ± 1.60 % of VO2max in ATL and at 85.25 ± 1.10 % of VO2max in NATL. AnT was reached at 87.57 ± 1.30 % of VO2max in ATL and 84.64 ± 1.10 % of VO2max in NATL. Hetlelid et al. [13] found AnT in well-trained and recreationally trained athletes to be at 90 and 83 % of VO2max, respectively. Mickelson et al. [20] found AnT in elite athletes at 83 % VO2max. Our results coincide with previous studies confirming higher anaerobic capacities in athletes.
Pearson correlations for AnT and FATmin in ATL and NATL were very high (r = 0.99, p < 0.01, 95 % CI 0.99 to 1.00 and r = 0.97, p < 0.01, 95 % CI 0.91 to 0.98), respectively. Large effect size explained 98.01 % of variance in ATL demonstrating high strength of connection. The corresponding effect size in NATL explained 94.09 % of variance.
Previous studies examining the variability of substrates utilization at high intensities did not report intensities and total fat and CHO oxidation values when RQ ≥1. Goedecke at al. [11] investigated substrate oxidation at three different intensities and reported RQ of 0.97 at last stage equaling 70 % peak power output. Van Loon et al. [29] performed measurements at different intensities, with the last stage corresponding to 72 % VO2max. RQ was not reported, but it was stated that fat was contributing up to 25 % to total energy production. Therefore, we estimated that last stage RQ was 0.92 ± 0.05. Stepto et al. [28] performed testing at 86 % VO2max and reported RQ = 0.92. Coyle et al. [7] performed testing at 80 % VO2max without reporting RQ but confirmed “high lactate threshold,” leading to an assumption that exercise intensity was under AnT. Romijn et al. [25] performed testing at 85 % VO2max and reported RQ of 0.91. Hetlelid et al. [13] performed testing at 94 and 89 % of VO2max and reported RQ of 0.88 and 0.95, respectively. The results of these studies suggest that, at the highest measured intensities, subjects were continuously under RQ = 1 or AnT, which may explain why fat oxidation was present.
To the best of our knowledge, there have been no studies on CHO utilization at intensities corresponding to AnT. In our study, ATL had CHO oxidation of 4.47 ± 1.24 g min−1 when RQ = 1.00 contributing to 97.91 ± 1.02 % of total energy production. NATL had 4.17 ± 0.95 g min−1 CHO utilization (RQ = 1.00) and 96.99 ± 2.21 % contribution to total energy expenditure. Rehrer et al. [24] found an average CHO oxidation to be 2.50 g min−1 at 70 % VO2max in sedentary population, but did not report RQ. Stepto et al. [28] reported CHO oxidation of 4.95 g min−1 while cycling at 86 % VO2max (RQ = 0.92) which is higher than in our study. Romijn et al. [25] reported CHO oxidation of 3.22 g min−1 in male cyclists at 85 % VO2max and RQ = 0.91. Hetlelid et al. [13] reported CHO oxidation of 3.61 g min−1 (RQ = 0.88) at 94 % VO2max and 3.79 g min−1 (RQ = 0.95) at 89 % VO2max in well-trained and recreationally trained athletes, respectively. Our data is in agreement with these studies; nevertheless, a direct comparison with other studies is difficult due to differences in factors affecting CHO utilization (diet, pre-training meal, age, sex, weather conditions, and testing methodology).
In our study, we used a GXT treadmill protocol with 2-min stages and a constant incline of 1 %. With this type of protocol, we aimed to compensate for the lack of air resistance while running on a treadmill and to obtain more accurate and detailed sample data [6, 18]. The average duration of the exercise stage was 16 min in ATL and 12 min in NATL. Numerous authors recommend that tests for VO2max should last no longer as 12 min, as prolonged tests could lead to inconsistent results [4, 23]. We noticed that athletes could not achieve VO2max within 12 min due to their high endurance capacity, making short test stages more preferable. Finally, this type of protocol is highly correlative to running economy and actual VO2max consumption with outdoor running. This provided additional reassurance that the protocol used is suitable for accurate measurement of VO2max, correlation assessment, and physiological testing of athletes in their natural environment [6, 18, 22].
A mechanism behind lipid oxidation at high intensities has not been fully elucidated. With increased intensity, there is a gradual shift from fat as a primary fuel source to CHO, until complete cessation of fat as a fuel for high-intensity exercise. This point, called FATmin, highly correlates with AnT in our study. It is important to note two things: (1) AnT and FATmin are interchangeable because they occur at the same individual point and (2) the point at which they occur depends on several factors such as training level and activity type, sex, age, and genetics which could explain both inter-subject and inter-study differences [16].
Additional benefits for athletes and coaches can be surmised from this study. The size of the glycogen storage depends on the muscle size, with approximately 500 g locally available (additional 100 g globally available in liver), corresponding to approximately 3000 kcal of produced energy. Since there are no lipids available at intensities above AnT, CHO storage would be depleted after approximately 2 h, contributing towards the effect known as “hitting the wall,” therefore limiting not only athletes but also every subject performing at this level.
We consider as a key limitation of this study a rather small sample size which could affect our ability to estimate a causal relationship. Further studies with larger sample sizes including different types of subjects (sedentary, obese, different sports) would allow investigators to further understand how high-intensity exercise and the lipid oxidation are related. Other limitations include lack of control over the pre-test nutritional habits of the subjects which could have affected fat and CHO oxidation levels. High or low CHO diets could have some impact on total oxidation rates of the substrates making this question open for further studies.