Determinants of maximal whole‐body fat oxidation in elite cross‐country skiers: Role of skeletal muscle mitochondria

Abstract

Elite endurance athletes possess a high capacity for whole-body maximal fat oxidation (MFO). The aim was to investigate the determinants of a high MFO in endurance athletes. The hypotheses were that augmented MFO in endurance athletes is related to concomitantly increments of skeletal muscle mitochondrial volume density (Mito ) and mitochondrial fatty acid oxidation (FAO ), that is, quantitative mitochondrial adaptations as well as intrinsic FAO per mitochondria, that is, qualitative adaptations. Eight competitive male cross-country skiers and eight untrained controls were compared in the study. A graded exercise test was performed to determine MFO, the intensity where MFO occurs (Fat ), and . Skeletal muscle biopsies were obtained to determine Mito (electron microscopy), FAO , and OXPHOS (high-resolution respirometry). The following were higher (P < 0.05) in endurance athletes compared to controls: MFO (mean [95% confidence intervals]) (0.60 g/min [0.50-0.70] vs 0.32 [0.24-0.39]), Fat (46% [44-47] vs 35 [34-37]), (71 mL/min/kg [69-72] vs 48 [47-49]), Mito (7.8% [7.2-8.5] vs 6.0 [5.3-6.8]), FAO (34 pmol/s/mg muscle ww [27-40] vs 21 [17-25]), and OXPHOS (108 pmol/s/mg muscle ww [104-112] vs 69 [68-71]). Intrinsic FAO (4.0 pmol/s/mg muscle w.w/Mito [2.7-5.3] vs 3.3 [2.7-3.9]) and OXPHOS (14 pmol/s/mg muscle ww/Mito [13-15] vs 11 [10-13]) were, however, similar in the endurance athletes and untrained controls. MFO and Mito correlated (r = 0.504, P < 0.05) in the endurance athletes. A strong correlation between Mito and MFO suggests that expansion of Mito might be rate-limiting for MFO in the endurance athletes. In contrast, intrinsic mitochondrial changes were not associated with augmented MFO.

Comparison of Mitochondrial Respiration in M. triceps brachii and M. vastus lateralis Between Elite Cross-Country Skiers and Physically Active Controls

“…In comparison, both groups examined in our study were on a high level, and even CON closely matched the highly trained group of Jacobs and Lundby (2013) in terms of absolute trueV˙O 2peak values (4.83 vs. 4.82 L⋅min -1 ). Our results also contrast previous findings that found higher P ETF and P CI+CII in XC skiers compared to untrained participants (Dandanell et al, 2018). However, the participants in CON in the present study had higher trueV˙O 2peak than the untrained participants in the study by Dandanell et al (2018) (57 vs. 48 mL⋅min -1 kg -1 ).…”

“…Our results also contrast previous findings that found higher P ETF and P CI+CII in XC skiers compared to untrained participants (Dandanell et al, 2018). However, the participants in CON in the present study had higher trueV˙O 2peak than the untrained participants in the study by Dandanell et al (2018) (57 vs. 48 mL⋅min -1 kg -1 ). The lack of difference in mitochondrial respiration for M. vastus lateralis between groups may imply that there is a ceiling level in respiration after which more training and a higher fitness level do not lead to higher mitochondrial respiration.…”

“…However, these previous results include some important nuances. For example, respiratory states excluding the electron input from P CII , mitochondrial respiration appear to be the same for both trained and untrained participants (Mettauer et al, 2001; Zoll et al, 2002; Daussin et al, 2008), although higher respiration have been reported in trained participants (Jacobs and Lundby, 2013; Dandanell et al, 2018). Furthermore, when comparing four different groups, ranging from trained to elite participants, differences were only found when comparing the least and best trained groups (Jacobs and Lundby, 2013).…”

Are Alterations in Skeletal Muscle Mitochondria a Cause or Consequence of Insulin Resistance?

“…While highly trained endurance athletes also have increased IMCL stores, they have normal insulin sensitivity. This has been attributed to their increased ability to metabolise fat, which has been correlated to their increased skeletal muscle mitochondrial content compared to controls [116]. This aligns with the observation that exercise can prevent fatty acid induced insulin resistance through an increase in fatty acid oxidation, as well as increased triglyceride (TG) synthesis and a reduction in bioactive fatty acid metabolites such as DAG and ceramide species [117].…”

The relationship between peak fat oxidation and prolonged double‐poling endurance exercise performance

“…11,12 Likewise, the values of RS and ES are similar to those of recreationally active males and of endurance-trained males, as well as CC-skiers, respectively. 43,44 Whole-body fat oxidation rates during exercise have previously been proposed to be closely tied to resting plasma FFA concentrations in male athletes. 45,46 However, no associations between resting plasma FFA concentrations and PFO or Fat max were found in the present study.…”