Investigation of Skeletal Muscle Mitochondrial Function Following an Ultramarathon: A Case Study in Monozygotic Twins

Abstract

The primary objective of this case study was to investigate the changes in skeletal muscle mitochondria that occur in response to a 100-km ultramarathon in monozygotic twins. Secondarily, we sought to determine if periodically consuming a low-carbohydrate, high-fat diet (LCHFD) during training would affect these changes. To accomplish these objectives, one pair of male monozygotic twins ran 100 km side-by-side on treadmills. During 8 weeks of training leading up to this event, twin 1 (T1) and twin 2 (T2) consumed a calorie and macronutrient-matched, high-carbohydrate diet, with the exception of 2 consecutive days per week for the final 4 weeks where T2 consumed a calorie and protein-matched LCHFD. Training load was matched between the subjects, most of which they completed together. The subjects were extremely well-matched in VO2 max (61.2 and 60.3 ml/kg/min), BMI (20.4 and 20.6 kg/m2), and body mass (62 and 61.9 kg). During the 100-km run, food and fluid intake were matched, as well as the pace of the run. The run took 16 hours and 3 minutes to complete. Blood draws and muscle biopsies from the vastus lateralis were collected 4 weeks prior to the run, as well as 4- and 52-hours post-run. During the last 4 weeks of training, T2 lost an average of ~1 kg of body weight (BW) over the two days of LCHFD each week (comparing BW of 4 consecutive Mondays to Wednesdays, p=0.02), which was regained two days after reintroduction of carbohydrate, consistent with fluctuating glycogen levels. Though T2 trained with periodically low glycogen levels, respiratory-exchange ratio was no different from T1 throughout the 100-km run. Four hours post-run, state 3, uncoupled, and fatty acid-supported mitochondrial respiration were reduced in T1 by 22.3%, 34.1% and 25.7%, respectively, and by 28.4%, 49.4% and 39.9% in T2, which was associated with a decrease of 15-20% in citrate synthase (CS) activity. Both twins exhibited decreases in mitochondrial complex proteins, though this was more marked in T2. Blood CPK, CRP, and AST levels were also elevated post-run. Possibly the most novel finding from this study was that 52 hours post-run, respiratory capacity, mitochondrial proteins, CS activity and blood markers showed signs of rapid recovery. It is currently unclear by what mechanism mitochondrial respiration and content was reduced 4 hours post-run, or how it recovered in 2 days. Interestingly, maximal mitochondrial ROS production (glutamate, malate and succinate-supported) was decreased post-run at both timepoints but was increased compared to baseline with the addition of ADP (12.5 μM – 100 μM). This suggests that the mitochondria were less sensitive to ADP-induced reductions in ROS. Neither diet resulted in better preservation of mitochondrial function 4-hours post-run, though T2 showed some signs of supercompensation 52 hours-post run. This study is valuable because it provides evidence of an acute reduction in mitochondrial content and function following ultra-endurance activity, followed by rapid recovery. This study was approved by the Brigham Young University Institutional Review Board and conformed to the principles of human experimentation set by the Declaration of Helsinki.