For decades mitochondria have been seen as nothing more than microscopic cellular powerhouses. However, more recent findings have demonstrated that mitochondria do not only power the cell. They also play critical roles in balancing cell death and survival, the aging process, many disease phenotypes, and the physiological adaptations to endurance training.

Interestingly, mitochondria are the only organelles in animal cells with their own discrete genome, attributed to their endosymbiotic origin. Thus, while we inherit our chromosomal DNA from both our parents, our inherited mitochondrial DNA comes exclusively from our mothers. This maternal inheritance of mitochondrial DNA, combined with the high mutation rate of mitochondrial DNA, allows us to track our maternal lineage back through generations. You can envision these mitochondrial DNA lineages as a giant tree, where the clustered groups on the different branches make up different haplotypes, known as haplogroups. These haplogroups arose in geographically localized populations, and their distribution across the world has allowed investigators to reconstruct the origins and the ancient migrations of women across the globe as depicted in the image below:

Regional Radiation of Human mtDNAs from their Origin in Africa - Source - Wallace DC. Mitochondrial DNA variation in human radiation and disease. *Cell*.

It is well accepted that one of the main determinants of the individual variation in endurance performance is the metabolic properties of skeletal muscle, particularly its mitochondrial oxidative potential, which is coded by mitochondrial DNA passed down through the maternal lineage. This material DNA codes for some of the most essential polypeptides of the mitochondrial energy generating system, most notably OXPHOS, which generates cellular energy by the oxidation of dietary calories with oxygen. As electrons move down the electron transport chain, the energy released pumps protons out across the inner mitochondrial membrane to generate a proton electrochemical gradient, which the ATP synthase enzyme can employ to drive ATP synthesis. Therefore, the mitochondrial genome provides a few candidate genes for the study of elite endurance athletic status.

Because mitochondrial DNA genes have a central role in OXPHOS expression, different haplotypes and functional variants in mitochondrial DNA can have massive impacts on human physiology and exercise performance. For example, the efficiency with which the electron transport chain generates the proton gradient and by which the proton gradient is converted into ATP is referred to as the coupling efficiency, and humans can differ in their coupling efficiency due to mitochondrial DNA polymorphisms. Since a dietary calorie is a unit of heat, every calorie burned by the mitochondrion generates one calorie of body heat. Tightly coupled mitochondria generate the maximum ATP and minimum heat per calorie burned and thus could be beneficial in warmer climates, while loosely coupled mitochondria must burn more calories for the same amount of ATP, generating more heat, and could be of benefit in colder climates. The importance of heat generation per unit of energy created will be discussed shortly.

Emergent Performance Lab

Inter-individual Differences In Mitochondrial…

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