Exercise training increases skeletal muscle mitochondrial volume density by enlargement of existing mitochondria and not de novo biogenesis.
Level 4 - case-series / case-control
Single-arm prospective before-and-after study without a control group
PubMed 28580772 · doi:10.1111/apha.12905
What was done
Twenty-one healthy males underwent 6 weeks of endurance training. Skeletal muscle biopsy samples were collected before and after the training intervention. Transmission electron microscopy was utilized to measure mitochondrial volume density (Mito VD) and the number of mitochondrial profiles per area. Biochemical assays, western blotting, and high-resolution respirometry were performed to evaluate specific mitochondrial enzymatic and respiratory capacities.
What was found
Mito VD increased by 55 ± 9% (P < 0.001), while the number of mitochondrial profiles per area of muscle remained unchanged. Citrate synthase (CS) activity increased by 44 ± 12% (P < 0.001), but no functional changes occurred in oxidative phosphorylation capacity (OXPHOS, CI+II P) or cytochrome c oxidase (COX) activity. Prior to training, Mito VD correlated with CS (r = 0.58, P = 0.01), OXPHOS (R = 0.58, P = 0.01), and COX (R = 0.52, P = 0.02); after training, Mito VD correlated solely with CS activity (R = 0.49, P = 0.04). Intrinsic respiratory capacity decreased (P < 0.05) when normalized to Mito VD, but not when normalized to CS activity.
Why it matters
This study demonstrates that initial training-induced increases in mitochondrial volume density occur through the expansion of existing mitochondria rather than de novo formation. It also confirms that citrate synthase activity is an appropriate surrogate biomarker for tracking changes in mitochondrial volume density.
Limits
The study was an uncontrolled before-and-after study with a small sample (n = 21) restricted entirely to healthy males. Results may not generalize to females, older adults, or clinical cohorts, and adaptations beyond 6 weeks of training remain unmeasured.
Cited by
- supports Physical exercise increases the number, size, and health of mitochondria in the body.