# Mitochondrial respiratory capacity

Mitochondrial respiratory capacity is the top speed at which your mitochondria can push oxygen through their electron transport chain (ETC) to make energy, when fuel and ADP are plentiful. It is not the same as mitochondrial density, which is just how many you have. The gold-standard measure is done outside the body, by high-resolution respirometry on permeabilized muscle fibers. One reading (state-3, or OXPHOS) captures ATP-linked flux. Another, using an uncoupler called FCCP (the ETS reading), reveals the absolute ceiling of electron transfer. A few things set that ceiling: the activity of complexes I through IV, the inner-membrane surface area, and the supply of electron donors (NADH, FADH₂). The ceiling falls with age, partly from remodeling of the inner-membrane folds (cristae) and complex I trouble. That decline tracks with drops in VO2max, insulin sensitivity, and physical function. The good news: aerobic training and caloric restriction raise it, even in older adults.

## Sources

- Gnaiger E. (2009). Capacity of oxidative phosphorylation in human skeletal muscle: new perspectives of mitochondrial physiology. International Journal of Biochemistry and Cell Biology. https://doi.org/10.1016/j.biocel.2009.03.013

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