Mixed
Training with low muscle glycogen availability augments the activation of signaling proteins and the resulting phenotypic adaptations for mitochondrial biogenesis compared to training with normal glycogen.
If you want to maximize mitochondrial growth, you can try training with lower muscle glycogen. This might mean doing your workout before eating a large carb-heavy meal, or doing a second session later in the day when glycogen is depleted. This state of 'low fuel' signals your body to build more mitochondria more aggressively than when you are fully fueled. However, this may reduce your performance output during the workout itself.
Of recent interest are the observations that commencing endurance-based exercise with low muscle glycogen availability results in greater activation of many signaling proteins with putative roles in mitochondrial biogenesis compared with when the same exercise is undertaken with normal glycogen concentration... When exercise sessions are repeated (i.e., training) in the face of low glycogen availability, the phenotypic adaptations resulting from such interventions are also augmented.
Why this rating
Based on specific human studies cited (Hawley and Morton 2014), but the paper is a review.
Source
Molecular Basis of Exercise-Induced Skeletal Muscle Mitochondrial Biogenesis: Historical Advances, Current Knowledge, and Future Challenges
Christopher G. R. Perry et al. · Cold Spring Harbor Perspectives in Medicine · 2017
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