Mixed
Training with low muscle glycogen or low exogenous glucose availability (train-low) augments specific metabolic adaptations, such as mitochondrial enzyme activity and fat oxidation, compared to training with normal/high glycogen (train-high).
To boost metabolic adaptations like mitochondrial density and fat oxidation, intentionally start some training sessions with low glycogen. This can be done by training fasted, training after a previous session, or withholding carbs during/after exercise. Expect lower power output during these specific sessions, but recognize that this 'train-low' state triggers stronger metabolic signals than training with full glycogen stores.
Taken collectively, the results of these studies (1, 23, 32, 42, 53, 73, 104) demonstrate that, independent of prior training status, short-term (3–10 wk) training programs in which a portion of workouts are commenced with either low muscle glycogen and/or low exogenous glucose availability augment training adaptation (i.e., they increase the maximal activities of selected enzymes involved in carbohydrate and/or lipid metabolism and promote mitochondrial biogenesis) to a greater extent than when all workouts are undertaken with normal or elevated glycogen stores.
Why this rating
Based on multiple human studies cited (Hansen, Morton, Hulston, etc.) showing consistent metabolic markers, though performance outcomes can be compromised.
Source
Nutritional modulation of training-induced skeletal muscle adaptations
John A. Hawley et al. · Journal of Applied Physiology · 2010
DOI 10.1152/japplphysiol.00949.2010
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This is one finding among thousands. Every one is graded and traced to its source, so you can see what the evidence actually supports. Browse the research →