Macro partitioning
Endurance training progressively increases fat oxidation during exercise by shifting substrate reliance from plasma free fatty acids to intramuscular triglycerides (IMTG), with significant metabolic adaptations occurring as early as 5 days of training.
If you start an endurance training program, expect your body to become more efficient at burning fat within the first week, even before you see weight loss. This happens by shifting fuel use from blood fats to stored fat inside your muscles. Stick with it; the metabolic shift is real and measurable early on.
We conclude that long-term training induces a progressive increase in fat utilization mediated by a greater oxidation of fats from intramuscular sources and a reduction in glucose oxidation. Initial changes are present as early as 5D and occur before increases in muscle maximal mitochondrial enzyme activity.
Why this rating
Randomized controlled trial with stable isotope tracers, rigorous methodology, and clear statistical significance (P<0.05).
Source
Effects of training duration on substrate turnover and oxidation during exercise
Stuart M. Phillips et al. · Journal of Applied Physiology · 1996
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