When you wake up in the morning, your glycogen stores are slightly depleted because
you're in a fasted state, having not eaten for several hours.
This has several implications. Fat oxidation increases, but ability to resistance train
decreases because of low glycogen.
The significance of this is that cardio at a lower heart rate (~60%) in the morning will
allow you to burn a higher percentage of fat for energy, keep you energetic all day, and
anecdotally raise your metabolism throughout the day.
Fasted cardio has also been shown to be more beneficial in staving off fat gain in a
caloric surplus than cardio after a meal, even when calories are held constant
http://www.ncbi.nlm.nih.gov/pubmed/20837645 (possibly because it does a better job
increasing insulin sensitivity).
Unfortunately, resistance training ability will suffer and should be best kept for later in
the day. Studies have also shown that people are strongest in the evenings.
Conclusion - Cardio early, resistance training late.
Full story:
https://www.quora.com/What-is-the-best-time-of-day-to-exercise?no_redirect=1
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* Origin: The Hobby Line! BBS (999:1/1)