How long is one sleeping before fasted cardio? Some sleep 5-6 hours and may take a nap later in the day, others sleep over 8 hours.
How long before bedtime does the subject eat, specifically protein?
If one has protein shortly before bed, doesn't sleep as long hours and starts cardio soon after waking up (as opposed to lying in bed, watching the news, long drive to the gym, etc) their results and implications of fasted cardio might be much different than otherwise.
That's why I'm bewildered that the countless fasted vs fed cardio debates never touch on these factors, at least not the ones I've read. It's usually a bunch of young skinny guys quoting some study to say that fasted cardio has no benefits and is harmful, the studies which don't control sleep hours, bedtime meal timing etc.
Also there's not a specific heart rate that applies because your heart rate slowly decreases as you age, a certain heart rate for a 20 year old implies lower intensity than the same heart rate for a 40 year old.