Using an elliptical today, I wondered again about something I’ve noticed before - the little graph of heart rate shows that exercising at the lower heart rate is associated with fat burn, and higher rate is cardio.
If your heart is working harder, why wouldn’t that also be a fat burning exercise? or is it just relative - at lower rates, the main effect is on fat, and at higher rates the main effect is on cardio?
The latter, but only kind of. Metabolizing stored fat requirse that it be freed from storage first, and that requires oxygen. At higher intensities, fat can’t be freed fast enough to supply your energy demands, so energy has to come from somewhere else.
You end up burning a higher percentage of calories from fat at lower intensity exercise, but that doesn’t mean that you won’t be burning more fat during a higher intensity activity, and recovering from it as well.
The science of the various heart rate zones you’ll see everywhere is mildly suspect.