Which form of cardio is best for burning fat whilst most importantly retaining muscle mass? Will i retain more of my muscle mass from a high intensity workout (ie fast run/sprints) or from a low intensity workout (ie slow run)?

Also is it best to do cardio first thing in the morning on an empty stomach or havin eating an hour or so before?

I'v read the best time to do cardio for fat burning purposes is to do it without eating. This makes sense as the body would burn fat for energy as there is no food inside you, however i have also read that when performing cardio it is best to eat before and after your workout if you want to retain muscle.

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I have shy'd away from cardio in the past because i have lossed too juch muscle when doing cardio, however i want to loose a little fat but my main aim is to retain the muscle i have worked so hard for!