@Cuba1hr @Dimitrios-Kanellopoulos I think Polar simply looks at your heart rate distribution and uses established equations on fat/carbs as primary fuel source based on intensity.
The fun part is: if you rely on default heart rate zones set by Polar or yet another age-based formula, this number is probably off, and by a lot. You need to be below your aerobic threshold to use fat as a primary fuel and in a steady state activity. Go harder than necessary, or in an interval stop-go fashion, and you’ll be relying on alactic or anaerobic pathways.
Check out Uphill Athlete. They have multiple ways to test for aerobic and anaerobic thresholds. Once you have these, set your Suunto 9 heart rate zones based on their recommendation and do long runs or bike sessions (working up to 90-120 minutes a pop, longer sessions being really good at forcing your body to rely on fats and conserve carbs) below your aerobic threshold. That would build up an aerobic base and force all sorts of metabolism enhancing adaptations.
Between consistent exercise and clean, reasonable diet, you’d be in decent shape in no time.