Boy, there are a lot of unknowns. For starters - muscle weighs more than fat. A body composition test is more accurate for fat loss - which is what most people are striving for.
How old is your friend? Past age 40, things slow down (or in my case, metabolism goes backwards!) so we have to be very conscience of what goes into our mouths.
Aerobic exercise varies. The LSD type (long slow duration) is said to burn fat and intervals burns carbs. While that's true, intervals generally burn more calories because of the afterburn effect - you continue burning calories after you quit exercising. Plus, you burn more while you're at the higher end of the aerobic zone, although you can't hold onto that heart rate for a long time.
A heart rate monitor is very helpful for this.
And yes, mt biking requires more upper body strength - although I don't see it as necessarily a body building exercise. For me, road biking is much more aerobic - but I'm still a bit of a chicken off road! I've been a roadie for a long time & need to get the mantra "skidding is bad" out of my head!
I do spinning when I can't ride & see people barely breaking a sweat. Is your friend really working during spinning class? There should be a puddle of sweat underneath her bike.



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