Let me rephrase...building muscle/tone is necessary in order to boost metabolism.
Again, I really don't agree. And I promise, I'm not shooting ya down. Just discussing here.
There are plenty of ways of boosting your metabolic output without increasing muscle mass.
What do you think happens when people are dieting (energy deficit), do some HIIT, and elevate EPOC? No muscle was built, but metabolic output was certainly increased for an extended period of time.
Besides, even IF you built up a bunch of muscle while dieting, the latest figures I've seen suggest that each additional pound of muscle burns an extra 6 calories.
Well geez, if that's the case, even a man (which is hormonally dispositioned for putting on muscle a lot more easily than a woman) who is in an energy surplus isn't going to impact his metabolism significantly by adding muscle.
Don't read me wrong... relative to the other tissues in the body, muscle is more metabolically active. In terms of muscle mass playing a significant role in speeding up metabolic output.... I think it's very over-stated by many "gurus."
Not suggesting you are said guru.
ETA: How do you build muscle tone?