I agree with both Scott and Thomas.
Lean muscle mass at baseline (ie without being on a muscle building routine) is a prized commodity. I would not give it up lightly. It not only makes you more of a powerful athlete, it can keep you healthy through strength, as well as lean due to its high thermogenic effect. And, if I’m not mistaken, it will also boost your VO2 max, because of all that muscle consuming oxygen. As Scott points out, I would truly review your need to reduce muscle mass, perhaps talk to some trusted friends to see if they think it would be reasonable.
If you would like some prescriptive advice on how, specifically, to lose muscle, try the following ~6 month cycle:
Put a pause on any dedicated strength training.
Start building an aerobic base with progressive, long, Z1/Z2 distance running/hiking, as delineated in UA books.
Prioritize 2 long workouts per week, building up to 4-8hrs each, all at low intensity (eg hiking, long bike rides, run/walks).
Key workouts will be those over 3hr long, because past this point your body, in general, will ramp up using your own muscle as an energy source, in order of up to 1/10 – 1/5 of the calories you burn.
I want your body to be like: “hey, if I had less muscle, all this constant, easy movement would a lot easier!”
Paradoxically, I would continue to eat plenty of protein, say the usual rec of about 20-25G per meal or per 3hrs. I don’t have any specific science saying this will help or hinder muscle loss. I presume for rapid muscle loss you could cut out most of your protein, but this is not recommended, and is likely unhealthy.
Regarding overall calories–would just keep the same amount you are now, or just eat to hunger with wholesome foods, if your have the resources and time to make it work.
(unique goals like this may warrant consultation with a nutritionist, because sometimes it hard to know when to stop, once the muscle loss gets ramped up, and you may need an objective outsider to tell you when to sop or slow down)
Plan B: just drink tea with a little milk for a few months (and chain smoke cigarettes)–I think this is what christian bale did to lose 60lbs for the machinist. (JK–don’t do this)
