Do you lose muscle faster on a 500 calorie daily deficit or from halting all resistance training?
An interesting question.
To keep my answer as simple and manageable as possible, I’ll need to ignore the contribution of genetics, training history and age, along with a whole host of relevant factors, hoping that the result will contain enough meat on the bone for a decent meal (i.e. answer).
Let’s think about it a little.
Other than their usefulness in locomotion (movement), your muscles serve as easily accessible pools of reserve amino acids in times of food scarcity (the other source of easily accessible energy, about 1/2 kg. of stored glycogen, less than 2000 kcals worth, will be used up in the first few days of your starvation diet).
For a normally active person of average weight, we’ll guess that a minimum of 2000 kcals are required for functional weight maintenance. A 500 kcal diet will leave you with a daily deficit of roughly 1500 kcals. [1 gram of protein furnishes about 4 kcals while 1 gram of fat is worth 9 kcals of energy].
So making a wild guesstimate (again, not taking into consideration the influence of your personal respiratory profile, i.e. how your body preferentially draws energy or the rate at which your metabolism slows) and doing a little bit of creative accounting (2/3 of the calorie deficit paid for from amino acid resources and 1/3 from fat stores), after the first few days, you are looking at an admittedly arbitrary derived payout amount of approx. 0.25kg lean tissue loss/day. However, through much personal experimentation and experience, this muscle loss can be greatly ameliorated through proper nutrition and a high protein diet.
Having had many accidents and cumulative injuries over a long and active life, with more than a few surgeries to mend damaged bones, muscles and connective tissue, I can reliably attest that major signs of atrophy do not appear for a good 2–3 weeks after a body part is totally immobilized, after that, you will be surprised how fast muscle disappears before it reaches a steady state!
So, unless you plan spending your time chalking up bedrest while handcuffed to your bed, I would say that the level of calorie deficit given is going to play the leading role in lean tissue loss during a situation where you are allowed normal movement and daily activity.