<human>: A great friend of mine and I are a bit at odds with how we come to understand this concept, so I thought I'd ask here and hopefully get some good information on the topic to finally settle mine and my friend's understanding.

One argument is that at a calorie deficit, one can gain muscle while losing weight.  While doing this, a lot of protein will be needed to help repair the muscles being "worked out".

The other argument is that at a calorie deficit, one's body will simply increase its size or decrease its size until there is a balance between the calories being burned and the amount being taken in.  This means muscles that may require more calories than their share of calories at the calorie deficit will decrease in size/atrophy while muscles that are in surplus calories in their share of calories at the calorie deficit will become larger.

If the second argument is true, could it be possible that any muscle gains be at the cost of other muscles in your body that are not being used?

Can you provide a brief overview of the text?
<bot>: read the title - calorie deficit = lose weight and gain muscle OR calorie deficit = overall body size will "level out" to accommodate the new calorie budget.