i’m aware that technically at any level of sustained calorie deficit there is some degree of muscle loss, but at about how big of a deficit does there start to be a more significant loss that not even strength training can prevent?
You can lose weight on a calorie deficit and gain muscle mass. The research and lots of personal experiences show that. (I can pull the research up later, just being lazy right now :). Body recomposition
But it takes several things:
Quite a balancing act frankly.
Even at a deficit you will mantain muscle mass (or most of it) if you keep working out. The body gives a wider priority to fat as an energy source when you give it the stimulus that you need protein. It’s better to eat a lot of it as well, since that way you could even get bigger, but I think eating a decent amount will still be enough to mantain for the most part.
It likely depends on your current amount of muscle mass, the size deficit (% of bodyweight/week) and the activities you are doing throughout your deficit.
You’re absolutely right that muscle loss occurs. With ample resistance training and eating enough protein, you can maximize muscle mass retention and body fat loss. I don’t have specific numbers, but that is my experience with clients.
In my experience, it’s not just the deficit. It’s what you’re consuming in the deficit. I’ve been able to build muscle and strength while losing weight by heavily focusing on protein and hydration. I think if you are eating a low calorie diet that is lower in protein (my natural state bc I’m vegetarian so my diet tends towards a lot of leafy greens) you can see notable muscle mass loss quite quickly
“Individuals performing RT to build LM should avoid prolonged energy deficiency, and individuals performing RT to preserve LM during weight loss should avoid energy deficits >500 kcal day-1.”
https://pubmed.ncbi.nlm.nih.gov/34623696/
“Our results showed that, during a marked energy deficit, consumption of a diet containing 2.4 g protein · kg(-1) · d(-1) was more effective than consumption of a diet containing 1.2 g protein · kg(-1) · d(-1) in promoting increases in LBM and losses of fat mass when combined with a high volume of resistance and anaerobic exercise. “
https://pubmed.ncbi.nlm.nih.gov/26817506/
“In aspiring female physique athletes, a higher protein diet is superior to a lower protein diet in terms of increasing fat-free mass in conjunction with a resistance training program.”
https://pubmed.ncbi.nlm.nih.gov/29405780/
Most likely yes, but I wouldn’t worry too much about it, muscle memory works very well. As soon as you switch back to maintenance/surplus you’ll gain back the muscle sooner than you’d think. And reverse diet and a clean bulk is a good way to minimize the bodyfat you’re going to put back on.
According to DEXA scans (and backed up by how I looked and felt), I lost almost as much muscle as I did fat (roughly 60/40) over a 1 year period eating at a 500 calorie/day deficit. Took me twice as long to gain all that muscle back as it did for me to lose it.