Any calorie deficit of 500 under BMR and the body will not just consume the fat stores but also muscle. That’s what I learned on the bodybuilding forums at least. Some people went too high on the deficit and ended up losing some muscle definition
It's more likely to happen if your deficit below TDEE is greater than 500 Calories. If you're 500 below BMR, you're almost definitionally losing weight at an unsustainably unhealthy rate, like much greater than 1% of your bodyweight lost per week.
