It’s a bit of an involved subject, but here’s the short version.
We don’t recommend having a caloric deficit higher than 15% off your BMR. For example, if you need 1875 calories a day, you need at least 85% of that from food (1593 calories). If you go lower than that, you risk crashing your metabolism and going into starvation mode. This will stall ketosis, and it’s subsequent fat. Our bodies are smart and know to reduce output when faced with too large of a deficit.
If you dive deeper into the science of it, you’ll also run into issues of high fasting insulin that keeps fat cells from releasing energy, even with low glucose levels. Thus you end up with no glucose, no fat, and no ketones to run your cells and everything slows down.