It’s a bit of an involved subject but the short version is: we don’t recommend having a caloric deficit higher than 15% off your BMR. So if you need 1875 calories a day, get 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. Our bodies are smart and know to reduce output when faced with too large of a deficit.
If you do a deep dive into the science of it you’ll also run into issues where people will have high fasting insulin which keeps fat cells from releasing energy, even if the person has low glucose levels, thus you end up with no glucose, no fat, and no ketones to run your cells and everything slows down.