Why is consuming enough calories in a day so important during intermittent fasting?

I understand that it’s important to get within the healthy range of daily calories on intermittent fasting days, but can’t the body start manufacturing ketones for energy? I know that it takes some adaptation for the body to go from carb-burning to fat-burning, but what exactly happens if you eat below your calorie range for a day?I can understand why doing this long term would be bad for your body, but I don’t quite understand why it’s bad if you do it occasionally. via /r/intermittentfasting https://ift.tt/2Fug3if https://ift.tt/eA8V8J