We can digest fats and sugars because the sugars become fats and allow us to weather a bad time with low quality food, but long term it is a higher oxidation load. Healthy carbs always are loaded with vitamins and phytochemicals that help mop up that oxidation.
But as a doctor told me when I was 20, and quite badly underweight - being underweight causes other things to go wrong and if it's happening, you need to pile on the carbs and get the blubber back before you return to a normal diet.
It shows that in a healthy human body there needs to be times that prompt autophagy and that those times you need to be ketogenic so it hits the fat and not the muscle. Muscle readily releases sugar but only about 20% of the mass is sugar the rest is waste. And the amino acids that get oxidised to sugar obviously leave behind oxidative radicals in the process, and ammonia.
We need to starve ourselves a bit from time to time, but if you get rapidly below healthy, stop it immediately and get your glow back.