Ah, my thyroid and auto immune disease does the opposite to me. I was 9.5 stone for most of my adult life, apart from two pregnancies, and when this started I have put on over three stone.
Perhaps we should donate a pint of blood to each other, I will lose weight and you can put it on.
Bodies need calories to expend energy. Calories from sugar in the form of glucose, fructose, lactose, sucrose, dextrose etc. If there are no sugars, then the body will start to burn fat via ketosis.
Once you get to 45 there are cells which don't renew hence the 45 eye, and you find that older people end up wearing glasses or having laser treatment.
There are good fats and bad fats as in saturated and unsaturated.
https://www.nutrition.org.uk/healthyliving/helpingyoueatwell/fats.html?limit=1&start=2