I've been 80% fat, 15% protein, 5% Carbs for the past five and a half years. I began this for weight loss (took off the extra 50-70lbs I was carrying around) but reached my goal after the first 18 months. I continued with this way of eating because my whole body felt better. I do use sugar substitutes such as Allulose, Eurithrytol, Monkfruit, and Stevia (Stevia tastes pretty bad but it's very sweet!) and to date, have had no problems with them.
Warning: If buying in the local supermarkets, don't be fooled by the bottles of sweeteners calling themselves 'Stevia' or some derivative of that. Looking at the labels, there is 1% Stevia, and 99% Malitol inside. Stevia won't raise your blood sugar or spike your insulin. Malitol will...
Every 5-6 months I'll break the diet for a week, eating breads, pastries, potatoes, regular ice cream, pasta, etc., etc. All the things I've cut from my usual diet. But after 4-5 days of eating this way again, my knees start to hurt. Arthritic pains in my hand start up again. I feel bloated after meals... even 'normal' sized ones. Body aches and pains increase. So I go back on the diet.
In less than a week those pains have disappeared. I'll stick with this lifestyle another five and a half years. 🙂