The SBMI said I am in the "normal" weight range, but because of my diet (LC/keto) I will be underweight. No I won't, and if the diet were not sustainable, how did I get here and maintain my weight for several years when I was never able to maintain my weight on the Mediterranean or any other diet?

I played around with the calculator and subtracted 30 yrs from my age, and my SBMI was higher, so they appear to believe the dogma that gaining weight as you age is "normal". It is now common, but is it optimal?