I im a slow metaboliser, but like you was underweight all my life despite eating like a pig. I had very low body fat percentage. I had extremely high calcium levels on a hair test, which indicated a calcium shell. Id suggest a hair test to help determine mineral balance.

Anyway, last year I followed recommendations from the hair test to avoid anything with calcium and vitamin K, and started supplementing vitamin E, and various other combinations. I took 'super adrenal fatigue' and 'adrenal rebuilder', and trace elements 'vitamin E plus' and lived on basically pea protein powder fruit drinks, grapefruit and egg sandwiches. In those three months I gained a stone in weight, had energy and suffered barely any digestive problems.

I totally believe now that when you correct your mineral imbalances your body finds its healthy weight, irrespective of calorie intake or exercise.