Seven years at the end of November with a diagnosis of 61-62. Reduced to 47 by Feb 2014 simply by immediately cutting all obvious carbs, and down to 40 by that summer by eating a "proper" LCHF/ketogenic diet and using a meter. I've remained at low pre-diabetic/high end of normal level since then apart from a few blips due to other health issues. On metformin for first three years, then GP finally declared I no longer needed it.
What I eat now is much closer to the normal fat, moderate carb food I was brought up on and it's definitely sustainable long term and I much prefer it. (All I basically had to do was cut out those unnecessary carbs again...
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