Re. Diabetes, I think in some cases restricting carbs can be fatal as diabetics can suffer from ketoacidosis, where the blood turns acidic due to an uncontrollable amount of ketones present.

Also, if blood sugar drops too low, hypoglycaemia is a very real and potentially fatal threat.

In this case, a keto diet high in fat may not be ideal - an emphasis on low-GI carbs wound be safer. Perhaps it may be possible to ease into a keto diet slowly but the amount of time and monitoring may be a bit much to bare.

To prevent diabetes, keto all the way but it's a little bit tricky if you're already there and have been for some time.