1800 calories a day isn't going to be a major challenge to his insulin system, no matter how high the glycemic index, even in the long term. What he might have

1800 calories a day isn't going to be a major challenge to his insulin system, no matter how high the glycemic index, even in the long term. What he might have to deal with is the increased hunger from lack of satiety signal (leptin, et al), which would then require more will power to avoid overeating, a precious commodity. Better to spend under your caloric limit by eating foods with bulk (broccoli, carrots, etc) and some strategic fats (a sprinkle of cheese on your salad, nuts for a snack), thereby maximizing the satiety signal you get for those 1800 calories.