Well, because when your body shoots a slug of insulin into your blood after you eat lots of protein, it drives your blood sugar low
. People who drink high concentration whey protein
shakes (that are low
in sugar and carbs) they hit a wall, because the insulinogenic effect of the high protein promotes the injection of insulin, which then dramatically reduces the few grams of glucose you have circulating in your blood, and then you feel weak, cold, shivery, etc.