In his blog, the other Richard Feinman reported that a high protein diet can reduce haemoglobin A1c levels.
A 2005 study in the Journal of the American Dietetic Association reported the opposite. According to that study, high a high carbohydrate diet was (paradoxically) found to reduce haemoglobin A1c levels while a high protein diet reduced blood pressure but not haemoglobin A1c.
So, who is right? I don’t know. I would want to take a closer look at the available research before drawing a conclusion. One possibility comes to mind. Not all carbohydrates are created equal. Glucose and fructose are two common components of modern diets. Today we consume very high quantities of high fructose foods (largely because they taste good) compared to our recent ancestors. It is possible that the half dozen people on the high carbohydrate diet were consuming mostly high glucose foods, and crowded the high fructose foods out of their diet. The people on the high protein diet, on the other hand, may have cheated on their diet occasionally. When they did, they may have consumed high fructose foods.