One of our group members reported eating a bowlful of cooked barley with 3/4 cup of berries for breakfast. When I asked him what his average blood glucose is on that type of diet, he said 83 mg/dL (equivalent to 4.6 mmol/l), which is well within the non-diabetic normal range. I applauded him for his success but also pointed out that he is an exception. Most of us would have blood glucose numbers skyrocket with that much carbohydrate in one meal! His question then was:

“So then why does the McDougall Diet work? Why does the starch/rice diet work? Insulin is the fat storage hormone. On zero-carb keto, I gained weight because my protein intake raised my glucose via gluconeogenesis. Now I am eating rice and other whole foods and my insulin is not adding [not sure what he means by this] while I have actually lost 25 pounds with no exercise. How come?”

That has been, and still is, a mystery to me! I cannot discount the stories from that minority of diabetics who report reversing diabetes to non-diabetic normal numbers while eating a high-carb low- or no-fat vegan diet. I also believe those who tell me their success in reversing diabetes came from eating a low-carb high-fat moderate-protein (LCHF) diet, whether vegan, vegetarian, or Adventist non-vegetarian. I know from my own experience that LCHF is what has worked and is working to lower my A1C and daily blood glucose and triglycerides, and that eating whole grains and legumes and bananas and sweet potatoes did not lower my numbers. Even when I was eating a totally whole foods diet of farm fresh produce delivered to my door once a week, accompanied by brown rice and lentils and garbanzos and rolled oats and unsweetened soy milk!

I eventually came to the realization that it was not important for me to know why one approach works for some people and the opposite approach works for other people. It is only important that I come to accept and believe other people’s stories of their personal experiences. There is no one right way that works for everyone!

One of the best magazine articles on why different approaches work is this one:

How 5 People With Diabetes are Rocking Their A1c While Eating 5 Different Ways
One thing all of these rockstars have in common is consistency. They found what worked for them and they stick with it! Diabetes takes effort, no one can deny that. But choosing a way of eating and paying attention to how your body reacts allows YOU to be in control, not diabetes. Many people will push their way of eating, insisting it’s the only way—but here you see that there are many ways to reach the same goal—health and happiness.
https://www.diabetesdaily.com/blog/how-5-people-with-diabetes-are-rocking-their-a1c-while-eating-5-different-ways-433569/

But if you still feel compelled to solve the mysteries of the universe, I invite you to peruse some of Dr. Fung’s full articles on this topic:

Dr Jason Fung’s articles

Dr. Fung is a Toronto-based kidney specialist, having graduated from the University of Toronto and finishing his medical specialty at the University of California, Los Angeles in 2001. He is the author of the bestsellers The Obesity Code and The Complete Guide to Fasting. He has pioneered the use of therapeutic fasting for weight loss and type 2 diabetes reversal in his IDM [Intensive Dietary Management] clinic. However, he is primarily a kidney doctor (nephrologist) and weight loss specialist, not an endocrinologist specializing in diabetes, though he has certainly helped many diabetics in his practice.

The Short Version

In summary, Dr. Fung says, “If a high-fibre, high-unrefined-carbohydrate diet lowers insulin, then we should expect all the benefits of reduced weight and reduction in all of the manifestations of hyperinsulinemia—type 2 diabetes, hypertension, heart disease, etc. If a low-carb high-fat diet also lowers insulin, then we should see all the same results. And guess what? I don’t really care if you eat LCHF or Pritikin—as long as those insulin levels come down, I don’t care. Why? Because you will derive the same health benefits of lowering insulin. This will be highly beneficial in all the diseases of high insulin (metabolic syndrome).”

The Supporting Articles

The Carbohydrate-Insulin Hypothesis is Wrong
The link from insulin to obesity seems very solid. Giving insulin to people causes weight gain and taking it away leads to weight loss. However, it is the link from carbohydrates to insulin that is incomplete.  There are many things that can lead to increase in insulin, as well as many things that can lead to a decrease in insulin.
https://intensivedietarymanagement.com/carbohydrate-insulin-hypothesis-wrong-hormonal-obesity-vi/

How Insulin Works – Hormonal Obesity VII 
In technical terms, insulin suppresses gluconeogenesis (new glucose production in the liver). Since insulin tells the body to store sugar, you want to turn off the sugar-burning process.
https://idmprogram.com/insulin-works-hormonal-obesity-vii/

Thoughts on the Kempner Rice Diet
The truth, of course, is that the Rice Diet is only one more example of the unrealistic monotonous diets that occasionally gain faddish supporters. The potato diet, Optifast, Slimfast, the grapefruit diet, etc. If you only eat one thing, it quickly becomes monotonous and all pleasure is lost in eating. So you will eat only if absolutely necessary to avoid starvation because, well, it’s gross. Kempner would, on occasion, whip his patients to help compliance. The problem is that once you start eating normally, all the weight comes back.
https://intensivedietarymanagement.com/thoughts-on-the-kempner-rice-diet/

Thoughts on the Pritikin Diet
There is only one type of “magic”—lowering insulin results in benefits in a high-insulin disease state. But there are two different ways to get there. Both Pritikin and LCHF work fine.
https://intensivedietarymanagement.com/thoughts-on-the-pritikin-diet/