by Marion Nestle
Aug 9 2007

Insulin, Calories, and Body Weight

This question just in: “I absolutely loved your book “What To Eat.” I work at a book store and I constantly recommend it to people there. But I had one question I can’t seem to find the answer to anywhere. I’m a type 1 diabetic, and for years I’ve counted carbs and given myself insulin to match up only to carbs. As far as insulin is concerned, fats and proteins make little to no difference. For that reason, I always assumed that the low carb diets would be effective to lose weight, since they can diminish your insulin use so dramatically. So, I guess my question is, Why don’t fats and proteins affect your blood sugar as much as carbohydrates? The same question put another way might be, Why don’t all calories affect blood sugar? Or maybe, Does insulin use correlate strongly to weight gain/loss?”

Here’s how I see this: Insulin responds to calories from any source, but the rise in blood glucose following a meal depends mainly on the amount of carbohydrate you eat. This is because starches and sugars are metabolized (broken down) almost completely to glucose, whereas only some parts of some proteins break down to components that can be used to form glucose. With fats, only the glycerol portion—which is a small part of fat–can be made into glucose. High insulin levels push glucose into cells where they form carbohydrates and can also be synthesized into fat and stored. Alas, calories from any source contribute to weight gain.