The reason, I believe, why this is a healthier diet than a low-fat one is because it keeps my insulin levels down. When you eat a lot of sugary foods or easily digestible carbs (like rice or pasta) your body has to produce lots of insulin to suppress the rapid increase in your blood sugar levels. Eating fat or protein doesn’t have anything like the same effect on your blood sugars, and therefore your insulin levels. Because insulin is also a fat promoting hormone, advising people to eat lots of starchy foods (which is still standard dietary advice) is likely to make them fatter, not slimmer.