I think the Diet doctor has it right. It's hard to say from an observational study what was going on, of the 5 percent with the lowest carbohydrate intake, how may were actually on an Atkins type plan? A lot of the time in observation studies, "low carb" just means a higher percentage of the person's calories came as fat. Take the standard diet and just replace some of the non-fat calories with fat, and when it comes to water soluble vitamins, minerals etc. it really is becoming less nutrient dense. The problem with the argument vs. high fat diets being less nutrient dense isn't that they don't have a point--even just switching from bread to donuts, what non-fat soluble nutrition is in there is diluted by the fat and sugar. It's just that there's no end to the broccoli, spinach, peppers, liver etc. that can be added in to a high fat diet (hopefully not to donuts), so the real problem is the lack of those nutrients, not the presence of non-carbohydrate calories.