I've spent a fair amount of time thinking about the recent Internet dust up between Dr M Eades and A Colpo. Two very bright individuals with strong ego's. Each with a vested interest, for very different reasons, in the existence, or lack there of, of a metabolic advantage in very low carb ketogenic diets (VLCKD). The basic problem for each of them is that each of their arguments can be, and have been negated. There are a number of studies that categorically find that any and all weight loss is a matter of caloric balance. There are other studies that find the exact opposite. To assert that there is a universal advantage to VLCKD's, is to invite a "black swan". In other words, all it takes to negate the argument, is a single person for whom the advantage does not exist. The same holds true for the opposite argument, one person who shows a metabolic advantage on a VLCKD, negates the argument that caloric balance is all that matters.
So what are we left with? A paradox? Not quite.
Each argument assumes that the only independent variable is diet. When dealing with human beings however, things like diet do not exist in a vacuum. One of the big, if not the biggest variable in determining if someone will show a metabolic advantage with a low carb diet or not, is insulin sensitivity. If you are resistant to insulin, and/or have hyperinsulemia, then I have little doubt that carbohydrate restriction will allow you to eat more calories and still loose weight, then would be possible on a standard high carb low fat diet. Hence the existence of a metabolic advantage. If, however, you are insulin sensitive, a low carb diet will not affect your weight loss or gain beyond caloric balance. --No metabolic advantage. (In fact I've seen two studies that show that insulin sensitive people do better on low fat diets)
So what we really should be asking is how do we tell who is going to derive the most benefit from low carb diets. If you and your doctor are willing, go and have some lab work done. A standard lipid panel, A1C, and fasting glucose, would get the job done. The standard lipid panel is going to provide you with your HDL and triglyceride numbers. Ideally when you divide your triglyceride number by your HDL number it should be less than 2, 3 is ok but not great, 4 is not good, and 6 is really not good. Your fasting glucose should be less than 100, and your A1C less than 6. Depending on where you are in your progression with insulin resistance, the Tg/HDL ratio will be the first place it will show, followed in order by high fasting glucose and then elevated A1C. If your glucose is over 126,and/or your A1C is over 7, you probably have or are well on your way to diabetes, and your doctor has probably already scheduled you for more tests. If any of the following apply to you, you can benefit by cutting down on your carb intake; fasting glucose is over 95; A1C is 6 or more; TG/HDL ratio is above 2. The farther you are from these values, the more you need to cut your carbs. If you are diabetic, or very close, then you may need to down to less than 50g of carbs, if you are just a little off, then somewhere between 100 - 150g will probably do the trick.