Hi Jenrose,
It's a very valid point you make about WeightWatchers and similar weight-loss diet plans. Because their primary aim is to reduce weight, these regimes concentrate on reducing the food that has the highest calorie value, which is fat. But the foods they recommend are often quite high in sugar because sugar has fewer calories than fat.
But for a diabetic, sugar converts into glucose and raises blood sugar levels, fat doesn't. Similarly carbohydrate converts into glucose, but these diets are less keen on carb reduction because again carbs have fewer calories than fats.