No, I don't believe that sugar sub's will DIRECTLY hinder your fat loss. What I have come accross in my studies and experience is that it can hinder how well you stick to your diet. Generally when consuming lots of sugar subs like Splenda your getting the sweet tastes but not the cals/carbs so your body keeps/starts craving processed carbs and sugar. Which in return can increase your appetite and increase your chances of cheating or failing. I've always found that the stricter I am and better the quality of food is, the less I crave or even think about junk. It just increases your chances of success at a faster pace.