A step further than GFCF, the GFCFSF Sugar Free (gluten-free, casein-free, soy-free, sugar free) This diet presumes the body reacts to soy in much the same way it reacts to gluten so removing soy is also thought to be beneficial. Sugar can lead to inflammation, hyperactivity, and it is thought to feed yeast, so often parents embarking on this diet will remove sugar as well.