So a sweet refers to food of a high sugar content- candy, biscuits, chocolate etc. A sweet fix refers to the eating of these sweet foods, with a fix being the portion of the sugary food we eat. So in the article you have seen it is saying that "the more sugar we eat, the more the brain tells us that we must eat more sugar".
A fix can be used to refer a craving for any type of food or drink, or can be extended to further to things like movie genre or hobby. With fix meaning an intake, e.g. "I need to get my fix of exercise", " I need a tea fix". It originates from an idea of addiction, that you require this specific thing in order to get back to a normal state.
Hope this helps