It's rate of appearance in the blood provides a direct measure of the amount of triglyceride hydrolyzed in the body. Of course exercise greatly affects the mobilization of FFA in to the blood stream. During prolonged sub maximal exercise FFA oxidation is about 10 fold from a resting state and reesterification of FFA is also decreased. Sure the oxidation rate of FFA increased but lactate production also increases. Lactate decreases the FFA mobilization rate and increases FFA reesterification rate.
Carbohydrates then become an increasingly important source of fuel.
However, during prolonged sub maximal exercise blood lactate levels are very low, thus not affecting FFA mobilization significantly. Thus, carbohydrates are used much less as a fuel and oxidated fats become the most abundant source of energy. Well, to answer this we must next understand that if our body does not have enough calories to meet the demands of exercise and basal metabolism it must start breaking down stores. Of course, as we have seen, fat stores are broken down to meet these energy demands. However, the body is not selective in its needs, it also breaks down protein and carbohydrate stores.