This means your muscles will have to resort to fat or protein for energy. This is referred to as 'hitting the wall', quite a common phenomena for those who have run marathons.
Carbohydrate taken during exercise can overcome the above.
Trained muscles will take up the glucose for extra energy, helping you to maintain your pace at the end of a race and therefore better your time. Since glucose is brain food, you will ultimately 'feel' better and put in more effort. Fluids are much easier to consume than solids. Solid food may cause gastric upset or sit in your stomach, leaving you feeling 'heavy'. 30gms beginning of race at medium intensity, 60gms or more towards end, especially if high intensity.