The red blood cells use glucose as an immediate energy source. Glucose is absorbed into the bloodstream from the intestines, where the carbohydrates are broken down into simple sugars like glucose, fructose and beta-galactose. One part of glucose is converted into the needed energy, while the rest of glucose is stored as glycogen in muscles and in the liver.
The structure of fructose is different from glucose and it can only be metabolized by the liver. Hence, unlike glucose, fructose can only go into the liver and if large amounts of fructose are consumed, they will be transformed by the liver into VLDL (very low density lipoproteins), which is responsible for the risk of coronary artery disease.
Hence, since the metabolic functions of fructose are restricted, glucose becomes the selected immediate source of energy.