To understand how the process of how fruit sugars get used by the body differs, first let’s see how the body uses glucose.
How Does the Body Use Glucose and When Is Fat Stored?
If blood glucose levels are low, the body uses the glucose it gets from foods and burns it immediately for energy. This is one of the reasons why after a workout, the body utilizes carbohydrates so efficiently. Now, assuming that there is no immediate need for energy, glucose is then into glycogen and stored in the liver or the muscles. The liver can hold roughly 100 grams of glycogen but the muscles, depending on how muscular you are, may store between 200-400 grams. The key point to remember here however is the following: The glycogen from muscles can only supply energy to the muscles when they are contracting (so muscle glycogen gets depleted badly during a weight training workout). Liver glycogen however can supply energy to the entire body. It is key to remember this in order to understand how fructose does not help with fat loss.
The way that the body gets fat with an excess of carbohydrates is that if all of the glycogen stores in the body are full, then the extra glucose is converted to fat by the liver and stored as adipose tissue (bodyfat), probably in your buns and thighs or around your waist.
Why Is Fructose Different?
Now that you understand how glucose is used and how fat can be stored in situations where all glycogen levels are full, let's go back to the fruits. What happens with fructose is that the muscles do not have the enzyme required to turn fructose into glycogen. The liver does so fructose replenishes the liver. It does not take much to replenish a liver of glycogen as it can hold around 100 grams only. Therefore, if you eat too many fruits, you will fill up your liver glycogen and this causes the body to release an enzyme called phosphofructokinase signaling the body that glycogen stores are full. Since the liver has to supply energy for the whole body, the body uses its glycogen stores as the fuel gauge. When the tank is full, so to speak, that is when any extra fuel gets stored away. Because of this, I suggest that fruits be limited and even be eliminated if following an aggressive fat loss diet. If you want to eat some fruits on a fat loss diet, I recommend you eat a serving of low sugar fruits like apples or strawberries in the morning with breakfast and maybe another serving with the post workout meal.
By the way, if you are wondering why most fruits can be so low in GI and still cause so much damage is because fructose leaves the liver as fat and fat does not raise insulin levels. Bummer!
Now, I don't want you to think that I am anti-fruit because that is not the case. Fruits are great when on a muscle building diet and also they are valuable for hardgainers. All I am saying is that when on an aggresive fat loss plan, like the one used for competitive bodybuilding for example, you need to limit the fruits.