Our bodies can only absorb monosaccharides (glucose, galactose, or fructose), the single units of sugars and starches. Once absorbed through the small intestines into the portal vein, and then circulated into the bloodstream through the liver as blood glucose, our bodies can put glucose to work in three ways:
1. It can burn the glucose immediately for energy if blood glucose levels are not at a stable level of 20 grams blood borne glucose circulating per hour.
2. If it is not needed for energy immediately, then it is converted into glycogen in the liver or muscles. The liver has the capacity to store 100 grams of glycogen. The muscles have the capacity to store between 250-400 grams of glycogen, depending on muscle mass and physical condition. Liver glycogen supplies energy for the entire body. Muscle glycogen only supplies energy to muscles.
3. If the body has an excess of glucose, and all of the glycogen stores are full, the surplus glucose is converted to fat by the liver and stored as adipose tissue (bodyfat) around the body. If needed, fatty acids can be burned as fuel (BUT the fat cannot be converted back to glucose).
Now that we have outlined how our bodies use glucose, we will discuss why fruit (fructose or fruit sugar) is detrimental in an attempt to maximize fat loss. Since muscles have the specific purpose of contraction, they have a limited number of enzymes for glycogen synthesis. Muscle only has the necessary enzymes to convert glucose (and nothing else) into glycogen. The liver, however, is able to make glycogen from fructose, lactate, glycerol, alanine, and other three-carbon metabolites. Muscle glycogen, which is similar in structure to starch, is an amylopectin (branched chained polymer containing hundreds of glucose units). Unlike muscles, which can only supply energy to themselves through the stored 250-400 grams of glycogen, the liver is responsible for supplying energy to the entire body.
If you have fruit, fruit juice, or any of its derivatives, the following conditions occur:
Referring to the three ways the body uses glucose, assuming that blood glucose levels are adequate, the glucose will then be stored as glycogen. Muscle does not have the necessary enzymes to synthesize fructose into glycogen; therefore the liver converts this fructose into liver glycogen. It would only take three, 8-ounce glasses of orange juice to fully replenish liver glycogen stores. Since the liver is responsible for supplying energy to the entire body, once its stores are full, a rate limiting enzyme in glucose metabolism which is responsible for signaling the body to store glucose as glycogen or convert it to fat (phosphofructokinase), signals the body that all stores are full. If the glycogen stores are signaled as full, then the third way our body uses excess glucose is to convert it to fatty acids and store as adipose tissue. In essence, fruit sugar is easily converted to fat.
Many may be asking why then is fruit low on the glycemic index? If it does not cause a sudden release of insulin, then how could it ever be a poor food choice? Once the fructose (fruit sugar) enters the liver and liver glycogen is already full, then it can not be used by the muscles for glycogen or energy production. It is converted to fat and released back into the bloodstream to be stored as adipose tissue. The low glycemic response is based on the fact that fructose leaves the liver as fat, and fat does not raise insulin levels.
This is the biochemistry behind the recommendations to limit fruit in your diet. As mentioned, fruit is a very nutritious food full of vitamins, minerals, fiber, and low in calories and fat. If your goal is to exclusively to minimize bodyfat, then it is advisable that you consume more complex carbohydrates, which will go to replenishing muscle glycogen stores rather than fruit, which will only replenish liver glycogen stores, and is useless in muscle glycogen replenishment.