Login or Register

What Is the Function of Fructose?

Nov 30, 2011 | By Sydney Hornby, M.D.
What Is the Function of Fructose?
Photo Credit Jupiterimages/Brand X Pictures/Getty Images

Fructose is one of the three simple sugars that form the building blocks for most of the carbohydrates in the normal human diet. Moderate amounts of fructose are found in whole fruits, and large amounts in honey and processed sweeteners. Humans are capable of metabolizing fructose, but its metabolic functions are more limited than those of the other simple sugars. The limited metabolic fates of fructose have led some researchers to suggest that consuming large amounts of it may pose health hazards.

Simple Sugars

Glucose, fructose and beta-galactose are the three simple sugars at the core of human carbohydrate consumption. Almost all of the carbohydrates in the human diet are formed from chains of these three molecules. For example, sucrose, or cane sugar, consists of a molecule of glucose and a molecule of fructose linked together. Lactose, or milk sugar, is formed from a molecule of glucose and a molecule of beta-galactose linked together. Amylopectin, which is a type of starch found in many plant foods, is formed from thousands of glucose molecules linked in a branching chain. All carbohydrates are broken down into simple sugars by the intestines, and it is these simple sugars that are adsorbed into the blood.

Sources of Fructose

Whole fruits contain a moderate amount of fructose, both by itself and as a part of sucrose. According to the USDA food standard reference database, for example, a small banana contains about 6 grams of total fructose. By contrast, a 20 oz. bottle of cola sweetened with high-fructose corn syrup, which is 55 percent, fructose, contains about 36 grams of fructose. Sweeteners used commercially, such as sucrose, honey and high-fructose corn syrup, contain large amounts of fructose because, according to the Asia Pacific Journal of Clinical Nutrition website, fructose is more than 50 percent sweeter than glucose.

Functions of Glucose and Beta-Galactose

Glucose can be used by all human tissue as a source of energy, and some cells, such as red blood cells, can only use glucose, which is so important as an energy source that it is stored as glycogen in muscles and in the liver. Glycogen consists of thousands of glucose molecules linked in a branching chain, similar to amylopectin. When large amounts of glucose enter the blood from the intestines, some will be used for immediate energy needs throughout the body, some will be stored as glycogen and any that is left over will be made into fat by the liver. Beta-galactose is structurally very similar to glucose and can be converted into it by most tissues, so for metabolic purposes, it is essentially the same as glucose.

Functions of Fructose

In contrast to beta-galactose, fructose is structurally different from glucose and under normal circumstances can only be metabolized by the liver. Fructose can be converted into glucose by the liver, but this conversion only takes place when large amounts of fructose are consumed after periods of prolonged starvation, as is discussed in the paper "Intermediate Metabolism of Fructose," published in the American Journal of Clinical Nutrition. This paper also describes how fructose itself cannot be stored as glycogen, although small amounts of fructose do increase the rate at which glucose is converted into glycogen. These facts have led some researchers to hypothesize that while moderate amounts of fructose will likely be used to meet the liver's immediate energy needs, large amounts of it are likely to be converted into fat by the liver, simply because, in contrast to glucose, there isn't anywhere else for fructose to go. If this hypothesis is true, then consuming large amounts of fructose could have adverse health effects because fat synthesized by the liver is released as very low density lipoproteins, or VLDL, a type of cholesterol that greatly increases the risk of coronary artery disease.

The Fructose Controversy

Studies have been conducted to test the hypothesis that heavy fructose consumption can lead to increased VLDL in the blood. A typical example, published in the June 2009 issue of the American Journal of Clinical Nutrition, found that high fructose consumption increased VLDL by 51 percent in normal individuals and 110 percent in those at high risk for metabolic disease. The Calorie Control Council, an organization representing the low-calorie and reduced-fat food and beverage industry, disputes the finding of this and other similar studies on their Fructose Information Center website, claiming that the consumption of fructose in the studies does not accurately reflect the way fructose is actually consumed by most people.

References

Article reviewed by OmahaTyppo Last updated on: Nov 30, 2011

Must see: Photo Galleries

Member Comments