All carbohydrates -- from table sugar and bread to fruits and vegetables -- are made up of simple sugars, or monosaccharides. Fructose is one of three simple sugars. Most carbohydrate foods, to some degree, contain fructose and supply about 10 percent of the calories in the average diet. The liver breaks down fructose in food to yield energy in a process biochemically different than the liver's handling of glucose. Packed with water and fiber as it occurs in fruit, fructose is an ideal energy source. As with any calorie, carbohydrate or otherwise, too much fructose is converted by the liver and stored as fat.
Dietary Sources
The major source of dietary fructose is the disaccharide sucrose, a two-sugar molecule that when cleaved in the intestine, releases equal amounts of fructose and glucose. Sucrose, and therefore fructose, is found in most carbohydrate-containing foods, including grains, beans, vegetables, fruit and natural sweeteners.
In its free form, fructose is the major component of the artificial sweetener high-fructose corn syrup, which is 55 percent fructose and 45 percent glucose. High fructose corn syrup is used in colas, candies and baked goods. Free fructose is also found in many fruits and in honey.
In its free form, fructose is the major component of the artificial sweetener high-fructose corn syrup, which is 55 percent fructose and 45 percent glucose. High fructose corn syrup is used in colas, candies and baked goods. Free fructose is also found in many fruits and in honey.
Metabolism
Ingested fructose is absorbed by the small intestine and transported to the liver, which processes most of the dietary fructose. For fructose metabolism to occur, fructose must first be chemically altered by the enzyme fructokinase. In addition to its location in the liver, this enzyme is found in the kidney and small intestine.
As with the other simple sugars, fructose metabolism results in the formation of energy in the form of ATP, with pyruvate and Acetyl-CoA as intermediaries. Acetyl-CoA enters the Krebs cycle, also called the TCA cycle, or citric acid cycle. By cellular respiration and in the presence of oxygen, the metabolism of all calories converge, their carbon skeletons are converted to carbon dioxide and water with the production of ATP, the cell's energy currency.
As with the other simple sugars, fructose metabolism results in the formation of energy in the form of ATP, with pyruvate and Acetyl-CoA as intermediaries. Acetyl-CoA enters the Krebs cycle, also called the TCA cycle, or citric acid cycle. By cellular respiration and in the presence of oxygen, the metabolism of all calories converge, their carbon skeletons are converted to carbon dioxide and water with the production of ATP, the cell's energy currency.
Glucose vs. Fructose
Although the end-product of glucose and fructose metabolism is the same -- that is, energy -- how each is metabolized differs somewhat.
The entry of fructose into cells is not insulin-dependent, unlike glucose into certain tissues, including muscle and fat cells. The function of insulin is to increase the synthesis of fats and to inhibit their breakdown, resulting in weight gain. In contrast to glucose, fructose does not promote the secretion of insulin.
Additionally, the rate of fructose metabolism is more rapid than that of glucose, because fructose metabolites bypass the major rate-limiting step of glyolysis. The result is that fructose can be converted to energy at a more rapid rate than glucose.
The entry of fructose into cells is not insulin-dependent, unlike glucose into certain tissues, including muscle and fat cells. The function of insulin is to increase the synthesis of fats and to inhibit their breakdown, resulting in weight gain. In contrast to glucose, fructose does not promote the secretion of insulin.
Additionally, the rate of fructose metabolism is more rapid than that of glucose, because fructose metabolites bypass the major rate-limiting step of glyolysis. The result is that fructose can be converted to energy at a more rapid rate than glucose.
Fat Metabolism
As with any calorie, too much fructose is converted by the liver and stored as fat. In the presence of excessive Acetyl-CoA, liver metabolism gears in the direction of fat synthesis. Fat is stored as triacylglycerol and serves as the body's major stored form of energy. The other major form is liver glycogen, a long molecule of stored sugar. After a feeding, the body stores enough glycogen for 12 hours of fasting. By comparison, the body has enough stored fat for three weeks or more.
Health Tips
Not all sugar is created equal. The sugars, both fructose and glucose, found in fruits and vegetables are packed with water, fiber, vitamins and minerals, and are a preferential source of fuel. Fruits and vegetables are good carbs. On the other hand, sweeteners, whether natural or artificial, lack nutrition and are empty calories that have little or no place in the healthy diet. Opt for water over soft drinks, snack on fruits instead of candy, and avoid packaged and processed foods, as these are the very foods that are loaded with sugar.
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