The Metabolism Of Fructose

Fructose is a sugar in fruits such as apples, berries and melons and in some root vegetables. It has the same chemical formula as glucose but differs in its structure. Fructose is used as an artificial sweetener because it has a much sweeter taste than glucose or other simple sugars. A review in "Nutrition and Metabolism" reports that fructose had been consumed in amounts of approximately 20 g per day up to the latter 20th century when typical consumption of fructose rose to between 80 g and 100 g per day. There is some evidence to support the view that excessive intake of fructose is a risk factor for obesity and type 2 diabetes.

Intestinal Digestion

Similar to glucose, fructose is transported from the small intestine directly to the liver. If it is in the form of sucrose, which is a disaccharide composed of glucose and fructose, it is first split into the monosaccharides, glucose and fructose in intestinal cells prior to transport to the liver. A study in the "Journal of the American Dietetic Association" reports that intestinal absorption of fructose was incomplete, causing gastrointestinal distress in up to 50 percent of healthy subjects that consumed 25 g of fructose.

Liver Metabolism

In contrast to glucose, fructose is entirely metabolized in the liver. That means that fructose is not a direct energy source for muscles or the brain. Once in the liver, fructose is immediately converted to intermediates that enter into pathways that produce energy, glucose, stored glucose as glycogen or triglycerides. Therefore, if fructose is present it is metabolized preferentially to glucose, and any excess will be directly converted to fat. Accumulation of fat in the liver leads to insulin resistance. A review in the "American Journal of Clinical Nutrition" reports that studies in humans do show that consumption of high levels of fructose increases the rates of fat production, especially in people who are already insulin resistant or have type 2 diabetes.

Insulin Response

Also in contrast to glucose, fructose does not increase insulin release. Because of this, fructose has a lower glycemic index and was once proposed to be a better carbohydrate source than glucose. In fact, the review in the "American Journal of Clinical Nutrition" reports that low doses of fructose improve glucose metabolism. However, when insulin is increased in the blood, it acts on areas in the brain and suppresses appetite. Fructose does not increase the levels of the hormone leptin either, which also acts as an appetite suppressant. The review in the "American Journal of Clinical Nutrition" reports that some studies in humans show that excessive intake of fructose increases energy intake and body weight.

Fat Metabolism

Fructose promotes conversion of carbohydrates into fats mainly in the liver. A study in the June 2010 issue of "Diabetes and Metabolism" reports that excessive fructose intake increased liver fat production and circulating levels of very low density lipoproteins, also known as VLDLs in healthy men. An excess in circulating VLDLs is a risk factor for type 2 diabetes and cardiovascular disease.

Controversy

The majority of evidence supporting that fructose intake is detrimental to metabolism and increases the risk for obesity and type 2 diabetes comes from studies that used very high doses of fructose. A report in "Physiological Reviews" concludes that there is not good evidence that moderate fructose intake has adverse metabolic consequences. A report in "Obesity Reviews" concludes that the evidence that particular sources of carbohydrates have detrimental effects on metabolism is lacking.