Fructose, or fruit sugar, is a simple ketonic monosaccharide found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It exists in foods either as a monosaccharide (free fructose) or as a unit of a disaccharide (sucrose). It is one of the three dietary monosaccharides, along with glucose and galactose that are absorbed directly into the bloodstream during digestion.
Main use of fructose in the body is to form glycogen.
Free fructose is absorbed directly by the intestine. When fructose is consumed in the form of sucrose, it is digested (broken down) and then absorbed as free fructose. As sucrose comes into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit, which are then each absorbed. After absorption, it enters the hepatic portal vein and is directed toward the liver.
Although the metabolism of fructose and glucose share many of the same intermediate structures, they have very different metabolic fates in human metabolism. Fructose is metabolized almost completely in the liver in humans, and is directed toward replenishment of liver glycogen and triglyceride synthesis, while much of dietary glucose passes through the liver and goes to skeletal muscle, where it is metabolized to CO2, H2O and ATP, and to fat cells where it is metabolized primarily to glycerol phosphate for triglyceride synthesis as well as energy production. Also, glucose is an insulin secretagogue (promoting secretion), well as fructose is not and can in fact lower circulating insulin.
Fructose is often recommended for diabetics because it does not trigger the production of insulin by pancreatic β cells, probably because β cells have low levels of GLUT5, although the net effect for both diabetics and non-diabetics is debated. Fructose has a low glycemic index of 19 ± 2, compared with 100 for glucose and 68 ± 5 for sucrose. Fructose is also 73% sweeter than sucrose at room temperature, so diabetics can use less of it. Studies show that fructose consumed before a meal may even lessen the glycemic response of the meal. Fructose-sweetened food and beverage products cause less of a rise in blood glucose levels than do those manufactured with sucrose or glucose.
Honey is a good source of fructose.
Natural sources of fructose include fruits, vegetables (including sugar cane), and honey. Fructose is often further concentrated from these sources. The highest dietary sources of fructose, besides pure crystalline fructose, are foods containing table sugar (sucrose), high-fructose corn syrup, agave nectar, honey, molasses, maple syrup, fruit and fruit juices, as these have the highest percentages of fructose (including fructose in sucrose) per serving compared to other common foods and ingredients. Fructose exists in foods either as a free monosaccharide or bound to glucose as sucrose, a disaccharide. Fructose, glucose, and sucrose may all be present in a food; however, different foods will have varying levels of each of these three sugars.
Fructose is also used commercially in foods and beverages because of its low cost but primarily because of its high relative sweetness. Fructose is the sweetest of all naturally occurring carbohydrates and can also enhance other flavors in the system.
There are no adverse health effects for the lack of fructose in the diet.
Fructose is not an essential nutrient, which means you do not need to get it from food to be healthy, there are no adverse health effects for its lack in the diet.
Consuming large amounts of fructose increases the likelihood of weight gain.
Studies indicate that there may be an increased risk of cardiovascular disease from a high intake of fructose.
The negative side effects of fructose are mostly attributed to over consumption of fructose containing foods or worse still, pure fructose. Below are the negative health effects of fructose:
Fructose absorption occurs in the small intestine via the GLUT-5 (fructose only) transporter, and the GLUT2 transporter, for which it competes with glucose and galactose. Over-consumption of fructose, inhibition of GLUT2 by other phytochemicals, such as flavonoids, or other issues, may result in delivery of unabsorbed fructose into the large intestine, which will cause more water to be drawn into the large intestine through the process of osmosis causing diarrhea.
In addition, the excessive fructose becomes a source of nutrients for the gut flora resulting in a higher production of short chain fatty acids, hydrogen, carbon dioxide and other gases due to fermentation. This increase of gas causes gastrointestinal side effects that mimic irritable bowel syndrome.
Excess fructose consumption has been hypothesized to be a cause of insulin resistance, elevated LDL cholesterol and triglycerides. In preliminary research, fructose consumption was correlated with obesity. Fructose encourages visceral adipose tissue deposition in humans.
Studies indicate that there may be an increased risk of cardiovascular disease from a high intake of fructose. Studies have associated high fructose consumption with increased incidence of hypertension, both acutely and in the long term in subjects without a history of hypertension. The mechanistic link for this is thought to be the increased production of uric acid, a known predictor for hypertension. The elevation in uric acid is due to unregulated phosphorylation leading to depletion of ATP and subsequent ADP degradation to uric acid.
Compared with consumption of high glucose beverages, drinking high-fructose beverages with meals results in lower circulating insulin and leptin levels, and higher ghrelin levels after the meal. Since leptin and insulin decrease appetite and ghrelin increases appetite, researchers are of the view that eating large amounts of fructose increases the likelihood of weight gain.
Excessive fructose consumption may also contribute to the development of non-alcoholic fatty liver disease.
The World Health Organization recommends that both adults and children reduce the intake of free sugars to less than 10% of total energy intake. A reduction to below 5% of total energy intake brings additional health benefits, especially in what regards dental caries (cavities in the teeth).
Sugar found naturally in milk, fruit and vegetables doesn't count as free sugars. We don't need to cut down on these sugars, but remember that they are included in the "total sugar" count.