Fructose, also known as levulose, is a naturally occurring monosaccharide that can be found in fruits and honey. About twice as sweet as table sugar, and with a lower glycemic index, it can be used as a natural substitute for table sugar by people wanting to cut calories or maintain healthy blood sugar levels. For these reasons, it is sometimes used commercially in prepared cakes, cookies, and other sweets. Caution must be used when using fruit sugar in home cooking, however, as it has different physical and chemical properties from table sugar, and cannot always be substituted, in equal amounts, in standard recipes.
Monosaccharides are the simplest forms of sugar, each consisting of a single sugar molecule. There are a number of monosaccharides, both synthetic and naturally-occurring, but the only monosaccharides found in food are fructose, glucose, and galactose. The simple sugars are often found as bonded pairs, in which case they become disaccharides — such as sucrose, maltose, and lactose. Sugar molecules can also bond into long chains, known as polysaccharides or complex carbohydrates. From a nutritional standpoint, complex carbohydrates can be considered the most important form of sugar in the diet, as they take longer to break down in the digestive system, and create more stable blood sugar levels than quickly processed simple sugars.
The chemical formula for monosaccharides generally involves some multiple of CH2O. In a typical monosaccharide, the carbon atoms form a chain, in which, each carbon except one is bonded to a hydroxyl group. The single carbon that does not bond with a hydroxyl group is instead double bonded to an oxygen molecule, forming a carbonyl group. The location of the carbonyl group subdivides the monosaccharides into ketone sugars and aldehyde sugars. A laboratory test known as Seliwanoff's test can be used to chemically determine whether a particular sugar is a ketone sugar, such as fructose, or an aldehyde sugar, such as glucose or galactose.
While fruit sugar and honey are generally regarded as safe, excessive consumption can lead to hyperuricemia, a condition characterized by elevated levels of uric acid in the blood. There are also certain digestive disorders that are associated with difficulty processing, or absorbing, dietary fruit sugar. Fructose malabsorption is a disorder in which the small intestine lacks the ability to absorb that particular sugar, leading to a concentration of the sugar in the digestive system. Symptoms and testing for this condition are similar to those of lactose intolerance, and treatment often involves removing trigger foods from the diet.
A more serious condition is hereditary fructose intolerance (HFI), a genetic disorder involving a deficiency in the liver enzymes required for the digestion of fructose. Symptoms generally include severe gastrointestinal discomfort, dehydration, convulsions, and sweating. If left untreated, HFI can lead to permanent liver and kidney damage, and even death. While HFI is much more serious than fructose malabsorption, the treatment is similar, and usually centers around a careful avoidance of any foods containing fruit sugar or its derivatives.