Levulose

Levulose, more commonly known today as fructose, is a naturally occurring monosaccharide. It is a ketohexose with the molecular formula C6H12O6 and is the sweetest of the common natural sugars. In solution fructose rotates plane-polarized light to the left, a property that historically gave rise to the name levulose.

Levulose occurs naturally in many fruits, honey, and some vegetables. Plants produce it via photosynthesis, and it is released when carbohydrate-containing foods are broken down. In industry, glucose isomerase converts glucose into fructose to produce high-fructose corn syrup, a widely used sweetener. Fructose is a reducing sugar and, unlike glucose, exists predominantly in its ketose form in solution.

Biological role and metabolism: In humans, fructose is absorbed in the intestine and transported to the liver, where it is phosphorylated to fructose-1-phosphate by fructokinase and cleaved by aldolase B into dihydroxyacetone phosphate and glyceraldehyde, feeding glycolysis and subsequent metabolic pathways. Its hepatic metabolism differs from that of glucose and can contribute to lipogenesis under certain dietary conditions.

Health considerations: Fructose-containing sweeteners are calorically similar to other sugars, but their metabolism has been discussed for potential distinct effects on metabolic health. High intake of fructose-containing sugars has been associated, in some studies, with obesity, insulin resistance, and non-alcoholic fatty liver disease, though results depend on overall diet and context. Rare inherited disorders include essential fructosuria (fructokinase deficiency) and hereditary fructose intolerance (aldolase B deficiency).