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KEY WORDS: inulin, oligofructose, colon, physiology, fermentation
ABSTRACT
Fructan is a general term used for any carbohydrate in which one or more fructosylfructose link constitutes the majority of osidic bonds. This review focuses on the fate of inulin-type fructans (namely native chicory inulin, oligofructose produced by the partial enzymatic hydrolysis of chicory inulin, and synthetic fructans produced by enzymatic synthesis from sucrose) in the gastrointestinal tract, as well as on their systemic physiological effects on mineral absorption, carbohydrate and lipid metabolism, hormone balance, and nitrogen homeostasis. The scientific evidence for the functional claims of inulin-type fructans is discussed, as well as their potential application in risk reduction of disease, namely constipation, infectious diarrhea, cancer, osteoporosis, atherosclerotic cardiovascular disease, obesity, and non-insulin dependent diabetes.
INTRODUCTION
Definition, Chemistry, and Biosynthetic Pathways Fructan is a general name used for any carbohydrate in which one or more fructosyl-fructose link constitutes the majority of osidic bonds. Fructans are linear or branched fructose (oligo)polymers, which are either (Beta)-2, 1 -linked inulins or (Beta)-2,6-linked levans. Inulin has been defined as a polydisperse carbohydrate material consisting mainly, if not exclusively, of (Beta)-(2- 1 ) fructosyl-fructose links. It is mainly of plant origin, whereas some fungi and many bacteria are the major producers of levans (37, 116). Because they are synthesized from sucrose, by repeated fructosyl transfer from a fructosyl donor, both inulins and levans usually, but not always, have a terminal glucose unit. Indeed, the enzymes generally considered to be involved in plant fructans synthesis are sucrose-sucrose fructosyltransferases (EC 2.4.1.99), which catalyze fructosyl transfer from one sucrose molecule to another, leading to 1-kestose (glucosyl-1,2-fructosyl-1,2fructose). Chain elongation is mediated by either IF- or 6F-fructan-fructanfructosyltransferase (EC 2.4.1.100), leading to inulins and levans, respectively. Fungal and bacterial inulins and levans are generally assumed to be synthesized without a trisaccharide as intermediate, but by sequential transfer of fructosyl residues from sucrose as fructosyl donor to the growing inulin and levan chains by inulosucrase (sucrose 1 F-fructosyltransferase) and levansucrase (sucrose 6Ffructosyltransferase), respectively (99). They have a much higher degree of polymerization (DP) (up to 100,000) than plant inulin (DP up to 150).
Plant Sources
Fructan-containing plant species are found in a number of mono- and dicotyledonous families, such as Liliaceae, Amaryllidaceae, Gramineae, and Compositae....