FRUCTOSE

Fructose

Fructose

Fructose is a hexose that is present widely in the diet as free fructose, in the disaccharide, sucrose, and in fructans, which are chains of fructose with a terminal glucose. Fructans are also known as inulins (when the number of fructose molecules, or degree of polymerization or DP, is =10), or fructo-oligosaccharide (FOS) or oligofructose (when the DP is <10). Fructose has created much interest because of its possible role in contributing to the obesity epidemic in the USA and for its postulated role in inducing functional gut symptoms. Symptoms such as bloating, abdominal distension, discomfort, pain, and altered bowel habits are often described as “functional” gut symptoms since their cause is usually related to alterations in the function of the gut and enteric nervous system rather than being manifestations of structural abnormalities.

Discussion

Fructose malabsorption (FM) is defined as the incomplete absorption in the small intestine of dietary free fructose, with subsequent delivery of fructose to the distal small bowel and colon. FM is generally detected by breath hydrogen testing after an oral load of fructose, since bacteria present in those regions will rapidly ferment the malabsorbed fructose with subsequent hydrogen generation and appearance in the breath. In reality, FM represents the presentation of fructose to luminal bacteria before it is absorbed. This can be the result of three interdependent means— a reduced absorptive capability of the small intestinal epithelium, fast transit through the little bowel, and bacterial overgrowth in the distal little bowel (SBBO). Differentiation of these means is not very simple in the individual persevering, even with the concomitant use of lactulose or glucose wind testing. The ability of the human small intestine to absorb fructose is limited, especially when compared with the rapidity and completeness of glucose absorption. Thus, when 50 g fructose is ingested without other food, up to 80% of people will incompletely absorb the fructose, as demonstrated by breath hydrogen testing. Absorption is enhanced by co-ingestion with glucose, since glucose uptake stimulates additional transport pathways for fructose absorption in the small intestinal epithelial cell. Because of this, the fructose released from the hydrolysis of sucrose is generally completely absorbed.

There appears to be a spectrum of absorptive capacity of free fructose (that is, without glucose present) across populations from very poor to efficient, although reasons for the differences are complex and not completely understood. An important concept is that FM is probably a physiologically normal phenomenon, not ...