Evidence-based Dietary Management of Functional Gastrointestinal Symptoms: The FODMAP Approach

Peter R Gibson; Susan J Shepherd

Disclosures

J Gastroenterol Hepatol. 2010;25(2):252-258. 

In This Article

FODMAPs in the Diet

While all FODMAPs are potentially important in the genesis of symptoms (summary of food sources of FODMAP are listed in Table 1), the relative contribution of different subgroups of FODMAPs varies across ethnic and dietary groups due to the dose delivered in the diet. In North American and Western European diets, fructose and fructans are by far the most widespread in the diet and therefore the ones to which nearly all patients with IBS are exposed in their everyday diet. In addition, fructose is important because its absorption in the small intestine varies widely, its significance in dietary intervention will consequently vary widely among different people, and because it is often accompanied in food by sorbitol. An understanding of fructose and fructans are therefore critical to appropriate implementation of the diet.

Fructose is presented to the intestinal lumen as a free hexose in foods or following hydrolysis of sucrose. It is present in fruits, honey, and high fructose corn syrup. It is absorbed across the small intestinal epithelium via two mechanisms (reviewed in detail elsewhere[22]). First, free fructose is taken up by a facultative transporter, GLUT-5, that is present throughout the small intestine. This is of low capacity. Secondly, when present with glucose, fructose is taken up more efficiently, a response that is believed to be related to the insertion of GLUT-2 into the apical membrane of the enterocyte. Thus, fructose malabsorption manifests when free fructose (i.e. in excess of glucose) is in the lumen. This is the reason why fructose supplied in the form of sucrose is only malabsorbed if sucrase activity is diminished.

The ability to absorb free fructose varies widely across individuals. If fructose absorption is efficient in an individual, then dietary restriction of foods rich in free fructose should be unnecessary. It is therefore desirable to identify those who completely absorb a load of fructose. This is effectively done by breath hydrogen testing, preferably with a moderately high dose of fructose (35 g), although the evidence base for the dose that should be tested is minimal.[23,24]

Fructans are linear or branched fructose polymers and are the naturally occurring storage carbohydrates of a variety of vegetables, including onions, garlic and artichokes, fruits such as bananas, and in cereals.[25,26] Wheat is a major source of fructans in the diet, and contains 1–4% fructans on solid matter.[27] Additional sources of fructans are inulin (mostly as a long-chain fructan) and FOS, which are increasingly being added to foods for their putative prebiotic effects. Because the small intestine lacks hydrolases capable of breaking fructose-fructose bonds, and fructans cannot be transported across the epithelium, they are not absorbed at all. Formal examination of this has confirmed that 34–90% of ingested fructans can be recovered from small intestinal output in subjects with an ileostomy.[7,26,28–30] Lower yields, particularly of the short-chain fructans, are likely to be due to microbial degradation by the microflora colonizing the distal small intestine.[7,28]

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