Facilitating effect of amino acids on fructose and sorbitol absorption in children

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Review article: biological mechanisms for symptom causation by individual FODMAP subgroups - the case for a more personalised approach to dietary restriction

BACKGROUND

Due to the paucity of targeted therapy for irritable bowel syndrome (IBS), many patients turn to dietary modifications for symptom management. The combination of five subgroups of poorly absorbed and rapidly fermented carbohydrates-fructans, galacto-oligosaccharides, lactose, excess fructose and polyols-are thought to trigger gastrointestinal symptoms and are referred to collectively as "FODMAPs".

AIMS

To examine the biological plausibility and mechanisms by which foods high in specific FODMAP subgroups cause symptoms, and to use this information to explore the possibility of targeting select dietary components to allow for a more personalised approach to dietary adjustment METHODS: Recent literature was analysed via search databases including Medline, PubMed and Scopus.

RESULTS

Lactose, fructans and galacto-oligosaccharides have strong biologic plausibility for symptom generation due to lack of hydrolases resulting in distention from osmosis and rapid fermentation. However, excess fructose and polyols may only cause symptoms in specific individuals when consumed in high doses, but this remains to be established. There is evidence to suggest that certain patient characteristics such as ethnicity may predict response to lactose, but differentiation of other subgroups is difficult prior to dietary manipulation.

CONCLUSIONS

While some clear mechanisms of action for symptom generation have been established, further research is needed to understand which patients will respond to specific FODMAP subgroup restriction. We suggest that clinicians consider in some patients a tailored, personalised "bottom-up" approach to the low-FODMAP diet, such as dietary restriction relevant to the patients' ethnicity, symptom profile and usual dietary intake.

13C Breath Tests in Infections and Beyond

Stable isotope labeled compounds are widely used as diagnostic probes in medicine. These diagnostic stable isotope probes are now being expanded in their scope, to provide precise indications of the presence or absence of etiologically significant change in metabolism due to a specific disease. This concept exploits a labeled tracer probe that is a specifically designed substrate of a “gateway” enzyme in a discrete metabolic pathway, whose turnover can be measured by monitoring unidirectional precursor product mass flow. An example of such a probe is the 13C-urea breath test, where labeled urea is given to patients with H. pylori infection. Another example of this kind of probe is used to study the tripeptide glutathione (glu-cys-gly, GSH), which is the most abundant cellular thiol, and protects cells from the toxic effects of reactive oxygen species. Within the gamma glutamyl cycle, 5-oxoproline (L-pyroglutamic acid) is a metabolite generated during GSH catabolism, and is metabolized to glutamic acid by 5-oxoprolinase. This enzyme can also utilize the substrate L-2-oxothiazolidone-4-carboxylate (OTC), to generate intracellular cysteine, which is beneficial to the cell. Thus, labeled (13C) OTC would, under enzymatic attack yield cysteine and 13CO2, and can thus track the state and capacity of glutathione metabolism. Similarly, stable isotope labeled probes can be used to track the activity of the rate of homocysteine clearance, lymphocyte CD26, and liver CYP (cytochrome P450) enzyme activity. In the future, these applications should be able to titrate, in vivo, the characteristics of various specific enzyme systems in the body and their response to stress or infection as well as to treatment regimes.

Fructose malabsorption

Incomplete intestinal absorption of fructose might lead to abdominal complaints such as pain, flatulence and diarrhoea. Whether defect fructose transporters such as GLUT5 or GLUT2 are involved in the pathogenesis of fructose malabsorption is a matter of debate. The hydrogen production by colonic bacteria is used for diagnosis with the hydrogen breath test. However, the appropriate fructose test dose for correct diagnosis is unclear. Subjects with fructose malabsorption show increased breath hydrogen levels and abdominal symptoms after fructose administration but do not report any symptoms when fructose is given together with glucose. This beneficial effect of glucose, however, cannot be explained yet but might be used for clinical care of these subjects.

The malabsorption of commonly occurring mono and disaccharides: levels of investigation and differential diagnoses

BACKGROUND

Adverse food reactions (AFR) have has recently attracted increased attention from the media and are now more commonly reported by patients. Its classification, diagnostic evaluation, and treatment are complex and present a considerable challenge in clinical practice. Non-immune-mediated types of food intolerance have a cumulative prevalence of 30% to 40%, while true (immune-mediated) food allergies affect only 2% to 5% of the German population.

METHOD

We selectively searched the literature for pertinent publications on carbohydrate malabsorption, with special attention to published guidelines and position papers.

RESULTS

Carbohydrate intolerance can be the result of a rare, systemic metabolic defect (e.g., fructose intolerance, with a prevalence of 1 in 25,000 persons) or of gastrointestinal carbohydrate malabsorption. The malabsorption of simple carbohydrates is the most common type of non-immune-mediated food intolerance, affecting 20% to 30% of the European population. This condition is caused either by deficient digestion of lactose or by malabsorption of fructose and/or sorbitol. Half of all cases of gastrointestinal carbohydrate intolerance have nonspecific manifestations, with a differential diagnosis including irritable bowel syndrome, intolerance reactions, chronic infections, bacterial overgrowth, drug side effects, and other diseases. The diagnostic evaluation includes a nutritional history, an H2 breath test, ultrasonography, endoscopy, and stool culture.

CONCLUSION

The goals of treatment for carbohydrate malabsorption are to eliminate the intake of the responsible carbohydrate substance or reduce it to a tolerable amount and to assure the physiological nutritional composition of the patient's diet. In parallel with these goals, the patient should receive extensive information about the condition, and any underlying disease should be adequately treated.

Lactose and fructose malabsorption in children with recurrent abdominal pain: results of double-blinded testing

AIM

To investigate malabsorption of lactose and fructose as causes of recurrent abdominal pain (RAP).

METHODS

In 220 children (128 girls, mean age 8,8 [4.1-16.0] years) with RAP, hydrogen breath tests (H(2) BT; abnormal if ΔH(2) > 30 ppm) were performed with lactose and fructose. Disappearance of RAP with elimination, recurrence with provocation and disappearance with re-elimination, followed by a 6-month pain-free follow-up, were considered indicative of a causal relation with RAP. For definite proof, a double-blinded placebo-controlled (DBPC) provocation was performed.

RESULTS

Malabsorption of lactose was found in 57 of 210, of fructose in 79 of 121 patients. Pain disappeared upon elimination in 24/38 patients with lactose malabsorption, and in 32/49 with fructose malabsorption. Open provocation with lactose and fructose was positive in 7/23 and 13/31 patients. DBPC provocation in 6/7 and 8/13 patients was negative in all. However, several children continued to report abdominal symptoms upon intake of milk or fructose.

CONCLUSION

Lactose intolerance nor fructose intolerance could be established as causes of RAP, according to preset criteria including elimination, open provocation and DBPC provocation. However, in clinical practice, persistent feeling of intolerance in some patients should be taken seriously and could warrant extended elimination with repeated challenges.

Glutamine supplementation in sick children: is it beneficial?

The purpose of this review is to provide a critical appraisal of the literature on Glutamine (Gln) supplementation in various conditions or illnesses that affect children, from neonates to adolescents. First, a general overview of the proposed mechanisms for the beneficial effects of Gln is provided, and subsequently clinical studies are discussed. Despite safety, studies are conflicting, partly due to different effects of enteral and parenteral Gln supplementation. Further insufficient evidence is available on the benefits of Gln supplementation in pediatric patients. This includes premature infants, infants with gastrointestinal disease, children with Crohn's disease, short bowel syndrome, malnutrition/diarrhea, cancer, severe burns/trauma, Duchenne muscular dystrophy, sickle cell anemia, cystic fibrosis, and type 1 diabetes. Moreover, methodological issues have been noted in some studies. Further mechanistic data is needed along with large randomized controlled trials in select populations of sick children, who may eventually benefit from supplemental Gln.

Intestinal fructose transport and malabsorption in humans

Fructose is a hexose sugar that is being increasingly consumed in its monosaccharide form. Patients who exhibit fructose malabsorption can present with gastrointestinal symptoms that include chronic diarrhea and abdominal pain. However, with no clearly established gastrointestinal mechanism for fructose malabsorption, patient analysis by the proxy of a breath hydrogen test (BHT) is controversial. The major transporter for fructose in intestinal epithelial cells is thought to be the facilitative transporter GLUT5. Consistent with a facilitative transport system, we show here by analysis of past studies on healthy adults that there is a significant relationship between fructose malabsorption and fructose dose (r = 0.86, P < 0.001). Thus there is a dose-dependent and limited absorption capacity even in healthy individuals. Changes in fructose malabsorption with age have been observed in human infants, and this may parallel the developmental regulation of GLUT5 expression. Moreover, a GLUT5 knockout mouse has displayed the hallmarks associated with profound fructose malabsorption. Fructose malabsorption appears to be partially modulated by the amount of glucose ingested. Although solvent drag and passive diffusion have been proposed to explain the effect of glucose on fructose malabsorption, this could possibly be a result of the facilitative transporter GLUT2. GLUT5 and GLUT2 mRNA have been shown to be rapidly upregulated by the presence of fructose and GLUT2 mRNA is also upregulated by glucose, but in humans the distribution and role of GLUT2 in the brush border membrane are yet to be definitively decided. Understanding the relative roles of these transporters in humans will be crucial for establishing a mechanistic basis for fructose malabsorption in gastrointestinal patients.

Health implications of fructose consumption: A review of recent data

This paper reviews evidence in the context of current research linking dietary fructose to health risk markers.Fructose intake has recently received considerable media attention, most of which has been negative. The assertion has been that dietary fructose is less satiating and more lipogenic than other sugars. However, no fully relevant data have been presented to account for a direct link between dietary fructose intake and health risk markers such as obesity, triglyceride accumulation and insulin resistance in humans. First: a re-evaluation of published epidemiological studies concerning the consumption of dietary fructose or mainly high fructose corn syrup shows that most of such studies have been cross-sectional or based on passive inaccurate surveillance, especially in children and adolescents, and thus have not established direct causal links. Second: research evidence of the short or acute term satiating power or increasing food intake after fructose consumption as compared to that resulting from normal patterns of sugar consumption, such as sucrose, remains inconclusive. Third: the results of longer-term intervention studies depend mainly on the type of sugar used for comparison. Typically aspartame, glucose, or sucrose is used and no negative effects are found when sucrose is used as a control group.Negative conclusions have been drawn from studies in rodents or in humans attempting to elucidate the mechanisms and biological pathways underlying fructose consumption by using unrealistically high fructose amounts.The issue of dietary fructose and health is linked to the quantity consumed, which is the same issue for any macro- or micro nutrients. It has been considered that moderate fructose consumption of ≤50g/day or ~10% of energy has no deleterious effect on lipid and glucose control and of ≤100g/day does not influence body weight. No fully relevant data account for a direct link between moderate dietary fructose intake and health risk markers.

Comparison of breath testing with fructose and high fructose corn syrups in health and IBS

Although incomplete fructose absorption has been implicated to cause gastrointestinal symptoms, foods containing high fructose corn syrup (HFCS) contain glucose. Glucose increases fructose absorption in healthy subjects. Our hypothesis was that fructose intolerance is less prevalent after HFCS consumption compared to fructose alone in healthy subjects and irritable bowel syndrome (IBS). Breath hydrogen levels and gastrointestinal symptoms were assessed after 40 g of fructose (12% solution) prepared either in water or as HFCS, administered in double-blind randomized order on 2 days in 20 healthy subjects and 30 patients with IBS. Gastrointestinal symptoms were recorded on 100-mm Visual Analogue Scales. Breath hydrogen excretion was more frequently abnormal (P < 0.01) after fructose (68%) than HFCS (26%) in controls and patients. Fructose intolerance (i.e. abnormal breath test and symptoms) was more prevalent after fructose than HFCS in healthy subjects (25% vs. 0%, P = 0.002) and patients (40% vs. 7%, P = 0.062). Scores for several symptoms (e.g. bloating r = 0.35) were correlated (P < or = 0.01) to peak breath hydrogen excretion after fructose but not HFCS; in the fructose group, this association did not differ between healthy subjects and patients. Symptoms were not significantly different after fructose compared to HFCS. Fructose intolerance is more prevalent with fructose alone than with HFCS in health and in IBS. The prevalence of fructose intolerance is not significantly different between health and IBS. Current methods for identifying fructose intolerance should be modified to more closely reproduce fructose ingestion in daily life.

High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise

A recent study from our laboratory has shown that a mixture of glucose and fructose ingested at a rate of 1·8 g/min leads to peak oxidation rates of approximately 1·3 g/min and results in approximately 55 % higher exogenous carbohydrate (CHO) oxidation rates compared with the ingestion of an isocaloric amount of glucose. The aim of the present study was to investigate whether a mixture of glucose and fructose when ingested at a high rate (2·4 g/min) would lead to even higher exogenous CHO oxidation rates (>1·3 g/min).Eight trained male cyclists (VO2max: 68±1 ml/kg per min) cycled on three different occasions for 150 min at 50 % of maximal power output (60±1 % VO2max) and consumed either water (WAT) or a CHO solution providing 1·2 g/min glucose (GLU) or 1.2 g/min glucose+1·2 g/min fructose (GLU+FRUC). Peak exogenous CHO oxidation rates were higher (P<0·01) in the GLU+FRUC trial compared with the GLU trial (1·75 (se 0·11) and 1·06 (se 0·05) g/min, respectively). Furthermore, exogenous CHO oxidation rates during the last 90 min of exercise were approximately 50 % higher (P<0·05) in GLU+FRUC compared with GLU (1·49 (se 0·08) and 0·99 (se 0·06) g/min, respectively). The results demonstrate that when a mixture of glucose and fructose is ingested at high rates (2·4 g/min) during 150 min of cycling exercise, exogenous CHO oxidation rates reach peak values of approximately 1·75 g/min.

Review article: fructose malabsorption and the bigger picture

Fructose is found widely in the diet as a free hexose, as the disaccharide, sucrose and in a polymerized form (fructans). Free fructose has limited absorption in the small intestine, with up to one half of the population unable to completely absorb a load of 25 g. Average daily intake of fructose varies from 11 to 54 g around the world. Fructans are not hydrolysed or absorbed in the small intestine. The physiological consequences of their malabsorption include increasing osmotic load, providing substrate for rapid bacterial fermentation, changing gastrointestinal motility, promoting mucosal biofilm and altering the profile of bacteria. These effects are additive with other short-chain poorly absorbed carbohydrates such as sorbitol. The clinical significance of these events depends upon the response of the bowel to such changes; they have a higher chance of inducing symptoms in patients with functional gut disorders than asymptomatic subjects. Restricting dietary intake of free fructose and/or fructans may have durable symptomatic benefits in a high proportion of patients with functional gut disorders, but high quality evidence is lacking. It is proposed that confusion over the clinical relevance of fructose malabsorption may be reduced by regarding it not as an abnormality but as a physiological process offering an opportunity to improve functional gastrointestinal symptoms by dietary change.

Fructose malabsorption and symptoms of irritable bowel syndrome: guidelines for effective dietary management

Dietary fructose induces abdominal symptoms in patients with fructose malabsorption, but there are no published guidelines on its dietary management. The objective was to retrospectively evaluate a potentially successful diet therapy in patients with irritable bowel syndrome and fructose malabsorption. Tables detailing the content of fructose and fructans in foods were constructed. A dietary strategy comprising avoidance of foods containing substantial free fructose and short-chain fructans, limitation of the total dietary fructose load, encouragement of foods in which glucose was balanced with fructose, and co-ingestion of free glucose to balance excess free fructose was devised. Sixty-two consecutively referred patients with irritable bowel syndrome and fructose malabsorption on breath hydrogen testing underwent dietary instruction. Dietary adherence and effect on abdominal symptoms were evaluated via telephone interview 2 to 40 months (median 14 months) later. Response to the diet was defined as improvement of all symptoms by at least 5 points on a -10- to 10-point scale. Forty-eight patients (77%) adhered to the diet always or frequently. Forty-six (74%) of all patients responded positively in all abdominal symptoms. Positive response overall was significantly better in those adherent than nonadherent (85% vs 36%; P<0.01), as was improvement in individual symptoms (P<0.01 for all symptoms). This comprehensive fructose malabsorption dietary therapy achieves a high level of sustained adherence and good symptomatic response.

Frutose em humanos: efeitos metabólicos, utilização clínica e erros inatos associados

Revisa-se o metabolismo da frutose e do sorbitol, suas principais indicações e conseqüências decorrentes do uso inadequado. A frutose é um importante carboidrato da dieta, sendo encontrada principalmente nas frutas e vegetais, e é produzida no organismo a partir da glicose pela via do sorbitol. A frutose é conhecida pelos erros inatos do seu metabolismo, cujas manifestações clínicas são potencialmente graves, e por seu uso como substituta da glicose na dieta de diabéticos, visto não depender da insulina para o seu metabolismo. Nos últimos anos, especialmente em países desenvolvidos, seu consumo tem aumentado acentuadamente em virtude do emprego como adoçante em produtos industrializados. Porém, o uso excessivo de frutose não é isento de efeitos adversos, representados pelo aumento de triglicerídios e de colesterol no sangue. O conhecimento dos níveis sangüíneos normais é importante tanto para estabelecer a quantidade segura a ser administrada, como para permitir avaliar doenças metabólicas associadas à frutose.

Dietary fructose and gastrointestinal symptoms: a review

It has been proposed that fructose may cause or aggravate symptoms in patients with functional gastrointestinal disorders. Fructose is commonly used to sweeten processed foods, and the prevalence of incomplete fructose absorption (25 g, 10%) in healthy subjects is as high as 50%. The only controlled study that has been performed did not demonstrate a higher prevalence of fructose-induced gastrointestinal symptoms or incomplete fructose absorption in patients with functional gastrointestinal disorders. The amount and concentration of fructose used to evaluate absorption by breath testing has varied among studies. Moreover, dietary sources of fructose usually contain glucose, which increases fructose absorption in healthy subjects. Thus, breath testing with fructose alone may not reflect fructose ingestion under normal circumstances. Given these limitations, we suggest that a practical, empirical approach to testing in patients with suspected incomplete fructose absorption is to restrict fructose ingestion. Additional controlled studies are needed to clarify the relation between incomplete fructose absorption and symptoms, assess the effects of co-ingestion of other sugars on fructose absorption, and evaluate the effects of eliminating sugars from the diet on gastrointestinal symptoms.

Effects of a beverage containing an enzymatically induced-viscosity dietary fiber, with or without fructose, on the postprandial glycemic response to a high glycemic index food in humans

OBJECTIVE

Dietary supplementation with guar gum or fructose has been reported to reduce the postprandial glycemic response to an oral glucose challenge. As a result of the poor palatability of most foods containing guar gum, a novel low-viscosity beverage with guar gum was developed that becomes viscous in vivo through an enzymatic induction. The primary study objective was to determine the effect of an amylase-induced viscosity (I-V) product, with or without supplemental fructose, on the postprandial glycemic response to a high glycemic index test meal in healthy nondiabetic subjects.

DESIGN

The study was a four-treatment, placebo-controlled, double-blind, randomized block protocol.

SETTING

The study was performed at Glycaemic Index Testing, Inc., Toronto, Ontario, Canada.

SUBJECTS

A total of 30 healthy nondiabetic volunteers (13 male, 17 female, mean+/-s.e.m. age of 51+/-3 y and body mass index of 24.2+/-0.4 kg/m(2)) participated in the study.

INTERVENTION

In the morning after an overnight fast, subjects participated in four 3-h meal glucose tolerance tests on separate occasions. The test meals contained 50 g of available carbohydrate from maltodextrin and white bread (control) or the same meal with either 5 g of guar gum (3.6 g galactomannan), 5 g of fructose, or 5 g of guar gum +5 g of fructose.

RESULTS

Treatments containing guar gum had a reduced (P<0.01) baseline-adjusted peak glucose response and incremental area under the glucose curve. In contrast to previous studies, fructose increased (P<0.05) the baseline-adjusted peak glucose concentration.

CONCLUSIONS

Guar gum incorporated into an amylase I-V product provided a means to stabilize blood glucose levels by reducing the early phase excursion and then by appropriately maintaining the later phase excursion in healthy nondiabetic humans.

Nutritional management in diarrhoeal disease

Adequate nutritional intervention in diarrhoeal disease in children is crucial in obtaining optimal control of a disorder that may become life-threatening. During recent years, important advances have been made in our understanding of the pathophysiology of diarrhoeal states, in the formulation of oral rehydration solutions and in the role of micro- and macronutrients in diarrhoeal disorders. This chapter outlines some of the relevant concepts in the pathophysiology of diarrhoeal disease and provides a rationale for nutritional intervention. Guidelines for nutritional management in the settings of acute-onset diarrhoea, post-enteritis protracted diarrhoea and chronic non-specific diarrhoea are provided, mostly based on controlled clinical trials and meta-analyses of evidence-based medicine.

Chronic nonspecific diarrhea of childhood: pathophysiology and management

Chronic nonspecific diarrhea, or toddler's diarrhea, is a frequently encountered disorder of defecation in otherwise healthy children. Although the precise pathophysiology remains to be elucidated, evidence suggests that toddler's diarrhea primarily is a gut motility disorder, modulated by dietary factors. Although the role of low-fat diets has since long been established, the liberal consumption of fruit juices and soft drinks is considered an equally important factor. Normalization of the child's diet, especially with regard to fat, fiber, fluids, and fruit juices, usually suffices to attain resolution of the diarrhea.

Evaluation of 13CO2 breath tests for the detection of fructose malabsorption

Breath hydrogen (H2) studies have made clear that small intestinal absorption of fructose is limited, especially in toddlers. Malabsorption of fructose may be a cause of recurrent abdominal pain and chronic nonspecific diarrhea (toddler's diarrhea). Fructose absorption is facilitated by equimolar doses of glucose and, as we have found, amino acids (especially L-alanine); the mechanism underlying this effect remains unclear. To study fructose absorption in a more direct way, we combined breath H2 studies with breath 13CO2 studies. Gastric emptying was studied by using L-glycine-1-13C in 4 children from 12.1 to 16.0 years of age. After 25 gm of fructose and 27.5 gm of glucose, when given together, gastric emptying was significantly (p<0.05) slower than with either sugar alone. In a second series of experiments, 5 children from 12.0 to 15.9 years of age were tested with 25 gm of fructose, alone and with equimolar doses of glucose and L-alanine, and 4 younger children from 3.1 to 6.1 years of age were tested with 2 gm/kg (max 37.5 gm) fructose, alone or with an equimolar dose of L-alanine. All fructose solutions were enriched with 15 mg of D-fructose-13C-6. In all 9 children, fructose was malabsorbed as judged by breath H2 increases > or = 20 ppm, and the addition of glucose or L-alanine resulted in significantly lower breath H2 increases (p < or = 0.005 for glucose, p < or = 0.001 for alanine). In contrast, the addition of alanine or glucose did not change the pattern of breath 13CO2 excretion in the 5 older children, whereas in the 4 younger children (with relatively higher doses), L-alanine addition resulted in significantly lower increases in breath 13CO2. In the latter group, for each time point, breath H2 and 13CO2 concentrations after fructose were compared with those after fructose plus L-alanine; in 20 out of 24 points, both H2 and 13CO2 were higher after fructose. These results suggest that 13CO2 not only originated from the oxidation of absorbed substrate but also, at least in part, from colonic bacterial metabolism. For the detection of [correction of or] fructose malabsorption--as opposed to, for instance, lactose--the 13CO2 breath test seems to be of limited value.