Intestinal fructose transport and malabsorption in humans

Oral xylose isomerase decreases breath hydrogen excretion and improves gastrointestinal symptoms in fructose malabsorption - a double-blind, placebo-controlled study

BACKGROUND

Incomplete resorption of fructose results in increased colonic hydrogen production and is a frequent cause of abdominal symptoms. The only treatment available is diet.

AIM

To study whether orally administered xylose isomerase (XI), an enzyme that catalyses the reversible isomerisation of glucose and fructose, can decrease breath hydrogen excretion in patients with fructose malabsorption.

METHODS

Patients received 25 g fructose in 100 mL water together with either placebo or XI capsules. Primary endpoint was the reduction in breath hydrogen excretion, as assessed by the area under the breath hydrogen curve over 4 h (AUC). A secondary endpoint was the reduction in abdominal pain, bloating and nausea assessed on a visual analogue scale (VAS, range: 0-10). A P value <0.05 was considered statistically significant.

RESULTS

Sixty-five patients in whom fructose malabsorption had been diagnosed by positive breath hydrogen test within the previous year, were included in the study [15 males, 50 females; mean age 43.3 (s.d. = 14.4), range: 21-73 years]. The median AUC was 885 ppm/240 min in the XI group compared to 2071 ppm/240 min in the placebo group (P = 0.00). Median scores for abdominal pain (0.7 vs. 1.3) and nausea (0.2 vs. 0.6), but not for bloating (P = 0.053), were significantly improved after XI (P = 0.009 and P = 0.005) as compared with placebo.

CONCLUSIONS

Oral administration of xylose isomerase significantly decreased breath hydrogen excretion after ingestion of a watery fructose solution. Nausea and abdominal pain were significantly improved by xylose isomerase.

Fructose consumption and cancer: is there a connection?

PURPOSE OF REVIEW

Cancer cell metabolism is characterized by high rates of glucose uptake and anaerobic glycolysis. Sugar consumption has increased dramatically in the industrialized world, with refined fructose intake skyrocketing upwards in the USA over the past 30 years. Fructose provides an alternative carbon source for glycolysis, entering downstream of glucose and bypassing two key rate-limiting steps. Considering that glycolysis is the major pathway which fuels cancer growth, this review will focus on regulation and flux of glucose versus fructose through this pathway, and consider whether epidemiologic and experimental data support a mechanism whereby fructose might potentiate cancer growth in transformed cells.(Figure is included in full-text article.)

RECENT FINDINGS

Fructose intake is associated with increased risk of pancreatic and small intestinal cancers, and possibly others. Fructose promotes flux through the pentose phosphate, which enhances protein synthesis and may indirectly increase tumor growth. Fructose treatment is associated with more aggressive cancer behavior and may promote metastasis.

SUMMARY

Whereas glucose favors overall growth kinetics, fructose enhances protein synthesis and appears to promote a more aggressive cancer phenotype. Fructose has become ubiquitous in our food supply, with the highest consumers being teens and young adults. Therefore, understanding the potential health consequences of fructose and its role in chronic disease development is of critical importance.

Reliability of symptom analysis during carbohydrate hydrogen-breath tests

PURPOSE OF REVIEW

Relevance of symptom analysis during hydrogen breath test (HBT) for establishing a clinical diagnosis of sugar intolerance is reviewed.

RECENT FINDINGS

Evaluation of symptoms developed in response to the ingestion of 50 g lactose could represent a simple screening test to select patients for lactose intolerance testing. Patients who do not develop symptoms do not need to be referred for HBT. In addition, symptoms reported by patients during a negative HBT cannot be at all times attributed to a false-negative test; instead, a 'nocebo' effect is likely to be implicated. On the other hand, in a double-blind randomized study, a dose of 25 g fructose was suggested as the most appropriate for testing individuals with suspected fructose malabsorption, whereas symptom reliability to diagnose fructose intolerance was inaccurate.

SUMMARY

Whereas the development of symptoms after a positive HBT may indicate sugar intolerance, it is still not clear whether the absence of symptoms after sugar malabsorption gives any indication as to the role of that sugar in the genesis of patient's complaints. Further studies should evaluate whether the disappearance of symptoms with a sugar-restricted diet after a positive HBT is a better diagnostic criterion of sugar intolerance than the development of symptoms.

Diarrhea caused by carbohydrate malabsorption

This article will focus on the role of the colon in the pathogenesis of diarrhea in carbohydrate malabsorption or physiologically incomplete absorption of carbohydrates, and on the most common manifestation of carbohydrate malabsorption, lactose malabsorption. In addition, incomplete fructose absorption, the role of carbohydrate malabsorption in other malabsorptive diseases, and congenital defects that lead to malabsorption will be covered. The article concludes with a section on diagnostic tools to evaluate carbohydrate malabsorption.

Honey--a novel antidiabetic agent

Diabetes mellitus remains a burden worldwide in spite of the availability of numerous antidiabetic drugs. Honey is a natural substance produced by bees from nectar. Several evidence-based health benefits have been ascribed to honey in the recent years. In this review article, we highlight findings which demonstrate the beneficial or potential effects of honey in the gastrointestinal tract (GIT), on the gut microbiota, in the liver, in the pancreas and how these effects could improve glycemic control and metabolic derangements. In healthy subjects or patients with impaired glucose tolerance or diabetes mellitus, various studies revealed that honey reduced blood glucose or was more tolerable than most common sugars or sweeteners. Pre-clinical studies provided more convincing evidence in support of honey as a potential antidiabetic agent than clinical studies did. The not-too-impressive clinical data could mainly be attributed to poor study designs or due to the fact that the clinical studies were preliminary. Based on the key constituents of honey, the possible mechanisms of action of antidiabetic effect of honey are proposed. The paper also highlights the potential impacts and future perspectives on the use of honey as an antidiabetic agent. It makes recommendations for further clinical studies on the potential antidiabetic effect of honey. This review provides insight on the potential use of honey, especially as a complementary agent, in the management of diabetes mellitus. Hence, it is very important to have well-designed, randomized controlled clinical trials that investigate the reproducibility (or otherwise) of these experimental data in diabetic human subjects.

Natural versus commercial carbohydrate supplementation and endurance running performance

Background

We examined the metabolic, performance and gastrointestinal (GI) effects of supplementation with a natural food product (raisins) compared to a commercial product (sport chews).

Methods

Eleven male (29.3 ± 7.9 yrs; mean and SD) runners completed three randomized trials (raisins, chews and water only) separated by seven days. Each trial consisted of 80-min (75%VO₂max) treadmill running followed by a 5-km time trial (TT). Heart rate (HR), respiratory exchange ratio (RER), blood lactate, serum free fatty acids (FFA), glycerol and insulin, plasma glucose and creatine kinase, GI symptoms and rating of perceived exertion (RPE) were recorded every 20-min. We employed a within-subject two-way analysis of variance (ANOVA) for repeated measures with a Fisher’s post hoc analysis to determine significant differences.

Results

VO₂, HR, lactate, glycerol and RPE did not differ due to treatment. Average plasma glucose was maintained at resting levels (5.3 ± 0.4 mmol·L^-1) during the sub-maximal exercise bout (5.9 ± 0.6, 5.7 ± 0.6 and 5.5 ± 0.5 mmol·L^-1 for chews, raisins and water respectively), and was significantly higher with chews than water only. RER and % of non-protein macronutrient oxidation derived from carbohydrate was highest with chews, followed by raisins and water was the lowest (74.4 ± 6.4, 70.0 ± 7.0 and 65.1 ± 8.7% for chews, raisins and water respectively) during the sub-maximal exercise period. Serum FFA was higher in the water treatment versus both raisins and chews at 80 min of sub-maximal exercise. Serum insulin was higher with the chews than both raisins and water (5.1 ± 2.0, 3.1 ± 0.8, 1.9 ± 0.6 uU·ml^-1 for chews, raisins and water respectively). Plasma creatine kinase, corrected for baseline values, for the last 40 min of the sub-maximal exercise bout, was higher with raisins compared to other treatments. The TT was faster for both carbohydrate supplements (20.6 ± 2.6, 20.7 ± 2.5, 21.6 ± 2.7 min for raisin, chews and water respectively). GI disturbance was mild for all treatments.

Conclusion

Raisins and chews promoted higher carbohydrate oxidation and improved running performance compared to water only. Running performance was similar between the raisins and chews, with no significant GI differences.

Absorption and bioavailability of glucosamine in the rat

The objective of this study was to determine reasons behind the low oral (p.o.) bioavailability of glucosamine. By using male Sprague-Dawley rats, the movement of glucosamine through everted gut, the effect of dose and glucose, and inhibition of a glucose transporter (GLUT2) by quercetin were studied. Glucosamine pharmacokinetics and the effect of dosing, route of administration, food and antibiotic to eradicate gut microflora was also studied. Both in vitro and in vivo studies demonstrated linear absorption kinetics for glucosamine. Absorption from duodenum was the greatest. Glucose had no effect on the transport, whereas quercetin significantly reduced the extent of glucosamine transport. Intraperitoneal doses were completely absorbed, whereas p.o. doses demonstrated low bioavailability, indicating the gut as the site of presystemic loss. Food had no significant effect on glucosamine pharmacokinetics. Antibiotic treatment resulted in strong trends towards increased bioavailability with significant increase in fecal recovery. Incubation of glucosamine with faeces resulted in a significant loss. Glucosamine's low bioavailability is, at least in part, due to its dependence on a transport-facilitated absorption and presystemic loss brought about by the gut microflora.

Congenital diarrheal disorders: an updated diagnostic approach

Congenital diarrheal disorders (CDDs) are a group of inherited enteropathies with a typical onset early in the life. Infants with these disorders have frequently chronic diarrhea of sufficient severity to require parenteral nutrition. For most CDDs the disease-gene is known and molecular analysis may contribute to an unequivocal diagnosis. We review CDDs on the basis of the genetic defect, focusing on the significant contribution of molecular analysis in the complex, multistep diagnostic work-up.

Circadian clock genes and implications for intestinal nutrient uptake

There has recently been increasing interest in the phenomenon of circadian rhythmicity. We have used circadian rhythms as a means to understanding the regulation of glucose absorption in the intestine. We and others have previously demonstrated rhythmicity in intestinal glucose uptake, mediated by rhythmicity in the expression of the sodium glucose cotransporter 1. Rhythmicity of clock gene expression was subsequently confirmed in the intestine, a phenomenon also demonstrated in other viscera. Clock genes have since been shown via a combination of in vitro and in vivo techniques to play a role in the transcriptional regulation of key absorptive proteins.

Fructose might contribute to the hypoglycemic effect of honey

Honey is a natural substance with many medicinal properties, including antibacterial, hepatoprotective, hypoglycemic, antioxidant and antihypertensive effects. It reduces hyperglycemia in diabetic rats and humans. However, the mechanism(s) of its hypoglycemic effect remain(s) unknown. Honey comprises many constituents, making it difficult to ascertain which component(s) contribute(s) to its hypoglycemic effect. Nevertheless, available evidence indicates that honey consists of predominantly fructose and glucose. The objective of this review is to summarize findings which indicate that fructose exerts a hypoglycemic effect. The data show that glucose and fructose exert a synergistic effect in the gastrointestinal tract and pancreas. This synergistic effect might enhance intestinal fructose absorption and/or stimulate insulin secretion. The results indicate that fructose enhances hepatic glucose uptake and glycogen synthesis and storage via activation of hepatic glucokinase and glycogen synthase, respectively. The data also demonstrate the beneficial effects of fructose on glycemic control, glucose- and appetite-regulating hormones, body weight, food intake, oxidation of carbohydrate and energy expenditure. In view of the similarities of these effects of fructose with those of honey, the evidence may support the role of fructose in honey in mediating the hypoglycemic effect of honey.

SLC5 and SLC2 transporters in epithelia-cellular role and molecular mechanisms

Members of the SLC5 and SLC2 family are prominently involved in epithelial sugar transport. SGLT1 (sodium-glucose transporter) and SGLT2, as representatives of the former, mediate sodium-dependent uptake of sugars into intestinal and renal cells. GLUT2 (glucose transporter), as representative of the latter, facilitates the sodium-independent exit of sugars from cells. SGLT has played a major role in the formulation and experimental proof for the existence of sodium cotransport systems. Based on the sequence data and biochemical and biophysical analyses, the role of extramembranous loops in sugar and inhibitor binding can be delineated. Crystal structures and homology modeling of SGLT reveal that the sugar translocation involves operation of two hydrophobic gates and intermediate exofacial and endofacial occluded states of the carrier in an alternating access model. The same basic model is proposed for GLUT1. Studies on GLUT1 have pioneered the isolation of eukaryotic transporters by biochemical methods and the development of transport kinetics and transporter models. For GLUT1, results from extensive mutagenesis, cysteine substitution and accessibility studies can be incorporated into a homology model with a barrel-like structure in which accessibility to the extracellular and intracellular medium is altered by pinching movements of some of the helices. For SGLT1 and GLUT1, the extensive hydrophilic and hydrophobic interactions between sugars and binding sites of the various intramembrane helices occur and lead to different substrate specificities and inhibitor affinities of the two transporters. A complex network of regulatory steps adapts the transport activity to the needs of the body.

Fructose malabsorption and intolerance: effects of fructose with and without simultaneous glucose ingestion

Concern exists that increasing fructose consumption, particularly in the form of high-fructose corn syrup, is resulting in increasing rates of fructose intolerance and aggravation of clinical symptoms in individuals with irritable bowel syndrome. Most clinical trials designed to test this hypothesis have used pure fructose, a form not commonly found in the food supply, often in quantities and concentrations that exceed typical fructose intake levels. In addition, the amount of fructose provided in tests for malabsorption, which is thought to be a key cause of intolerance, often exceeds the normal physiological absorption capacity for this sugar. To help health professionals accurately identify and treat this condition, this article reviews clinical data related to understanding fructose malabsorption and intolerance (i.e., malabsorption that manifests with symptoms) relative to usual fructose and other carbohydrate intake. Because simultaneous consumption of glucose attenuates fructose malabsorption, information on the fructose and glucose content of foods, beverages, and ingredients representing a variety of food categories is provided.