Fructose transporters GLUT5 and GLUT2 expression in adult patients with fructose intolerance

Assessment of dietary interventions including low fermentable oligosaccharides, disaccharides, monosaccharides, and polyols diet as management for fructose intolerance

OBJECTIVES

Abdominal pain remains one of the most common referral reasons to pediatric gastroenterology. Dietary intolerances are often considered but due to various factors are hardly pursued. We observed that diet review in large number of children with abdominal pain was high in sugary foods which led to food intolerance investigation and dietary intervention.

METHODS

A retrospective review was conducted of patients presenting with abdominal pain, diarrhea, or vomiting and negative GI evaluation, who underwent fructose breath testing. Patients younger than 20 years old who were seen between June 1, 2018 and March 1, 2021 were included. Statistical analysis was performed in R.

RESULTS

There were 110 pediatric patients during the study period who underwent fructose breath testing, with 31% male and 69% female. The average age was 12.14 ± 4.01 years, and the average BMI was 21.21 ± 6.12. Abdominal pain was the most common presenting symptom (74.5%) followed by diarrhea and vomiting. Seventy-seven patients (70%) had a positive fructose breath test and were diagnosed with dietary intolerance to fructose. The 56 (67.5%) of those patients experienced symptoms during the breath test. Forty-three patients improved with dietary intervention. Twenty-seven on low fermentable oligosaccharides, disaccharides, monosaccharides, and polyols diet and 16 on other diets.

CONCLUSIONS

Based on analysis of our cohort of children with abdominal pain and high incidence of fructose intolerance as well as improvement in symptoms, following dietary changes, this condition should be considered and treated. Further investigation is needed to improve diagnostic testing but also into understanding mechanisms behind symptom presentation in this population.

Beyond butyrate: microbial fiber metabolism supporting colonic epithelial homeostasis

Human gut bacteria produce metabolites that support energy and carbon metabolism of colonic epithelial cells. While butyrate is commonly considered the primary fuel, it alone cannot meet all the carbon requirements for cellular synthetic functions. Glucose, delivered via circulation or microbial metabolism, serves as a universal carbon source for synthetic processes like DNA, RNA, protein, and lipid production. Detailed knowledge of epithelial carbon and energy metabolism is particularly relevant for epithelial regeneration in digestive and metabolic diseases, such as inflammatory bowel disease and type 2 diabetes. Here, we review the production and role of different colonic microbial metabolites in energy and carbon metabolism of colonocytes, also critically evaluating the common perception that butyrate is the preferred fuel.

GLUT5: structure, functions, diseases and potential applications

Glucose transporter 5 (GLUT5) is a membrane transporter that specifically transports fructose and plays a key role in dietary fructose uptake and metabolism. In recent years, a high fructose diet has occupied an important position in the daily intake of human beings, resulting in a significant increase in the incidence of obesity and metabolic diseases worldwide. Over the past few decades, GLUT5 has been well understood to play a significant role in the pathogenesis of human digestive diseases. Recently, the role of GLUT5 in human cancer has received widespread attention, and a large number of studies have focused on exploring the effects of changes in GLUT5 expression levels on cancer cell survival, metabolism and metastasis. However, due to various difficulties and shortcomings, the molecular structure and mechanism of GLUT5 have not been fully elucidated, which to some extent prevents us from revealing the relationship between GLUT5 expression and cell carcinogenesis at the protein molecular level. In this review, we summarize the current understanding of the structure and function of mammalian GLUT5 and its relationship to intestinal diseases and cancer and suggest that GLUT5 may be an important target for cancer therapy.

Genetic Variants in Carbohydrate Digestive Enzyme and Transport Genes Associated with Risk of Irritable Bowel Syndrome

Irritable Bowel Syndrome (IBS) is characterized by abdominal pain and alterations in bowel pattern, such as constipation (IBS-C), diarrhea (IBS-D), or mixed (IBS-M). Since malabsorption of ingested carbohydrates (CHO) can cause abdominal symptoms that closely mimic those of IBS, identifying genetic mutations in CHO digestive enzymes associated with IBS symptoms is critical to ascertain IBS pathophysiology. Through candidate gene association studies, we identify several common variants in TREH, SI, SLC5A1 and SLC2A5 that are associated with IBS symptoms. By investigating rare recessive Mendelian or oligogenic inheritance patterns, we identify case-exclusive rare deleterious variation in known disease genes (SI, LCT, ALDOB, and SLC5A1) as well as candidate disease genes (MGAM and SLC5A2), providing potential evidence of monogenic or oligogenic inheritance in a subset of IBS cases. Finally, our data highlight that moderate to severe IBS-associated gastrointestinal symptoms are often observed in IBS cases carrying one or more of deleterious rare variants.

The inhibition of intestinal glucose absorption by oat-derived avenanthramides

Avenanthramides are phenolic compounds unique to oats and may contribute to health-promoting properties associated with oat consumption. This study used Xenopus laevis oocytes expressing the glucose transporters, glucose transporter 2 (GLUT2) or sodium-glucose transport protein 1 (SGLT1) and human Caco-2 cells models to investigate the effect of oat avenanthramides on human intestinal glucose transporters. The presence of avenanthramide reduced the glucose uptake in a dose-dependent manner in Caco-2 cells. Glucose uptake in oocytes expressing either GLUT2 or SGLT1 was nullified by oat avenanthramide. There was no significant difference between the inhibition potencies of avenanthramides C and B. Thus, our results suggest that avenanthramides may contribute to the antidiabetic properties of oats. PRACTICAL APPLICATIONS: The present research focus on the antidiabetic properties of avenanthramides, which are unique phenolic compounds found in oats. Inhibiting the activities of the glucose transport proteins expressed in the small intestine is a known strategy to improve the control of postprandial glucose level. We therefore examined the inhibitory effects of avenanthramides on two glucose transporters, glucose transporter 2 and sodium-glucose transport protein 1, predominantly found in the small intestine using the human small intestinal cell model Caco-2 cell line and by heterologously expressing these two transporters in the Xenopus laevis oocytes. Based on our results, we have confirmed for the first time that the glucose uptake is indeed inhibited by the presence of avenanthramides, suggesting the possibility of incorporating avenanthramides in foods to enhance postprandial glucose response, and ultimately improve the management of diabetes. Therefore, future research could consider utilizing this evidence in the development of diabetic-friendly functional foods or nutraceuticals containing avenanthramides.

Gene variants of the SLC2A5 gene encoding GLUT5, the major fructose transporter, do not contribute to clinical presentation of acquired fructose malabsorption

Background

While role of ALDOB-related gene variants for hereditary fructose intolerance is well established, contribution of gene variants for acquired fructose malabsorption (e.g. SLC2A5, GLUT5) is not well understood.

Methods

Patients referred to fructose breath test were further selected to identify those having acquired fructose malabsorption. Molecular analysis of genomic DNA included (I) exclusion of 3 main ALDOB gene variants causing hereditary fructose intolerance and (II) sequencing analysis of SLC2A5 gene comprising complete coding region, at least 20 bp of adjacent intronic regions and 700 bp of proximal promoter.

Results

Among 494 patients, 35 individuals with acquired fructose malabsorption were identified based on pathological fructose-breath test and normal lactose-breath test. Thirty four of them (97%) had negative tissue anti-transglutaminase and/or deamidated gliadin antibodies in their medical records. Molecular analysis of SLC2A5 gene of all 35 subjects identified 5 frequent and 5 singular gene variants mostly in noncoding regions (promoter and intron). Allele frequencies of gene variants were similar to those reported in public databases strongly implying that none of them was associated with acquired fructose malabsorption.

Conclusions

Gene variants of coding exons, adjacent intronic regions and proximal promoter region of SLC2A5 gene are unlikely to contribute to genetic predisposition of acquired fructose malabsorption.

Fructose intolerance is not associated with malabsorption in patients with functional gastrointestinal disorders

BACKGROUND

Symptoms following fructose ingestion, or fructose intolerance, are common in patients with functional gastrointestinal disorders (FGID) and are generally attributed to intestinal malabsorption. The relationships between absorption, symptoms, and intestinal gas production following fructose ingestion were studied in patients with FGID.

METHODS

Thirty FGID patients ingested a single dose of fructose 35 g or water in a randomized, double-blind, crossover study. Blood and breath gas samples were collected, and gastrointestinal symptoms rated. Plasma fructose metabolites and short-chain fatty acids were quantified by targeted liquid chromatography-tandem mass spectrometry. Patients were classified as fructose intolerant or tolerant based on symptoms following fructose ingestion.

KEY RESULTS

The median (IQR) areas under the curve of fructose plasma concentrations within the first 2 h (AUC_0-2 h ) after fructose ingestion were similar for patients with and without fructose intolerance (578 (70) µM·h vs. 564 (240) µM·h, respectively, p = 0.39), as well as for the main fructose metabolites. There were no statistically significant correlations between the AUC_0-2 h of fructose or its metabolites concentrations and the AUCs of symptoms, breath hydrogen, and breath methane. However, the AUCs of symptoms correlated significantly and positively with the AUC_0-2 h of hydrogen and methane breath concentrations (r = 0.73, r = 0.62, respectively), and the AUCs of hydrogen and methane concentrations were greater in the fructose-intolerant than in the fructose-tolerant patients after fructose ingestion (p ≤ 0.02).

CONCLUSIONS & INFERENCES

Fructose intolerance in FGID is not related to post-ingestion plasma concentrations of fructose and its metabolites. Factors other than malabsorption, such as altered gut microbiota or sensory function, may be important mechanisms.

GLUT5 is a determinant of dietary fructose-mediated exacerbation of experimental colitis

The Western diet has been suggested to contribute to the rising incidence of inflammatory bowel diseases. This has led to the hypothesis that fructose, a component of the Western diet, could play a role in the pathogenesis of inflammatory bowel diseases. A high-fructose diet is known to exacerbate experimental colitis. This study tested whether the expression of GLUT5, the fructose transporter, is a determinant of the severity of experimental colitis during elevated fructose consumption and whether ileal inflammation is associated with altered GLUT5 expression in Crohn's disease. Studies in genetically engineered mice showed that in comparison to Glut5^+/+ mice, feeding a 15 kcal% fructose diet to Glut5^-/- mice led to worse dextran sodium sulfate (DSS)-induced colitis. This effect was associated with elevated levels of colonic fructose and a shift in the fecal microbiota in Glut5^-/- mice. Importantly, treatment with broad-spectrum antibiotics protected against the worsening of colitis mediated by dietary fructose in Glut5^-/- mice. Gene expression analysis revealed that GLUT5 levels are reduced in the intestines of patients with ileal Crohn's disease. Moreover, levels of GLUT5 negatively correlated with expression of proinflammatory mediators in these samples. Collectively, these results demonstrate that dietary constituent (fructose)-host gene (GLUT5) interactions can shape the colonic microbiota, thereby impacting the severity of colitis.NEW & NOTEWORTHY This study provides the first evidence that reduced levels of GLUT5, the fructose transporter, worsen experimental colitis upon fructose feeding, an effect mediated by changes in the gut microbiota. Moreover, GLUT5 expression is reduced in Crohn's ileitis. Overall, these findings demonstrate the importance of interactions between dietary fructose and host GLUT5 as determinants of both the composition of colonic microbiota and severity of experimental colitis.

Expression of the fructose transporter GLUT5 in patients with fructose malabsorption

BACKGROUND

 Patients with abdominal symptoms are frequently diagnosed with fructose malabsorption (FM). Fructose is absorbed by monosaccharide transporters located in the brush border of the human small intestine. The aim of this study was to investigate the histoanatomical distribution of the main fructose transporter GLUT5.

MATERIALS AND METHODS

 We studied 223 patients diagnosed with FM by a hydrogen breath test and grouped according to their response to a fructose-free diet. The control group were 42 healthy individuals and 29 patients with celiac disease (CD). The fructose breath test was done with 50 g fructose. The expression of Glut5 in duodenal biopsy specimens was studied by immunohistochemistry. The Kruskal-Wallis-test and Mann-Whitney U-test were used to carry out the statistical analysis.

RESULTS

 The histoanatomical expression pattern of GLUT5 did not differ significantly between those patients with FM who responded completely to a fructose-free diet (n = 183) and healthy individuals (n = 42); nor did it correlate to H2 production measured in fructose breath testing. In patients with FM, the GLUT5 expression pattern did not differ between those individuals responding to a fructose-free diet and those who did not. However, GLUT5 expression pattern was significantly different in patients with CD (n = 29) compared to patients with FM and to healthy individuals (p = 0.009).

CONCLUSION

 GLUT5 expression patterns are not be related to adult patients with FM. However, in secondary malabsorption, a decreased GLUT5 expression was found. Further investigation is needed to understand the essential factors in FM and the influence on functional gastrointestinal disorders.

Gastrointestinal Food Allergies and Intolerances

Adverse reactions to food include immune-mediated food allergies, celiac disease, and nonimmune-mediated food intolerances. Differentiating between these many disorders is important to guide us toward appropriate testing and management. Double-blind placebo-controlled food challenges are the gold standard for food allergy diagnosis but are difficult and time-consuming. In place of this, strong clinical history, other supportive tests, and oral food challenges are helpful. Some commonly available tests for food allergy and intolerances lack sufficient evidence for efficacy. Food intolerance diagnosis is largely based on history and supported by symptom improvement with appropriate dietary manipulation.

A comprehensive overview of substrate specificity of glycoside hydrolases and transporters in the small intestine : "A gut feeling"

The human body is able to process and transport a complex variety of carbohydrates, unlocking their nutritional value as energy source or as important building block. The endogenous glycosyl hydrolases (glycosidases) and glycosyl transporter proteins located in the enterocytes of the small intestine play a crucial role in this process and digest and/or transport nutritional sugars based on their structural features. It is for these reasons that glycosidases and glycosyl transporters are interesting therapeutic targets to combat sugar related diseases (such as diabetes) or to improve drug delivery. In this review we provide a detailed overview focused on the molecular structure of the substrates involved as a solid base to start from and to fuel research in the area of therapeutics and diagnostics.

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.

Can fructose influence the development of obesity mediated through hypothalamic alterations?

Epidemiological data from the last decades point to an exponential growth in the number of obese people. Different behavioral factors, mainly associated with food consumption, appear to contribute significantly to its development. Concomitant with increased obesity rates, an increase in the consumption of fructose has been observed; therefore, fructose consumption has been implicated as an important obesogenic factor. However, changes in brain activity due to fructose consumption are possible, especially in relation to hypothalamic satiety mechanisms. In addition, the obese state may provide an environment of chronic inflammation and further contribute to the discontinuation of satiety mechanisms in the hypothalamus. We briefly review the intrinsic alterations to the increased adipose tissue, its connections with the hypothalamus in the control of energy signaling mechanisms and, consequently, the participation of fructose as a co-adjuvant or trigger. Presenting the current context with clinical trials involving human and animal studies, we seek to contribute to a better understanding of the role of fructose in the progression of obesity.

Fructose and irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic disorder characterised by recurrent abdominal pain or discomfort and transit disturbances with heterogeneous pathophysiological mechanisms. The link between food and gastrointestinal (GI) symptoms is often reported by patients with IBS and the role of fructose has recently been highlighted. Fructose malabsorption can easily be assessed by hydrogen and/or methane breath test in response to 25 g fructose; and its prevalence is about 22 % in patients with IBS. The mechanism of fructose-related symptoms is incompletely understood. Osmotic load, fermentation and visceral hypersensitivity are likely to participate in GI symptoms in the IBS population and may be triggered or worsened by fructose. A low-fructose diet could be integrated in the overall treatment strategy, but its role and implication in the improvement of IBS symptoms should be evaluated. In the present review, we discuss fructose malabsorption in adult patients with IBS and the interest of a low-fructose diet in order to underline the important role of fructose in IBS.

Giardia intestinalis and Fructose Malabsorption: A Frequent Association

Nowadays, scientific studies are emerging on the possible etiological role of intestinal parasites in functional digestive disorders. Our study was carried out with healthy individuals (control group; n = 82) and symptomatic patients with lactose or fructose malabsorption, including positive (malabsorbers; n = 213) and negative (absorbers; n = 56) breath test, being analyzed for the presence of intestinal parasites. A high parasitic prevalence was observed in malabsorbers (41.8%), exclusively due to single-cell eukaryotes but not helminths. Giardia intestinalis was the predominant parasite in cases of abnormal absorption (26.5%), significantly associated with fructose malabsorption and doubling the probability of developing this pathology. Within controls, Blastocystis sp. (13.4%) was almost the only parasite, being the second among patients (12.6%), and Cryptosporidium parvum, the last species of clinical relevance, was detected exclusively in two malabsorbers (0.9%). The consumption of ecological food and professions with direct contact with humans arose as risk factors of parasitism. A diagnosis of carbohydrate malabsorption in adulthood is the starting point, making the search for the primary cause necessary. Accurate parasitological diagnosis should be considered another tool in the clinical routine for patients with recurrent symptoms, since their condition may be reversible with adequate therapeutic intervention.

Age-related intestinal monosaccharides transporters expression and villus surface area increase in broiler and layer chickens

In the chicken small intestine, glucose is mainly transported by the apically located sodium/glucose cotransporter 1 (SGLT1) and the basolaterally located glucose transporter 2 (GLUT2). Fructose is transported by the apically located glucose transporter 5 (GLUT5) and similarly by GLUT2. During the early post-hatching period, the intestinal villus surface area (VSA) should be considered as an important factor related to the monosaccharide absorption capacity. Our objective here was to study intestinal monosaccharide absorption by analyzing the effects of age, diet, and breed on monosaccharide transporters and the VSA. The mRNA expression patterns of SGLT1, GLUT2 and GLUT5 genes in broiler and layer chickens were measured from the day of hatching to day 28 using the absolute quantitative real-time PCR. Both the intestinal mRNA expression levels of these genes and the VSA were affected by age. The mRNA expression levels of SGLT1 and GLUT2 were significantly increased from day 1 to day 3 and then decreased from day 3 to day 28. The expression levels of GLUT5 decreased from day 1 to day 7. The broiler chickens VSAs were significantly larger than those of the layer chickens from days 7 to 28. The effect of diet on the gene expression patterns of these monosaccharide transporters and the VSA were not significant. Our results suggest that the expression levels of these monosaccharide transporters are increased rapidly at the beginning of intestinal growth to meet the demands for monosaccharides to support the fast growth of the chick before day 7. As intestinal maturation and VSA increased, the expression levels of these monosaccharide genes decreased to a certain expression level to maintain the intestinal transport capacity and the absorption balance of all other nutrients.

Dapagliflozin Prevents NOX- and SGLT2-Dependent Oxidative Stress in Lens Cells Exposed to Fructose-Induced Diabetes Mellitus

Purpose: Cataracts in patients with diabetes mellitus (DM) are a major cause of blindness in developed and developing countries. This study aims to examine whether the generation of reactive oxygen species (ROS) via the increased expression of glucose transporters (GLUTs) and the receptor for advanced glycation end products (RAGE) influences the cataract development in DM. Methods: Lens epithelial cells (LECs) were isolated during cataract surgery from patients without DM or with DM, but without diabetic retinopathy. In a rat model, fructose (10% fructose, 8 or 12 weeks) with or without dapagliflozin (1.2 mg/day, 2 weeks) treatment did induce DM, as verified by blood pressure and serum parameter measurements. Immunofluorescence stainings and immunoblottings were used to quantify the protein levels. Endogenous O₂˙¯ production in the LECs was determined in vivo with dihydroethidium stainings. Results: We investigated that GLUT levels in LECs differed significantly, thus leading to the direct enhancement of RAGE-associated superoxide generation in DM patients with cataracts. Superoxide production was significantly higher in LECs from rats with fructose-induced type 2 DM, whereas treatment with the sodium-glucose cotransporter 2 (SGLT2) inhibitor dapagliflozin prevented this effect in fructose-fed rats. Protein expression levels of the sodium/glucose cotransporter 2 (SGLT2), GLUT1, GLUT5, the nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase subunit p67-phox, NOX2/4 and RAGE were upregulated in fructose-fed animals, whereas dapagliflozin treatment reversed these effects. Conclusions: In rats with fructose-induced DM, dapagliflozin downregulates RAGE-induced NADPH oxidase expression in LECs via the inactivation of GLUTs and a reduction in ROS generation. These novel findings suggest that the SGLT2 inhibitor dapagliflozin may be a candidate for the pharmacological prevention of cataracts in patients with DM.

ChREBP deficiency leads to diarrhea-predominant irritable bowel syndrome

OBJECTIVE

Fructose malabsorption is a common digestive disorder in which absorption of fructose in the small intestine is impaired. An abnormality of the main intestinal fructose transporter proteins has been proposed as a cause for fructose malabsorption. However the underlying molecular mechanism for this remains unclear. In this study, we investigated whether carbohydrate response element-binding protein (ChREBP) plays a role in intestinal fructose absorption through the regulation of genes involved in fructose transport and metabolism and ion transport.

METHODS

Wild type (WT) and Chrebp knockout (KO) mice (6 or 8 weeks old) were fed a control diet (55% starch, 15% maltodextrin 10) or high-fructose diet (HFrD, 60% fructose, 10% starch) for 3-12 days. Body weight and food intake were measured, signs of fructose malabsorption were monitored, and the expression of genes involved in fructose transport/metabolism and ion transport was evaluated. Furthermore, transient transfection and chromatin immunoprecipitation were performed to show the direct interaction between ChREBP and carbohydrate response elements in the promoter of Slc2A5, which encodes the fructose transporter GLUT5.

RESULTS

Chrebp KO mice fed the control diet maintained a constant body weight, whereas those fed a HFrD showed significant weight loss within 3-5 days. In addition, Chrebp KO mice fed the HFrD exhibited a markedly distended cecum and proximal colon containing both fluid and gas, suggesting incomplete fructose absorption. Fructose-induced increases of genes involved in fructose transport (GLUT5), fructose metabolism (fructokinase, aldolase B, triokinase, and lactate dehydrogenase), and gluconeogenesis (glucose-6-phosphatase and fructose-1,6-bisphosphatase) were observed in the intestine of WT but not of Chrebp KO mice. Moreover the Na⁺/H⁺ exchanger NHE3, which is involved in Na⁺ and water absorption in the intestine, was significantly decreased in HFrD-fed Chrebp KO mice. Consistent with this finding, the high-fructose diet-fed Chrebp KO mice developed severe diarrhea. Results of chromatin immunoprecipitation assays showed a direct interaction of ChREBP with the Glut5 promoter, but not the Nhe3 promoter, in the small intestine. Ectopic co-expression of ChREBP and its heterodimer partner Max-like protein X activated the Glut5 promoter in Caco-2BBE cells.

CONCLUSIONS

ChREBP plays a key role in the dietary fructose transport as well as conversion into lactate and glucose through direct transcriptional control of genes involved in fructose transport, fructolysis, and gluconeogenesis. Moreover, ablation of Chrebp results in a severe diarrhea in mice fed a high-fructose diet, which is associated with the insufficient induction of GLUT5 in the intestine.

Intestinal Absorption of Fructose

Increased understanding of fructose metabolism, which begins with uptake via the intestine, is important because fructose now constitutes a physiologically significant portion of human diets and is associated with increased incidence of certain cancers and metabolic diseases. New insights in our knowledge of intestinal fructose absorption mediated by the facilitative glucose transporter GLUT5 in the apical membrane and by GLUT2 in the basolateral membrane are reviewed. We begin with studies related to structure as well as ligand binding, then revisit the controversial proposition that apical GLUT2 is the main mediator of intestinal fructose absorption. The review then describes how dietary fructose may be sensed by intestinal cells to affect the expression and activity of transporters and fructolytic enzymes, to interact with the transport of certain minerals and electrolytes, and to regulate portal and peripheral fructosemia and glycemia. Finally, it discusses the potential contributions of dietary fructose to gastrointestinal diseases and to the gut microbiome.

Fibromyalgia Syndrome: A Metabolic Approach Grounded in Biochemistry for the Remission of Symptoms

Fibromyalgia syndrome (FMS) is a chronic, complex, and heterogeneous disorder of still poorly understood etiopathophysiology associated with important musculoskeletal widespread pain, fatigue, non-restorative sleep, and mood disturbances. It is estimated to afflict 2–3% of the worldwide population, with clean prevalence among women. The objective of this paper is to propose a novel treatment for symptomatic remission of FMS, grounded in biochemistry and consisting in the withdrawal from the diet of molecules that can indirectly trigger the symptoms. The hypothesis develops from the evidence that low serotonin levels are involved in FMS. Serotonin is synthesized starting from the essential amino acid tryptophan. The presence of non-absorbed molecules in the gut, primarily fructose, reduces tryptophan absorption. Low tryptophan absorption leads to low serotonin synthesis that triggers FMS symptoms. Moreover not-absorbed sugars could also produce a microbiota deterioration activating a positive feedback loop: the increasing microbiota deterioration reduces the functionality of absorption both of fructose and tryptophan in the gut, entering a vicious circle. The therapeutic idea is to sustain serotonin synthesis allowing the proper tryptophan absorption. The core of the cure treatment is the exclusion from the diet of some carbohydrates and the marked reduction of some others. The main target is the limitation of total dietary fructose as marked as possible. It could be an effective strategy to get the remission of symptoms acting on the impaired biochemical pathways. The straying from the treatment is expected to cause the reappear of the symptoms.

Identification of essential amino acids for glucose transporter 5 (GLUT5)-mediated fructose transport

Intestinal fructose uptake is mainly mediated by glucose transporter 5 (GLUT5/SLC2A5). Its closest relative, GLUT7, is also expressed in the intestine but does not transport fructose. For rat Glut5, a change of glutamine to glutamic acid at codon 166 (p.Q166E) has been reported to alter the substrate-binding specificity by shifting Glut5-mediated transport from fructose to glucose. Using chimeric proteins of GLUT5 and GLUT7, here we identified amino acid residues of GLUT5 that define its substrate specificity. The proteins were expressed in NIH-3T3 fibroblasts, and their activities were determined by fructose radiotracer flux. We divided the human GLUT5 sequence into 26 fragments and then replaced each fragment with the corresponding region in GLUT7. All fragments that yielded reduced fructose uptake were analyzed further by assessing the role of individual amino acid residues. Various positions in the first extracellular loop, in the fifth, seventh, eighth, ninth, and tenth transmembrane domains (TMDs), and in the regions between the ninth and tenth TMDs and tenth and 11th TMDs were identified as being important for proper fructose uptake. Although the p.Q167E change did not render the human protein into a glucose transporter, molecular dynamics simulations revealed a drastic change in the dynamics and a movement of the intracellular loop connecting the sixth and seventh TMDs, which covers the exit of the ligand. Finally, we generated a GLUT7-GLUT5 chimera consisting of the N-terminal part of GLUT7 and the C-terminal part of GLUT5. Although this chimera was inactive, we demonstrate fructose transport after introduction of four amino acids derived from GLUT5.

The sedentary (r)evolution: Have we lost our metabolic flexibility?

During the course of evolution, up until the agricultural revolution, environmental fluctuations forced the human species to develop a flexible metabolism in order to adapt its energy needs to various climate, seasonal and vegetation conditions. Metabolic flexibility safeguarded human survival independent of food availability. In modern times, humans switched their primal lifestyle towards a constant availability of energy-dense, yet often nutrient-deficient, foods, persistent psycho-emotional stressors and a lack of exercise. As a result, humans progressively gain metabolic disorders, such as the metabolic syndrome, type 2 diabetes, non-alcoholic fatty liver disease, certain types of cancer, cardiovascular disease and Alzheimer´s disease, wherever the sedentary lifestyle spreads in the world. For more than 2.5 million years, our capability to store fat for times of food shortage was an outstanding survival advantage. Nowadays, the same survival strategy in a completely altered surrounding is responsible for a constant accumulation of body fat. In this article, we argue that the metabolic disease epidemic is largely based on a deficit in metabolic flexibility. We hypothesize that the modern energetic inflexibility, typically displayed by symptoms of neuroglycopenia, can be reversed by re-cultivating suppressed metabolic programs, which became obsolete in an affluent environment, particularly the ability to easily switch to ketone body and fat oxidation. In a simplified model, the basic metabolic programs of humans' primal hunter-gatherer lifestyle are opposed to the current sedentary lifestyle. Those metabolic programs, which are chronically neglected in modern surroundings, are identified and conclusions for the prevention of chronic metabolic diseases are drawn.

The sedentary (r)evolution: Have we lost our metabolic flexibility?

During the course of evolution, up until the agricultural revolution, environmental fluctuations forced the human species to develop a flexible metabolism in order to adapt its energy needs to various climate, seasonal and vegetation conditions. Metabolic flexibility safeguarded human survival independent of food availability. In modern times, humans switched their primal lifestyle towards a constant availability of energy-dense, yet often nutrient-deficient, foods, persistent psycho-emotional stressors and a lack of exercise. As a result, humans progressively gain metabolic disorders, such as the metabolic syndrome, type 2 diabetes, non-alcoholic fatty liver disease, certain types of cancer, cardiovascular disease and Alzheimer´s disease, wherever the sedentary lifestyle spreads in the world. For more than 2.5 million years, our capability to store fat for times of food shortage was an outstanding survival advantage. Nowadays, the same survival strategy in a completely altered surrounding is responsible for a constant accumulation of body fat. In this article, we argue that the metabolic disease epidemic is largely based on a deficit in metabolic flexibility. We hypothesize that the modern energetic inflexibility, typically displayed by symptoms of neuroglycopenia, can be reversed by re-cultivating suppressed metabolic programs, which became obsolete in an affluent environment, particularly the ability to easily switch to ketone body and fat oxidation. In a simplified model, the basic metabolic programs of humans’ primal hunter-gatherer lifestyle are opposed to the current sedentary lifestyle. Those metabolic programs, which are chronically neglected in modern surroundings, are identified and conclusions for the prevention of chronic metabolic diseases are drawn.

Current and emergent pharmacologic treatments for irritable bowel syndrome with diarrhea: evidence-based treatment in practice

Irritable bowel syndrome with diarrhea (IBS-D) is a common, chronic functional gastrointestinal disorder with symptoms that can be distressing for patients and often result in substantially impaired quality of life. This review focuses on providing clinicians with information on practical, evidence-based treatment for IBS-D. Current therapies commonly used for the treatment of IBS-D, including pharmacologic and nonpharmacologic interventions, are briefly reviewed, followed by discussion of the emergent pharmacologic treatments (rifaximin and eluxadoline) and medical foods (IBgard® and EnteraGam®). Given the lack of a standard treatment algorithm for IBS-D and the emergence of new pharmacologic therapies, treatment needs to be tailored to the individual patient and take into account the severity of disease. In this context, the latter part of this manuscript examines how treatments for IBS-D can be used in clinical practice by presenting three patient case scenarios with varying degrees of IBS-D severity. For each case, the patient's medical history and clinical presentation are related to the Rome Foundation multidimensional clinical profile (MDCP) and potential treatment options with current and emergent therapies are reviewed. The interplay of gastrointestinal symptoms and their psychosocial impact, as well as the importance of a patient-centered approach to therapy, are discussed. Consideration is given to the potential need for combination therapies and how emergent treatments could fit into the treatment pathway for mild, moderate, and severe cases of IBS-D in clinical practice.

Adding glucose to food and solutions to enhance fructose absorption is not effective in preventing fructose-induced functional gastrointestinal symptoms: randomised controlled trials in patients with fructose malabsorption

BACKGROUND

In healthy individuals, the absorption of fructose in excess of glucose in solution is enhanced by the addition of glucose. The present study aimed to assess the effects of glucose addition to fructose or fructans on absorption patterns and genesis of gastrointestinal symptoms in patients with functional bowel disorders.

METHODS

Randomised, blinded, cross-over studies were performed in healthy subjects and functional bowel disorder patients with fructose malabsorption. The area-under-the-curve (AUC) was determined for breath hydrogen and symptom responses to: (i) six sugar solutions (fructose in solution) (glucose; sucrose; fructose; fructose + glucose; fructan; fructan + glucose) and (ii) whole foods (fructose in foods) containing fructose in excess of glucose given with and without additional glucose. Intake of fermentable short chain carbohydrates (FODMAPs; fermentable, oligo-, di-, monosaccharides and polyols) was controlled.

RESULTS

For the fructose in solution study, in 26 patients with functional bowel disorders, breath hydrogen was reduced after glucose was added to fructose compared to fructose alone [mean (SD) AUC 92 (107) versus 859 (980) ppm 4 h^-1 , respectively; P = 0.034). Glucose had no effect on breath hydrogen response to fructans (P = 1.000). The six healthy controls showed breath hydrogen patterns similar to those with functional bowel disorders. No differences in symptoms were experienced with the addition of glucose, except more nausea when glucose was added to fructose (P = 0.049). In the fructose in foods study, glucose addition to whole foods containing fructose in excess of glucose in nine patients with functional bowel disorders and nine healthy controls had no significant effect on breath hydrogen production or symptom response.

CONCLUSIONS

The absence of a favourable response on symptoms does not support the concomitant intake of glucose with foods high in either fructose or fructans in patients with functional bowel disorders.

Use of the Biphasic (13)C-Sucrose/Glucose Breath Test to Assess Sucrose Maldigestion in Adults with Functional Bowel Disorders

Sucrase insufficiency has been observed in children with of functional bowel disorders (FBD) and symptoms of dietary carbohydrate intolerance may be indistinguishable from those of FBD. A two-phase (13)C-sucrose/(13)C-glucose breath test ((13)C-S/GBT) was used to assess sucrase activity because disaccharidase assays are seldom performed in adults. When (13)C-sucrose is hydrolyzed to liberate monosaccharides, oxidation to (13)CO2 is a proportional indicator of sucrase activity. Subsequently, (13)C-glucose oxidation rate was determined after a secondary substrate ingestion (superdose) to adjust for individual habitus effects (Phase II). (13)CO2 enrichment recovery ratio from (13)C-sucrose and secondary (13)C-glucose loads reflect the individualized sucrase activity [Coefficient of Glucose Oxidation for Sucrose (CGO-S)]. To determine if sucrase insufficiency could be a factor in FBD, (13)C-S/GBT was validated using subjects with known sucrase gene mutation status by comparing (13)CO2-breath enrichment with plasma (13)C-glucose enrichment. (13)C-S/GBT was used to assess sucrose digestion in FBD patients and asymptomatic controls. (13)CO2-breath enrichment correlated with the appearance of (13)C-sucrose-derived glucose in plasma (r (2) = 0.80). Mean, control group CGO-S-enrichment outcomes were 1.01 at 60', 0.92 at 75', and 0.96 at mean 60'-75' with normal CGO-S defined as >0.85 (95% C.I.). In contrast, FBD patients demonstrated lower CGO-S values of 0.77 at 60', 0.77 at 75', and 0.76 at mean 60'-75' (Chi Square: 6.55; p < 0.01), which points to sucrose maldigestion as a cause of FBD.

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.

Dietary sugars: their detection by the gut-brain axis and their peripheral and central effects in health and diseases

Background

Substantial increases in dietary sugar intake together with the increasing prevalence of obesity worldwide, as well as the parallels found between sugar overconsumption and drug abuse, have motivated research on the adverse effects of sugars on health and eating behaviour. Given that the gut–brain axis depends on multiple interactions between peripheral and central signals, and because these signals are interdependent, it is crucial to have a holistic view about dietary sugar effects on health.

Methods

Recent data on the effects of dietary sugars (i.e. sucrose, glucose, and fructose) at both peripheral and central levels and their interactions will be critically discussed in order to improve our understanding of the effects of sugars on health and diseases. This will contribute to the development of more efficient strategies for the prevention and treatment for obesity and associated co-morbidities.

Results

This review highlights opposing effects of glucose and fructose on metabolism and eating behaviour. Peripheral glucose and fructose sensing may influence eating behaviour by sweet-tasting mechanisms in the mouth and gut, and by glucose-sensing mechanisms in the gut. Glucose may impact brain reward regions and eating behaviour directly by crossing the blood–brain barrier, and indirectly by peripheral neural input and by oral and intestinal sweet taste/sugar-sensing mechanisms, whereas those promoted by fructose orally ingested seem to rely only on these indirect mechanisms.

Conclusions

Given the discrepancies between studies regarding the metabolic effects of sugars, more studies using physiological experimental conditions and in animal models closer to humans are needed. Additional studies directly comparing the effects of sucrose, glucose, and fructose should be performed to elucidate possible differences between these sugars on the reward circuitry.

Fermentable oligosaccharides, disaccharides, monosaccharides and polyols: role in irritable bowel syndrome

Irritable bowel syndrome (IBS) was previously left poorly treated despite its high prevalence and cost. Over the past decade, significant research has been conducted providing new dietary strategies for IBS management. The 'low fermentable oligosaccharides, disaccharides, monosaccharides and polyols diet' has shown symptom improvement in 68-76% of patients. Randomized, controlled trials have now proven its efficacy. The diet, low in poorly absorbed and fermentable carbohydrates, uses dietary restriction and re-challenge to determine individual tolerance to various short-chain carbohydrates. However there may be potential detrimental effects of the diet in the long term, due to potential changes to the gastrointestinal microbiota. Appropriate dietary education and management of the diet is imperative. Future research should focus on the relevance of changes to the microbiota and ways to liberalize the dietary restrictions.

Fructose-induced symptoms beyond malabsorption in FGID

The dietary carbohydrate fructose can be incompletely absorbed in the small intestine and is sometimes associated with gastrointestinal symptoms that include motility disturbances and abdominal pain. Fructose malabsorption has been well documented in variable but similar proportions of healthy and populations with functional gastrointestinal disorders. Recent work into the expression of the main intestinal fructose transporter proteins highlight that our understanding of the mechanistic basis for fructose malabsorption and how it differentiates in gastrointestinal patients is incomplete. Until we have further mechanistic insight, restricting dietary fructose intake and other poorly absorbed short-chain carbohydrates and polyols remains an efficacious approach for managing functional gastrointestinal symptoms.

Dietary fructose intolerance, fructan intolerance and FODMAPs

Dietary intolerances to fructose, fructans and FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) are common, yet poorly recognized and managed. Over the last decade, they have come to the forefront because of new knowledge on the mechanisms and treatment of these conditions. Patients with these problems often present with unexplained bloating, belching, distension, gas, abdominal pain, or diarrhea. Here, we have examined the most up-to-date research on these food-related intolerances, discussed controversies, and have provided some guidelines for the dietary management of these conditions. Breath testing for carbohydrate intolerance appears to be standardized and essential for the diagnosis and management of these conditions, especially in the Western population. While current research shows that the FODMAP diet may be effective in treating some patients with irritable bowel syndrome, additional research is needed to identify more foods items that are high in FODMAPs, and to assess the long-term efficacy and safety of dietary interventions.