Fructose transport by rat intestinal brush border membrane vesicles. Effect of high fructose diet followed by return to standard diet

Cerebral Fructose Metabolism as a Potential Mechanism Driving Alzheimer's Disease

The loss of cognitive function in Alzheimer's disease is pathologically linked with neurofibrillary tangles, amyloid deposition, and loss of neuronal communication. Cerebral insulin resistance and mitochondrial dysfunction have emerged as important contributors to pathogenesis supporting our hypothesis that cerebral fructose metabolism is a key initiating pathway for Alzheimer's disease. Fructose is unique among nutrients because it activates a survival pathway to protect animals from starvation by lowering energy in cells in association with adenosine monophosphate degradation to uric acid. The fall in energy from fructose metabolism stimulates foraging and food intake while reducing energy and oxygen needs by decreasing mitochondrial function, stimulating glycolysis, and inducing insulin resistance. When fructose metabolism is overactivated systemically, such as from excessive fructose intake, this can lead to obesity and diabetes. Herein, we present evidence that Alzheimer's disease may be driven by overactivation of cerebral fructose metabolism, in which the source of fructose is largely from endogenous production in the brain. Thus, the reduction in mitochondrial energy production is hampered by neuronal glycolysis that is inadequate, resulting in progressive loss of cerebral energy levels required for neurons to remain functional and viable. In essence, we propose that Alzheimer's disease is a modern disease driven by changes in dietary lifestyle in which fructose can disrupt cerebral metabolism and neuronal function. Inhibition of intracerebral fructose metabolism could provide a novel way to prevent and treat this disease.

Suppressive effect of nobiletin and epicatechin gallate on fructose uptake in human intestinal epithelial Caco-2 cells

Inhibition of excessive fructose intake in the small intestine could alleviate fructose-induced diseases such as hypertension and non-alcoholic fatty liver disease. We examined the effect of phytochemicals on fructose uptake using human intestinal epithelial-like Caco-2 cells which express the fructose transporter, GLUT5. Among 35 phytochemicals tested, five, including nobiletin and epicatechin gallate (ECg), markedly inhibited fructose uptake. Nobiletin and ECg also inhibited the uptake of glucose but not of L-leucine or Gly-Sar, suggesting an inhibitory effect specific to monosaccharide transporters. Kinetic analysis further suggested that this reduction in fructose uptake was associated with a decrease in the apparent number of cell-surface GLUT5 molecules, and not with a change in the affinity of GLUT5 for fructose. Lastly, nobiletin and ECg suppressed the permeation of fructose across Caco-2 cell monolayers. These findings suggest that nobiletin and ECg are good candidates for preventing diseases caused by excessive fructose intake.

Fructose-induced increases in expression of intestinal fructolytic and gluconeogenic genes are regulated by GLUT5 and KHK

Marked increases in fructose consumption have been tightly linked to metabolic diseases. One-third of ingested fructose is metabolized in the small intestine, but the underlying mechanisms regulating expression of fructose-metabolizing enzymes are not known. We used genetic mouse models to test the hypothesis that fructose absorption via glucose transporter protein, member 5 (GLUT5), metabolism via ketohexokinase (KHK), as well as GLUT5 trafficking to the apical membrane via the Ras-related protein in brain 11a (Rab11a)-dependent endosomes are required for the regulation of intestinal fructolytic and gluconeogenic enzymes. Fructose feeding increased the intestinal mRNA and protein expression of these enzymes in the small intestine of adult wild-type (WT) mice compared with those gavage fed with lysine or glucose. Fructose did not increase expression of these enzymes in the GLUT5 knockout (KO) mice. Blocking intracellular fructose metabolism by KHK ablation also prevented fructose-induced upregulation. Glycolytic hexokinase I expression was similar between WT and GLUT5- or KHK-KO mice and did not vary with feeding solution. Gavage feeding with the fructose-specific metabolite glyceraldehyde did not increase enzyme expression, suggesting that signaling occurs before the hydrolysis of fructose to three-carbon compounds. Impeding GLUT5 trafficking to the apical membrane using intestinal epithelial cell-specific Rab11a-KO mice impaired fructose-induced upregulation. KHK expression was uniformly distributed along the villus but was localized mainly in the basal region of the cytosol of enterocytes. The feedforward upregulation of fructolytic and gluconeogenic enzymes specifically requires GLUT5 and KHK and may proactively enhance the intestine's ability to process anticipated increases in dietary fructose concentrations.

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.

Regulation of D-fructose transporter GLUT5 in the ileum of spontaneously hypertensive rats

Abnormalities in carbohydrate metabolism and the insulin resistance status have been associated with hypertension. We have previously described alterations in the sodium-coupled sugar absorption in an experimental model of hypertension; in the present work, we studied the regulation of the sodium-independent, GLUT5-facilitated D-fructose intestinal transport in this pathology. Spontaneously hypertensive rats (SHR) and their normotensive, genetic control Wistar-Kyoto (WKY) rats, were used. Kinetic studies, carried out in ileal brush-border membrane vesicles (BBMVs), revealed a significant reduction (P < 0.05) in the maximal rate of transport (Vmax) for D-fructose in SHR, which, on the other hand, showed unaltered values for the Michaelis constant (Km) and the diffusion constant (Kd). Immunoblotting analysis revealed the existence of lower (P< 0.05) levels of GLUT5 in apical membranes from SHR, this reduction being similar to that of Vmax. Similarly, Northern blot studies on the abundance of GLUT5 mRNA from ileal enterocytes showed a decrease (P< 0.05) in hypertensive rats, following the same pattern mentioned above. Therefore, the impaired D-fructose intestinal absorption is another feature of SHR, and this decrease in D-fructose uptake correlates with a reduction in the abundance of the apical GLUT5 transporter, which is controlled at a transcriptional level.

Signaling cascade of ANG II-induced inhibition of alpha-MG uptake in renal proximal tubule cells

ANG II and Na+-glucose cotransporter have been reported to be associated with the onset of diverse renal diseases. However, the effect of ANG II on Na+-glucose cotransporter activity was not elucidated. The effects of ANG II on alpha-methyl-D-[14C]glucopyranoside (alpha-MG) uptake and its related signal pathways were examined in the primary cultured rabbit renal proximal tubule cells (PTCs). ANG II (>2 h; >10(-9) M) inhibited alpha-MG uptake in a time- and concentration-dependent manner and decreased the protein level of Na+-glucose cotransporters, the expression of which was abrogated by both actinomycin D and cycloheximide exposure. ANG II-induced inhibition of alpha-MG uptake was blocked by losartan, an ANG II type 1 (AT1) receptor blocker, but not by PD-123319, an ANG II type 2 receptor blocker. ANG II-induced inhibition of alpha-MG uptake was blocked by genistein, herbimycin A [tyrosine kinase (TK) inhibitors], mepacrine, and AACOCF3 (phospholipase A2 inhibitors), suggesting the role of TK phosphorylation and arachidonic acid (AA). Indeed, ANG II increased AA release, which was blocked by losartan or TK inhibitors. The effects of ANG II on AA release and alpha-MG uptake also were abolished by staurosporine and bisindolylmaleimide I (protein kinase C inhibitors) or PD-98059 (p44/42 MAPK inhibitor), but not SB-203580 (p38 MAPK inhibitor), respectively. Indeed, ANG II increased p44/42 MAPK activity. ANG II-induced activation of p44/42 MAPK was blocked by staurosporine. In conclusion, ANG II inhibited alpha-MG uptake via PKC-MAPK-cPLA2 signal cascade through the AT1 receptor in the PTCs.

Rapid insertion of GLUT2 into the rat jejunal brush-border membrane promoted by glucagon-like peptide 2

A possible role for GLUT2 transiently expressed in the rat jejunal brush-border membrane (BBM) under the influence of glucagon-like peptide 2 (GLP-2) was investigated using in vivo perfusion of the intestinal lumen as well as isolation of membrane proteins and immunohistochemistry. A 1 h vascular infusion of GLP-2 in vivo doubled the rate of fructose absorption and this increase could be blocked by luminal phloretin. Immunohistochemistry of frozen sections of rat jejunum showed the expression of GLUT2 in both the basolateral and BBMs of mature enterocytes. Perfusion of the intestinal lumen with 50 mM D-glucose or vascular infusion of 800 pM GLP-2 for 1 h increased the expression of GLUT2 in the BBM. Quantification of these changes using Western blotting of biotinylated surface-exposed protein showed a doubling of the expression of GLUT2 in the BBM, but the effects of glucose and GLP-2 were not additive. These results indicate that vascular GLP-2 can promote the insertion of GLUT2 into the rat jejunal BBM providing a low-affinity/high-capacity route of entry for dietary hexoses.

Cloning and functional characterization of the mouse fructose transporter, GLUT5

Mouse GLUT5 cDNA and a 7.7-kb genomic fragment have been isolated and characterized. The cDNA sequence suggests mouse GLUT5 is composed of 501 amino acids, and has 69-88% amino acid identity with human, rat, and rabbit GLUT5. Expression of mouse GLUT5 cRNA in Xenopus laevis oocytes showed that GLUT5 mediated fructose transport, with a K(t) of 13 mM. Northern blot studies detected GLUT5 mRNA expression in mouse small intestine, kidney, and testis, with transcript sizes of approximately 2.1, 2.1, and 2.8 kb, respectively. 5'Rapid Amplification of cDNA Ends (5'RACE) determined that the differences in transcript sizes occurred because GLUT5 possessed alternative transcriptional initiation sites in somatic and germ cells. In agreement with studies in rats and rabbits, mouse small intestinal GLUT5 mRNA expression levels were increased following exposure to a 65% fructose-enriched diet. In addition, developmental studies showed a significant increase in GLUT5 mRNA expression levels in adult mouse testis when compared to prepubertal mouse testis. To begin to identify the cis-acting domains responsible for GLUT5 expression characteristics, a 7.7-kb GLUT5 genomic fragment was isolated from a mouse lambda fix11 library and sequenced. The clone contained exons 1-4 and 5' flanking regions. Moreover, caudal homeobox gene (CdxA), upstream stimulatory factor (USF), and sex-determining region of Y (SRY) binding sites were identified in the 5' flanking region that may be responsible for GLUT5's expression characteristics: tissue distribution, sensitivity to dietary fructose in the small intestine, and developmental expression in the testis.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Stimulation of fructose transport across the intestinal brush-border membrane by PMA is mediated by GLUT2 and dynamically regulated by protein kinase C

Perfusion of rat jejunum in vitro with PMA increased fructose transport by 70% compared with control values and was blocked by the protein kinase C (PKC) inhibitor chelerythrine. The brush-border membrane contained both the fructose transporters GLUT5 and GLUT2; the presence of the latter was confirmed by luminal biotinylation. PMA increased the GLUT2 level 4-fold within minutes, so that the level was comparable with that of the basolateral membrane, but had no effect on GLUT5 level. GLUT2 was functional, accessible to luminal fructose and could be inhibited selectively by phloretin to permit determination of GLUT2- and GLUT5-mediated transport components. The 4-fold increase in GLUT2 level induced by PMA was matched by a 4-fold increase in GLUT2-mediated transport: there was a compensatory fall in the GLUT5-mediated rate. The pattern of dynamic trafficking was seen only for GLUT2, not GLUT5 or SGLT1, implying that GLUT2 trafficks to the brush-border membrane by a different pathway. Trafficking of GLUT2 to the brush-border membrane correlated with activation of PKC betaII, implying that this isoenzyme is likely to control trafficking. Since PKC is activated by endogenous hormones, GLUT2 levels in vivo are 3-4-fold those in vitro; moreover, because PKC is inactivated as soon as intestine is excised, GLUT2 is lost from the brush-border within minutes in vitro. It is therefore difficult to detect GLUT2 in most in vitro preparations and its role in intestinal sugar absorption across the brush-border membrane has accordingly been overlooked.

Regulation of GLUT5, GLUT2 and intestinal brush-border fructose absorption by the extracellular signal-regulated kinase, p38 mitogen-activated kinase and phosphatidylinositol 3-kinase intracellular signalling pathways: implications for adaptation to diabetes

We have investigated the role of the extracellular signal-regulated kinase (ERK), p38 and phosphatidylinositol 3-kinase (PI 3-kinase) pathways in the regulation of intestinal fructose transport. Different combinations of anisomycin, PD98059 and wortmannin had very different effects on fructose transport in perfused isolated loops of rat jejunum. Transport was stimulated maximally by anisomycin (2 microM) and blocked by SB203580 (20 microM), confirming involvement of the p38 pathway. PD98059 (50 microM) alone had little effect on fructose transport. However, it had a dramatic effect on stimulation by anisomycin, diminishing the K(a) 50-fold from 1 microM to 20 nM to show that the ERK pathway restrains the p38 pathway. The K(a) for diabetic jejunum was 30 nM and PD98059 had no effect. Transport in the presence of anisomycin was 3.4-fold that for anisomycin plus PD98059 plus wortmannin. Transport was mediated by both GLUT5 and GLUT2. In general, GLUT2 levels increased up to 4-fold within minutes and with only minimal changes in GLUT5 or SGLT1 levels, demonstrating that GLUT2 trafficks by a rapid trafficking pathway distinct from that of GLUT5 and SGLT1. GLUT2 intrinsic activity was regulated over a 9-fold range. It is concluded that there is extensive cross-talk between the ERK, p38 and PI 3-kinase pathways in their control of brush-border fructose transport by modulation of both the levels and intrinsic activities of GLUT5 and GLUT2. The potential of the intracellular signalling pathways to regulate short-term nutrient transport during the assimilation of a meal and longer-term adaptation to diabetes and hyperglycaemia is discussed.

The small intestinal fructose transporters: site of dietary perception and evidence for diurnal and fructose sensitive control elements

To obtain an insight into the mechanisms responsible for GLUT5 diurnality and fructose responsiveness, rats were gavaged at 9:00 AM or 6:00 PM with 1 g of fructose in the presence or absence of cycloheximide. After 4 h of fructose exposure, GLUT5 mRNA and protein levels increased 2-3.5-fold above the natural diurnal levels of expression. In situ hybridization and immunochemical analysis of GLUT5 mRNA and protein demonstrated that both diurnality and fructose responsiveness was confined to mature enterocytes. The protein synthesis inhibitor, cycloheximide, blunted the diurnal and fructose driven increase in GLUT5 mRNA expression in the morning, but had minimal effect on the pattern of expression in the evening. This differential sensitivity of intestinal GLUT5 mRNA to de novo protein synthesis may reflect the increasing presence of diurnal and fructose sensitive control factors during the day. Following vehicle gavage, Cycloheximide was more effective in reducing GLUT5 protein expression levels in the morning when compared to the evening. These data suggest that the turnover of GLUT5 protein may be diurnally influenced.

Advances in nutrition and gastroenterology: summary of the 1997 A.S.P.E.N. Research Workshop

BACKGROUND

The 1997 A.S.P.E.N. Research Workshop was held at the annual meeting in San Francisco, on January 26, 1997. The workshop focused on advances in clinical and basic research involving the interface between nutrient and luminal gastroenterology.

METHODS

Presentations on the genetic regulation of gastrointestinal development, the molecular biology of small intestinal adaptation, the effect of nutrition support on intestinal mucosal mass, the relationship between nutrition and gastrointestinal motility, nutrient absorption, and gastrointestinal tract substrate metabolism were made by the preeminent leaders in the field.

RESULTS

The investigators presented an insightful analysis of each topic by reviewing data from their own laboratories and the published literature.

CONCLUSIONS

This workshop underscored the important interactions between nutrition and luminal gastroenterology at the basic science, metabolic/physiologic, and clinical levels. The integration of presentations from the different disciplines provided a unique interaction of information and ideas to advance our understanding of nutrition and gastrointestinal tract.

Responses of glucose absorption and fructose absorption to prostaglandin E2 in intestinal loops of sheep

This study was designed to clarify whether the anti-absorptive action of prostaglandin E2 (PGE2) on glucose absorption was specific or non-specific to the glucose absorptive process, by investigating its effect on fructose absorption which has a different mechanism from that of glucose absorption. The effect of PGE2 on mucus secretion was also evaluated as one of the non-specific inhibiting factors. PGE2 significantly stimulated the secretion of water and mucus, and the absorptions of glucose and fructose were inhibited 49.3 per cent and 31.1 per cent respectively at the highest dose. However, fructose absorption was not inhibited by lower doses of PGE2, although the secretion of fluid and mucus was stimulated significantly and glucose absorption was inhibited significantly at the lower doses.

The regulation of GLUT5 and GLUT2 activity in the adaptation of intestinal brush-border fructose transport in diabetes

The adaptation of d-fructose transport in rat jejunum to experimental diabetes has been studied. In vivo and in vitro perfusions of intact jejunum with d-fructose revealed the appearance of a phloretin-sensitive transporter in the brush-border membrane of streptozotocin-diabetic rats which was not detectable in normal rats. The nature of the transporters involved was investigated by Western blotting and by d-fructose transport studies using highly purified brush-border and basolateral membrane vesicles. GLUT5, the major transporter in the brush-border membrane of normal rats, was not inhibited by d-glucose or phloretin. In contrast, GLUT2, the major transporter in the basolateral membrane of normal rats, was strongly inhibited by both D-glucose and phloretin. In brush-border membrane vesicles from diabetic rats, GLUT5 levels were significantly enhanced; moreover the presence of GLUT2 was readily detectable and increased markedly as diabetes progressed. The differences in stereospecificity between GLUT2 and GLUT5 were used to show that both transporters contributed to the overall enhancement of d-fructose transport measured in brush-border membrane vesicles and in vitro isolated loops from diabetic rats. However, overall d-fructose uptake in vivo was diminished. The underlying mechanisms and functional consequences are discussed.

Characterization of the rabbit intestinal fructose transporter (GLUT5)

Recent studies suggest that the jejunal/kidney-type facilitative glucose transporter (GLUT5) functions as a high-affinity D-fructose transporter. However, its precise role in the small intestine is not clear. In an attempt to identify the fructose transporter in the small intestine, we measured fructose uptake in Xenopus oocytes expressing jejunal mRNA from five species (rat, mouse, rabbit, hamster and guinea-pig). Only jejunal mRNA from the rabbit significantly increased fructose uptake. We also cloned a rabbit GLUT5 cDNA from a jejunal library The predicted amino acid sequence of the 487-residue rabbit GLUT5 showed 72.3 and 67.1% identity with human and rat GLUT5 respectively. Northern-blot analysis revealed GLUT5 transcripts in rabbit duodenum, jejunum and, to a lesser extent, kidney. After separation of rabbit jejunal mRNA on a sucrose density gradient, the fractions that conferred D-fructose transport activity in oocytes also hybridized with rabbit GLUT5 cDNA. Hybrid depletion of jejunal mRNA with a GLUT5 antisense oligonucleotide markedly inhibited the mRNA-induced fructose uptake in oocytes. Immunoblot analysis indicated that GLUT5 (49 kDa) is located in the brush-border membrane of rabbit intestinal epithelial cells. Xenopus oocytes injected with rabbit GLUT5 cRNA exhibited fructose uptake activity with a Km of 11 mM for D-fructose. D-Fructose transport by GLUT5 was significantly inhibited by D-glucose and D-galactose. D-Fructose uptake in brush-border membrane vesicles shows a Km similar to that of GLUT5, but was not inhibited by D-glucose or D-galactose. Finally, cytochalasin B photolabelled a 49 kDa protein in rabbit brush-border-membrane preparations that was immunoprecipitated by antibodies to GLUT5. Our results suggest that GLUT5 functions as a fructose transporter in rabbit small intestine. However, biochemical properties of fructose transport in Xenopus oocytes injected with GLUT5 cRNA differed from those in rabbit jejunal vesicles.

GLUT2 is the transporter for fructose across the rat intestinal basolateral membrane

BACKGROUND

The exact roles of disaccharidases and GLUT5 in the brush border membrane and GLUT2 in the basolateral membrane in the absorption of fructose across the intestine have not been fully determined. This paper describes characterization of fructose transport across the jejunal basolateral membrane using isolated membrane vesicles.

METHODS

Transport of fructose was measured using rapid filtration of vesicles. Luminal perfusion in vivo with glucose and fructose before vesicle preparation was used to assess modulation of GLUT2 activity. Western blotting measured the abundance of GLUT2 in the membrane.

RESULTS

The maximal rate of transport for fructose was 1100 pmol/mg protein/s and the Michaelis constant was 16 mmol/L. Fructose and glucose could completely inhibit the transport of each other. Perfusion of the intestinal lumen with fructose or glucose saline for 4 hours produced a fourfold increase in maximal fructose transport.

CONCLUSIONS

These data indicate that the one transport protein, GLUT2, is responsible for moving both fructose and glucose out of the enterocyte across the basolateral membrane under basal conditions. The activity of this, or a closely related carrier, is rapidly upregulated by the presence of hexoses in the intestinal lumen, explaining the potentiation of fructose absorption by luminal glucose and obviating any need to involve apical disaccharidases.

Fructose and related food carbohydrates. Sources, intake, absorption, and clinical implications

It is possible to point out subjects consuming considerable quantities of fructose and sorbitol, and the intake seems to be increasing both from added and natural sources. Studies of the absorption of fructose in animals are inconsistent, and the mechanisms of fructose uptake seem to vary in accordance with the species. In most species fructose absorption takes place by a specific carrier (facilitated transport), but it may be active in the rat. In vitro studies of human intestine are very scarce; there is no evidence of active intestinal fructose transport in the human intestine. By means of hydrogen breath tests, a very low absorption capacity for fructose given as the free monosaccharide has been found in humans. Fructose given as sucrose or in equimolar combinations with glucose is well absorbed, and only fructose in excess of glucose is malabsorbed. On this basis it is hypothesized that two different uptake mechanisms for fructose are present in the human intestine. One of these may be a disaccharidase-related uptake system. Sorbitol ingestion may aggravate malabsorption of fructose given as the monosaccharide; it is not known whether a specific mechanism is involved. In children and adults with functional bowel distress the absorption capacities for fructose may not differ from those of healthy individuals, but malabsorption of fructose and/or sorbitol may be the cause of or aggravate abdominal symptoms. Fructose polymers (fructans) are also subject to increasing nutritional interest. Fructans are not absorbed in the small intestine but are strongly fermented in the large bowel. Fructans may be of potential benefit for large-bowel function and blood glucose regulation.