Characterization of bovine glucose transporter 1 kinetics and substrate specificities in Xenopus oocytes

Glucose is an essential substrate for lactose synthesis and an important energy source in milk production. Glucose uptake in the mammary gland, therefore, plays a critical role in milk synthesis. Facilitative glucose transporters (GLUT) mediate glucose uptake in the mammary gland. Glucose transporter 1 (GLUT1) is the major facilitative glucose transporter expressed in the bovine mammary gland and has been shown to localize to the basolateral membrane of mammary epithelial cells. Glucose transporter 1 is, therefore, thought to play a major role in glucose uptake during lactation. The objective of this study was to determine the transport kinetic properties and substrate specificity of bovine GLUT1 using the Xenopus oocyte model. Bovine GLUT1 (bGLUT1) was expressed in Xenopus oocytes by microinjection of in vitro transcribed cRNA and was found to be localized to the plasma membrane, which resulted in increased glucose uptake. This bGLUT1-mediated glucose uptake was dramatically inhibited by specific facilitative glucose transport inhibitors, cytochalasin B, and phloretin. Kinetic analysis of bovine and human GLUT1 was conducted under zero-trans conditions using radio-labeled 2-deoxy-D-glucose and the principles of Michaelis-Menten kinetics. Bovine GLUT1 exhibited a Michaelis constant (K(m)) of 9.8 ± 3.0mM for 2-deoxy-d-glucose, similar to 11.7 ± 3.7 mM for human GLUT1. Transport by bGLUT1 was inhibited by mannose and galactose, but not fructose, indicating that bGLUT1 may also be able to transport mannose and galactose. Our data provides functional insight into the transport properties of bGLUT1 in taking up glucose across mammary epithelial cells for milk synthesis.     

Co-consumption of sugars or ethanol and glucose in a Saccharomyces cerevisiae strain deleted in the HXK2 gene

In previous studies it was shown that deletion of the HXK2 gene in Saccharomyces cerevisiae yields a strain that hardly produces ethanol and grows almost exclusively oxidatively in the presence of abundant glucose. This paper reports on physiological studies on the hxk2 deletion strain on mixtures of glucose/sucrose, glucose/galactose, glucose/maltose and glucose/ethanol in aerobic batch cultures. The hxk2 deletion strain co-consumed galactose and sucrose, together with glucose. In addition, co-consumption of glucose and ethanol was observed during the early exponential growth phase. In S.cerevisiae, co-consumption of ethanol and glucose (in the presence of abundant glucose) has never been reported before. The specific respiration rate of the hxk2 deletion strain growing on the glucose/ethanol mixture was 900 micromol.min(-1).(g protein)(-1), which is four to five times higher than that of the hxk2 deletion strain growing oxidatively on glucose, three times higher than its parent growing on ethanol (when respiration is fully derepressed) and is almost 10 times higher than its parent growing on glucose (when respiration is repressed). This indicates that the hxk2 deletion strain has a strongly enhanced oxidative capacity when grown on a mixture of glucose and ethanol.     

Tiliroside, a glycosidic flavonoid, inhibits carbohydrate digestion and glucose absorption in the gastrointestinal tract

Scope Recent studies have reported that tiliroside, a glycosidic flavonoid, possesses anti‐diabetic activities. In the present study, we investigated the effects of tiliroside on carbohydrate digestion and absorption in the gastrointestinal tract. Methods and results This study showed that tiliroside inhibits pancreatic α‐amylase (IC₅₀ = 0.28 mM) in vitro. Tiliroside was found as a noncompetitive inhibitor of α‐amylase with K_i values of 84.2 μM. In male ICR mice, the increase in postprandial plasma glucose levels was significantly suppressed in the tiliroside‐administered group. Tiliroside treatment also suppressed hyperinsulinemia after starch administration. Tiliroside administration inhibited the increase of plasma glucose levels in an oral glucose tolerance test, but not in an intraperitoneal glucose tolerance test. In human intestinal Caco‐2 cells, the addition of tiliroside caused a significant dose‐dependent inhibition of glucose uptake. The inhibitory effects of both sodium‐dependent glucose transporter 1 (SGLT1) and glucose transporter 2 (GLUT2) inhibitors (phlorizin and phloretin, respectively) on glucose uptake were significantly inhibited in the presence of tiliroside, suggesting that tiliroside inhibited glucose uptake mediated by both SGLT1 and GLUT2. Conclusion These findings indicate that the anti‐diabetic effects of tiliroside are at least partially mediated through inhibitory effects on carbohydrate digestion and glucose uptake in the gastrointestinal tract.     

Molecular cloning of CIF1, a yeast gene necessary for growth on glucose.

The cif1 mutation of Saccharomyces cerevisiae (Navon et al., Biochemistry 18, 4487-4499, 1979) causes inability to grow on glucose and absence of catabolite inactivation. We have cloned the CIF1 gene by complementation of function and located it in a 2.75 kb SphI-BstEII fragment situated at ca. 18 kb centromere distal of LYS2 and ca. 80 kb centromere proximal of TYR1 on chromosome II. Southern analysis demonstrated that CIF1 is present in a single copy in the yeast genome. Northern analysis revealed that the corresponding mRNA of 1.8 kb is more abundant in cells grown on galactose than in those grown on glucose. A protein of ca. 54 kDa was predicted from the open reading frame in the sequenced fragment. In strains carrying the cif1 mutation the intracellular concentration of ATP decreased immediately after addition of glucose while the intracellular concentration of cAMP did not increase. cAMP concentration increased in response to galactose or 2,4-dinitrophenol. Disruption of BCY1 or overexpression of CDC25 in a cif1 background did not restore growth on glucose, suggesting that the absence of cAMP signal is not the primary cause of lack of growth on glucose. Complementation tests showed that cif1 is not allelic to fdp1 although the two genes seem to be functionally related.     

Changes in the concentrations of glucose and galactose in the peripheral blood of sucking piglets

Changes in the concentrations of glucose and galactose were measured in the peripheral blood of ten piglets after they had ingested milk during a natural sucking. In addition, the mild stress associated with the experimental procedure was determined by sampling nine fasted piglets over a period of 9 to 12 min. During this period there was a significant increase in the concentration of glucose in the blood of the piglets but no change in the concentration of galactose. After milk ingestion during a natural sucking the concentrations of both glucose and galactose increased from 5.7 mM and 19 microM to reach peak values of 7.7 mM and 122 microM, respectively, by 30 to 35 min. The concentrations of glucose and galactose returned to initial values in 60-80 min and 80-100 min, respectively, after sucking. Since the change in the concentration of galactose in the peripheral blood was much lower than the change in the concentration of glucose, we conclude that galactose was rapidly removed by the livers of sucking piglets. However, after the ingestion of milk the percentage increase (from initial to peak values) in the concentration of galactose in the blood was much larger (650%) than the increase in the concentration of glucose (43%). Thus, we propose that the determination of galactose in the peripheral blood may provide a qualitative method for monitoring the digestion and absorption of milk lactose in sucking piglets.     

Tea catechins modulate the glucose transport system in 3T3-L1 adipocytes

In this study, we investigated the effects of tea catechins on the translocation of glucose transporter (GLUT) 4 in 3T3-L1 adipocytes. We found that the ethyl acetate fraction of green tea extract, containing abundant catechins, most decreased insulin-induced glucose uptake activity in 3T3-L1 cells. When the cells were treated with 50 μM catechins in the absence or presence of insulin for 30 min, nongallate-type catechins increased glucose uptake activity without insulin, whereas gallate-type catechins decreased insulin-induced glucose uptake activity. (-)-Epicatechin (EC) and (-)-epigallocatechin (EGC), nongallate-type catechins, increased glucose uptake activity in the dose- and time-dependent manner, whereas (-)-catechin 3-gallate (Cg) and (-)-epigallocatechin 3-gallate (EGCg), gallate-type catechins, decreased insulin-induced glucose uptake activity in the dose- and time-dependent manner. When the cells were treated with 50 μM catechins for 30 min, EC and EGC promoted GLUT4 translocation, whereas Cg and EGCg decreased the insulin-induced translocation in the cells. EC and EGC increased phosphorylation of PKCλ/ζ without phosphorylation of insulin receptor (IR) and Akt. Wortmannin and LY294002, inhibitors for phosphatidylinositol 3'-kinase (PI3K), decreased EC- and EGC-induced glucose uptake activity in the cells. Cg and EGCg decreased phosphorylation of PKCλ/ζ in the presence of insulin without affecting insulin-induced phosphorylation of IR, and Akt. Therefore, EC and EGC promote the translocation of GLUT4 through activation of PI3K, and Cg and EGCg inhibit insulin-induced translocation of GLUT4 by the insulin signaling pathway in 3T3-L1 cells.     

Stimulation of glucose uptake by Musa sp. (cv. elakki bale) flower and pseudostem extracts in Ehrlich ascites tumor cells

BACKGROUND: Glucose uptake study plays a major role in diabetes research. Impaired glucose uptake has been implicated in the development of hyperglycemia during diabetes. Banana plant is known for its anti‐diabetic properties and our earlier report revealed that banana flower and pseudostem of Musa sp. cv. elakki bale is beneficial during diabetes in rat models. The present study was designed to evaluate the potential effect of banana flower and pseudostem extracts on glucose uptake in Ehrlich ascites tumor (EAT) cells using 2‐[N‐(7‐nitrobenz‐2‐oxa‐1,3‐diazol‐4‐yl)amino]‐2‐deoxy‐D ‐glucose (2‐NBDG), a fluorescent analogue of 2‐deoxyglucose. RESULTS: Methanol and aqueous extracts of banana flower and pseudostem were more potent in promoting glucose uptake in EAT cells, in comparison to acetone and ethanol extracts. At 20 µg dosage, highest net glucose uptake was observed in aqueous extracts of banana flower (18.17 ± 0.43 nmol L^?1) and pseudostem (19.69 ± 0.41 nmol L^?1). Total polyphenol content was higher in methanol (9.031 ± 0.036 g kg^?1) and aqueous (6.862 ± 0.024 g kg^?1) extracts of banana flower compared to pseudostem, which were 0.442 ± 0.006 and 0.811 ± 0.011 g kg^?1, respectively. CONCLUSION: Banana flower and pseudostem extracts are able to promote glucose uptake into the cells, presumably through glucose transporters 1 and 3, which could be beneficial in diabetes. Glucose uptake is likely promoted by phenolic acids besides other bioactives. It can be hypothesized that consumption of nutraceutical‐rich extract of banana flower and pseudostem could replace some amount of insulin being taken for diabetes. Copyright © 2011 Society of Chemical Industry     

Expression and regulation of glucose transporters in the bovine mammary gland

Glucose is the primary precursor for the synthesis of lactose, which controls milk volume by maintaining the osmolarity of milk. Glucose uptake in the mammary gland plays a key role in milk production. Glucose transport across the plasma membranes of mammalian cells is carried out by 2 distinct processes: facilitative transport, mediated by a family of facilitative glucose transporters (GLUT); and sodium-dependent transport, mediated by the Na+/glucose cotransporters (SGLT). Transport kinetic studies indicate that glucose transport across the plasma membrane of the lactating bovine mammary epithelial cell has a K(m) value of 8.29 mM for 3-O-methyl-D-glucose and can be inhibited by both cytochalasin-B and phloretin, indicating a facilitative transport process. This is consistent with the observation that in the lactating bovine mammary gland, GLUT1 is the predominant glucose transporter. However, the bovine lactating mammary gland also expresses GLUT3, GLUT4, GLUT5, GLUT8, GLUT12, and sodium-dependent SGLT1 and SGLT2 at different levels. Studies of protein expression and cellular and subcellular localizations of these transporters are needed to address their physiological functions in the mammary gland. From late pregnancy to early lactation, expression of GLUT1, GLUT8, GLUT12, SGLT1, and SGLT2 mRNA increases from at least 5-fold to several hundred-fold, suggesting that these transporters may be regulated by lactogenic hormones and have roles in milk synthesis. The GLUT1 protein is detected in lactating mammary epithelial cells. Its expression level decreases from early to late lactation stages and becomes barely detectable in the nonlactating gland. Both GLUT1 mRNA and protein levels in the lactating mammary gland are not significantly affected by exogenous bovine growth hormone, and, in addition, GLUT1 mRNA does not appear to be affected by leptin.     

Short communication: Protein kinase C regulates glucose uptake and mRNA expression of glucose transporter (GLUT) 1 and GLUT8 in lactating bovine mammary epithelial cells

The aim of this study was to determine the role of protein kinase C (PKC) in regulating glucose uptake in lactating bovine mammary epithelial cells (BMEC). The BMEC were cultured and treated with different concentrations of phorbol 12-myristate 13-acetate (PMA;0, 10, 25, 50, 100, and 200 ng/mL), the classic activator of PKC, for 48 h. Compared with the cells with no PMA treatment, 50 and 100 ng of PMA/mL significantly stimulated the glucose uptake of the BMEC, whereas the glucose uptake by the cells treated with the lowest and the highest amounts of PMA (25 and 200 ng/mL, respectively) did not show a significant difference. Consistently, the mRNA expression of glucose transporter (GLUT) 1 and 8 showed a similar pattern of increase under the treatments of PMA. Furthermore, when the cells were pretreated with GF1090203X (0, 0.25, 0.5, 1, and 2 μM), an inhibitor of PKC, for 30 min before exposed to PMA (50 ng/mL), the PMA-induced glucose uptake and GLUT1 and GLUT8 expression were decreased by GF1090203X in a dose-dependent manner. These results demonstrate that PKC is involved in the regulation of glucose uptake by BMEC, and this function may work, at least partly, through upregulating the expression of GLUT1 and GLUT8.     

Short communication: change in plasma ghrelin in dairy cows following an intravenous glucose challenge

Ghrelin is an endogenous ligand of the growth hormone secretagogue receptor and a potent orexigenic (appetite-stimulating) agent in humans and rodents, but little is known about its effect in dairy cows. Ten multiparous dairy cows 35 d in milk were subjected to an i.v. glucose challenge (300 mg of d-glucose/kg of body weight). Before infusion and at regular intervals after infusion, plasma glucose, insulin, nonesterified fatty acids (NEFA), growth hormone, epinephrine, and ghrelin concentrations were monitored. Plasma insulin rose (27.2 mU/L at 10 min) and NEFA, epinephrine, and ghrelin declined (nadir = 0.22 mmol/L, 22.2 μg/L, and 272 μg/L at 31, 13, and 22 min, respectively) after the glucose infusion. Ghrelin declined for 22 min before returning to suprabasal levels at approximately 75 min postinfusion. Sequential changes of the hormones and metabolites suggested a glucose transporter, type 2- and glucose transporter, type 4-mediated disposal of glucose, and an insulin-mediated reduction in NEFA. Ghrelin and epinephrine declined after glucose infusion and before the insulin peak, but the effect of insulin as a controlling factor in the hyperglycemic reduction in these hormones cannot be discounted. The post-nadir surge in ghrelin may be regulated by the decline in circulating concentrations of glucose and NEFA (an energy-deficit signal). The profile of change in plasma ghrelin in lactating dairy cows after a glucose challenge was similar to that in monogastric animals.     

Insulin stimulates glucose uptake via a phosphatidylinositide 3-kinase-linked signaling pathway in bovine mammary epithelial cells

The aim of this study was to investigate the effects of insulin on glucose uptake in lactating bovine mammary epithelial cells (BMEC). Primary BMEC were cultured in Dulbecco’s modified Eagle’s medium/nutrient mixture F-12 and treated with different levels of insulin (0, 5, 50, and 500 ng/mL) for 48 h after a 24-h starvation without fetal calf serum. Compared with the control cells (0 ng of insulin/mL), cell proliferation was enhanced by insulin treatment at all tested levels. Insulin significantly increased glucose uptake at a concentration of 500 ng/mL. In addition, the protein synthesis inhibitor cycloheximide (0.5 mg/mL) counteracted the insulin-elevated glucose uptake, thereby suggesting that newly synthesized transporter protein might take part in the insulin-induced glucose uptake. Furthermore, pretreatment of the cells with SB203580, an inhibitor of p38 mitogen-activated protein kinase, did not influence the insulin-induced glucose uptake, but LY294002, a specific inhibitor of phosphatidylinositide 3-kinase, significantly reduced the insulin-stimulated glucose uptake. These results indicated that insulin-induced glucose uptake in BMEC may involve the phosphatidylinositide 3-kinase- but not mitogen-activated protein kinase-mediated signaling pathways.     

Effects of feeding whole cottonseed to lactating dairy cows on glucose and palmitate metabolism

Eight lactating Holstein dairy cows were fed corn silage-based diets with or without whole cottonseed at 18.5% of the dietary dry matter. At 42 days postpartum, a pulse injection of 100 mg glucose/kg body weight was given intravenously and plasma glucose concentration was monitored for 45 min. At 50 d postpartum, biopsies of adipose tissue and mammary tissue were taken and tissue slices were incubated in vitro with either uniform carbon-14 glucose or 1-carbon-14 palmitate. Basal concentration of plasma glucose was not affected by diet, although apparent distribution volume of glucose did seem to decrease due to feeding whole cottonseed. Feeding whole cottonseed decreased uptake of glucose and palmitate in both adipose tissue and mammary tissue and also decreased oxidation of glucose to carbon dioxide in both tissues. Palmitate oxidation was not affected by diet. Incorporation of carbon-14 from glucose into adipose tissue lipids was decreased in cows fed whole cottonseed. Results indicate that fat supplementation in the form of whole cottonseed may decrease palmitate incorporation and glucose utilization for glycerol and reducing equivalent synthesis in both adipose tissue and mammary tissue of lactating dairy cows.     

Maltose Transport by Brewer's Yeasts in Brewer's Wort

The kinetics of maltose transport by two industrial yeasts were studied. The ale and lager strain each showed both high and low affinity transport. For the lager strain, maltose transport was only weakly inhibited by maltotriose, sucrose and trehalose, suggesting that its dominant maltose transporter is the maltose‐specific type coded by MALx1 genes. For the ale strain, maltose transport was strongly inhibited by maltotriose, sucrose and trehalose, suggesting that its dominant maltose transporter may be the AGT1‐encoded type that also carries these sugars. Also glucose inhibited transport by the ale strain more than that by the lager strain. Instantaneous inhibition by ethanol at concentrations met in brewery fermentations was moderate (about 25% at 50 g ethanol · L^?1). The apparent Vmax for high affinity transport increased about 100‐fold between 0 and 30°C, whereas the Km (3 ± 1 mM) was constant. Standard activities of maltose transport and maltase were followed through pilot fermentations of 11–24°P worts. Rapid (20 s) measurements of the zero‐trans‐rate of maltose uptake were also made with each day's yeast (rapidly harvested and washed) in reaction mixtures containing the same day's wort labelled with tracer ¹⁴C‐maltose. Results suggested that maltose uptake is the dominant factor controlling the rate of maltose utilization in these wort fermentations.     

Nonlinear pharmacokinetics and conversion to glucose of intravenous sodium propionate in dairy cattle

Three doses of sodium propionate (.75, 1.5, and 3.0 mmol/kg) were administered intravenously to 6, 8, and 14 dairy cows. Using first order kinetic analysis, the apparent plasma half-life increased significantly with increasing propionate dose. The apparent increase of propionate half-life with increasing propionate dose was attributed to saturation of uptake and disposal mechanisms. Using the nonlinear mathematical model of Henri-Michaelis-Menten for propionate concentrations at 3.0 mmol/kg, propionate half-life was significantly shorter than that obtained with the first order kinetic model. The Michaelis constant was 4.0 mM, the maximal rate of concentration decrease was .55 mM/min, half-life was 4.8 min, and distribution volume was .37 L/kg. Plasma glucose concentrations increased following all doses of propionate. The maximal increase in glucose concentration occurred earliest for the lowest dose and latest for the highest dose and increased in magnitude with increasing propionate dose. The plasma glucose response to intravenous propionate has been suggested as a measure of liver function in ruminants. Of the three propionate doses tested, the 3.0 mmol/kg dose appeared to saturate the uptake and disposal mechanisms of healthy liver and should be the most satisfactory dose for observing the plasma glucose response to injected propionate.     

Relationship of portal-drained viscera and liver net flux of glucose, lactate, volatile fatty acids, and nitrogen metabolites to milk production in the ewe

The objectives of this study were, first, to determine the relationship between hepatic glucose release and milk production and, second, to determine the relationship between net hepatic uptake of gluconeogenic precursors and milk production. Nine multiparous ewes were individually penned and fed an alfalfa hay-based diet for ad libitum intake. Catheters were surgically placed in the portal vein, a branch of the hepatic vein, a mesenteric vein, and the abdominal aorta. Metabolite fluxes across the portal-drained viscera and liver were subsequently measured at 1, 3, 6, and 10 wk after parturition. Net hepatic glucose release, net hepatic lactate uptake, and net hepatic propionate uptake increased with increased milk production. Hepatic oxygen consumption increased with increased net hepatic glucose release. Net hepatic glucose release increased with increased hepatic propionate uptake and tended to increase with increases in metabolized amino acid and lactate uptakes. The observed increases in oxygen consumption by the portal-drained viscera with increased milk production were probably the result of increased nutrient flux. Increased hepatic oxygen consumption with increased milk production was probably due to increased glucose and urea synthesis.     

High-affinity glucose uptake in Saccharomyces cerevisiae is not dependent on the presence of glucose-phosphorylating enzymes

Glucose uptake in Saccharomyces cerevisiae is believed to consist of two kinetically distinguishable components, the affinity of which is modulated during growth on glucose. It has been reported that triple hexose-kinase deletion mutants do not exhibit high-affinity glucose uptake. This raises the question of whether and how high-affinity glucose uptake is related to the presence of glucose-phosphorylating enzymes. In this study the kinetics of glucose uptake in both wild-type cells and cells of hexose-kinase deletion mutants, grown on either glycerol or galactose, were determined using a rapid-uptake method. In wild-type cells glucose uptake measured over either 5 s or 200 ms exhibited high affinity. In contrast, in cells of hexose-kinase deletion mutants the apparent affinity of glucose uptake was dependent on the time scale during which uptake was measured. Measurements on the 5-s scale showed apparent low-affinity uptake whereas measurements on the 200-ms scale showed high-affinity uptake. The affinity and maximal rate of the latter were comparable to those in wild-type cells. Using a simple model for a symmetrical facilitator, it was possible to simulate the experimentally determined relation between apparent affinity and the time scale used. The results suggest that high-affinity glucose transport is not necessarily dependent on the presence of glucose-phosphorylating enzymes. Apparent low-affinity uptake kinetics can arise as a consequence of an insufficient rate of removal of intracellular free glucose by phosphorylation. This study underlines the need to differentiate between influences of the translocator and of metabolism on the apparent kinetics of sugar uptake in yeast.     

蛋白棒对健康人餐后血糖、糖耐量影响及其血糖生成指数研究

目的：探讨蛋白棒对健康人餐后血糖、糖耐量影响和血糖生成指数。方法：按WS/T652——2019对符合要求的12个健康志愿者进行了蛋白棒对健康人餐后血糖、糖耐量影响和血糖生成指数研究,包括3次独立试食试验,其中葡萄糖2次,蛋白棒1 次。结果：进食蛋白棒餐后2小时内血糖变化很小,餐后30 min血糖达到峰值5.8mmol/L,最低为餐后15 min的5.4mmol/L;餐后血糖应答曲线平坦,波幅仅0.4mmol/L;且餐后15 min、30 min、45 min、60 min血糖和IAUC均明显低于相应的葡萄糖餐后血糖和IAUC,而餐后 120 min 血糖则明显高于相应的葡萄糖餐后血糖,以上差异均有统计学意义(P＜0.01或P＜0.05)。蛋白棒的GI值为23。结论：蛋白棒GI值为23,低于55,属于低GI食物,具有明显降低健康人餐后血糖波动和糖耐量的作用,能很好维持餐后血糖稳定。     

Molecular mechanisms of the naringin low uptake by intestinal Caco-2 cells

Naringin, the main flavanone of grapefruit, was reported to display numerous biological effects: antioxidant, hypocholesteremic, anti-atherogenic and favoring drug absorption. Naringin absorption mechanisms were studied in Caco-2 cells (TC7 clone). We investigated the possible involvement of several membrane transporters implicated in polyphenolic compounds intestinal transport (sodium-dependent glucose transporter 1, monocarboxylate transporter, multidrug-associated resistance proteins 1 and 2, and P-glycoprotein). Naringin was poorly absorbed by Caco-2 cells, according to its low value of apparent permeability coefficient (P(app) = 8.1 +/- 0.9 x 10(-8) cm/s). In the presence of verapamil, a specific inhibitor of P-glycoprotein, cellular uptake was increased by almost threefold after 5 min, and P(app) was doubled after 30 min. Our results indicated the involvement of P-glycoprotein, an ATP-driven efflux pump, capable of transporting naringin from the Caco-2 cell to the apical side. This phenomenon could explain, at least in part, the low absorption of this flavanone at the upper intestinal level.