figure 4: hydrolysis of sucrose glucose , fructose sucrase
figure 5: intestinal sugar transport proteins
fructose exists in foods either monosaccharide (free fructose) or unit of disaccharide (sucrose). free fructose absorbed directly intestine. when fructose consumed in form of sucrose, digested (broken down) , absorbed free fructose. sucrose comes contact membrane of small intestine, enzyme sucrase catalyzes cleavage of sucrose yield 1 glucose unit , 1 fructose unit, each absorbed. after absorption, enters hepatic portal vein , directed toward liver.
the mechanism of fructose absorption in small intestine not understood. evidence suggests active transport, because fructose uptake has been shown occur against concentration gradient. however, majority of research supports claim fructose absorption occurs on mucosal membrane via facilitated transport involving glut5 transport proteins. since concentration of fructose higher in lumen, fructose able flow down concentration gradient enterocytes, assisted transport proteins. fructose may transported out of enterocyte across basolateral membrane either glut2 or glut5, although glut2 transporter has greater capacity transporting fructose, and, therefore, majority of fructose transported out of enterocyte through glut2.
capacity , rate of absorption
the absorption capacity fructose in monosaccharide form ranges less 5 g 50 g (per individual serving) , adapts changes in dietary fructose intake. studies show greatest absorption rate occurs when glucose , fructose administered in equal quantities. when fructose ingested part of disaccharide sucrose, absorption capacity higher because fructose exists in 1:1 ratio glucose. appears glut5 transfer rate may saturated @ low levels, , absorption increased through joint absorption glucose. 1 proposed mechanism phenomenon glucose-dependent cotransport of fructose. in addition, fructose transfer activity increases dietary fructose intake. presence of fructose in lumen causes increased mrna transcription of glut5, leading increased transport proteins. high-fructose diets (>2.4 g/kg body wt) increase transport proteins within 3 days of intake.
malabsorption
several studies have measured intestinal absorption of fructose using hydrogen breath test. these studies indicate fructose not absorbed in small intestine. when fructose not absorbed in small intestine, transported large intestine, fermented colonic flora. hydrogen produced during fermentation process , dissolves blood of portal vein. hydrogen transported lungs, exchanged across lungs , measurable hydrogen breath test. colonic flora produces carbon dioxide, short-chain fatty acids, organic acids, , trace gases in presence of unabsorbed fructose. presence of gases , organic acids in large intestine causes gastrointestinal symptoms such bloating, diarrhea, flatulence, , gastrointestial pain exercise after consumption can exacerbate these symptoms decreasing transit time in small intestine, resulting in greater amount of fructose emptied large intestine.
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