The digestion of carbohydrates liberates oligosaccharides, which are further hydrolyzed to free mono- and disaccharides. The increase in blood glucose after a test dose of a carbohydrate compared with that after an equivalent amount of glucose (as glucose or from a reference starchy food) is known as the glycemic index. Glucose and galactose have an index of 1 (or 100%), as do lactose, maltose, isomaltose, and trehalose, which give rise to these monosaccharides on hydrolysis. Fructose and the sugar alcohols are absorbed less rapidly and have a lower glycemic index, as does sucrose. The glycemic index of starch varies between near 1 (or 100%) and near 0 as a result of variable rates of hydrolysis, and that of nonstarch polysaccharides is 0. Foods that have a low glycemic index are considered to be more beneficial since they cause less fluctuation in insulin secretion. Resistant starch and nonstarch polysaccharides provide substrates for bacterial fermentation in the large intestine, and the resultant butyrate and other short-chain fatty acids provide a significant source of fuel for intestinal enterocytes. There is evidence that butyrate also has antiproliferative activity, and so provides protection against colorectal cancer.

Amylases Catalyze the Hydrolysis of Starch

 The hydrolysis of starch is catalyzed by salivary and pancreatic amylases, which catalyze random hydrolysis of α(1 → 4) glycoside bonds, yielding dextrins, then a mixture of glucose, maltose, and maltotriose and small branched dextrins (from the branchpoints in amylopectin).

Disaccharidases Are Brush Border Enzymes

 The disaccharidases, maltase, sucrase-isomaltase (a bifunctional enzyme catalyzing hydrolysis of sucrose and isomaltose), lactase, and trehalase are located on the brush border of the intestinal mucosal cells, where the resultant monosaccharides and those directly ingested are absorbed. Congenital deficiency of lactase occurs rarely in infants, leading to lactose intolerance and failure to thrive when fed on breast milk or normal infant formula. Congenital deficiency of sucrase-isomaltase occurs among the Inuit, leading to sucrose intolerance, with persistent diarrhea and failure to thrive when the diet contains sucrose.

In most mammals, and most human beings, lactase activity begins to fall after weaning and is almost completely lost by late adolescence, leading to lactose intolerance. Lactose remains in the intestinal lumen, where it is a substrate for bacterial fermentation to lactate, resulting in abdominal dis comfort and diarrhea after consumption of relatively large amounts. In two population groups, people of north European origin and nomadic tribes of sub-Saharan Africa and Arabia, lactase persists after weaning and into adult life. Marine mammals secrete a high-fat milk that contains no carbohydrate, and their pups lack lactase.

There Are Two Separate Mechanisms for the Absorption of Monosaccharides in the Small Intestine

Glucose and galactose are absorbed by a sodium-dependent process. They are carried by the same transport protein (SGLT 1) and compete with each other for intestinal absorption (Figure 1). Other monosaccharides are absorbed by carrier-mediated diffusion. Because they are not actively transported, fructose and sugar alcohols are only absorbed down their concentration gradient, and after a moderately high intake, some may remain in the intestinal lumen, acting as a substrate for bacterial fermentation. Large intakes of fructose and sugar alcohols can lead to osmotic diarrhea.

Fig1. Transport of glucose, fructose, and galactose across the intestinal epithelium. The SGLT 1 transporter is coupled to the Na+-K+ pump, allowing glucose and galactose to be transported against their concentration gradients. The GLUT 5 Na+-independent facilitative transporter allows fructose, as well as glucose and galactose, to be transported down their concentration gradients. Exit from the cell for all sugars is via the GLUT 2 facilitative transporter.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تعلن تأجيل مؤتمر معالم الحياة الطيبة عند أهل البيت (عليهم السلام) الدولي الثاني

قسم العلاقات العامة يقيم أمسية رمضانية ثقافية لطلبة الأقسام الداخلية في جامعة بابل

قسم الشؤون الفكرية ينظم ختمة قرآنية رمضانية في دولة تشاد بإفريقيا

ضمن مشروع بيوت النور.. المجمع العلمي ينظّم أمسية قرآنية رمضانية في كربلاء

المزيد

الآخبار الصحية

الماء المملح.. فائدة حقيقية أم "صيحة زائفة"؟

عادة بسيطة تدعم الصحة وتحافظ على الوزن المثالي

الغذاء النباتي يقلل مخاطر السرطان.. مع استثناء مهم

الحليب قليل أم كامل الدسم.. أيهما أفضل؟

المزيد

الآخبار التكنلوجية

أنثروبيك تتحدى البنتاغون.. وترامب يأمر بوقف التعامل معها

ميزة جديدة من "إنستغرام" لحماية المراهقين من الانتحار

الأولى من نوعها.. "تقنية مذهلة" في هاتف سامسونغ الجديد

لماذا تشعر المرأة بالألم أكثر من الرجل؟.. دراسة تجيب

المزيد

مواضيع ذات صلة

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