28.10
炭水化物の吸収
炭水化物は、体の主なエネルギー源となる必須の主要栄養素です。炭水化物の消化は口の中で始まり、唾液アミラーゼがデンプンなどの複合炭水化物を部分的に小さなオリゴ糖に分解します。この機械的および酵素的な活動により、炭水化物は消化管でさらに処理される準備が整います。
飲み込まれた後、部分的に消化された炭水化…
食事性デンプンなどの炭水化物の消化は、咀嚼と唾液アミラーゼの作用により口の中で始まります。
これらの炭水化物は、唾液アミラーゼによって部分的に消化されてオリゴ糖にされた後、糜糞に取り込まれ、十二指腸に渡されます。
ここで、膵臓アミラーゼは残りのデンプンをより小さなオリゴ糖に分解し、腸のブラシ境界酵素によって単糖に変換されます。
これらの単糖類(主にグルコース、フルクトース、ガラクトース)は、空腸と回腸の内側を覆う腸細胞による吸収の準備が整っています。
腸細胞の頂端では、グルコースとガラクトースはナトリウムイオンによる二次活性輸送によって入ります。
ナトリウム-グルコーストランスポーターは、1つのグルコース分子と2つのナトリウムイオンの結合部位を持っており、細胞内輸送のために占める必要があります。
ガラクトースも細胞への侵入に同じシンポーターを使用しますが、フルクトースは促進拡散を使用します。
腸細胞内に入ると、すべての単糖類は、促進拡散を介して基底外側膜を通って絨毛の毛細血管に輸送されます。
最終的に、毛細血管は単糖類を肝臓の門脈を介して肝臓に運びます。
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Q1: Where does carbohydrate digestion begin and what enzyme starts the process?
Carbohydrate digestion begins in the mouth with chewing and the action of salivary amylase, which partially breaks down dietary starch into oligosaccharides. This mechanical and enzymatic activity prepares carbohydrates for further processing. The partially digested carbohydrates are then incorporated into chyme and passed into the duodenum for continued breakdown.
Q2: What happens to carbohydrates when they enter the stomach?
When chyme enters the stomach, the acidic environment inactivates salivary amylase, halting carbohydrate digestion temporarily. This pause in enzymatic activity occurs until the partially digested carbohydrates move into the duodenum, where pancreatic amylase resumes the breakdown process in the alkaline environment.
Q3: How do brush border enzymes complete carbohydrate digestion?
Brush border enzymes—maltase, sucrase, and lactase—are located on the microvilli of enterocytes and catalyze the final breakdown of disaccharides into monosaccharides at the site of absorption. Maltase splits maltose into glucose, sucrase hydrolyzes sucrose into glucose and fructose, and lactase breaks down lactose into glucose and galactose. By the time carbohydrates reach the jejunum, they are almost entirely reduced to monosaccharides.
Q4: How do glucose and galactose enter enterocytes differently from fructose?
Glucose and galactose are absorbed via secondary active transport using sodium-glucose symporters at the apical surface of enterocytes, requiring two sodium ions and one glucose or galactose molecule to bind for cellular entry. Fructose, however, enters through facilitated diffusion mediated by GLUT5 transporters, which does not require energy and relies on a concentration gradient between the intestinal lumen and enterocyte cytoplasm.
Q5: What role does the sodium-potassium pump play in monosaccharide absorption?
The sodium-potassium pump at the basolateral membrane maintains the sodium gradient essential for secondary active transport of glucose and galactose. It expels sodium ions from the cell in exchange for potassium ions, creating the concentration difference that drives sodium-glucose symporters to function. This indirect support is critical for efficient glucose and galactose uptake at the apical membrane.
Q6: How are absorbed monosaccharides transported from enterocytes into the bloodstream?
Once inside enterocytes, all monosaccharides are transported across the basolateral membrane into capillaries of the villi via GLUT2 transporters using facilitated diffusion. This process does not require additional energy and ensures rapid transfer into the bloodstream. The capillaries then carry monosaccharides via the hepatic portal vein to the liver for metabolism, storage, or distribution to peripheral tissues.
Q7: How is the small intestine structurally adapted for efficient carbohydrate absorption?
The small intestine maximizes carbohydrate absorption through villi and microvilli that significantly increase surface area for nutrient uptake. A dense capillary network within villi enables swift transport of absorbed nutrients into the bloodstream. Brush border enzymes are strategically positioned on microvilli to immediately hydrolyze disaccharides into monosaccharides at the site of absorption, minimizing nutrient loss and supporting metabolic states of the body the absorptive state.