Absorption Of Carbohydrates Introduction

Carbohydrates are essential macronutrients that provide energy for the human body. After ingestion, carbohydrates undergo digestion, breaking down complex molecules into simpler monosaccharides, which are then absorbed in the small intestine. The absorption process is complex, involving various mechanisms, transport proteins, and enzymes. Understanding the biochemical processes of carbohydrate absorption is crucial for nurses to comprehend patients’ nutritional needs, metabolic functions, and possible malabsorption disorders.

Absorption of Carbohydrates

Carbohydrate digestion is essentially completed in the small intestine, where all complex dietary carbohydrates, such as starch and glycogen, along with disaccharides, are ultimately broken down into simpler monosaccharides. The small intestine is the primary site for absorbing these monosaccharides, the final products of carbohydrate digestion.

Rate of Absorption in the Small Intestine

The absorption of monosaccharides in the small intestine is not uniform; it diminishes from the proximal jejunum to the distal ileum. The proximal jejunum shows an absorption rate three times greater than that of the distal ileum, highlighting the region’s critical role in carbohydrate absorption.

Absorption of Disaccharides

Interestingly, some disaccharides that escape digestion can still enter the cells lining the intestinal lumen through a process known as pinocytosis. Within these cells, they are hydrolyzed. However, carbohydrates higher than monosaccharides cannot be directly absorbed into the bloodstream under normal health conditions. If these larger molecules are administered parenterally, they are eliminated as foreign bodies.

Cori Theory About the Rate of Absorption for Different Carbohydrates

The rate of absorption of different carbohydrates varies, as studied by Cori in experiments on rats. Using glucose absorption as the baseline (100), other sugars demonstrated the following comparative rates:

• Galactose (110) > Glucose (100) > Fructose (43) > Mannose (19) > Xylose (15) > Arabinose (9)

These findings illustrate that glucose and galactose are absorbed rapidly, while fructose and mannose are absorbed at intermediate rates, and pentoses are absorbed slowly. Notably, galactose is absorbed more quickly than glucose.

Mechanisms of Absorption

There are two primary mechanisms for carbohydrate absorption:

1. Simple Diffusion: This process depends on sugar concentration gradients between the intestinal lumen, mucosal cells, and blood plasma. All monosaccharides are absorbed to some extent by passive diffusion.
2. Active Transport Mechanisms:
   • Glucose and Galactose: Both are absorbed very rapidly, suggesting an active transport mechanism that requires energy.
   • Fructose: While its absorption is faster than pentoses, it is slower compared to glucose and galactose. Fructose is absorbed through facilitated transport, which involves specific transport mechanisms beyond simple diffusion.

Wilson and Crane’s Hypothesis of Active Transport

Wilson and Crane proposed that sugars transported actively share common chemical features. For a sugar to be actively transported, it must:

1. Have a six-membered ring.
2. Have one or more carbon atoms attached to C5.
3. Possess an –OH group at C2 with the same stereochemical configuration as in D-glucose. An –OH group and a hydroxymethyl or methyl group on the pyranose ring are essential structural requirements for active transport.

Role of Energy: The energy required for active transport is provided by ATP. The interaction between the sodium-dependent sugar carrier and the sodium pumps facilitates sugar concentration within the cell without any back leakage into the lumen. Sodium binding by the carrier protein is a prerequisite for glucose binding, altering the protein’s conformation and enabling glucose absorption. An analogous carrier protein likely exists for D-galactose, suggesting a cotransport system.

Evidences for Cotransport System of Glucose Absorption

Several pieces of evidence support the cotransport system for glucose absorption:

• The dependence of glucose’s active transport on sodium ions has been demonstrated by replacing sodium with potassium or lithium in the bathing fluid of isolated rat intestine loops, significantly reducing or halting glucose transport.
• Drugs like strophanthin and ouabain, which inhibit the sodium pump, also inhibit the active transport of sugars.
• Substances that prevent the liberation of metabolic energy, such as dinitrophenol (DNP), inhibit active sugar transport.
• Phloridzin, a glycoside, likely inhibits glucose transport by displacing sodium from its binding site, preventing glucose from being bound and transported.

Absorption of Other Sugars

• Sugars like D-fructose and D-mannose are absorbed via facilitated transport, which requires a carrier protein but does not require energy.
• Sugars such as pentoses and L-isomers of glucose and galactose are absorbed passively through simple diffusion.

Facilitated Transport vs. Active Transport

While both facilitated and active transport share similarities, such as involving carrier proteins, specificity, and saturable carriers, they also have key differences:

• Facilitated Transport: Can act bidirectionally and does not require energy.
• Active Transport: Always occurs against an electrical or chemical gradient and requires energy.

Mechanism of Facilitated Transport: Ping-Pong Mechanism

Facilitated transport is explained by the “Ping-Pong” mechanism:

• Carrier Protein Conformations: The carrier protein exists in two primary conformations, depending on solute concentration: Pong state and Ping state.
• In the Pong state, the carrier is exposed to high solute concentrations, allowing solute molecules to bind to specific sites on the carrier protein within the cell’s lipid bilayer.
• A conformational change to the Ping state discharges the solute on the side favoring new equilibrium.
• The empty carrier protein reverts to the Pong state, completing the cycle.

Factors Determining Facilitated Transport:

The rate at which solutes enter a cell through facilitated transport is influenced by:

• Concentration gradient across the membrane.
• The amount of available carrier protein.
• The rapidity of solute-carrier interaction.
• The speed of conformational changes between the Ping and Pong states.

Factors Influencing the Rate of Absorption

1. State of the Mucous Membrane and Contact Time:
   • Unhealthy mucous membranes hinder absorption. Similarly, a shorter contact time due to hurried bowel movements reduces absorption.
2. Hormones:
   • Thyroid Hormones: Increase absorption of hexoses by acting directly on the intestinal mucosa.
   • Adrenal Cortex: Absorption decreases in cases of adrenal cortical deficiency due to reduced sodium concentration in body fluids.
   • Anterior Pituitary: Influences absorption through its effect on thyroid function. Hyperpituitarism can induce thyroid overactivity.
   • Insulin: Has no impact on glucose absorption.
3. Vitamins:
   • Absorption decreases with deficiencies in B-vitamins, including thiamine, pyridoxine, and pantothenic acid.
4. Inherited Enzyme Deficiencies:
   • Deficiencies in enzymes such as sucrase and lactase can interfere with the hydrolysis and absorption of the respective disaccharides.

Clinical Aspects: Defects in Carbohydrate Digestion and Absorption

1. Lactase Deficiency:
   • Infants with lactase deficiency exhibit lactose intolerance. Symptoms include diarrhea, flatulence, abdominal cramps, and distension due to unhydrolyzed lactose accumulation and subsequent bacterial fermentation in the intestine.
2. Types of Lactase Deficiency:
   • Inherited Lactase Deficiency: A rare disorder manifesting soon after birth with symptoms like diarrhea, lactosuria, and electrolyte disturbances. Treatment involves a lactose-free diet.
   • Primary Low Lactase Activity: A common syndrome, especially among non-white populations, characterized by a gradual decline in lactase activity later in life.
   • Secondary Low Lactase Deficiency: Occurs due to gastrointestinal conditions like tropical and non-tropical sprue, kwashiorkor, colitis, or chronic gastroenteritis.
3. Sucrase Deficiency:
   • Inherited deficiency of sucrase and isomaltase manifests with symptoms similar to lactase deficiency following ingestion of sucrose-containing foods.
4. Disacchariduria:
   • An increase in the excretion of disaccharides, observed in patients with disaccharidase deficiency or intestinal damage (e.g., sprue, celiac disease).
5. Monosaccharide Malabsorption:
   • Inherited disorders with slow absorption of glucose and/or galactose, possibly due to a lack of necessary carrier proteins for their absorption.

Conclusion

Carbohydrate absorption in the small intestine is a multifaceted process involving both passive and active transport mechanisms. Various factors, including hormones, enzymes, and inherited conditions, influence the rate and efficiency of absorption. Understanding these processes is crucial for nurses to effectively manage and address patients’ nutritional needs and related disorders.