Effect of food and GI motility on absorption

Effect of GI tract contents on absorption

The presence of food masses in the digestive tract lumen has a multidirectional effect on drug pharmacokinetics. On the one hand, food creates a physical mechanical barrier, blocking full contact between drug molecules and the absorptive surface of the mucosa. In addition, certain nutrients can enhance peristalsis, shortening the time the drug remains in the optimal zone for absorption. On the other hand, the digestive process is accompanied by a powerful stimulation of blood flow in the intestinal wall, which, conversely, can intensify drug penetration into the systemic circulation.

The basic rule of pharmacokinetics states that food slows down the evacuation of gastric contents. As a result, the drug enters the small intestine—the primary site of absorption—later, which decreases the absorption rate of most drugs.

A significant exception to this rule is highly lipophilic compounds. Dietary fats act as a powerful stimulant for bile release. Bile acids ensure proper emulsification and dissolution of fat-soluble molecules. This is why the absorption of spironolactone (Spironolactonum), griseofulvin (Griseofulvinum), and fat-soluble vitamins increases significantly when taken with fatty meals.

Chemical and metabolic interactions

Food components can enter into direct physicochemical reactions with active drug molecules, altering their bioavailability. A classic example is the interaction of calcium ions, abundantly present in dairy products, with tetracycline antibiotics. This results in the formation of stable, insoluble chelate complexes that pass through the GI tract in transit without being absorbed. A similar precipitation effect is demonstrated by strong tea: tannins bind plant alkaloids such as papaverine (Papaverinum) and codeine (Codeinum).

Metabolic interaction (“Grapefruit juice effect”) A special place is occupied by clinically significant interactions at the level of enterocyte enzyme systems. Citrus fruits (grapefruit, pomelo, lime) contain specific substances—furanocoumarins (e.g., dihydroxybergamottin).

1. Furanocoumarins act as irreversible inhibitors of the intestinal cytochrome P450 isozyme — CYP3A4.
2. As a result, presystemic metabolism of substrate drugs directly in the intestinal wall is suppressed.
3. The bioavailability and plasma concentration of drugs (nifedipine — Nifedipinum, cyclosporine — Cyclosporinum, statins) rise sharply and uncontrollably.
4. The risk of severe toxic effects and overdose increases manifold, which is why these beverages are strictly contraindicated during such therapy.

Role of gastric and intestinal motility

The rate of gastric emptying is a critical factor determining how quickly a drug reaches the duodenum. Physiologically, liquids leave the stomach the fastest. Solid food is retained longer, with the rate of evacuation depending on its chemical composition (carbohydrates leave faster than proteins, and fats are evacuated the slowest). High caloric content, extreme food temperatures (very hot or icy), and high chyme acidity slow down this process. Large pieces of food, conversely, stimulate mechanoreceptors and accelerate emptying.

Pharmacological regulation of motility:

• Acceleration: Prokinetics (metoclopramide) and macrolides stimulate rapid delivery of chyme to the intestine. This leads to the rapid absorption of weak bases (propranolol).
• Slowdown: M-anticholinergics relax smooth muscle, causing a delayed onset of the therapeutic effect.

Peristalsis is of particular importance for slowly absorbed drugs, such as iron preparations or cardiac glycosides. When prokinetics are taken, chyme moves too quickly, the drug does not have time to be absorbed, and its bioavailability drops. Conversely, the inhibition of peristalsis by atropine (Atropinum) prolongs transit time, extends contact with the mucosa, and can lead to the complete absorption of even toxic doses of digoxin.

Microbiome and drug interactions at the absorption stage

The normal GI microflora actively participates in drug metabolism even before they enter the systemic circulation. A striking clinical example is digoxin (Digoxinum). In a healthy intestine, the bacterium Eubacterium lentum inactivates about 40% of the administered dose of this glycoside. If a patient is prescribed a course of antibiotics, the flora is suppressed, digoxin metabolism in the intestinal lumen ceases, and the absorption of the active fraction increases sharply. Given the narrow therapeutic window of digoxin, this is fraught with severe cardiac glycoside intoxication.

Drug-drug interactions:

• Adsorbents: Activated charcoal (Carbo activatus) non-specifically binds molecules to its surface, suppressing the absorption of any simultaneously administered drugs.
• Antacids: Aluminum, calcium, and magnesium preparations form non-absorbable chelates with fluoroquinolones. In addition, by increasing gastric juice pH, antacids impair the ionization and absorption of weak acids while simultaneously improving the absorption of weak bases.
• Transport systems: Blockade of P-glycoprotein (responsible for pumping xenobiotics back into the lumen) contributes to the abnormal accumulation of drugs.

To avoid conflicts at the absorption stage, an interval of at least 2 hours must be observed between the administration of interacting agents.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Effect of changes in gastric pH on the degree of ionization of weak acids and bases
• Role of the efflux transport system (P-glycoprotein) in the intestinal barrier function
• Mechanisms of absorption of fat-soluble vitamins and the role of micelle formation
• Changes in monoamine oxidase (MAO) substrate bioavailability during food interactions

Other topics in "General Pharmacology"

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