How food and pH affect the action of drugs in the body

Medications can have powerful, beneficial, harmful and unusual effects on our bodies. This whole area is called pharmacodynamics. Pharmacokinetics are often divided into:

Absorption - how the drug enters the body

Distribution - where does the drug go in the body

Metabolism - how the drug is chemically broken down

Elimination - how the drug leaves the body

Most drugs are broken down and broken down by liver enzymes, through processes that biochemists refer to as metabolism in the test tube, these chemical reactions are strongly affected by acidity. But because they occur in the blood, acid-alkaline balance usually has a secondary role in the metabolism of most medications. Absorption describes how quickly and comprehensively a drug enters the body, and elimination describes how quickly and effectively the body gets rid of the drug, but metabolism plays a key role in how long a drug stays at potential therapeutic levels in the blood.

Hydrogen has a lot of potential

The 14-point pH scale is logarithmic and confusing.                                                                                                                                                       The strongest acids are actually represented by the smallest numbers. The strongest acid has a pH of 1, 7 neutral, and numbers greater than 7 represent alkalis. The typical pH for different bodily fluids is:

Blood plasma 7.3-7.45

Cerebrospinal fluid 7.30-7.36

Stomach 1-3 (too acidic)

Small intestine 4-7 (mild to neutral acidic)

Urine 5.8-7.5 (mild acidic to slightly alkaline)

Carbon dioxide levels in the blood affect pH, with higher levels making the blood more acidic. Excess carbon dioxide can be eliminated by rapid breathing, which shows that body systems other than the digestive system help regulate the acid-alkaline balance in the body. The pH of the contents of the gastrointestinal tract and urinary system shows much greater difference than in the blood. Eating acidic foods, especially orange juice and other citrus products, soft drinks, and vitamin C supplements, can reduce the pH and increase the acidity of stomach contents. High-protein diets can also increase the pH of the stomach to some extent. Foods that are able to significantly reduce stomach acidity are very few but many antacid medications can. Histamine 2 inhibitors (Pepcid/famotidine, Tagamet/cimetidine, Zantac/ranitidine) inhibit the release of histamine into the stomach. Because histamine release stimulates stomach cells to produce and release acid, histamine 2 inhibitors reduce stomach acidity, usually for several hours after consumption. Proton pump inhibitors (Prilosec/omeprazole, Nexium/esomeprazole, Prevacid/lansoprazole) block acid formation and release by stomach cells. They tend to cause larger and longer changes in stomach pH compared to histamine blockers, and can reduce acidity by up to 97%, for most of the day.

Take a look at pKa

What chemists call the acid dissociation constant of a molecule, is the pH of the solution in which that molecule is found in a 50% neutral molecular form and 50% ionized forms. The pKa numbers are exactly the same as pH, where smaller numbers are more acidic. For example, the pKa value of methylphenidate (Ritalin, Concerta) is 9.5, making it mild to moderate alkaline. This means that in a neutral solution (pH 7), not much methylphenidate is ionized.  In an acidic environment, less methylphenidate will be ionized.

What does this have to do with absorption or excretion?

For absorption to occur, the molecule must travel from the contents of the digestive system, through the cell membrane, and the neutral molecules move more easily through the fats and proteins that make up the cell membranes. Charged particles struggle to cross cell membranes. However, in some cases, the cell can expend energy to pump ions from the digestive system into the cell. Moderate essential medications such as methylphenidate are absorbed more easily when in a solution close to pKa. If stomach contents become more acidic due to a diet rich in acids, non-ionizing methylphenidate is present less frequently, and is absorbed less. On the other hand, due to how the kidneys work, the most important factor for elimination is not ionization and crossing of fatty membranes.  Instead, what matters is how ionized the molecule in the blood is relative to urine, which determines how easily it spreads from one solution to another. Increasing the acidity of urine by diet increases the rate of excretion of a molecule such as methylphenidate. Acidity can therefore be a double blow to many drugs: they are absorbed less, but are excreted more quickly, both of which contribute to lower levels of the drug in the blood.

But what happens in real life?

Why don't everyone's drug levels suddenly change due to a high-acid diet?  For one thing, the average pH of the stomach is already far from the pKa of most drugs, so that very little absorption occurs there For absorption, the surface area is usually a factor greater than acidity. Most drug absorption occurs in the small intestine, and even if the small intestine begins more acidic than normal, there will be enough space for the drug to be absorbed later in the intestinal tract, where the pH is closer to the pKa of the drug. The properties of the drug also affect absorption. Slow-release or long-acting drugs are often either wrapped in a casing or locked in a chemical matrix, which takes a long time to dissolve. This makes them less susceptible to changes in stomach acidity, however, some sophisticated coatings, such as Mydayis, a slow-release form of amphetamine, are designed to dissolve only at a certain pH. This may make them more sensitive than other slow-release tablets to changes in stomach acidity.