How does the intestinal pH affect absorption?

The intestinal pH affects absorption by determining the ionisation state of substances, influencing their solubility and permeability.

The pH level in the intestines plays a crucial role in the absorption of nutrients and drugs. This is because the pH level can affect the ionisation state of substances, which in turn influences their solubility and permeability. The ionisation state refers to whether a substance is in its ionised or non-ionised form. In general, non-ionised substances are more readily absorbed because they are more lipophilic (fat-soluble) and can easily pass through the lipid-rich cell membranes of the intestinal lining.

The pH of the intestines varies along its length, with the stomach being highly acidic (pH 1-2), the small intestine slightly alkaline (pH 7-8), and the large intestine mildly acidic to neutral (pH 5-7). This pH gradient allows for the optimal absorption of different substances at different points in the digestive tract. For instance, weak acids, which are non-ionised and more absorbable in an acidic environment, are primarily absorbed in the stomach. On the other hand, weak bases, which are non-ionised and more absorbable in an alkaline environment, are primarily absorbed in the small intestine.

Moreover, the pH level can also affect the stability of substances. Some drugs and nutrients are sensitive to pH and can degrade in an unfavourable pH environment, reducing their bioavailability. For example, certain medications are coated with an enteric coating that resists dissolution in the acidic stomach but dissolves in the alkaline small intestine, ensuring that the drug is released and absorbed at the right location.

In conclusion, the intestinal pH is a key factor in absorption, affecting the ionisation, solubility, permeability, and stability of substances. Understanding this can help in the design of drug delivery systems and in the management of nutrient absorption disorders.