Stomach Acid, Absorption And Why Some Nutrients Slip

Absorption of most nutrients holds up well with age. A small number depend on stomach acid, and those are the ones affected when acid production falls.

What stomach acid does beyond digestion

Acid denatures proteins and activates digestive enzymes, but it also releases certain minerals from the food matrix into a form the intestine can absorb.

It provides a barrier against swallowed bacteria, limiting how many survive into the small intestine where they would compete for nutrients.

Stomach cells also produce intrinsic factor, a protein required for vitamin B12 to be absorbed at the far end of the small intestine.

Why acid production falls

Ageing alone reduces acid output less than was once believed. The larger contributor is atrophic gastritis, where the acid-producing lining is progressively lost.

Long-standing infection with the bacterium associated with ulcers is a common cause, and an autoimmune form destroys the same cells through a different route.

Acid-suppressing medication produces the same functional situation deliberately, and these drugs are frequently continued for years beyond the original indication.

The nutrients that depend on it

Vitamin B12 requires both acid to release it from food protein and intrinsic factor to carry it, so low acid impairs the first step even when intrinsic factor is present.

Iron in plant foods is absorbed considerably better in an acidic environment, so reduced acid disproportionately affects people eating little meat.

Calcium absorption from some forms and magnesium absorption are also affected, though the practical significance varies and is debated.

Why the deficiency takes years to show

The liver holds a substantial store of vitamin B12, which is why a problem with absorption can take several years to produce symptoms.

Early features are non-specific: fatigue, low mood, unsteadiness or tingling, all of which are readily attributed to ageing or to another condition.

Neurological effects can appear before any change in the blood count, so a normal full blood count does not rule the problem out.

How this is handled clinically

Where deficiency is suspected, the assessment involves specific blood tests, sometimes including markers that reflect cellular availability rather than blood levels alone.

Where acid-suppressing medication is involved, the question is usually whether it is still needed, which is a periodic review rather than an automatic stop.

None of this supports self-treating with supplements, since correcting one deficiency without identifying the cause can mask the very signals used to diagnose it.