If a person does not consume alcohol where would the alcohol (that this enzyme breaks down) come from?

If a person does not consume alcohol, the enzyme that typically breaks down alcohol, called alcohol dehydrogenase, would still be present in their body. However, in the absence of ingested alcohol, this enzyme would not have its typical substrate to act upon.

That being said, the body does produce small amounts of alcohol (ethanol) naturally through various processes. One such source is the endogenous production through the normal digestive process. Bacteria in the gut can ferment carbohydrates to produce ethanol as a byproduct. Another source is from the metabolism of certain biological substances, such as sugars and fats, although this would be in very small amounts compared to what would be produced through fermentation in the gut or through ingesting alcoholic beverages.

In summary, if a person does not consume alcohol, the body can still produce small amounts of it through other internal biological processes