They can be produced from an incomplete metabolism of carbohydrates, fats, and proteins.
Humans produce about 1–1.5 mmoles of H+ per kilogram per day.
[1] Most nonvolatile acids are excreted by the kidneys.
Lactic acid is usually completely metabolized by the body, and is thus not excreted from the body.
The following reactions result in nonvolatile acids: