NOTE: Two concepts: acidic food and acid-forming food are often confused. Urine pH is affected by acid-forming food and not by acidic food. (read more..)
The fact that acid-ash foods or acid-forming foods make urine more acidic is well-established by scientific studies. (1)
Unlike blood pH, the normal values of pH in urine range widely between 4.6pH and 8.0pH. However, it may differ slightly between laboratories. (2) Fluctuations of the urine pH within this range indicate that the kidneys do their job making sure that blood pH stays within its narrow range.
How does it work?
After food has been digested and absorbed, the remaining ash makes an impact on the acid-base level within the body. A number of the body’s balancing mechanisms, however, ensure that blood pH remains at the optimal level.
One of these mechanisms is via the kidneys which remove through urine the excess of particles (ash) which could disrupt the delicate pH balance in the body.
As a result of the removal of ash through kidneys, urine becomes: (3, 4)
- less alkaline/more acidic (in other words pH drops) when foods contain more acid-ash or
- more alkaline/less acidic when foods contain more alkaline ash.
Calculating the amount of acid in urine can be even determined using mathematical formulas (such as Net Renal Acid Excretion NEA and Potential renal acid load PRAL), which use nutrient data from ingested foods. (5, 6)
Please note, however, that the above does not prove that the alkaline diet works which the myth question usually relates to.