Acidic foods and urine pH

Does a diet high in acidic foods make the urine acidic?

Pawel Malczewski
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Short summary

The food acidity (the pH of the actual food) measured outside the body does not influence the acid-base balance inside the body or urine. For a quick answer click here.

Explanation

There are two concepts that are often confused which need clarification: acidic food and acid-forming food. (read more..) While the acid-forming foods refer to its acid-ash contents and this concept is used in the controversial alkaline diet, the acidic foods simply relate to the pH of the food before consumption.

It has been well established that acid-ash absorbed from the diet makes the urine more acidic/less alkaline.
Urine pH is affected by acid-forming food and not by acidic food.

Here is how it works:

While acid-forming foods have proven to lower the pH of the urine, acidic foods don’t necessarily have the same effect. What determines the urine acid level (or urine pH) is “ash”.

Acid-ash causes urine pH to drop while base-ash causes urine pH to rise.

Acidic foods” may be acid-forming or base-forming depending on the net charge of the ash that it contains. (1)Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009 Nov;24(11):1835-40. Available here.

For instance, acidic food which can be tested for its acidity (pH) outside of the body (before it is processed by the body) may contain high base-ash contents (alkaline/base-forming minerals). (2)Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009 Nov;24(11):1835-40. Available here.

A good example is an acidic citrus fruit – lemon, which has pH of about 2 (neutral pH is 7).

After ingestion, lemon/lemon juice, which is highly acidic, is mixed with substantial amounts of more potent stomach acid with a pH of 1.5-3.6. (3)Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009 Nov;24(11):1835-40. Available here. (4)Hoehn KN, Marieb EN. Human anatomy & physiology. 8th Edition. 2010. ISBN 0-8053-9591-1. Available here. (5)Advanced BioTech. pH Scale Comparisons. Available here. (6)MedlinePlus. Stomach acid test. Available here.

When the lemon/lemon juice reaches the intestine, the mixture of stomach juices and digested lemon is neutralized by bile secretions. It then gets broken down further until the remaining minerals (ash) get absorbed by the blood system.

As an analogy, imagine a gold panning technique where a wide, shallow pan submerged in water is shaken to sort gold from the gravel and dirt. Gold is heavier than stones and dirt so it drops to the bottom of the pan and the rest of the material simply flows out of the pan. Imagine that the gold which stays in the pan after all the stones and dirt have been removed represents the inorganic ash that stays within the body causing specific changes to the body’s acid-base balance. The rest of the food is just dispersed and utilized in numerous body functions or excreted.

If ash is acidic, in order to prevent changes in blood acidity, the kidneys filter these acid-forming minerals and dump it in urine which is then excreted. The presence of these acid-forming minerals in urine makes it more acidic. (7)Merc Manual. Acidosis. Available here.

Lemons, even though they are acidic in nature, when broken down and the organic material is removed, contain a majority of alkalizing minerals (base-ash). This residue causes the pH of urine to rise rather than drop, making it less acidic/more alkaline.

Conclusion

Back to top
Acidic food indicates only that the pH of the food itself is lower than 7 and it can be either acid-forming or base-forming depending on the ash contents of the food.

The ash content of the food is independent of how acidic the food is when measured before ingestion.
Acid-forming food contains substances which, after processing by the body, have the potential of changing the body’s acidity. Removing acid through urine which changes the pH of urine insures that the body acidity stays constant.

References   [ + ]

1. Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009 Nov;24(11):1835-40. Available here.
2. Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009 Nov;24(11):1835-40. Available here.
3. Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009 Nov;24(11):1835-40. Available here.
4. Hoehn KN, Marieb EN. Human anatomy & physiology. 8th Edition. 2010. ISBN 0-8053-9591-1. Available here.
5. Advanced BioTech. pH Scale Comparisons. Available here.
6. MedlinePlus. Stomach acid test. Available here.
7. Merc Manual. Acidosis. Available here.

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