World Kidney Day, March 14th 2013
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Kidneys health >> How do kidneys work? >> What are urinary wastes?
How do kidneys work?
What are urinary wastes?
Urinary wastes come mainly from food breakdown and metabolism in the body.

Two major wastes :
  • Urea : Urea is the major waste resulting from protein metabolism, accounting for about half of total urinary wastes. Most of the urea is eliminated through urine, and thus the amount of urea in the urine is often used as a marker of protein intake.

  • Sodium : Sodium is an essential mineral, coming exclusively from food. It has a key role in maintaining body fluid volume. Sodium can also be eliminated through perspiration or bowel elimination, but the major part is through urination. The kidneys can adapt sodium elimination to a wide range of intakes, from virtually 0 to about 25g per day

›› In a modern diet comprising many processed foods sodium intake largely exceeds our daily requirements, and most dietary sodium must be excreted in urine. That’s why it is sometimes used as an indicator of excess sodium intake !


Other waste products are also excreted, but in a smaller proportion:


Ammonia : comes mainly from protein metabolism.

Uric acid : comes from the breakdown of compounds such as proteins that are ingested through foods.

Oxalate : part of the oxalate comes directly from food but the major part is a by-product of other substances such as vitamin C.

Minerals : calcium, magnesium, potassium, phosphorous… Their excretion is regulated by kidneys, depending on the internal mineral balance.