MadSci Network: Biochemistry

Re: If a person cannot drink salt water, why is it okay to drink sugar water.

Date: Sun Feb 27 19:04:59 2011
Posted By: Peter E. Hughes, Ph. D. Biochemistry, Faculty, Biochemistry,
Area of science: Biochemistry
ID: 1298347935.Bc

Hi Karl,

The major considerations involving osmolarity in biological systems are:

  1. The nature of the solute, its molecular size
  2. The nature of the membrane(s), porosity and receptors (transport)
  3. The concentration of the solute
  4. The volume(s) of the exchange materials, in this case, water containing a solute
As it turns out, we can drink salt water, in small volumes. Iíve done it many times while attempting to surf. Iím not real good at surfing so the ocean reminds me who is Boss by making me drink salt water and a certain amount of sand.

Salt is a very small molecule that dissociates in the stomach acid into virtually free sodium ions, which are even smaller. Sodium ions are small enough to permeate through the membranous pores of the stomach and the small intestine. Further, the osmolarity is rich enough to deplete the stomach wall of water, dehydrating the stomach wall. This makes many of us feel queasy.

Letís be careful to not downplay this osmolarity difficulty with sodium. It can also cause whole system dehydration with resultant multiple system shutdown.

Sugar solutions, greater than 6%, will also cause dehydration but by a different mechanism. Sugars are much larger than sodium ions and usually require a receptor to be absorbed. However, the osmolarity is still an issue and water will be extracted from the body in order to dilute it. This happens much slower and may not be noticed so quickly.

It is for this reason that the NFL dilutes its drinks to roughly 2% during a contest. Less than 6% should present no problem.

Thanks for your question!
Peter, Mad Scientist

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