Isotonic (‘iso’ means ‘the same’ and ‘tonic’ means ‘pressure’). When isotonic is used to describe solutions for human consumption, it means having the same fluid pressure as body fluids. Tears, blood plasma, lymph and other body fluids are all isotonic. Everything we eat and drink must be made isotonic before passing to the small intestine so the body can absorb it.
The fluid pressure of isotonic solutions across semi-permeable membranes, such as the walls of the small intestine, is generally referred to as having a particular osmolarity or tonicity. For the purposes of this discussion, osmolarity is measured in units referred to as milliosmoles/kilogram (mOs/kg). It can be said that when solutions on opposite sides of a membrane have the same osmolarity, they are iso-osmotic or iso-tonic. Normal human blood serum (the fluid portion without cells and platelets) has an osmolarity of 288 mOs/kg, as do most other fluids in the body. Therefore, in physiologic terms when a food solution’s osmolarity is the same, or nearly the same, as that of normal human blood serum, it is referred to as isotonic.
To obtain this state, when we ingest food, drink a beverage, or ingest most nutritional supplements, the stomach must secrete diluting juices, enzymes, hydrochloric acid, and at least one vitamin-binding protein to continue the process of digestion. Muscular contractions of the stomach mix the contents until a reasonably uniform solution is achieved. This action is a relatively time consuming process – about 40 minutes to 4 hours depending on the size of the meal. Once isotonic, the solutions of food (or chyme) can then enter the small intestine for absorption.
