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The Meaning of Isotonic in Physiologic Terms

Market America's Isotonix® nutritional formulas are an osmotic adjuvant system composed of a series of related formulas. All of the formulas may be used to manufacture products for use in human nutrition that, when prepared for ingestion as solutions are of the same osmotic pressure (or osmolarity) as most of the fluids in the human body. By definition, these solutions may be called isotonic "equal tonicity".

Specifically, the fluid pressures of the solutions across semi-permeable membranes (the walls of the small intestines, for example, are semi-permeable membranes) in biological systems are generally referred to as having a particular osmolarity. For the purposes of this discussion, the 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 isotonic, regardless of whether that osmolarity is 100 mOs/kg or 650 mOs/kg.

However, in human medicine that definition is considerably narrowed. Normal human blood serum (the fluid portion of the blood without cells and platelets) has an osmolarity of 288.6 mOs/kg, as do most other fluids in the body (notable exceptions being sweat and urine). Therefore, in human medicine when a solution's osmolarity is the same, or nearly the same as that of normal human blood serum, it is referred to as isotonic.

Black's Medical Dictionary defines isotonic as follows: "ISOTONIC is a term applied to solutions that have the same power of diffusion as one another. An isotonic solution in medicine is one that can be mixed with body fluids without causing any disturbance. Solutions that are weaker or stronger than the fluids with which they are intended to be mixed are known as hypotonic or hypertonic respectively."

In 1905, P. Carnot and A. Chassevant, French physiologists, reported that isotonic saline (0.9% sodium chloride) solutions administered orally to dogs emptied from the stomach more rapidly than either a hypotonic solution (water) or a hypertonic solution (3% sodium chloride). Over the following 91 years, this physiologic phenomenon has been repeatedly verified in many species of animals and in humans. Researchers at the University of Leeds, Guy's Hospital in London, Baylor University, and the Mayo Clinic, among others, have shown that not only do isotonic solutions leave the stomach faster, but also that the osmolarity of the stomach contents (its osmotic pressure) is one of the controlling mechanisms for the rate at which the stomach empties.

Micronutrients (vitamins and minerals) are most commonly consumed in solid (tablet or capsule) form. These solid forms must be broken down in the stomach and the active ingredients (micronutrients) must then dissolve in the gastric fluids in the stomach. This disintegration and dissolution process is often slower than any other process involved in the absorption of a micronutrient or a drug. When the active ingredients have dissolved, the body must adjust the concentration of the stomach contents to become an isotonic solution before the contents can proceed into the small intestine for absorption. If there are any foods or beverages in the stomach at the time the tablet or capsule is taken, the active ingredients will be delayed further in their transit to the small intestine until the food is properly prepared to leave the stomach.

Absorption into the bloodstream is a function of surface area, and the small intestine has a huge surface due to its length, its many folds, and the microvilli (minute finger-like projections) on intestinal epithelial cells. In comparison, the surface area of the stomach is very small, and the absorption that occurs there is negligible. The fastest and greatest absorption takes place in the small intestine.

Remington's Pharmaceutical Sciences states that, "When drugs and micronutrients are administered in solid dosage forms (capsule, tablets, etc.), the rate at which the drug is released in solution may be often the slower than any other process involved in absorption, so that dissolution becomes the rate-limiting factor on absorption." It says further: "The motility (motility refers to the movement or contractions of the stomach which cause it to empty) of the stomach is more important to the rate at which an orally administered drug is passed on to the small intestine than is the rate of absorption from the stomach itself, since for various reasons stated above, absorption from the stomach is of minor importance." In addition, it states that: "The average emptying time of the stomach is about 40 minutes, though it varies according to contents, reflux and physiological factors, and the action of certain autonomic drugs or disease. The effect of food to delay absorption is due in part to its action to prolong emptying time. The emptying time causes a delay in the absorption of the drug."

It should be clear from the information provided above that there is a strong scientific basis for Market America's Isotonix® nutritional supplements based on the osmotic adjuvant system. Even a casual search of the literature will reveal a large body of experimental data supporting these concepts. See the next page for some literature references.


  1. P.E. Turner, J. Tuomilehto, P. Happonen, A.E. LaVille, M. Shaikh, B. Lewis
  2. Metabolic studies on the hypolipidaemic effect of guar gum. Atherosclerosis 81: 145-150, 1990.
  3. J.L. Whyte, R. McArthur, D. Topping, P. Mestel. Oat bran lowers cholesterol in mildly hypercholesterolemic men. Journal of the American Dietetic Association 92 (4): 446-449, 1992.
  4. J.W. Anderson and A.E. Slesel. Hypocholesterolemic effects of oats products. Advances in Experimental Biology and Medicine 270: 17-36, 1990.
  5. C.L. Redard, P.A. Davis, S.O. Schneeman. Dietary fiber and gender: Effect on postprandial lipemia. American Journal of Clinical Nutrition 52: 337-345, 1990.
  6. B. Kashtan et al. Wheat bran and oat bran supplements: Effects on blood lipids and lipoproteins. American Journal of Clinical Nutrition 53: 976-980, 1992.
  7. J.M. Keenan, J.B. Wenzm, S. Meyers, C. Lipsin, Z.Q. Huang, Randomized, controlled, crossover trial of oat bran in hypercholesterolemic subjects. Journal of Family Practice 33 (6): 600-603, 1991.
  8. J.J. Cards, F.L. Robbins, C.W. Burgin, T.G. Bumgartner, R.W. Rice. The effects of protein on patients at risk for coronary heart disease without altering diet or lifestyle. Clinical Cardiology 11 (9): 589-594, 1988.
  9. D. Kritchevsky. Dietary fiber. Annual Review of Nutrition 8: 301-323, 1988.
  10. J.L. Slavin. Dietary fiber: Mechanisms or magic on disease prevention. Nutrition Today 36 (6): 6-10, 1990.
  11. P. Carnot and A. Chessevant. Modifications studies dans l.estomac et la duodenum par les solutions salines suivant leur concentration moleculaire le reflex regulteru du sphincter pylorique. C.R. Soc. Biol. Paris 58: 173-176, 1905.
  12. J.N. Hunt. Some properties of an alimentary osmoreceptor mechanism. Journal of Physiology (London) 132: 267-288, 1965.
  13. A.D. Koparkar, L.L. Augsburger, R.F. Shangraw. Intrinsic dissolution rates of tablet fillers and binders and their influence on the dissolution of drugs from tablet formulations. Pharmaceutical Research 7 (1): 80-85, 1990.
  14. J.C. Meeroff, V.L.W. Go, S.F. Phillips. Control of gastric emptying by osmolarity of duodenal contents in man. Gastroenterology 68: 1144-1151, 1975.
  15. Anonymous. Absorption, distribution, and execration. In: Remington's Pharmaceutical Sciences, 17th edition, Philadelphia College of Pharmacy and Science, 1975.
  16. D.Y. Graham, J.L. Smith, A.A. Bouvet. What happens to tablets in the stomach. Journal of Pharmaceutical Sciences 79 (5): 420-424, 1990.
  17. J.A. Seigel. Biphasic nature of gastric emptying. Gut 29: 35-39, 1988.