Glossary: Hydration 101

How well do you know hydration?

Hydration is in the news! Every day, we see or hear some story or another involving the unhealthy effects of becoming dehydrated. This is not going to go away, the world is heating up and it seems most of us are exposed to some sort of heat stress at some time in the year. When this happens it’s important we know about hydration, re-hydration and what to do about it.

No matter whether you’re interested in staying hydrated, getting re-hydrated, keeping your body cool or even the tonicity of your cells The Osmolality Lab™ is here to help! You should know that they all depend on how well your body retains water, and how well your body retains the water you put in it, depends on a physical attribute of the products you may use, something called “Osmolality” . The related terms below begin to explain how hydration works and what you need to know to work out which products are best…


A natural phenomenon in water containing fluids that drives the movement of water molecules from one area to another.

Osmotic Pressure

A physical characteristic of a water containing solution created by the presence of dissolved materials, called solutes. A difference in osmotic pressure between two areas defines the direction the water molecules will flow. Naturally, always from an area of lower solute concentration to that of an area of higher solute concentration, in a natural attempt to equalize the solute concentrations across the two areas.

Osmole (Osm)

The standard unit of osmotic pressure, defined as a one molal (chemical amount) concentration of an ion in solution. A milliosmole (mOsm) is 1/1000th of an osmole.


The effective concentration of all solutes dissolved in a solution in a non-biological system, i.e., a property of liquids like water, beverages or even biological fluids when measured outside the body, e.g., urine, and blood. Osmolality is expressed in units called osmoles (Osm) per kilogram (kg), i.e., Osm/kg of solution or mOSm/kg. Osmolalities within a specific range optimize fluid retention, which is a good situation to be in to stay hydrated or rehydrate if necessary.


Another measure of solute concentration for fluids but this time expressed in units of osmoles (Osm) per liter, i.e., Osm/l or mOsm/l. One can convert from one to the other if the density of the fluid is accurately known.


The method used to measure osmolality. An Osmometer takes advantage of the basic principle that dissolved solids have on the freezing point of a liquid, they lower it. Think putting ice on your driveway, the salt dissolves into the rain or snow and lowers the freezing point such that it does remain as ice. We measure the freezing point of the sample and compare it to known standards, with specific ion concentrations, representing specific osmolalities. Thus, if our sample is within the range of our standards we can accurately calculate the osmolality of that solution.


In biology, this term describes the ability of the fluid environment surrounding a cell to move water in or out of that cell by osmosis, thereby affecting its cellular volume. If the environment is isotonic this means the net water movement, although dynamic, is zero. Hypo-tonic means water will flow into the cell and hyper-tonic means water will flow out of the cell. The tonicity of the fluid in question will partly be determined by the osmolality of the fluid but also by the dynamics of the solutes with the cell membrane. Thus, tonicity can be considered as an effective osmolality for living cells.


The osmolality (or osmolarity) of a product is a function of the dissolved solids it contains. Electrolytes in the form of sodium, potassium or calcium salts and indeed carbs such as glucose, fructose and sucrose and even certain amino acids are examples of the types of solutes that contribute to osmolality.


Some solutes can have the opposite effect, i.e., they can cause you to expel more water than you take in, these are typically bad for hydration or to take if you are dehydrated. Examples of diuretics are caffeine, in sufficient quantities and alcohol.

Interested in learning more about hydration and osmolality? Check out our Hydration Education page here.

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