Composition of our blood: Plasma Osmolality

Plasma osmolality is a measure of the concentration of sodium, chloride, potassium, glucose and other ions (molecules with a positive or negative electric charge) in the blood. This osmolality is affected by changes in water content, increasing with dehydration and decreasing with overhydration. Normal plasma osmolality ranges from 275-300 mOsm/kg. This is similar to the osmolality of sport drinks.

The kidney plays a protective role in the hydration status of the body. It helps to regulate plasma osmolality by reabsorbing water when we are dehydrated, and increases its filtration rate when we are overhydrated. Although the composition of sweat varies from person to person, it is always hypotonic (less concentrated) compared to body fluids, and therefore sweat loss increases plasma osmolality.

Electrolyte Concentration (mmol/L)




– 144

– 80


– 4.9

– 8


– 5.2

– 4


– 2.1

– 4


100 –

– 70

Composition of Sport drinks

There are three types of sport drinks: isotonic, hypotonic, and hypertonic.

  • Most sport drinks are isotonic, and help to reduce the risk of water intoxication by replenishing fluids and electrolytes in a similar ratio to that normally found in the human body. Isotonic sport drinks, such as Powerade and Gatorade, contain similar concentrations of salt and sugar as in the blood.
  • Hypotonic drinks (<275 mOsm/kg), such as water, and Slazenger S1, are less concentrated, promote the rate of water uptake into the intestine and may be more effective in cases of severe dehydration where rapid rehydration is required. These drinks are not ideal for endurance exercise longer than 1 hour.
  • Hypertonic drinks (>300 mOsm/kg), such as fruit juice and chocolate milk, may cause gastric distress due to the high concentrations of sugar, and may exacerbate dehydration by stimulating less water absorption. These drinks are best suited for refueling and rehydrating after exercise.

Sport drinks are a convenient way to replace water, carbohydrate, and sodium losses during and after exercise. Replacing the large amounts of sodium lost in sweat may help endurance athletes to avoid hyponatremia. Sodium also helps to maintain extracellular fluid volume, and stimulate the uptake of water and glucose into the small intestine. Dilute glucose-electrolyte solutions, with a hypotonic osmolality compared to plasma, help to maximize the rate of water uptake.

Sport drinks are a great way to ensure both sodium and fluids are consumed, but these needs can also be met by eating salty foods and drinking water.

Studies show that optimal water and sodium uptake occurs in solutions containing 60-100 mmol/L of glucose, and 50-120 mmol/L of sodium.

The type of sugar in sport drinks may also have an effect on absorption. Research shows that glucose stimulates more net water and sodium absorption than does fructose. Maltodextrin, a glucose polymer often found in sport drinks, may be the most effectively absorbed. It is less sweet than glucose or fructose, and is often mixed with other sugars to increase palatability. Maltodextrin refers to a family of glucose polymers, is composed of between 3-19 molecules of glucose, and is commercially manufactured from potato, corn or rice.

How to make your own isotonic sport drink

1) Mix 200 mL of concentrated orange juice + 1 g (a pinch) of salt + 1 L water
2) Mix kool aid (to your taste) + 8 tsp sugar + ½ tsp salt + 250 mL water
3) Mix your favorite fruit juice + 8 tsp sugar + ½ tsp salt + 250 mL water

The “What” and the “Why” of Sport Drinks