Sweating during exercise causes loss of water and electrolytes from the body. Post-exercise, drinking fluids can help prevent dehydration and aid rehydration. Sports drinks have the added benefit of providing carbohydrate (CHO) as fuel plus electrolytes to help replace those lost in sweat, increasing the drive to drink and promoting water absorption.
Sports drinks are categorised according to their osmolality – an index of the concentration of a solution. In sports drinks, osmolality is dependent on the type and concentration of CHO and electrolytes. Blood plasma has an osmolality of 280-300mOsm/kg and any drink with the same osmolality is said to be in balance with the body’s own fluids, and therefore defined as isotonic.
Hypotonic drinks have an osmolality lower than blood, while hypertonic drinks have a higher osmolality than blood. By varying the osmolality, the core functional benefit of the drink can be manipulated to focus on hydration and/or energy provision. Isotonic and hypotonic drinks are used regularly by exercisers as they effectively transport fluid and CHO into the blood stream, and therefore help tackle two common causes of fatigue: dehydration and the depletion of the body’s CHO stores.
|
Sports drink |
Typical CHO/sodium content |
Recommendations |
|
Water |
0%
|
If sweat losses are small during short-duration (<45 min), low-intensity exercise, then water is fine. Water consumption post-exercise promotes diuresis (urine production), so add sodium (salted snacks, sports drinks or add salt to a post-exercise meal) to promote fluid absorption. |
|
Hypotonic ~50-270mOsm/kg |
<4% <50mg/100ml |
Useful when exercising at high intensities as they are easier to tolerate in warm conditions, where fluid consumption is the priority or when weight management is the primary goal. Ideal for exercise sessions lasting less than 60mins, when hydration is the primary goal. Examples: Lucozade Sport Lite, very diluted squash/fruit juice |
|
Isotonic 270-330mOsm/kg |
4-8% ~50-70mg/100ml |
The CHO content provides a readily available source of energy, which is ideal during intense exercise lasting longer than 60mins. Aids recovery from demanding exercise by replenishing CHO stores and restoring fluid balance. Examples: Isotonic sports drinks |
|
Hypertonic >330mOsm/kg |
>8%
|
Due to a higher CHO content, hypertonic drinks are absorbed slowly and are therefore not recommended during exercise. Useful when energy needs are high, e.g., after exhaustive exercise. Rarely contain salts and are therefore not suitable for rehydration. Examples: Fruit juice, energy and recovery drinks |
Terri Graham studied sport and exercise science at Bath University prior to completing a master's in sport and exercise nutrition at Loughborough University. She has delivered practical and educational nutrition advice within running, rugby union and league, and for the university’s sports development service.
This feature was first printed in the June/July 2011 issue of Fitpro Network magazine.
The views and opinions expressed in this article are those of the author and not of Fitness Professionals Ltd or Virtual Magazine. Consult a qualified health or fitness professional before making changes to your diet or exercise.
