Making Your Fluids Work Harder When You Work Hard

Water is an unassuming nutrient in the scheme of diet and fitness. Most of us focus on nutrients that give energy, build stronger bones and muscle, or have key metabolic action in our bodies. Water is the one that, among other roles, provides the foundation upon which all other nutrients perform. And when in a water deficit, our bodies have the least tolerance compared to all other nutrients.
For those undergoing intense exercise training, maintaining body water at adequate levels is important.
Interestingly, simply replacing sometimes isn’t enough to do the trick during high physical demands particularly if the effort is in a hot environment. Sometimes, having charged particles called electrolytes in the fluid is of benefit.
Which electrolytes do I need?
When we sweat to stay cool, we lose water and electrolytes or minerals that include sodium, chloride, and potassium. Calcium and magnesium are also lost to a lesser extent. Additionally trace minerals including zinc, iron, iodine, and others may be lost in very small amounts.

Among all of these, sodium has been demonstrated to be the most important. Besides replacing the losses, sodium in a rehydration beverage plays functional roles that include:
  • stimulating thirst to improve adequate fluid intake
  • facilitating fluid absorption when glucose is also present
  • promoting rehydration.
It has often been said by physiologists that “unless one replaces the sodium, one will not rehydrate completely.” Sodium can help restore the plasma volume, which is key to maintaining proper cardiovascular function and temperature regulation (cooling). Finally, although the research is limited, sodium in the beverage might help reduce the risk of muscle cramps when sweat loss is high during efforts sustained or repeated over a prolonged period of time. This includes endurance races like marathons or team sports like football or soccer.

Other electrolytes that are replaced in a hydration beverage are beneficial but the primary electrolyte that science supports that is critical to maximizing rehydration benefits is sodium. The statements “sports drink with low sodium” and “sodium-free sports drink” are oxymorons.
How much sodium for endurance and exercise in the heat?
Research shows that for hydration beverages containing roughly 50 mmol/L, rehydration is near complete. This is because 50 mmol/L approximates the average sodium content of sweat. By replacing the sodium, the deficit is corrected and fluid is kept in the body. In other words, sodium allows the body to hold onto the fluid. Otherwise the kidneys eliminate the incoming water as urine.

The adjacent table provides examples of sodium and potassium content in beverages used for recovery after exercise. Sweat is listed as a frame of reference. Ranges are listed in parentheses.
Fluid Sodium, mmol/L Sodium, mg/8 oz Potassium, mmol/L Potassium, mg/8 oz
Bottled water 0 0 0 0
Glaceau Vitamin Water 0 0 19 175
Sobe Lifewater® 5 25 5 to 10 50 to 90
Gatorade® 20 110 3 30
Milk 25 138 40 366
EAS Hydrate® 45 248 20 187
Sweat 50 (20 - 80) 276 (110 - 440) 4 (2 - 10) 37 (20 - 95)
Sweat sodium and other electrolytes can vary quite widely in the sweat. Currently there isn’t a practical or inexpensive and accurate way to measure losses in the sweat. This makes it hard to predict exactly what a person loses and what he or she needs to replace during exercise. For this reason, selecting a beverage that approximates the midrange for sweat sodium content is likely to work best for the majority of exercisers and athletes. In nearly all cases, it is not a problem if one over-replaces electrolytes. The body will simply excrete the extra over time. If a person has a sensitivity to sodium and has issues with blood pressure or kidney function, it would be wise to talk to the personal physician before using beverages to replace sodium during exercise.
With the guidance provided here, athletes and serious exercisers can make the water in their rehydration beverages work harder for them when it counts during exercise occasions.
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