Purpose: The creators of this study tried to find the best way to rehydrate after strenuous exercise in the heat. All the experiments were performed under controlled conditions in a laboratory in which researchers experimented with different rehydration techniques.
Because potassium is thought of to help rehydration after exercise because potassium is a major component of cytosol and ingestion of potassium after exercise might help retain water in the intracellular space, researchers set up an experiment to find out if that indeed was the case.
Type of study: This practice would make them intervention trials. No control subjects where used in these trials.
Research methodology: The experimenters used clear-cut definitions of dehydration and rehydration. For example, they defined dehydration as, ”a decrease in body mass of about 2% of initial value for a 70kg person.” After 30 minutes of the end of the exercise subjects were allowed to consume varying beverages with different volumes over 1 hour, after which no further food or beverage intake was allowed until the end of the study period. Measurements seemed also clear cut with Urine volume and composition, plasma measurements such as electrolyte balance, hormone levels, osmolality, blood and plasma volume throughout the study clearly defined.
It looks as the researchers did a good job in setting up the study by trying to measure “hydration” or “dehydration” from varying angles.
Summary of results: This particular study was set up by dehydrating eight male volunteers by 2.1% of body mass using intermittent cycle ergometer exercise in the heat. 45 minutes after the end of the exercise, researchers gave the subjects a glucose beverage (90mmol/l), a sodium beverage (NaCl 60mmol/l), a potassium beverage (25mmol/l KCl), and a beverage including all three electrolytes over a period of 30 minutes. The drink volume was equal to the amount of sweat lost in all experiments which was about 1.6L. Plasma volume was decreased by 4.4% in all dehydration experiments. Then researchers collected urine over six hours.
Smaller amounts of urine were collected for electrolyte drinks versus the electrolyte free drink. After researchers gave the subjects drinks, plasma volume increased in all trials, but interestingly, recovery was slower when KCl was consumed. But by the end of the study period, which was six hours after rehydration period there was no difference in an increase of plasma volume. The results were 5.1% for glucose- electrolyte beverage, 6.2% for NaCl beverage, 5.1% for KCl, and 2% for the drink containing all three electrolytes.
This research shows that potassium is not necessarily better in rehydration than other electrolyte containing drinks. In fact, the authors suggest that a rehydration drink should contain relatively high amounts of sodium, a minimum of 50mmol/L and some potassium. Further more the studies show that sodium is a superior medium to rehydrate compared to other electrolytes.
Critique: Researchers tackled this topic of rehydration from multiple angles, trying to cover as much ground as possible under controlled conditions of heat exposure. In general, they tried to measure as many data points possible before, dehydration, the rehydration period, and after rehydration was thought to be completed. Furthermore researchers state, that in order to optimally rehydrate for the next event, fluid losses through sweat should not only be replaced, by a surplus should be consumed.
There is only a couple of questions I have concerning the quality of this study. Why didn’t the researchers try to recruit more subjects for this study? Eight people seem to be a small number to draw conclusions for an entire population of athletes and weekend warriors alike. Why did researchers choose different molarities for their potassium study? (90mmol/L glucose, 60mmol/L NaCl, and 25mmol/L KCl) I understand it was a British study, but looking at the National Research councils recommendations of 60mmol/L for NaCl makes the intake short 5mmol/L of the suggested adequate intake. It gets even worse with potassium. The 25mmol/L administered are short of a whopping 95mmol/L of the suggested adequate daily intake. I would think that athletes incur an increased requirement electrolyte requirement because of their strenuous activities.
Implications for Future Research: Even though researchers did a good job with this series of study, future research could focus more on potassium for rehydration. I would think researchers could also use more test subjects. Trying this out on several sports teams during competition would be a great way to really get to the bottom of rehydration. Implications for the “weekend warrior” are clearly defined here. Rehydrate with more fluid than lost through perspiration, and make sure you use a NaCl-KCl medium to do that.
Maughan R., Leiper J., Shirreffs S. (1997). Factors influencing the restoration of fluid and electrolyte balance after exercises in the heat. Br J Sports Med. 31p.175-82