Scott Riewald, PhD, CSCS, NSCA-CPT
As seemingly more and more athletic events are contested in hot, humid environments, it is prudent for athletes to familiarize themselves with the process of heat acclimatization and how to prepare their bodies to perform optimally in this hostile environment. It has been well documented that sweat rate increases with heat acclimatization, helping to reduce the physiological strain on the body and improve cooling. Because of this increased sweat rate, the potential for heat illness and excessive mineral loss also increases, even if the body adjusts to decrease the concentration of these minerals in the sweat. Also, while some research has indeed shown that heat acclimatization leads to a decreased sodium concentration in sweat, research on the sweat concentrations of other minerals is lacking. Chinevere et al. (1) recently undertook a study in which they examined how mineral concentrations change in athletes during a period of 10 days. Their findings have implications on how athletes should approach heat acclimatization to ensure optimal performance and avoid mineral deficiencies that could arise with the increased sweat rate.
In their study, Chinevere et al. examined 8 healthy, athletic men (20.2 ± 2 years of age), who underwent 10 days of heat acclimatization. The subjects walked on a treadmill at a pace of 3.5 miles per hour for 100 minutes each day in an environmentally controlled chamber (temperature 45°C, relative humidity 20%). In some instances, the activity was stopped if core temperature exceeded 39.5°C or the subject was unable to continue at the predetermined pace. Subjects ate a set meal 2 hours before exercise each day and drank 250 milliliters of fluid 1 hour before exercise. After exercise, subjects drank enough fluids to return body weight to within 1% of the pre-exercise value. On days 1 and 10, sweat was collected from one arm of each subject and analyzed to compute sweat rate, mineral concentrations (sodium, potassium, calcium, copper, iron, magnesium and zinc), and estimates for whole-body mineral losses. Statistical analyses were performed to assess changes between the start and end of the acclimatization period.
WHAT DID THE STUDY FIND?
Chinevere et al. found changes in sweat rate and sweat mineral concentrations for almost every athlete, with the body adjusting sweat concentrations to reduce mineral losses that might be expected with increased sweat rates. Specifically, the study found the following:
- Sweat rate increased an average of 6% over the course of the acclimatization, although this increase was not significant.
- Sweat sodium concentrations on day 10 were decreased by 34% compared with day 1, similar to what has been reported in previous studies. Additionally, although the sweat rate increased during the same period, the estimated whole-body sodium losses still decreased by 29% during the testing period.
- Potassium concentration in the sweat did not change significantly during the 10-day period.
- Calcium, copper, and magnesium sweat concentrations decreased significantly (p < 0.05) during the 10-day testing period, as did the estimated bodily losses. These decreases were observed in 7 of the 8 subjects tested.
- Mean sweat concentrations of iron and zinc tended to decrease during the course of the acclimatization (75% and 23%, respectively), but the changes were not statistically significant. Decreases were observed in 5 of the 8 subjects for iron and 6 for zinc.
- Body weight did not change from day 1 to day 10, but heart rate and core body temperature did decrease significantly during the course of the testing period.
APPLICATIONS AND IMPLICATIONS
So what does this mean for you, the strength coach, and the athletes you train? First, the data present a fairly good picture of the time course of heat acclimatization and how mineral concentrations in sweat change as an athlete becomes acclimatized to the heat. It is especially important to note that the mineral/electrolyte losses are most pronounced at the start of the acclimatization process. Although sweat rates are lower during this period, the sweat mineral concentrations are still high. It is important to keep this in mind as you think about nutrition/hydration strategies and how to replenish these lost minerals.
Second, note that not all athletes will respond in the same way. Although most of the athletes trended toward lower mineral concentrations as the acclimatization progressed, several did not. Individualization and knowing your athletes is the key. Just as you design individualized strength training programs, you need to realize each athlete may respond to the heat and acclimatization in a slightly different way. Don't just take a “cookbook” approach to acclimatization and mineral replenishment rather tailor your recommendations and training to the individual.
Third, don't be fooled by the findings of this study and think that once an individual becomes acclimatized to the heat that mineral losses in sweat are unimportant. Yes, sweat concentrations of many minerals and electrolytes may decrease with acclimatization, but during a prolonged competition or a long practice, an athlete can still lose considerable amounts of these substances, setting the stage for ill-effects down the road. Be vigilant about ensuring your athletes replace what they use or lose. Keep these things in mind as you work with your athletes.
As with any study, there are some limitations to the interpretation of the results. For one, sweat mineral concentrations can vary depending on where the sweat is collected on the body (e.g., arm concentration may be higher compared to concentrations measured on the torso or legs). Additionally, sweat rates differ depending on the body part analyzed. Consequently, although sweat collected from the arm provides a good window into what is going on in the body, the results cannot necessarily be extrapolated to explain what is going on everywhere in the body. The whole body mineral loss calculations, therefore, should be viewed with this thought in mind, because there may be changes in other parts of the body that differ from what these investigators measured. Additionally, the acclimatization period was only 10 days. Other studies suggest complete acclimatization occurs over a slightly longer time frame, i.e., 14 days or more. Therefore, the results presented in this study may not truly reflect the full effects of heat acclimatization. Regardless, the results of this study show the importance of replenishing lost electrolytes and minerals that are lost in sweat, especially as the athlete is first starting the acclimatization process.