Filed under: Heat acclimation
Approximately 300 million people around the world have sickle cell trait (SCT), including approximately 9% of African Americans. It is a hereditary condition in which red blood cells are affected, but most people who have it never experience symptoms. (It is important to note that SCT is not the same as sickle cell disease [SCD]. Sickle cell disease [or sickle cell anemia] can lead to other serious clinical risks and can cause severe symptoms. Those with the SCD usually have a shorter lifespan.)
Individuals with SCT usually can participate in normal physical activity and sports, as SCT doesn’t seem to adversely affect performance. In fact, some studies have found that those with SCT excel in short-distance power activities such as sprinting and jumping.
While SCT is largely a benign condition, there have been related complications such as exertional rhabdomyolysis and exercise-related sudden death. They have been found in non-SCT individuals as well, but they occur at higher rates in those with SCT and are a “hot topic” in military, and civilian communities; the National Collegiate Athletic Association even requires screening for all its Division I and II athletes.
It has been suggested that those with SCT may be more prone to sudden death from dehydration, heat illness, and high-intensity exercise; however, these factors and the role of prevention standards, medications, and the use of dietary supplements are still being studied. In both military and civilian SCT populations, collapsing during exercise is most commonly observed during times runs and sprints within the first few weeks of conditioning. SCT Recruits who have difficulty passing the Physical Readiness Test are also at higher risk for collapse. Military leaders should be aware of safe training guidelines and take universal precautions. Effective prevention tactics include heat acclimatization, hydration, gradual physical conditioning, and addressing progressively worse symptoms early on.
All newborns in the United States are screened for both SCT and SCD as part of a public health imperative. Each military branch has its own policies regarding SCT. The Army does not screen for SCT but promotes universal precautions for all soldiers, whereas the Air Force, Navy, and Marines all screen for SCT after accession. Further testing and counseling may be done for those who are positive for SCT. If you are unsure about SCT and exercise, consult with your physician, especially if you are starting a new exercise routine.
Heat-related injuries are a threat to Warfighters, even those in top physical condition, deployed to extreme environments. Heat acclimatization is necessary to ensure that the health and performance of Warfighters is not compromised to a dangerous degree when exposed to heat stress.
Take it slow. For unacclimatized Warfighters, physical exertion should be limited in intensity and time. Allow 9-14 days of progressive heat exposure and exertion—more for less-fit Warfighters, less for more-fit Warfighters.
Don’t overdo it; don’t underdo it! Heat acclimatization requires exposure at least two hours per day (can be two one-hour segments) while engaged in a cardiovascular exercise (which should increase in intensity each day of the acclimatization period).
It’s all relative. The level of heat acclimatization achieved is relative to the exertion normally expended by the Warfighter. If light exertion is the norm, the level of heat acclimatization after two weeks will match that. If more strenuous exertion is called for, additional acclimatization and possibly improved fitness is required.
Work smart. If Warfighters must perform physical work during the acclimatization period, take advantage of the cooler hours during the morning, evening, and night.
Stay hydrated. Adequate water is essential. Heat acclimatization increases sweating and, therefore, water requirements. Dehydration rapidly degrades safety and performance, even for those who are already heat acclimatized or in top physical condition.
For a more detailed look at heat stress and acclimatization, read HPRC’s reports on managing heat exposure.
Science Daily reports on a new study published in the Journal of Applied Physiology which indicates that training in warm weather not only improves heat acclimation and performance in the heat, but also improves performance in cool conditions. Click here for more details about the study.
Runnersworld.com has an article that cites a new study from the Journal of Applied Physiology which reports that 10 days of extreme heat acclimation can improve performance by six to eight percent.
Click on link below to access article.
Heat-related injuries are a significant threat to the health and operational effectiveness of military members and their units. The human body’s response to heat stress is quite resilient if given several weeks for adaptation to occur. This process, called acclimatization, involves internal adjustments, in response to the outside environment, which improve heat tolerance. This adaptation can be fully achieved after 10 to 14 days of exposure to heat, but two-thirds, or even 75 percent, of the adaptation takes place within five days.
Myths and/or Claims
1) It is commonly believed that warfighters who are physically fit do not need to be heat acclimatized.
2) It is also assumed that older individuals are less heat tolerant than their younger counterparts.
3) Women are thought to need longer acclimatization time, since they are more vulnerable to heat illness.
1) Though fit warfighters acclimatize faster than less fit warfighters, a physically active person cannot be fully acclimatized without exposure to environmental heat stress.
2) Age has no effect on acclimatization. Research that controlled for body size and composition, aerobic fitness, hydration, degree of acclimatization, and chronological age showed little or no age-related decrements in one’s ability to manage or acclimate to extreme temperatures.
3) Nor does gender appear to be a factor: women were thought to need longer acclimatization, since they are more vulnerable to heat illness. However, women and men show equivalent thermoregulation during exercise when levels of fitness and acclimatization were controlled.
4) Heat-related injuries such as exertional heat illness remain a major cause of illness and occasional fatalities within the Armed Forces. However, as mentioned earlier, the human body’s can be resilient to heat stress if given several weeks to adapt.
Heat acclimatization adaptations may vanish after only a few weeks of inactivity (i.e., 18-28 days). The first adaptations to degrade are those that develop first: heart rate and other cardiovascular variables.
Summary for Military Translation
Studies have shown that acclimated soldiers suffer no detrimental effects of exertional heat stress, despite almost the same degree of heat strain. The Technical Bulletin-Medical 507 provides an evidence-based preventive program to protect military personnel from heat stress and associated adverse health effects. The recommended heat acclimatization strategies are to mimic the deployment climate, ensure adequate heat stress (i.e. by invoking sweating, having 4 to 14 days of heat exposure, and maintaining the daily duration of at least 100 minutes). It is also suggested that heat acclimatization start at least one month before deployment; and upon arrival, acclimatization should start slowly and build up by increasing heat and training volume as tolerance permits.
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