Summertime in the United States means the arrival of weather conditions that can pose a number of significant challenges to individuals performing outdoor activity and exercise. Indeed, summertime environmental conditions place considerable stress on the cardiovascular and thermoregulatory systems. For instance, hot and humid weather conditions during the summer months have been associated with increased adverse cardiac events. The fact that summers are getting longer and warmer, combined with a growing threat of extreme heatwave events, means that health and exercise professionals need to help their clients take extra precautions when exercising outdoors. This article outlines strategies for helping your clients perform outdoor summertime activities safely and effectively.

Hotter Summers and the Importance of Heat Adaptation

Regardless of your stance on global warming, it is abundantly clear that our summer months are getting warmer. However, of more concern from an exercise and health perspective, is the increased chances of extreme heatwave events. In fact, in the past 50 years, there has been a 100-fold increased likelihood of it being extremely hotter than normal on a given day. Collectively, these data mean our clients are facing, on average, more challenging heat exposure conditions when exercising outdoors, and quite possibly significantly more potent heat stress, than they are likely anticipating.

Exercising in a warmer summertime environment places clients at an elevated risk for heat injury and illness, along with greater chances for cardiac events, especially for older adults with underlying heart disease conditions. Following a preventive physiological adaptation strategy can help your clients lower their risk of adverse events. It is well known that repeated exposure to hot and humid environmental conditions, which elicit an increase in both core temperature and sweating, leads to numerous physiological adaptations that ultimately reduce the deleterious effects of heat stress. The process of physiological heat adaptation is called acclimatization when obtained in a natural setting (e.g., training outdoors in hot weather), and acclimation when obtained in an artificial setting (e.g., in an indoor sauna). The mechanisms of heat adaptation, such as increased plasma volume and capillary density, not only improve heat tolerance, but have the capacity to improve cardiorespiratory fitness and performance as well.

Evidence-based Heat Adaptation Strategies

One of the great features to heat adaptation is that it can be obtained in a relatively short timeframe and then fairly easily maintained for months thereafter. As with any training paradigm, getting the various components of programming (e.g., frequency and intensity) correct is important for it to be effective, safe and well-tolerated. The FITT formula—which consists of frequency, intensity, time (duration) and type of exercise—is commonly used to design exercise programs. The same approach can be applied to heat adaptation, and these FITT recommendations are presented in Table 1.

Once your clients have properly achieved heat adaption, they can maintain it across many months, provided intermittent reacclimation/acclimatization sessions are integrated into their overall routines. Research demonstrates that two heat adaptation sessions every few weeks is a sufficient environmental stimulus timeframe to maintain the beneficial physiological adaptations to heat stress. The FITT recommendations for maintaining heat adaptation are presented in Table 2.

Safe Hydration Practices

Adequate hydration also enhances heat tolerance. Maintaining adequate fluids during exercise can be a delicate balance. Drinking too little can lead to dehydration, which can lead to an elevated heart rate and body temperature. However, drinking too much plain water—out of fear of not drinking enough—could lead to hyponatremia (i.e., low sodium in the blood), a condition less well known and understood, but equally dangerous. Fortunately, the body is very good at handling and normalizing large variations in fluid intake. For this reason, severe hyponatremia and dehydration are rare and generally affect very specific high-risk populations during specific types of activities. Both conditions are highly preventable. To prevent dehydration and hyponatremia, the goal is to drink just the right amount of fluid and/or electrolytes. The following section details hydration recommendations for before, during and after exercise to maintain a state of normal hydration (the technical term is euhydration) and optimize heat tolerance.  

Hydration Before Exercise

Most clients will begin exercise with a normal hydration level (euhydrated) and likely have little need for a rigorous prehydration regimen. However, if fewer than eight to 12 hours have elapsed since the last intense training session, fluid intake has been inadequate or environmental conditions are expected to be hot/humid, a prehydration program may be beneficial. A client should begin prehydrating about four hours prior to an exercise session by slowly consuming approximately 5 to 7 mL (0.17 to 0.24 ounces) of fluid per 1 kg (2.2 lb) of body weight. If no urine is produced or if the urine is dark or highly concentrated, they should drink an additional 3 to 5 mL (0.10 to 0.17 ounces) of fluid per 1 kg (2.2 lb) of body weight two hours before exercise or physical activity. Consuming sodium-containing beverages or salted snacks can help exercisers retain fluids.

Hydration During Exercise

The goal of hydration during exercise is to minimize and prevent heat injury or performance-diminishing effects from dehydration or hyponatremia. Share the following guidelines with your clients:

  • Aim for a 1:1 fluid replacement to fluid loss ratio. Ideally, exercisers should consume the same amount of fluid that they lose in sweat. An easy way to assess post-exercise hydration is to compare pre- and post-exercise body weight. The goal is to avoid weight loss greater than 2%. While there is no one-size-fits-all recommendation, if determining individual needs is not feasible, urge your clients to aim to drink 0.4 to 0.8 liters per hour (8 to 16 ounces per hour), with the higher rate for faster, heavier clients in a hot/humid environment and the lower rate for slower, lighter clients in a cool environment. Because your clients will sweat at varying rates and exercise at different intensities, this range may not be appropriate for everyone. However, when individual assessment is not possible, this recommendation works for most individuals.
  • Drink fluids with sodium during prolonged exercise sessions. If an exercise session lasts longer than two hours or if the activity stimulates heavy sweat (and consequently, sodium) losses, the client should consider consuming a sports drink that contains elevated levels of sodium. In one study, researchers did not find a benefit from sports drinks that contain the 18 mmol/L (or 100 milligrams per 8 ounces) of sodium that is found in most sports drinks. For this reason, they concluded that higher levels are needed to prevent hyponatremia during prolonged exercise. Individuals exercising for prolonged periods in hot environments should consume sports drinks that contain 20 to 30 mEq/L (0.5 to 0.7 g/L) of sodium, 5 to 10% of carbohydrate, and 2 to 5 mEq/L (0.8 to 2.0 g/L) of potassium to stimulate thirst and to replace sweat losses. Alternatively, clients can consume extra sodium with meals and snacks prior to a lengthy exercise session or a day of extensive physical activity.
  • Drink carbohydrate-containing sports drinks to reduce fatigue. Clients exercising for longer than one hour should also consume carbohydrate with fluids. With prolonged exercise, muscle glycogen stores become depleted and blood glucose becomes a primary fuel source. To maintain performance levels and prevent fatigue, clients should choose drinks and snacks that provide about 30 to 60 grams of rapidly absorbed carbohydrate for every hour of training. This recommended carbohydrate concentration of about 6 to 8% provides the necessary fuel to support continued exercise in hot conditions. For those who would like more information on sports drinks, this graphic explains how to read a sports drink label.

Hydration After Exercise 

Following exercise, the goal is to correct any fluid imbalances that occurred from exercising in hot environmental conditions. This includes consuming water to restore hydration, carbohydrates to replenish glycogen stores, and electrolytes to speed rehydration. If the client has at least 12 hours to recover before the next workout in hot conditions, rehydration with usual meals and snacks and water should be adequate. The sodium in food will help retain the fluid and stimulate thirst. If rehydration needs to occur quickly, urge your clients to drink about 1.5 L of fluid for each kilogram (or 0.70 L of fluid for each pound) of body weight lost. This is enough to restore lost fluid and also compensate for increased urine output that occurs with rapid consumption of large amounts of fluid.

The ACE Mover MethodTM in Action: Help Clients Exercise Safely This Summer

As a health and exercise professional, you recognize that a hot/humid environment is the most stressful environment for exercising. If your clients are unprepared, exercising in these conditions can lead to heat illness, including heat exhaustion and heat stroke.

Are your clients prepared for the coming warmer weather conditions of summertime? Do they know how to adapt to the heat and hydrate properly? What are some cool tips you can share with them? The ACE Mover Method is a great approach to asking open-ended questions that help your clients identify how they feel about their preparedness for the warmer weather. Maybe they are unsure of how long it will take them to achieve heat adaptation, or the different types (active or passive) of heat that can be used for heat adaptation (see Table 1). Discuss any barriers that may be preventing your client from being adequately hydrated. For example, are they unsure of the various electrolyte and/or carbohydrate content of various sports drinks? Table 3 offers guidance on how to evaluate the quality of sports drinks.

ACE Certified Professionals can access and download more information on fluid replacement guidelines using the bonus tool button at the top of this page.

Finally, collaborate and use your knowledge and what they have shared with you to come together and develop the best plan moving forward. Even with proper heat adaptation and hydration practices, there may be times when it will still be too hot/humid to safely exercise outdoors. Collaborate with your clients by reviewing the heat index chart, below, to make informed decisions. Perhaps your clients need some guidance on proper clothing choices, such as always wearing lightweight, well-ventilated apparel and avoiding impermeable or nonbreathable garments.

Utilizing the ACE Mover Method sets you and your clients up for success because it helps them prepare for safe and effective summertime exercise, which is especially important given the overall warmer conditions and increased severity of heat waves that are expected to occur during the summer months.