Written By: 

Dillon Wells, UH Dietetic Intern

Edited By: Meredith Parmley, MS, RD, CSSD, LD, Memorial Hermann Rockets Sports Medicine Institute

What is Heat Strain?

“Hot” conditions go beyond what the temperature reading is on your weather app. This reading is often taken in the shade, and actual temperatures can be 15-25°F higher¹. The weather may also change throughout the day, or differing conditions can create different climates (Ex. Running on concrete vs. trail, or practicing on grass vs. artificial turf)¹. Sun exposure, humidity, and air flow also contribute to how temperature is perceived by the athlete. Finally, clothing and equipment for sport can limit the ability to dissipate heat through sweating and can raise core body temperature¹. For example, a football player wearing a helmet and pads will have a greater burden than a cross country runner in spandex shorts and a tank. On the other hand, certain clothing can be sun protective and aid in the burden thermal heat energy can have on the body¹.

Core body temperature is regulated by heat produced (from internal fuel utilization to perform exercise or externally through climate) and the ability to dissipate heat through the skin. When core temperature rises, blood vessels expand to direct more blood to the skin surface to shift heat and encourage heat loss through sweating². A greater energy demand is required of the body when training in the heat, and when paired with longer exposure to a hot climate, heat cannot dissipate as fast as it rises³.

Additionally, training redirects and prioritizes blood flow to the muscles being used rather than the skin to fuel the demands of work, creating even greater heat strain.

Thus, one of the greatest risks of heat strain is prolonged exposure to dangerous conditions. This is not limited to continuous exposure, as events with repeated bouts of exposure (Ex. Tennis matches, back to back tournament games) are prone to inadequate hydration and cooling which can produce a carryover effect, resulting in quicker elevation of core body temp in later events⁴. If cooling is not managed in longer events, the worst-case scenario is exertional heat stroke (EHS) occurs. EHS is an increase in core body temperature greater than 104.9°F for an extended period of time⁵. Symptoms include loss of balance and coordination, disorientation, fainting, confusion, unwarranted aggression, and seizures⁵.

How Heat Impacts Performance

Training in the heat can impact performance long before reaching the point of EHS. The effects are progressive in nature, as performance declines the longer an athlete is exposed to the heat. The aforementioned rise in core body temperature causes cardiovascular (CV) strain. This CV strain limits the ability of the heart to circulate blood and oxygen to the brain and muscle, resulting in declines in maximal intensity, voluntary drive, and quicker fatigue^2,6,7. As a result, maximal and submaximal work capacity as well as endurance performance are negatively affected^6,7.

There are also indirect impacts on performance. For example, gastrointestinal (GI) dysfunction limits the intensity at which an athlete can perform. Long durations of exercise in the heat are known to contribute to GI distress. There are many different symptoms and causes of GI issues, so pinpointing the culprit can be difficult. Potential reasons are decreased GI motility and backup of pre-workout nutrition, or inflammatory issues caused by bacterial translocation causing an immune response and malabsorption of nutrients². Both of these can be linked back to the redistribution of blood supply away from the GI tract for more important functions (i.e. working muscle and skin cooling)². 

Athletes most at risk of heat strain, diminished performance, and/or EHS are endurance runners, team sport athletes playing multiple games in a day, athletes playing on surfaces with greater reflective temperatures (Ex. artificial turf, concrete), and athletes whose sport requires helmets and other heavy protective gear. There are, however, some sports that may actually benefit from performing in a hot environment. Rises in core temperature also lend to higher muscle temperature, which is advantageous to short, explosive movements due to improved nutrient utilization and the ability to contract muscle². This could improve short distance sprinters and jumpers’ performance. Additionally, the impacts of dehydration from heat stress are also not as likely to become an issue, and may even provide an advantage in having lighter body weight and more efficient movement as long as dehydration is not excessive to the point of impaired health².

Planning for the Heat

Pre-Training

During exercise, the body sweats in order to keep cool. Sweat decreases fluid stores and total body mass through the loss of “water weight”. These losses are normal, and trying to hydrate to match 100% of the losses through sweat is not advised. Rather, performance decrements are seen beginning at 2% body mass loss, so the athlete’s hydration strategy should be to mitigate fluid losses and not cross below that 2% loss². Sweat rate varies from person to person and training session to session, so measuring body mass pre- and post-training for multiple sessions can help guide the athlete’s hydration plan. 

The best way to prevent a fluid deficit is to begin training in a hydrated state by monitoring hydration in the days leading up to the event. This can be assessed by ensuring a clear/pale yellow urine color. While not necessary, acute sodium loading (20-40mg/kg w/ 10mL/kg fluid) taken 1-2hr prior to training may aid in fluid retention and reduce urine output for those who have high sweat rates and sodium loss². For example, a 175lb athlete would take 1590-3180mg sodium with ~27oz of fluid.

During Training

As stated previously, the goal of hydrating during training is to prevent significant loss in body mass. If the athlete is consistently losing more than 2% body weight during training, their intake should increase during exercise. A common problem for athletes is the lack of desire to drink during training. Using cool beverages containing sodium, carbohydrates, or both increase palatability and all have been shown to increase thirst drive and voluntary fluid consumption⁸.

Additionally, palatable drinks may provide some performance benefits, whether direct or through the perception of less fatigue⁹. Carbohydrates will be covered more later, but consuming carbohydrates during training can provide significant improvements in most cases. While we don’t fully understand the effects of sodium replacement, consuming sodium during exercise may prevent hyponatremia (low sodium) - which is usually caused by overconsumption of fluid without a balance of electrolytes².

Post-Training

Post-training hydration strategies will largely vary and depend on the fluid deficit that was created, and if additional training is going to take place later in the day. For quick rehydration for multiple events in a day, the goal is to replace up to 150% of fluid losses. This will be more efficient if consumed with food and electrolytes to aid in fluid retention. Complete sodium restoration is not necessary, however, it can help with fluid retention and reducing urine production that can often occur when large volumes of fluid are consumed at one time². If there is a longer duration of time between training sessions, then normal drinking habits of ~15oz/lb of BM lost should be adequate to restore hydration⁸. For example, if an athlete lost 1.5lbs during training, then they would want to consume 22.5oz of fluids over 1-2 hours. If this athlete did have to train again later in the day, then they would want to consume 33.75oz of fluids over 1-2 hours.

Nutrition and Other Considerations

Carbs

Rather than energy availability, hydration status is more likely to cause fatigue during training in the heat. However, nutritional fueling strategies are well-studied and can aid performance in the heat. As mentioned previously, carbohydrates can improve the palatability of fluid and drive the desire to drink more during exercise. Carbohydrates can aid in fluid retention since carbohydrates are bound to a certain amount of water when stored as glycogen. Consuming carbohydrate during exercise maintains blood flow to the gut which can mitigate GI distress. When training in hotter temperatures, carbohydrates are used at a higher rate, which may indicate it would be advantageous to consume carbohydrates in the form of a sports drink (which would also include beneficial electrolytes)².

Recommendations for endurance athletes range from 30-90g CHO/hour, and can even be advantageous to athletes with short sprint bursts due to the ability of carbohydrates to excite the brain and decrease feelings of fatigue^9–11. 

Ice Slushees/slurries

Another potentially beneficial strategy is the consumption of semi-frozen or cold fluids. Ice slurries are similar to a slushy, usually prepared with electrolytes and other substances, and can manage elevations in core temperature while replacing fluids. Pre-training, ice slurries reduce core temperature and increase exercise capacity¹². During training, ice slurries can decrease sweating, which is the most efficient method to reduce core temperature. Consuming ice slurries during training likely would only produce benefits for cooling and performance in extremely hot (>90°F) and humid environments since sweat doesn’t evaporate as quickly to cool us in that type of environment¹².

There is also some evidence that menthol, a substance found in products like peppermint, could actually enhance performance by creating an artificial feeling of coldness². This has been shown to be most effective in continuous events of 20-70 min by decreasing perception of heat and ease of breathing, so it may be more relevant for longer distance runners and cyclists¹³. 

Adaptations and External Cooling

Outside of fueling, there are a few other strategies to reduce heat strain. Heat acclimation, a period of a few weeks spent training in the heat, produces adaptations that lead to improvements in heat tolerance and performance. Heat acclimation shows conclusive results that support its use^1–3,14. 

The next most effective will be adequate time in the shade if the opportunity presents itself to reduce core body temperature, or using cooling products like ice vests and towels to cool off¹⁵. Ice vests have been shown to reduce core body temperature when used as a pre-cooling strategy, causing a greater rise in temperature required to reach levels that would cause heat exhaustion¹⁵. If time is available, these products could be used during exercise, but for many sports this is not applicable and using strategies like dousing with cool water are more applicable. In the post exercise setting, ice vests have been shown to reduce core body temperature and could aid in reducing core body temperature after prolonged exposure to heat¹⁶.

Lastly, protecting skin from UV rays with proper clothing and/or sunscreen if longer sleeves would cause issues with sweat evaporation in events with greater internal heat production (ex. Football).

Sample Fueling Plan

Now to put it all together! Below is an example of a fueling plan for a 175lb football player participating in a 2-hour practice with no other training for the rest of the day.

Planning for training in the heat has many variables to consider, not all of which need to be implemented. These recommendations should be trialed in practice settings to determine which combination of strategies are tolerated and work best for the athlete before being implemented in a competition setting.

1 Nutritional Strategies for Training in the Heat. Tips to prepare for the summer.

2 What is Exertional Heat Stroke (EHS)?

3 Tips for fueling in the heat.

4 Other considerations.

5 Making your own fueling plan.

Have questions? Please feel free to talk to an Athlete Training and Health Performance Coach or Meredith Sorensen, Sports Dietitian, MS, RD, LD with the Memorial Hermann Rockets Sports Medicine Institute. Meredith can be reached at Meredith.Sorensen@memorialhermann.org or can be found on Instagram at @meredithdarcienutrition. 

References

1. Racinais S, Hosokawa Y, Akama T, et al. IOC consensus statement on recommendations and regulations for sport events in the heat. Br J Sports Med. 2023;57(1):8-25. doi:10.1136/bjsports-2022-105942 
2. McCubbin AJ, Allanson BA, Odgers JNC, et al. Sports Dietitians Australia Position Statement: Nutrition for Exercise in Hot Environments. International Journal of Sport Nutrition and Exercise Metabolism. 2020;30(1):83-98. doi:10.1123/ijsnem.2019-030
3. Febbraio MA, Snow RJ, Hargreaves M, Stathis CG, Martin IK, Carey MF. Muscle metabolism during exercise and heat stress in trained men: effect of acclimation. Journal of Applied Physiology. Published online February 1, 1994. doi:10.1152/jappl.1994.76.2.589
4. Bergeron MF. Hydration and thermal strain during tennis in the heat. Br J Sports Med. 2014;48(Suppl 1):i12-i17. doi:10.1136/bjsports-2013-093256
5. Miller KC, Casa DJ, Adams WM, et al. Roundtable on Preseason Heat Safety in Secondary School Athletics: Prehospital Care of Patients With Exertional Heat Stroke. J Athl Train. 2021;56(4):372-382. doi:10.4085/1062-6050-0173.20
6. González-Alonso J, Crandall CG, Johnson JM. The cardiovascular challenge of exercising in the heat. J Physiol. 2008;586(1):45-53. doi:10.1113/jphysiol.2007.142158
7. Trangmar SJ, González-Alonso J. Heat, Hydration and the Human Brain, Heart and Skeletal Muscles. Sports Med. 2019;49(Suppl 1):69-85. doi:10.1007/s40279-018-1033-y
8. Flavia Meyer, Zbigniew Szygula, Boguslaw Wilk. Fluid Balance, Hydration, and Athletic Performance. Taylor & Francis Group; 2019.
9. Carter JM, Jeukendrup AE, Jones DA. The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med Sci Sports Exerc. 2004;36(12):2107-2111. doi:10.1249/01.mss.0000147585.65709.6f
10. Superior Endurance Performance with Ingestion of Multiple Transportable Carbohydrates. Accessed December 4, 2022. https://oce.ovid.com/article/00005768-200802000-00012?relatedarticle=y
11. Krings BM, Peterson TJ, Shepherd BD, McAllister MJ, Smith JW. Effects of Carbohydrate Ingestion and Carbohydrate Mouth Rinse on Repeat Sprint Performance. Int J Sport Nutr Exerc Metab. 2017;27(3):204-212. doi:10.1123/ijsnem.2016-0321
12. Ice slush: effects, benefits and evidence. askerjeukendrup. Published August 5, 2022. Accessed June 30, 2023. https://www.mysportscience.com/post/ice-slush-effects-benefits-and-evidence
13. Can peppermint improve performance in the heat? askerjeukendrup. Published March 25, 2019. Accessed June 30, 2023. https://www.mysportscience.com/post/2019/03/25/can-peppermint-improve-performance-in-the-heat
14. Lorenzo S, Halliwill JR, Sawka MN, Minson CT. Heat acclimation improves exercise performance. J Appl Physiol (1985). 2010;109(4):1140-1147. doi:10.1152/japplphysiol.00495.2010
15. Racinais S, Ihsan M, Taylor L, et al. Hydration and cooling in elite athletes: relationship with performance, body mass loss and body temperatures during the Doha 2019 IAAF World Athletics Championships. Br J Sports Med. 2021;55(23):1335-1341. doi:10.1136/bjsports-2020-103613
16. Seeley AD, Sherman RA. An Ice Vest, but Not Single-Hand Cooling, Is Effective at Reducing Thermo-Physiological Strain During Exercise Recovery in the Heat. Front Sports Act Living. 2021;3:660910. doi:10.3389/fspor.2021.660910