Exercising in the heat

Exercising in the heat represents a number of challenges for an athlete, ranging from discomfort to severe health threats. This is particularly relevant this year, as we approach the summer Olympic games in Tokyo, where temperature and other ambient conditions can be severe and non-predictable.

Effects of heat on the body

Our bodies function best at temperatures between 35 and 39°C (1).

Exercise stress can result in exercise-induced gastrointestinal syndrome, which consists in decreased integrity and function of the gastrointestinal system. This condition is worsened when our core temperature goes up to 39°C. A compromised gut can, in turn, cause other issues such as bacteria entering the bloodstream, decreased nutrient absorption and decreased immune function (2).

Another effect of an increased body temperature is increased fatigue, impaired aerobic performance and impaired neuromuscular function – including a diminished capacity to produce force (3).

Heat stress due to environmental conditions and exercise can lead to exertional heat illness (EHI). See the table below for symptoms depending on the severity of EHI (3).

Sources of heat during exercise (i.e. how temperature goes up)

Our bodies produce heat as a by-product of metabolism. This is exacerbated when we exercise, as the contraction of our muscles produce even more heat.

Environmental conditions also affect our core body temperature. This is more obvious when exercising outside but remember indoor spaces can also heat up considerably during winter, especially when there are multiple people exercising at the same time. Let’s not forget that some exercise spaces are warmed up deliberately i.e. hot yoga).

Clothing and protective equipment can also be responsible for elevating an athlete’s body temperature. In addition, they can also prevent effective cooling down, as we will see later.

Thermoregulation (i.e. how temperature goes down)

Our bodies rely on two main mechanisms to lower temperature as it rises: increasing skin blood flow and sweat (1).

Skin blood flow

As core body temperature rises, the blood vessels in our skin expand to facilitate blood to move from our core to our skin. This heat can be given out to the environment in processes called convective and radiative cooling (2).

Sweat

If convective and radiative cooling are not enough, we start to sweat (2). The evaporation (not just the production) of sweat is the mechanism that provides effective cooling. This is known as evaporative cooling (4).

This means that both clothing and protective equipment can hinder effective thermoregulation, in addition to being potential sources of extra heat.

Other factors that prevent sweat evaporation are high humidity and lack of airflow.

Young athletes

Children and adolescents have different sweat rates and surface-to-body mass ratios, which results in more convective and radiative cooling and less evaporative cooling. Young athletes seem to be at higher risk of heat illness. This could be a combination of physiological differences, inadequate hydration, inappropriate clothing, etc. (4)

Older athletes

In older people, sweat mechanisms fire later than in younger individuals. In addition, their ability to redistribute blood flow to the skin is also compromised. These factors put older athletes at risk of heat-related illness, which can be exacerbated with other chronic diseases (4).

Other considerations

Anything that affects blood flow, sweating and core body temperature will impair our ability to thermoregulate.

Strategies for exercising in the heat

Fluid and electrolyte balance

Hydration is highly depending on factors such as the type and duration of sport, fluid availability and individual factors, including gastrointestinal tolerance and the rate of sweat production (2, 3).

Pre-exercise

Ideally, athletes should start exercising in a euhydrated state (meaning having replenished all previous fluid loses) (2).

To ensure adequate fluid and electrolyte balance, athletes can consider increasing sodium intake before exercising in the heat (2, 3), particularly if they sweat a lot or experience muscle cramps (3). The recommended dose is 20-40mg of sodium per kg of body mass taken with 10ml of fluid per kg of body mass 1-2 hours pre-exercise (2).

During exercise

Fluids taken during exercise, if any, should also account for electrolyte replenishment.

It is also important to note that excessive water intake devoid of electrolytes, may cause exercise-associated hyponatremia (2). This condition is defined as a plasma or serum sodium concentration of less than 135mmol per litre (5). It is important to note that hyponatremia does not always cause symptoms, and that if they occur, they are often non-specific. Examples include headache, dizziness, weakness, vomiting and fatigue (5).

Post-exercise

For adequate post-exercise rehydration it is recommended to consume fluids alongside a meal to enhance their retention (2).

Carbohydrate

To assist with water retention, it is recommended to consume fluids containing carbohydrate at a rate of 30-60g per hour for events lasting more than 1 hour and 90g per hour for events lasting more than 2.5 hours (3).

Menthol

Menthol activates cold receptors in the mouth and pharynx, generating a feeling of coolness for increased comfort in hot environments. This substance does not need to be ingested, as mouth rinses can be enough to produce the desired effect (2).

Glycerol

This alcohol also enhances fluid retention and reduces urine output. The recommended dose is 1.2-1.4g per kg of fat-free mass taken with 25ml of fluid per kg of fat-free mass 1.5-3 hours pre-exercise (2).

Glycerol can be combined with sodium for a more effective water retention protocol (2).

Fluid temperature

Cold or frozen fluids can help lower body temperature. Electrolytes, carbohydrates and glycerol lower the freezing point of liquids, yielding even colder beverages (2).

Heat acclimatisation and acclimation

Both heat acclimatisation and acclimation refer to training in hot environments to get the body used to heat stress. The difference is that acclimatisation refers to naturally hot environments (e.g. a tropical location) and acclimation to artificial environments (e.g. a lab) (2).

External cooling

The use of ice baths and cooling garments such as ice jackets before exercise can help with lowering body temperature (2, 3).

References

1. Balmain BN, Sabapathy S, Louis M, Morris NR. Aging and Thermoregulatory Control: The Clinical Implications of Exercising under Heat Stress in Older Individuals. BioMed research international. 2018;2018:8306154.
2. Alan JM, Bethanie AA, Joanne NCO, Michelle MC, Ricardo JSC, Gregory RC, et al. Sports Dietitians Australia Position Statement: Nutrition for Exercise in Hot Environments. International Journal of Sport Nutrition and Exercise Metabolism. 2020:1-1
3. Racinais S, Cocking S, Periard JD. Sports and environmental temperature: From warming-up to heating-up. Temperature (Austin, Tex). 2017;4(3):227-57.
4. Ben D, Nicholas AB, Mark T, Daniel RM, Kirsty JE-S. Nutrition for Special Populations: Young, Female, and Masters Athletes. International Journal of Sport Nutrition and Exercise Metabolism. 2019;29(2):220-7.
5. Knechtle B, Chlibkova D, Papadopoulou S, Mantzorou M, Rosemann T, Nikolaidis PT. Exercise-Associated Hyponatremia in Endurance and Ultra-Endurance Performance-Aspects of Sex, Race Location, Ambient Temperature, Sports Discipline, and Length of Performance: A Narrative Review. Medicina (Kaunas, Lithuania). 2019;55(9).

[Photo by Tom Roberts]

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