Feeling the heat? No sweat, it is all just a part of the process

Singapore marathoner Mok Ying Ren is drenched in sweat during the marathon at the SEA Games last week. Sweating is an important process because it allows excess heat to escape from our bodies during exercise.
Singapore marathoner Mok Ying Ren is drenched in sweat during the marathon at the SEA Games last week. Sweating is an important process because it allows excess heat to escape from our bodies during exercise.ST PHOTO: MARK CHEONG

The most common complaint we hear from athletes is that it is too hot to be running in Singapore.

One has to either run early in the morning before the sun rises or in the evening after the sun sets. Hardly do we get to do what we read about in running magazines - running on nice soft trails on cool summer days.

Given the potential health detriments of exercising in the heat, we will be dedicating the next three issues to discuss the aetiology and consequences of heat stress during exercise, as well as plausible strategies to prevent and prepare for it.

To understand the science behind heat stress, let us first understand how heat is generated and lost in our bodies during exercise.

HEAT PRODUCTION

Heat is produced when our muscles work hard to propel us forward. Muscle cells mix carbohydrates (or fats) that we consume with oxygen that we breathe in to create energy, of which only a small portion is used to move our bodies and the majority is released as heat.

The harder and longer we exercise, the more this process (called respiration) occurs, and more heat is produced as a by-product.

EXTERNAL HEAT GAIN

In addition to endogenous heat production, heat can be gained from the surroundings as well. This is obvious when exercising under the hot sun. The sun radiates thermal energy onto our bodies and that is why wearing a dark-coloured shirt will absorb more of the sun's heat.

In order to keep the body temperature at a safe level, the increase in heat gain and production is balanced by the following mechanisms.

  • Take precautions to guard against heat stress

  • With global warming and the increasingly warmer weather in Singapore due to the Urban Heat Island effect, there are growing health concerns over the safety of endurance races.

    Ambient temperature can be as high as 31 deg C even when the sun sets in the evening. This may pose serious health concerns if one exercises at high intensity under such weather conditions without proper preparations. Even elite runners can succumb to heat stress when they are not prepared for it.

    During exercise, there is an increase in body metabolic heat production as well as an increase in sweat production for evaporative heat loss.

    However, if the body's thermoregulatory responses fail to match the increased heat production, there will be a consequential rise in body core temperature, a condition known as exercise-induced hyperthermia (EIH).

    When exercising in hot weather, heat loss via skin surface vasodilation and sweat evaporation is restricted, and EIH can be further exacerbated.

    EIH is a known factor that impairs endurance performance in the heat. As the core temperature rises, the body experiences difficulty in sustaining the exercise intensity due to increased competition for resources and energy, while trying to regulate the amount of heat in the body.

    This presents a significant cardiovascular strain to the body as it strives to maintain the necessary blood supply to sustain the energy requirement of the exercising muscles.

    If left unchecked, EIH not only affects endurance performance, but may also lead to serious heat-related injuries such as heat exhaustion, heat cramps, and syncope (fainting).

    Severe heat stress may even result in potentially fatal illnesses such as heat stroke. Common symptoms of heatstroke includes nausea, unconsciousness and seizures.

    As such, it is important for those with a history of cardiovascular-related diseases to avoid strenuous activity in the heat and to always check with a doctor when uncertain.

    In the next article, we will be sharing some tips to prepare against potential heat stress while exercising, so look out for next week's article to learn more about the do's and don'ts when exercising in the heat.

CONVECTION

Do you often hear the whir of your laptop fan beginning when it gets too warm? The same mechanism is used to cool our bodies. Wind and cool air moving around the body helps to dissipate heat - that is why it feels so good to stand in front of a fan after a run.

To further enhance this process, also known as convection, blood vessels under our skin expand so that more blood flows to the skin. This allows internal heat to be brought to the skin surface to promote convective heat loss. That is why sometimes we appear to be in the "pink of health" after a bout of exercise.

EVAPORATION

When water is converted into vapour, it requires heat energy. Accordingly, our bodies are smart enough to harness this process by sweating.

Sweating places a thin layer of water on our skin which, upon evaporation, removes heat from our skin. That is why we sweat more when we run harder or when the weather is warmer. The body attempts to produce more sweat in the hope of removing excess heat through greater sweat evaporation.

However, this process can be limited when the ambient humidity (amount of water vapour already in the air) is high. Thus, heat loss via sweat evaporation is often less efficient in humid Singapore and exercising often leaves us sticky and sweaty.

EXERCISE-INDUCED HYPERTHERMIA

When there is an imbalance in the above processes, with more heat production/gain than heat loss, it can sometimes result in exercise-induced hyperthermia (overheating of the body).

Hyperthermia is a known issue that impairs endurance performance in the heat. We have not seen a marathon world record broken in the tropics, have we?

If left unchecked, hyperthermia can even result in serious health issues such as heat cramps, heat exhaustion and heat stroke.

We will discuss this further next week. For now, stay cool, stay safe.

A version of this article appeared in the print edition of The Straits Times on August 26, 2017, with the headline 'Feeling the heat? No sweat, it is all just a part of the process'. Print Edition | Subscribe