Heat, Hydration & Exercise

I’ve struggled cycling in the blistering heat of the Atacama Desert, shivered through 8 hour night-long ultra marathons in the Irish winter and suffered with altitude sickness in the beautiful but 3399m above sea level town of Cusco, Peru. Climate and weather conditions have always proved challenging in the world of sport. But this years Irish Summer has brought the country to its knees. Hose pipe bans and fines imposed for those caught using them has meant little relief from the heat either. And I’m not giving out about it. It’s been great. But tell that to the once green fields of Ireland (or corn field as my kids now call our lawn), my poor one year old garden shrubs (all but dead), and my kids that are playing in the same paddling pool of water since before the hose pipe ban came into place. The rise in temperature also coincided with me finally deciding to get my ass in gear with a more structured marathon training program – kind of bad timing as the physiological adaptations with taking up a training regime are difficult enough without throwing heat into the mix. So what are the factors to be considered with training in the heat?


Typically in Ireland we’re faced with problems of keeping warm while exercising but not in these last few months. The GAA have allowed water breaks during peak temperatures and additional water carriers during inter county games all in an attempt to reduce the incidence of heat stroke and exhaustion. The temperature of the skin can vary widely, depending on the environmental temperature, but the temperature of the deep tissues must be maintained within only a few degrees of the normal resting level of about 37°C. So the rate of heat gain by the body must be balanced by the rate of heat loss. Any imbalance will result in a change in body temperature. To limit the potentially harmful rise in core temperature, the rate of heat loss must be increased accordingly, and is done through sweating.

How does dehydration affect performance?

The simple truth is that no other nutritional intervention (at least legal) comes close to providing the performance-enhancing effects of being and staying well-hydrated. As little as 2% dehydration can impair physical performance by 10%. There are three dominant mechanisms that affect performance outcome:

  1. Dehydration-induced reductions in blood volume and cardiac output lead to increases in the relative cardiovascular and physiological stress of exercise. For anyone exercising in the heat in the last few weeks you may have noticed your heart right higher than normal for the same perceived effort- I certainly have.
  2. Hyperthermia: the normal thermoregulatory response to exercise becomes impaired resulting in an uncompensable increase in core temperature.
  3. Secondary to both previous mechanisms comes an increased reliance on carbohydrate for energy provision resulting in more rapid depletion of muscle glycogen.

How does dehydration occur?

During exercise, dehydration occurs in two different ways. Firstly an athlete takes in less fluid than is required to offset fluid losses through sweating. In other words the fluid in does not match the fluid out. Secondly sweat rates exceed the maximum rate of tolerable fluid ingestion. But remember sweating is also an essential part of thermoregulation, despite deodorant sprays attempts to block those embarrassing underarm sweat patches (for up to 48 hours some will claim).We cannot prevent the body from losing fluid as it is a necessary phenomenon to regulate body temperature but we can prevent or at least limit the body from becoming dehydrated. We do this by:

  • ensuring adequate hydration before exercise commences

  • replacing fluids lost during exercise
  • replacing fluids lost after exercise

Best practice for hydration in sport – before activity

Begin hydrating at least 4 hours before the training/match e.g. when you wake, drink a pint of water and 500mL thereafter every two hours. This should suffice. There is no need to be guzzling on a big bottle of water all day, and running to the toilet every half hour. There is no evidence that such large intakes provide any added benefit. But if you do not produce urine within 2 h, or the urine is dark or highly concentrated, a larger intake is needed. Your urine should be almost see through and clear and a simple way to determine hydration status.

Best practice for hydration in sport – during activity

When possible, fluid should be ingested at rates that most closely match sweating rate. Generally 250mL every 20 mins is a good guideline. Water is better than no fluid but research shows that a drink with carbohydrate and electrolytes is better than water alone. This can be quite individual and I have my own horror stories of over ingesting on carbohydrates during exercise. I stick with plain old fashioned water.

Best practice for hydration in sport – after activity

As well as a deficit in body water after dehydration through sweating, a deficit in electrolyte balance is generally present. Therefore we need to replace both water and salts lost in sweat. We should aim to drink approximately 1.5 L of fluid for each kg of body mass lost during activity. And this can be difficult. I once lost 3kg after a marathon which meant taking on board 4.5 L of fluid. For me that’s as challenging as the marathon itself. But the good news is this doesn’t mean pure water consumption. Normal meals and snacks with a sufficient volume of plain water will restore hydration levels, provided the food contains sufficient sodium to replace sweat losses. Drinks containing sodium such as sports drinks may be helpful, but many foods can supply the needed electrolytes.

Heat acclimatisation

The human body is extremely good at adapting: it lays down collagen around injured tendon in an attempt to make it stronger; for malnourished females with eating disorders it grows more surface hair in an attempt to retain more body heat. We see this every day and much like the body accommodating to altitude or cold conditions it can also get used to hot environments. But the amount and rate of heat acclimation depends on training status, duration of exposure, and rate of internal heat production. Repeated exposure to heat stress during exercise improves your ability to get rid of excess heat through an increase in sweat rate and that sweat becoming more dilute. When not used to exercising in the heat blood flow is redistributed from non essential regions to the skin to aid in heat dissipation. However as the body gets acclimated the blood flow to the skin is reduced leaving more blood available to the muscles. However this takes time. A 10-14 day period of heat acclimatisation is generally recommended. The initial few days of exercising in the heat generally lead to cardiovascular adaptations with the changes to the body’s ability to sweat differently taking longer. So maybe there’s value to those warm weather training camps after all. Get your sales pitch ready for the club manager for next season.

For more information on any of the issues addressed throughout this article please contact Rob at mccabephysiotherapy@gmail.com, Twitter , Instagram and Facebook or visit http://www.mccabephysiotherapy.com

Rob McCabe MISCP

Chartered Physiotherapist

MSc (pre reg) Physio, BSc Sport Science and Health, MSc Sports Physiotherapy, PG Dip Orthopaedic Medicine

Orchard House, Moorefield Rd, Newbridge, Co. Kildare


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