Dear Runners,

So at the beginning of the fall training cycle it is hotter than the blazes outside and in addition to trying to build your base and increase your speed you have to deal with the heat.  If you are an experienced runner, you know the routine and may be somewhat familiar with getting used to the heat.  You may even be somewhat or fully acclimated to the heat since it has been hot in Louisville for a while.  For this article I wanted to spend some time talking about the heat and the physiological toll that it takes on the body. 

Running in the heat changes the body.  And the first few runs in the hot weather often feel sluggish as the body is acclimating.  This week’s article touches on the different changes that take place in the body during this adaptation process. 

Running in the Heat

If you have been running for more than a season, you have likely encountered the different way your body feels when you are running in the heat versus the cooler temperatures and you may have wondered what was really going on physiologically.  Or maybe you just felt bad and didn’t like it. 

As you may have assumed, when we exercise our cardiovascular system delivers blood to both the muscles (to provide oxygen and fuel) and also the skin (to provide thermal regulation via heat removal).  During activity of short duration, low intensity, or in cooler temperatures, thermal regulation is less of an issue.  However, as ambient temperature, exercise intensity, and duration increases, thermal regulation becomes increasingly more difficult.  Simply put, these harsher conditions make the body work harder at keeping cool.

What happens is that the body regulates heat by changing the distribution of the blood delivered throughout the body.  Specifically, when it is hot outside, the skin will receive a greater portion of the blood flow and the muscles will receive less.  The blood vessels of the skin and the muscles vasodilate or widen to facilitate increased blood flow and other organs that are not undergoing such high demands will have their blood vessels vasoconstrict and the heart will begin to beat faster in an attempt to reestablish cardiovascular homeostasis.  So basically, when it’s hot outside, to put the body is in thermal homeostasis more blood goes to the skin leaving less blood for the muscles. 

This increased blood flow to the skin facilitates heat transfer in two ways.   First skin temperature is elevated thus facilitating convection and second the skin is cooled through sweating.  If those mechanisms can achieve thermal homeostasis then everything is fine, except there is still less blood available to the muscles which will cause decreased performance. 

However, when ambient temperature, intensity, or duration are too high relative to the body’s acclimation, then the body’s core temperature will continue to elevate leading to fatigue, decreased performance and possibly injury.  In this case, we need to worry about not only reduced performance, but we also need to be concerned about injury. 

Other Thermal Considerations

As you can imagine there are several variables that must be considered when trying to manage thermal regulation.  Many of these factors are external and include the impact of wind, cloud cover and clothing, to name a few. 


When we are running into the wind or the wind is coming from the side, there is a thermal benefit as the air crosses our skin in that it increases heat loss through convection (if the skin is dry), assuming that the air temperature is less than the skin temperature or evaporation (if the skin is sweaty).  However, if there is no wind, or if there is a tailwind that is equal to our pace then we are in a wind neutral environment which can lead to heat increase. 

But in general, as we all know, a slight breeze is a good thing and helps to keep us cool.  Just remember that direction is important too.  For instance, a breeze blowing at 5 MPH on the back of a runner who is doing 12 minute miles will basically yield no benefit.  However, that same breeze blowing from the side will have a nice impact. 


Clothing creates a microclimate around the body.  While running in heat this microclimate is further facilitated by a water vapor barrier that is created as we begin to sweat and that sweat begins to evaporate.  The thermal properties of clothes are measured in CLO (clothing) units where one CLO unit is the thermal equivalent of the amount of insulation that is provided by a piece of clothing which will keep a person comfortable at approximately 70°  F in low humidity and low wind.  The average person at rest needs to wear about 12 CLO units to remain comfortable.  However, a runner clocking a 10 minute mile only needs about 3 CLO units to remain comfortable while running. 

The Physiology of Getting Acclimated

There are three phases of heat acclimation which I call initial, comprehensive and transformative. 

During the initial phase of heat acclimation the runner will feel the biggest difference but is also subject to the greatest risk of heat illness.  During this time, the most noticeable physiological change is an increase in the plasma (blood) volume.   Yes, that’s right…Your body actually makes about one more pint of blood to help the body function during heat. 

Other changes that occur include cellular and molecular modifications to increase tolerance of heat.  With this increased blood volume the body is able to allocate a larger supply of blood to both the muscles as well as the skin.  If the temperatures are not too high then the body will be able to fully feed the muscles while fully regulating core temperature. 

As the acclimation process continues the flow of blood to the skin changes dramatically while central blood volume and blood pressure are increased.  In this case the blood pressure increase is a correction for the decrease we discussed previously.  In addition the body also undergoes glandular hypertrophy where the sweat glands increase in size.  While this is occurring, the skin temperature also increases to promote heat transfer through convection.  These secondary changes are more gradual but involve more parts of the body. 

In the third stage, full acclimation can occur which will actually lower skin temperature.  This condition may not appear in all athletes but may be present in runners who are indigenous to hotter climates. 

The Acclimation Process

To become acclimated does not mean that you can run forever in any conditions.  You will likely still incur performance degradation as a function of the heat.  However, it does mean that you can regulate your body temperature more efficiently.  So the question becomes, what do you actually need to do to get acclimated to the heat and how long does it take? 

In general significant changes in heart rate, blood volume, skin temperature and sweat rate will occur in just one week of training in heated conditions.  In fact, changes in heart rate can be seen in as little as 4 or 5 days.  However, full adaptation can take up to a month. 

It is also important to note that when running in conditions that consist of high intensity, high temperature, or high humidity most runners will have to modify their pace and/or duration to finish.  This means that while adaptations will occur for runners becoming conditioned to heat that does not mean that once conditioned that performance will be the same in high temperatures as they are in low temperatures. 

In general, to become acclimated to heat the best process is through daily training sessions lasting between 30 and 100 minutes.  Additional duration of each day’s session is not shown to be more effective than having shorter sessions on additional days.  As a result, a runner who is conditioned to running 30 miles per week would do well to have several sessions of 3-7 miles instead of a couple of sessions totaling 10 miles each.  Furthermore, for the purpose of heat acclimation sessions over 100 minutes are not shown to produce additional acclimation value but can be valuable from a muscular training maintenance perspective and also a mental training perspective.  Just be careful of heat injury.  

In addition, since one of the biggest adaptations that occurs is the increase in blood volume then the body needs additional nutrients.  During this transition phase, you should be drinking increased fluids in the form of water and juices.  This is in addition to eating an already healthy diet.  This combination will help give your body the fuel it needs to create the extra blood. 

A Really Cool Upside

If you have run for a few seasons then you have probably experienced that first run in fall (after the temperature becomes cooler) where you feel like you are flying but your are not exerting a great deal of effort.  Here’s why.  Remember the largest physiological adaptation from the initial phase of acclimation is an increase in blood volume?  This additional blood (about 1 pint) will remain in the body as long as the body remains acclimated to the heat. 

However, when it turns cold an interesting benefit occurs.  Since the body no longer needs to divert a significant volume of blood to the skin for cooling, all of that blood becomes available for the muscles!  This extra blood to the muscles feeds them with additional oxygen and fuel.  It is like giving the body an extra pint of blood and saying “have fun.”  Muscles love the extra fuel and oxygen and respond with increased performance (fast running). 

Unfortunately, the body becomes reconditioned to the cold in about the same time as it becomes conditioned to the heat.  So after about a week the majority of the benefit is gone. 

Running In the Heat – Management

In order to run in the heat there are several things runners need to consider and two of the most important revolve around managing hydration (fluid consumption minus fluid loss) and electrolytes (largely lost through sweating).


When we talk about managing hydration we are primarily trying to replace the fluids that the body loses through sweat.  To manage hydration, runners need to start by understanding their sweat rate.  The sweat rate is literally the amount of fluid your body loses during a run in a given period of time and that rate will vary from runner to runner and also fluctuate depending on ambient conditions such as temperature and humidity.  Fortunately calculating your sweat rate is pretty easy.  Here are the steps:

  1. Weigh yourself before a run.  It is best to weigh yourself naked as you will understand in a minute.
  2. Run for a fixed period of time, usually an hour (be sure to get dressed again first)
  3. Track the amount of water you consumed during the run (in ounces)
  4. Weigh yourself again after the run.  It’s best to dry off and weigh yourself naked so you don’t count the weight of the sweat in the clothes. 

Now to calculate the sweat rate, take your weight in step 1 and subtract your weight in step 4, convert that to ounces.  Now, add any water you consumed during your run, also in ounces. 

Here’s an example.  Let’s assume a runner weighed 150 pounds before a run, then went out and ran for an hour and came back in and had a new weight of 148.5 pounds.  That runner lost 1.5 pounds or 24 ounces.  Now, let’s also assume that runner also consumed 8 ounces of water during the run.  Then that runner’s sweat rate would be 24 (ounces lost) + 8 (ounces consumed) =32 ounces total per hour. 

Based on this example the runner should consume 32 ounces of water per hour or 8 ounces every 15 minutes to maintain equilibrium.  However, most people runners will not consume this volume of liquid.  In fact, when athletes drink ad libitum (as they see fit) they will typically enter a state of voluntary dehydration or underhydration.  Most athletes can only consume about 16 to 30 ounces of liquid per hour without feeling gastric distress or feeling bloated.  However, that is generally not a problem because some dehydration is acceptable during exercise. 

The key then becomes to understand your sweat rate so that you can balance your fluid intake to offset your sweat loss as closely as possible without suffering GI issues.   That is another helpful part of the training runs because you get to practice how much liquid your body needs and can tolerate.

I have found that sweat rates can really be surprising to people.  The example here only talks about a 1.5 pound loss.  However, I have seen several runners who routinely will lose 3 or 4 pounds during a hot and humid run.  If you are not consuming liquids during these runs you will also likely incur performance degradation as a result of dehydration that is in addition to your performance loss due to the heat.  So do the sweat test now so you know how your body performs. 


The second component of heat management centers on electrolyte replacement.  Okay, this is such a long topic that I could literally write note around this single topic alone and we don’t have room to do that.  The key here is that the electrolytes are needed to regulate several components of your body including extracellular and intracellular fluid content. 

Therefore replacement of only the water component of sweat is typically not enough to maintain performance.  This is particularly true when exercise duration exceeds one hour or when heat or humidity is increased.  Bottom line, when running in these conditions it is best to use a sports drink or electrolyte replacement. 

One final note on electrolyte replacement and that is a runner’s electrolyte replacement needs will be greater during the beginning phases of heat acclimation. During this initial acclimation phase the electrolyte concentration (particularly salt) in sweat is higher than in an acclimated athlete.  So the electrolyte loss is greater as well.  Often runners will notice a white, chalky substance on their face and body after a long run.  This can be particularly true when it first turns hot.  This is salt that is coming through the sweat.  As the body adapts this salt concentration typically reduces. 

A practical tip about electrolyte replacement and hydration…if you are running and fell that sloshing feeling in your belly like the liquid you drank is not being absorbed it may be caused by an electrolyte imbalance.  In other words, drink a sports drink and it will probably go away. 

Hopefully, this will help you have a good run and adapt to the new heat. 


Okay, sorry for the long note…time for a video.  This week’s video is designed to help you speak the vernacular of non-runners.  Since they often don’t understand our world, maybe this video will help to bridge the cultural divide.  Enjoy!  How to speak to non runners –

Run well…


Shawn Herbig is an RRCA Certified Running Coach.  In his day job he owns a market research and data analytics firm called IQS Research. 

© 2013 Shawn Herbig