Running in the heat: benefits, costs and how to adjust

Running in the heat benefits you in one specific way: do it regularly for a week or two and your body adapts, so hot runs get easier and you cope better on hot race days. Until then, heat costs you. Your heart rate climbs, your pace drops and the same run feels harder. Neither of those is a character flaw. It's physics with a pulse.

Below: what heat does to a run, what adapting buys you, how to adjust pace and fluids, and when the run is over. Plan it by hand or with a running coach app that also fits your lifting around it.

What running in the heat does to your body

Your muscles produce heat when they work. On a cool day, that heat escapes easily. On a hot day, your body has to push more blood to the skin and sweat more to get rid of it, and that blood is no longer available to your working muscles.

A review by Cheuvront and colleagues (2010) puts the main cost down to cardiovascular strain: high skin and core temperatures reduce VO2max, so any given pace becomes a higher relative effort. You either tire sooner at the same pace or slow down to keep the same feeling of effort. The same review notes that once dehydration passes about 2% of body mass, aerobic performance is consistently impaired, on top of the heat itself.

The race data says the same thing. An analysis of seven major marathons over decades of results (Ely et al., 2007) found a progressive slowing of marathon performance as the wet-bulb globe temperature (a heat index that includes humidity and sun) rose from 5 to 25°C. Top male finishers were 1.7% slower than the course record in the coolest conditions and 4.5% slower in the warmest, and slower runners were hit harder than the leaders.

If you're new to running, this is worth knowing before your first summer. A beginner plan built for the cooler months will feel harder in July, and that's the weather, not you going backwards. The fundamentals don't change, from easy pacing to running drills explained as a warm-up habit. The dose does.

Running in the heat benefits: what adapting buys you

A consensus statement by Racinais and colleagues (2015) calls heat acclimatization the most important intervention to reduce physiological strain and optimize performance in the heat, and says it should comprise repeated exercise-heat exposures over 1 to 2 weeks.

What changes, according to a review by Périard, Racinais and Sawka (2015):

• ›Improved sweating
• ›Improved skin blood flow
• ›Lower body temperatures
• ›Reduced cardiovascular strain
• ›Improved fluid balance

The same review says heat acclimation improves aerobic performance in warm-hot environments and "potentially" in temperate ones. That word matters.

A meta-analysis by Tyler and colleagues (2016) of 96 studies found heat adaptation had a moderately beneficial effect on exercise capacity and performance in the heat, with longer regimens more effective than shorter ones. Regimens under 14 days produced many useful adaptations, but the biggest effects came from regimens longer than 14 days.

Do the benefits carry over to cool weather?

Maybe. In one often-quoted study, 12 trained cyclists did a 10-day heat acclimation program and improved VO2max by 5% and time-trial performance by 6% in cool conditions, with no changes in a control group that trained in the cool (Lorenzo et al., 2010). That's a small study, in cyclists, under lab conditions. File "heat training makes you faster in autumn" under promising idea, not a sure thing. The well-supported benefit is coping better in the heat.

How to adjust your runs on hot days

Run by effort, not pace

Your normal easy pace is a cool-weather number. On a hot day, hold the effort and let the pace go. Easy runs stay conversational even if the pace on your watch looks embarrassing. Heart rate will drift higher than usual for the same pace, which is exactly the point of the Cheuvront review above.

For hard sessions, either shorten the reps, lengthen the recoveries, or move them to a cooler time. A tempo run that comes in slower than your spring splits, at the same effort, is still a tempo run.

Pick the time and the route

The NHS advises avoiding the sun between 11am and 3pm in hot weather (NHS). For runners, that usually means early morning or later in the evening. It also recommends light-colored, loose clothing. Pick shaded routes and loops that pass water.

Hills cost more in the heat because they push your effort up when you're already working harder to stay cool. Hill running explained is a summer skill as much as a strength one: hold the effort uphill, and walk if you need to.

Thick, humid air also makes breathing feel harder, even for healthy lungs. If you have asthma, follow the plan your doctor gave you. The basics of how to breathe when running with asthma or without it start in the same place: slow down until your breathing settles.

Warm up less, cool down more

A hot day does half your warm-up for you. Cut the easy jogging, keep the mobility and a few strides, and don't spend 20 minutes heating up before the session starts. Running warmup explained still applies; the dose just shrinks. A few short drills fit well here too.

After the run, walk in the shade for a few minutes rather than stopping dead in the sun. A proper running warm up and cool down routine matters more in summer, mainly because it gets you out of the heat gradually and gives you time to start drinking.

How much to drink when running in the heat

Fluid needs vary a lot between people, so the most useful numbers are ranges plus a way to find your own. These come from the joint position stand of the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada (Thomas, Erdman and Burke, 2016).

A few more points from the same document (Thomas, Erdman and Burke, 2016):

• ›Sweat rates range from 0.3 to 2.4 L per hour, depending on intensity, fitness, heat acclimatization and conditions. That's why one person's drinking plan is useless to another.
• ›Find your own number. Weigh yourself before and after a run, accounting for what you drank. A loss of 1 kg represents approximately 1 L of sweat.
• ›Sodium matters for heavy or long sweating. The position stand says sodium should be taken during exercise with high sweat rates (over 1.2 L per hour), salty sweat, or exercise lasting more than 2 hours.
• ›Cold drinks may help. Cold beverages may help reduce core temperature and improve performance in the heat.
• ›Don't overdo it. Drinking more than you sweat is the primary cause of hyponatremia (low blood sodium), and the position stand notes over-hydration is typically seen in recreational athletes. More is not automatically better.

For longer runs, carry fluid or plan a route past water. This is general guidance, not a personal prescription. If you have a heart, kidney or other medical condition, talk to your doctor about fluids and heat.

A simple heat-acclimatization plan

You don't need a heat chamber. You need a couple of weeks of sensible exposure, built into running you were already doing:

1. ›Start with easy runs in the warmth, not your hardest session. Keep them short at first.
2. ›Run in the warmth regularly over 1 to 2 weeks, in line with the consensus statement's 1 to 2 week window (Racinais et al., 2015).
3. ›Hold effort constant and let the pace come back as you adapt.
4. ›Keep the hard sessions cooler in that first week: early morning, shade, or indoors.
5. ›If a race is in the heat, arrive acclimatized rather than hoping for a cool day.

If you lift too, heat doesn't change the scheduling rules: keep heavy legs away from the day before your intervals or long run. It just means your hard runs may need the cooler time slots, so plan the gym around them.

Heat exhaustion and heatstroke: when to stop

This is the part that isn't about performance. The NHS lists these warning signs (NHS: heat exhaustion and heatstroke).

The symptoms of heat exhaustion include:

• ›tiredness
• ›dizziness
• ›headache
• ›feeling sick (nausea) or being sick (vomiting)
• ›excessive sweating and skin becoming pale and clammy (a change in skin colour can be harder to see on brown or black skin)
• ›cramps in the arms, legs and stomach
• ›a high temperature
• ›being very thirsty
• ›feeling irritable

The NHS says that if someone is showing signs of heat exhaustion, they need to be cooled down and given fluids, and they should start to cool down and feel better within 30 minutes. It also says: "Get help from NHS 111 now if: you or someone else have symptoms of heat exhaustion that you're struggling to treat or you need advice about."

The NHS says to call 999 now if you or someone else have signs of heatstroke, including:

• ›still being unwell after 30 minutes of resting in a cool place, being cooled and drinking fluids
• ›a very high temperature
• ›hot skin that's not sweating and might look red (this can be harder to see on brown or black skin)
• ›a fast heartbeat
• ›fast breathing or shortness of breath
• ›confusion and lack of coordination
• ›a seizure or fit
• ›loss of consciousness

In the NHS's words: "Heatstroke is a medical emergency. Get immediate medical help if someone has the symptoms of heatstroke." It also says: "Do not drive yourself to A&E." Outside the UK, call your local emergency number. And note the ACSM position stand on exertional heat illness warns that exertional heatstroke can affect seemingly healthy athletes even when the environment is relatively cool. Being fit doesn't make you immune.

The practical rule: stop when something feels wrong rather than tired.

How exrsz handles this

exrsz builds a periodized plan from your goal, schedule, level and equipment, with running and lifting balanced in one plan. It adapts each week to how you actually trained, so a hot week where your runs came in slower is treated as training that happened, and missed sessions get reshuffled rather than stacked up.

FAQ

What are the benefits of running in the heat?

Regular running in the heat leads to heat acclimatization: better sweating and skin blood flow, lower body temperatures and less cardiovascular strain (Périard et al., 2015). The main payoff is better performance in hot conditions, with some evidence of benefits in cool weather too.

How long does it take to acclimate to running in the heat?

A consensus statement recommends repeated exercise-heat exposures over 1 to 2 weeks (Racinais et al., 2015). A meta-analysis found the largest adaptations came from regimens longer than 14 days (Tyler et al., 2016).

How much water should I drink when running in hot weather?

It depends on your sweat rate. The ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position stand says a typical plan lands at 0.4 to 0.8 L per hour, aiming to keep fluid losses under 2% of body weight without drinking more than you sweat (Thomas, Erdman and Burke, 2016).

Should I slow down when running in the heat?

Yes. Run by effort rather than pace, because heat makes any given pace a higher relative effort (Cheuvront et al., 2010). Your pace returns as you acclimatize and the weather cools.

When should I stop running in the heat?

Stop if you feel dizzy, confused, sick or unwell in a way that isn't normal tiredness, then get cool and drink. The NHS says to call 999 for signs of heatstroke, such as a very high temperature, hot skin that isn't sweating, confusion or still being unwell after 30 minutes of cooling (NHS).