Running in the Heat: What Summer Does to Your Body

Yes, you can run in the heat. No, noon in August is not a personality test.

Is summer approaching and you're feeling more tired than usual? Is that 5K effort beginning to feel like a half marathon? The fatigue you're feeling isn't a funk, and it's probably not overtraining. You might be hitting a wall for a reason you're not considering: the heat.

Fun fact: heat interferes with your cognitive functions. So the same weather that slows you down also makes you worse at deciding whether to slow down. Nature has a sense of humor.

Once your core temperature climbs toward 100 degrees Fahrenheit and beyond, fatigue sets in and muscle activation decreases. Numerous studies show runners slow down as core temperature rises. Dr. Michael Bergeron's 2014 work found that heat degrades runners' physiology, perception of effort, and performance all at once. So what's happening?

Humidity

High humidity prevents sweat from evaporating, and evaporation is your main cooling system. Add clothing that traps sweat against your skin and you've limited your body's ability to shed heat. The result can be a climbing core temperature, which stresses the cardiovascular system. Light, breathable clothing helps; it lets sweat do its job.

A note on rhabdomyolysis (rhabdo), since it comes up in every scary summer-running article: rhabdo is a serious condition in which muscle tissue breaks down and releases its contents into the bloodstream. It's driven by extreme muscular overexertion, the kind you get from going far beyond what your body is trained for. Heat stress can compound the risk, but your outfit does not give you rhabdo. If you develop severe muscle pain, weakness, or dark (cola-colored) urine after a hard effort, that's an emergency-room situation, not a stretch-it-out situation.

Sweat

Your sweat rate climbs with the temperature, and sweat loss is fluid loss, which puts your heart under extra strain. You can't avoid sweating (nor should you want to), but on long or intense summer runs the goal is to keep the fluid deficit from getting out of hand. Drink to thirst on the run, and go in well hydrated. We wrote a whole piece on how much runners should drink, including why more is not always better.

Rehydration and electrolytes

Ever wonder why beverage companies push sports drinks on athletes? Rehydrating involves more than water. Long, sweaty efforts drain sodium along with fluid, and replacing fluid without any sodium can compound the problem. For runs over an hour in the heat, an electrolyte drink or salty snack earns its keep.

Which brings us to hyponatremia. Ever get a headache or nausea during or after a hot run and congratulate yourself for working hard? You may have been flirting with something worse. Hyponatremia is a dangerously low blood sodium level, and one way runners get there is drinking large volumes of plain water while sweating out salt. It can be deadly. Details in the hydration post.

The practical part

A few other things to consider before a summer run. Heat interacts badly with certain medications and medical conditions; if that might be you, ask your doctor before logging hot miles. Beyond that:

• Run early morning or late evening instead of midday.
• Swap a weekly road run for treadmill time when the forecast is hostile.
• Slow down on purpose. Your easy pace in July is slower than your easy pace in October, and that's physiology, not weakness.
• Cut the run short if you feel dizzy, confused, or stop sweating. Those are warning signs, not challenges.

If you're determined to let off some steam and do what frozen things do in summer, go for it. Just be wise about it.