Heat cramps: What you need to know

From the desk of
Robb Wolf
ScienceHeat cramps: What you need to know

If you’ve exercised on a hot day, you may have experienced heat cramps. These painful, sustained muscle contractions are a common form of heat-related illness. 

The term “heat cramps” is a little misleading, though. Heat doesn’t actually cause cramps directly; rather, it’s increased sweating — and particularly the accompanying loss of sodium — that is the leading contributor to these types of cramps.

Because heat cramps usually result from sweating, they’re relatively easy to prevent. You can reduce exertion when it’s hot out, and replace fluids and electrolytes (especially sodium) when you sweat.

I wrote this article to give you the complete rundown on heat cramps. We’ll cover how heat cramps present, how they’re different from muscle cramps, what causes them, and prevention.

What Are Heat Cramps?

Heat cramps are sustained, involuntary muscle spasms — usually in large muscle groups. They often occur in athletes exercising in the heat (tennis players, football players, etc.) but can affect anyone who sweats.

The term “heat cramping” was coined in the Industrial Era to describe the frequent cramps incurred by workers laboring in the heat. That’s an interesting bit of history that I’ll cover in a moment.

Besides painful muscle contractions, other heat cramp symptoms may include:

  • Nausea
  • Vomiting
  • Fatigue
  • Excessive sweating 
  • Weakness
  • Rapid heartbeat 

These symptoms don’t necessarily have the same cause as heat cramps (electrolyte deficiency due to sweating). Simply being outside too long in 100℉ or greater temperatures can cause heat exhaustion or heatstroke. Any condition of impaired thermoregulation can lead to a variety of complications. If you get hot enough for long enough, not even sweat can keep you cool.

Heat Cramps vs. Exercise-Associated Muscle Cramps

Researchers often differentiate between heat cramps and other muscle cramps incurred during exercise. These other muscle cramps, they argue, aren’t always heat-related because cramping during exercise isn’t always linked to core body temperature, and can occur even during cool-weather exercise.

For instance, you can sweat a ton while exercising in cold climates and, as a result, develop severe cramps. Personally, I think this is a solid argument to just call both types of cramps, “sweat cramps,” since they both share a common factor: loss of sodium and fluids through sweat.

It’s also not clear that exertion alone causes muscle cramps. Sure, marathoners tend to cramp more towards the end of a race, but is that because they’re fatigued or low on sodium? I lean towards the latter explanation. They often have two forces pushing down their sodium levels:

  1. Loss of fluids and electrolytes through sweat
  2. Overhydration with plain water (which dilutes blood sodium levels)

That’s a formula for muscle cramps. I’m not dismissing the influence of fatigue, but I believe low sodium is the more significant driver.

What Causes Heat Cramps?

As I covered earlier, sweat, not heat, is the primary cause of heat cramps. According to NIH StatPearls, heat cramps are caused by electrolyte deficiencies in sodium, chloride, potassium, and magnesium.

Because sodium is the primary solute lost through sweat, it’ll remain my focus in this article. But I’ve covered potassium and magnesium in the past. Check out those articles to understand their benefits and best sources.

Now, let’s zoom in on the evidence connecting salty sweat to heat cramps.

Why Sweating Salt Causes Heat Cramps

I recently came across this 2012 paper titled, “The role of sodium in ‘heat cramping,’” published in the journal Sports Medicine. In the paper, Dr. Randy Eichner presents three lines of evidence why salty sweating causes heat cramps:

#1: Historical evidence

During sea voyages, ships’ crew used to drink seawater to prevent heat cramps. Sounds tasty, right? I think I’ll stick with LMNT, which includes a science-backed ratio of sodium, potassium, and magnesium.

Moving on, during the Industrial Era:

  • Drinking saline reduced cramps in English factory workers.
  • Men building the Hoover Dam cramped when they replaced lost fluids with water only. When they replaced water AND sodium, the cramping improved.
  • Steelworkers experienced fewer cramps when supplementing with salt.

The takeaway? Salt deficits have been cramping up humankind for a long time.  

#2: Athletic evidence

Any sport that induces sweating can induce heat cramps. Let’s look at tennis, football, and triathlon, for instance.

First, tennis players with a history of heat cramping each lost 2.6 liters of water and 2.7 grams of sodium per hour. These losses interfere with muscle contraction since sodium regulates much of the nervous system. Also note that these athletes lost more sodium per hour than the measly 2.3 grams the US government recommends consuming per day.

College football players also sweat profusely during summer training. In one study, researchers split players into two groups: those who cramp frequently and those who cramp infrequently. Both groups lost around the same volume of sweat, but frequent crampers lost about twice as much sodium at 10.4 grams per day! The saltier the sweat, the more frequent the cramps.

Finally, triathletes who cramp frequently tend to have less sodium in their blood. This doesn’t necessarily disqualify the exertion-causes-cramps hypothesis, but it does suggest we should Stay Salty.

#3: Clinical evidence

It’s one thing to identify that frequent crampers have saltier sweat. It’s another thing to show that consuming salty fluids prevents and reverses cramping.

Well, it’s been shown. Consider the following:

  • When Harvard researchers studied heat cramping among Hoover Dam workers, they noted that all cases were symptom-free six hours after IV saline treatment.
  • Tennis players who consumed salt (or sports drinks containing salt) at the first signs of muscle twitching stayed cramp-free on the court. Similar effects hold true for other sports.
  • Medical professionals prevent cramping in dialysis patients by giving them fluids containing electrolytes.

Across the board, we see fewer cramps with proper salt replacement. Next, I’ll cover simple steps you can take to prevent heat cramps.

Preventing Heat Cramps

To prevent heat cramps, seek to:

  1. Stay cool
  2. Stay hydrated with fluids and sodium

Let’s review these tactics in more detail.

#1: Stay cool

When it’s hot or humid out, you sweat more. To reduce sweating and avoid heat cramps, do your best to stay cool. The cooler you are, the less your body needs to cool itself with sweat, the lower your risk for heat cramps.

You don’t need to burrow indoors like a naked mole-rat. Just save the heavy exertion for cooler conditions. Practically speaking, these could mean:

  • Moving your golf outing to a less sweltering day 
  • Jogging in the early morning instead of the early afternoon
  • Working out in an air conditioned room 
  • Spraying yourself with a cool misting fan while your friends watch enviously
  • Moving to a cooler climate (honey, I read an article today… we’re moving to Alaska!)

Let’s talk about hydration.

#2: Stay hydrated

Most people think staying hydrated is synonymous with drinking water. We’d all be healthier if we drank more H2O, right? Not quite.

If you only drink plain water, you’re neglecting a crucial input for energy production, mood regulation, immune health, and more: electrolytes. Sodium is especially essential to the hydration equation, but try not to neglect potassium or magnesium either.

The solution doesn’t have to be complicated: just replace the sodium! Salt your food and drink electrolyte water on sweaty days. That’s it.

We created LMNT to ensure people can hydrate healthily and conveniently. Each stick contains:

  • 1000 mg sodium
  • 200 mg potassium
  • 60 mg magnesium
  • Zero sugar, artificial flavors, or any other junk

Simply add one LMNT stick pack to 16-32 ounces of water to suit your taste, and drink when thirsty. We’ve heard from thousands of customers who credit LMNT for eliminating their cramps. Browse these reviews and see for yourself.

And if LMNT isn’t an option for you, here’s a guide to making your own electrolyte drink mix at home. It’s a little more prep and mess, but the important thing is to Stay Salty.

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