Hydration and Electrolytes for Hot-Weather Training
Updated: 24 hours ago
How much did you weigh when you walked into the gym this morning, and how much when you walked out? Almost nobody knows. Yet on a hot, humid day, that difference is the most useful hydration number you can collect, and it costs nothing but a bathroom scale.
In the Philippines, where I grew up swimming and playing volleyball, heat wasn't an event. It was the default. You learned quickly that the same session feels completely different in thick, humid air at 93°F than it does on a cool morning. If you train in a garage gym, an open-door box or outside in summer, you're dealing with the same reality.
Heat changes the rules. Your heart rate climbs faster, your sweat rate goes up, and your usual pacing stops being accurate. Hydration isn't about drinking as much as possible. It's about arriving ready, replacing what matters during the work and recovering well afterward.
IN THIS ARTICLE
Before: arrive ready, not waterlogged
Good hydration starts hours before the session, not in the parking lot. A widely used sports-medicine guideline is to drink roughly 5 to 7 milliliters per kilogram of body weight about four hours before exercise. For a 75 kg athlete, that's around 400 to 500 milliliters, which is simply a big glass or two of water with a meal.
A regular meal with some salt in it helps you hold on to that fluid. Pale-yellow urine is a reasonable, practical sign you're in a good place. Dark urine is a nudge to drink more; completely clear urine all day may mean you're overdoing it.
The other preparation is acclimatization. Your body adapts to heat over roughly 10 to 14 days of repeated exposure: you start sweating earlier, your sweat becomes a little less salty and your heart doesn't have to work as hard at a given pace. During the first week or two of a heat wave or a move to a hotter climate, reduce intensity and volume on purpose. Your fitness hasn't disappeared. Your cooling system is still catching up.
I didn't always respect this. Years into CrossFit, I went out on the first properly hot day of a summer and paced an interval workout the way I would have in spring. By the third round my heart was pounding, my legs felt fine and my head felt awful. I had to sit in the shade and watch the clock run. Nothing about my fitness had changed that week. The weather had.
During: drink with a plan, not with panic
For most class-length sessions under about an hour, drinking to thirst works well for healthy adults. Keep a bottle within reach, take sips between rounds or during transitions, and don't force down large volumes you don't want.
Longer sessions change the picture. Two-hour Saturday training blocks, outdoor team workouts and competition days in the heat call for a plan. Sweat rates vary a lot from person to person, and in hot conditions many athletes lose somewhere between half a liter and two liters an hour. A common guideline is to limit body-weight loss during a session to less than about 2 percent, because performance and heat tolerance tend to suffer beyond that point.
This is where electrolytes earn their place. Sweat carries sodium in particular, and some athletes, the ones who finish with white salt lines on their shirts, lose a lot of it. I'm one of those athletes, and I used to wonder why plain water never quite made me feel right after long summer sessions. For long or very sweaty work, a drink with sodium helps you retain fluid and reduces the risk of diluting your blood sodium by drinking plain water alone. If you use an electrolyte product, choose one that's third-party tested, especially if you compete.
There's also such a thing as drinking too much. Gaining weight over a session is a warning sign, not a badge of discipline. Overdrinking plain water during long events can lead to low blood sodium (hyponatremia), which can be dangerous. The goal is to replace, not to flood.
Coach's cue: In the heat, slow down early, drink with a plan and let the thermometer set the pace.
After: replace what the scale says you lost
This is where the bathroom scale comes back. Weigh yourself before and after a hot session, with minimal clothing and after using the restroom, and you get a rough estimate of fluid lost. A common recovery guideline is to drink about 1.25 to 1.5 liters of fluid for every kilogram lost, spread over the next few hours rather than chugged at once. The extra volume accounts for the fact that you keep losing some fluid as urine while you rehydrate.
Pair that fluid with sodium, either from food or an electrolyte drink. A normal recovery meal with some salt, carbohydrates and protein does a lot of the work for you. If your next session is within a few hours, be more deliberate. If it's tomorrow, regular meals and drinking to thirst will usually cover it.
Here's the simple sweat test I ask my athletes to run a few times during summer:
Weigh yourself before training with minimal clothing and an empty bladder.
Note how much you drink during the session.
Towel off and weigh yourself again right after.
Subtract the post-session weight from the pre-session weight, then add what you drank. One kilogram is roughly one liter.
Divide by session length in hours for your approximate sweat rate in those conditions.
Do it on a few different days and you'll know far more about your own needs than any generic chart can tell you. The first time most of my athletes run it, they're surprised. Many realize they've been drinking far less than they sweat.
When heat becomes a medical problem
Heat illness isn't something to push through. Stop, get to shade or air conditioning and cool down if you notice dizziness, nausea, headache, unusual fatigue, cramping or goosebumps and chills in the heat. Confusion, collapse, slurred speech or a very high body temperature are signs of a medical emergency; call emergency services and start cooling the person immediately, ideally with cold water.
Coaches, this applies to your class too. Adjust workouts on hot days, schedule water breaks and watch the athlete who always tries to prove something. If you have a heart condition, high blood pressure, kidney issues or take medication that affects fluid balance, speak with your doctor about training in the heat.
Adjusting the workout, not just the water bottle
The best hydration plan in the world doesn't make a hot day feel like a cool one. Train earlier or later in the day when you can. Cut volume by a round or two, extend rest periods and pace by effort rather than by the splits you hit in spring. Your heart rate will run higher at the same output, so a steady aerobic piece might need to be slower to stay truly easy. When I coach in summer, I'd rather see you finish a little under your spring numbers and feel good than chase them and fade.
Respect the details, even the unglamorous ones like weighing yourself after training. Keep showing up, but show up smart when the thermometer climbs.
If you want your engine work structured so you can build fitness through every season, with clear intensity targets you can adjust on hot days, Sphere Programming gives you that plan alongside your regular classes.
Your turn. Tried this? Tell me how it went in the comments below. I read every one, and your questions help shape what I write next. If it helped, tap the heart so it reaches more athletes.
Keep reading
Further reading: Journal of the International Society of Sports Nutrition
Coach Fritz Bautista · Head Coach, ATHLETESPHERE
CrossFit Level 2 Trainer (CF-L2) ✓ · CrossFit Programming Certificate · MBA · Digital Marketing Certified · Coaching athletes from all walks of life online
Comments