Electrolytes vs. Water: The Surprising Effect on Your Training Zones

I noticed something odd during a long run last summer. About forty minutes in, my heart rate started climbing—not because I was running faster, but while holding the exact same pace. What had been a comfortable Zone 2 effort at 135 bpm was now sitting at 152. I hadn't sped up. The temperature hadn't changed dramatically. My legs felt fine. But according to my watch, I'd drifted out of my target zone entirely and was grinding away in Zone 3—the so-called "black hole" where adaptation doesn't happen efficiently.

I blamed fitness. I blamed the heat. It didn't occur to me to blame the bottle of plain water I'd been sipping.

It turns out, what you drink during exercise has a far more direct effect on your heart rate zones than most people realize. And if you're training with heart rate targets—which is one of the best ways to improve your VO₂ max—this matters more than you'd think.

Why your heart rate drifts upward during exercise

Exercise physiologists have a term for what I experienced: cardiovascular drift. It describes a well-documented phenomenon where heart rate gradually increases during prolonged exercise even when your pace stays constant. At the same time, stroke volume—the amount of blood your heart pumps per beat—decreases. Your heart beats faster to compensate, trying to maintain the same cardiac output.

Several things drive this. As you exercise, your core temperature rises. Blood gets redirected to the skin for cooling. Plasma volume drops as you lose fluid through sweat. The result is less blood returning to the heart per beat, which forces a higher heart rate to keep up.

A systematic review published in BMJ Open Sport & Exercise Medicine described the phenomenon clearly: cardiovascular drift occurs after just 10–15 minutes of continuous moderate exercise and consists of a progressive increase in heart rate matched by a decrease in stroke volume. The effect is amplified by dehydration and heat.

Here's the part that surprised me: research on cyclists who exercised for two hours found that those who didn't consume fluids experienced a heart rate increase of about 10%, while those who drank enough to match their sweat rate saw only a 5% increase. Roughly half the drift was explained by dehydration alone.

This means the zone your watch assigns to your workout—the zone you're using to guide your training intensity—might be wrong. Not because the watch is broken, but because your cardiovascular system is responding to fluid loss rather than exercise intensity.

The heart rate penalty of dehydration

The numbers are remarkably consistent across studies. A systematic review examining the relationship between dehydration and heart rate found that for every 1% loss in body weight from fluid loss, heart rate increases by approximately 3 beats per minute during exercise.

That might not sound like much. But consider: a 75 kg person sweating at a moderate rate of 1 liter per hour will lose about 1.3% of body weight in a single hour. That's a roughly 4 bpm increase from dehydration alone. After two hours without adequate hydration, you're looking at 8–10 extra beats per minute sitting on top of your exercise heart rate—purely from fluid loss, not from working harder.

For someone whose Zone 2 ceiling is around 140 bpm, that's the difference between staying in the aerobic base-building zone and drifting into the moderate-intensity territory that most coaches consider unproductive. Your legs feel fine. Your breathing is controlled. But your heart rate says you're working harder than you actually are.

And this has a downstream effect on your VO₂ max readings. Since your Apple Watch estimates cardio fitness by correlating heart rate with pace, a dehydration-inflated heart rate during an outdoor run can make your fitness look worse than it actually is. It's one of those invisible factors that contributes to the frustrating week-to-week fluctuations that make tracking VO₂ max feel unreliable.

Where electrolytes change the equation

Here's where it gets interesting. Drinking water during exercise helps—but it doesn't solve the problem as well as you'd expect.

When you sweat, you lose both water and sodium. If you replace only the water, you dilute the sodium concentration in your blood. Your body responds by reducing its drive to retain fluid—you actually start excreting more of what you drink rather than absorbing it. It's counterintuitive: drinking water can leave you less hydrated than you'd think because your kidneys let more of it pass through.

A meta-analysis in Sports Medicine examining the effects of different drinks on plasma volume during exercise found that beverages containing electrolytes were more effective at maintaining central hydration than water alone. Electrolyte content was positively associated with better plasma volume preservation, regardless of whether the drink was hypotonic, isotonic, or hypertonic.

A systematic review with meta-analysis looked specifically at how fluid type affects heart rate during exercise. Their findings were striking: while plain water showed only a trend toward reducing exercise heart rate (about 4.6 bpm lower, but not statistically significant), isotonic electrolyte drinks produced a statistically significant reduction of about 7 bpm compared to no fluid. The difference between water and an electrolyte drink was meaningful.

The mechanism is straightforward. Sodium is the primary electrolyte in your extracellular fluid—the blood plasma that your heart is pumping. When you consume sodium alongside water, it maintains plasma osmolality (the concentration of dissolved particles in your blood), which does two things: it keeps your kidneys from dumping the water you just drank, and it helps retain fluid in the vascular space where it's actually useful for maintaining stroke volume.

A study published in the Journal of Athletic Training tested this directly by having active men exercise for three hours in the heat while consuming drinks matched to their sweat rate. The sodium-containing beverages preserved plasma volume at pre-exercise levels, while the sodium-free options did not—even though total fluid intake was identical.

What this means for your training zones

If you're training by heart rate—and you should be, if your goal is improving VO₂ max—hydration quality directly affects training quality.

Picture a typical 80/20 polarized training week. Most of your sessions are supposed to be in Zone 2: easy, conversational, aerobic base-building. The ceiling for Zone 2 is usually around 70–75% of max heart rate. For a 40-year-old with a max heart rate of 185 (using the HUNT formula), that's roughly 130–139 bpm.

Now add dehydration. Forty minutes into a warm-weather run, cardiac drift pushes your heart rate up 7–10 beats. Suddenly your "easy" run at the same pace sits at 145. Your watch logs it as Zone 3. You think you're building your aerobic base, but physiologically, your body is accumulating more fatigue than it should be—and not getting the recovery benefit that makes the 80/20 model work.

On the flip side, this also affects your hard sessions. If you're already partially dehydrated heading into a 4×4 interval workout, your heart rate hits the 90–95% zone earlier and with less actual cardiovascular stimulus. The internal negotiation arrives sooner because your perceived exertion climbs faster—not because you're pushing harder, but because your cardiovascular system is working with less plasma volume.

Proper hydration—specifically electrolyte-containing hydration—helps keep the gap between actual effort and measured heart rate small. Your zones stay honest. Your easy days stay easy. Your hard days deliver the right stimulus.

The practical takeaway

I've changed a few things since digging into this research, and the difference in my training quality has been noticeable.

For sessions under an hour in mild conditions, plain water is fine. The American College of Sports Medicine's position stand on fluid replacement notes that during exercise lasting less than about 90 minutes, water alone is generally sufficient. The cardiac drift from short sessions is manageable.

For anything longer—particularly Zone 2 runs of 60 minutes or more, which are the backbone of any VO₂ max improvement program—I add electrolytes. Not a sugary sports drink. Something with sodium in the range of 300–800 mg per liter, which aligns with the Gatorade Sports Science Institute's formulation guidelines for maintaining plasma volume.

For hard interval days, I prehydrate with an electrolyte drink about 45 minutes before the session. There's evidence that pre-exercise sodium loading expands plasma volume and reduces heart rate drift during subsequent exercise—essentially giving your cardiovascular system more headroom before cardiac drift kicks in.

A few other things I've noticed:

Hot days exaggerate everything. Sweat rates can exceed 1.5 liters per hour in heat, and sweat sodium concentration increases with exercise intensity. On warm days, the difference between water and an electrolyte drink is more pronounced. This is also when your Apple Watch readings are most likely to underestimate your fitness, because heat-driven cardiac drift inflates the heart rate relative to pace.

You can overshoot on water. Drinking large amounts of plain water during prolonged exercise can dilute blood sodium levels—a condition called hyponatremia. It's rare in recreational exercise, but the research on endurance athletes is clear that drinking to match sweat loss using sodium-containing fluids is safer and more effective than just guzzling water.

The effect is cumulative across sessions. If you train dehydrated on Tuesday and don't fully rehydrate before Thursday's workout, you're starting your next session at a deficit. A study comparing rehydration with water versus electrolyte beverages found that athletes retained about 77% of fluid from an electrolyte drink compared to only 58% from water. Over a week of training, that difference in fluid retention adds up.

The bigger picture

I used to think of hydration as a binary: drink enough water, check the box. What the research actually shows is more nuanced. The composition of what you drink affects your cardiovascular function in ways that directly interact with heart rate–based training—which is how most of us structure our work toward better VO₂ max.

This doesn't mean electrolytes are magic. They don't improve your cardiovascular fitness or raise your VO₂ max directly. What they do is keep your cardiovascular system operating closer to its actual capacity during exercise, so that the training zones you're targeting actually match the physiological stimulus your body receives.

The improvements that matter for longevity come from consistent training at the right intensities over months. Anything that helps you stay in the right zone—whether it's music to fight boredom, caffeine to lower perceived effort, or electrolytes to keep cardiac drift in check—is worth paying attention to.

It's a small adjustment. But in a training program where the difference between productive and unproductive work comes down to which heart rate zone you're actually in, small adjustments compound.

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Track your VO₂ max trend over time with VO2 Max Pro. The app syncs with Apple Health, notifies you when new readings come in, and translates your number into a biological age—so you can see whether your training is actually working.