The athlete's guide to hydration, heart rate, and HRV

Written by Rachael Schultz (opens in a new tab)

Medically reviewed by Ecler Ercole Jaqua, MD, MBA, DABOM (opens in a new tab)

<h3>Key Takeaways:</h3><ul><li>Hydration influences cardiovascular efficiency by maintaining blood volume, which allows the heart to circulate blood with fewer beats.</li><li>When dehydration reduces blood volume, the heart compensates by beating faster, raising resting heart rate and lowering HRV.</li><li>During exercise and high sweat loss, electrolytes help fluid move into and remain in circulation rather than being rapidly excreted.</li><li>Hydration is an easily controllable way to support heart rate and HRV, even when other variables fluctuate.</li></ul>

Dehydration can be more subtle than you think. Symptoms like fatigue, brain fog, and dark urine can be early signs that your body is out of balance.

Your tracker says your heart rate variability (HRV) dipped a bit overnight. Your resting heart rate (RHR) is up eight beats per minute. But you scaled back training yesterday, logged a solid eight hours of sleep, and didn’t drink alcohol. On paper, recovery should look fine… so what gives?This is where a lot of people get tripped up, because it’s easy to assume that if training, sleep, and nutrition are dialed in, hydration is just background noise. But your cardiovascular system doesn’t see it that way.Even mild dehydration — <a href="https://pubmed.ncbi.nlm.nih.gov/24736771/" rel="noopener noreferrer" target="_blank">as little as</a> 1–2% of the body’s total fluid — can reduce blood volume and force your heart to work harder to circulate blood. That extra workload shows up quickly in metrics like resting heart rate and HRV, even when nothing else in your routine has changed.When resting heart rate rises and HRV stays suppressed, it often reflects a state of increased physiological stress. Recovery slows. Training feels harder than it should and you notice a decrease in performance.The good news, says cardiovascular surgeon, former endurance athlete, and LMNT Partner <a href="https://www.drjeremylondon.com/" rel="noopener noreferrer" target="_blank">Jeremy London, MD</a>, is that hydration is one of the most controllable performance levers you have. <h2>How Dehydration Impacts Cardiovascular Efficiency </h2>Your cardiovascular system is a closed loop system, says Dr. London. You’ve got a pump (the heart), pipes (blood vessels), and fluid (blood volume). Change any one of those variables, and the whole system has to adapt.That’s why hydration matters more than most people think. When you’re hydrated, there’s plenty of blood volume, and the blood vessels are relaxed. Each heartbeat moves more blood forward, which means the heart doesn’t need to beat as often to meet demand, Dr. London explains. The opposite happens when you’re dehydrated. Even mild dehydration reduces circulating blood volume. When volume drops, the heart must beat more often. Blood vessels constrict and the heart speeds up to keep oxygen and nutrients moving where they’re needed.Walk around chronically underhydrated, and the system stays in a higher-effort state. Resting heart rate rises and HRV declines as the heart works a little harder to maintain baseline circulation. Over time, persistently elevated resting heart rate and reduced HRV are associated with poorer cardiovascular health.<h2>Resting Heart Rate </h2>Resting Heart Rate (RHR) is the number of times your heart beats per minute (bpm) while you're at rest and at ease. It’s a simple but powerful metric because it reflects how hard your heart has to work to circulate blood when you’re at rest.Most wearables measure RHR by monitoring your pulse overnight or during long periods of inactivity. You can also measure it manually by finding your pulse at your wrist or neck and counting beats for 60 seconds while seated and relaxed.<h3>Why RHR matters</h3>Resting Heart Rate gives you a snapshot of cardiovascular health and fitness. When your heart muscle is strong and efficient, it can pump a larger volume of blood with each beat, Dr. London explains. That means it doesn’t need to beat as often to meet the body’s baseline needs.When efficiency drops — from illness, stress, lack of training, or dehydration — the heart compensates by beating more frequently to maintain circulation. That adaptation works, but it comes at a cost. Persistently elevated resting heart rate <a href="https://www.sciencedirect.com/science/article/abs/pii/S1050173822000731?" rel="noopener noreferrer" target="_blank">increases your risk of</a> cardiovascular disease.Think of it like <a href="https://drjeremylondon.com/articles/what-does-your-resting-heart-rate-say-about-your-health" rel="noopener noreferrer" target="_blank">this</a>, says Dr. London: If one person has a resting heart rate of 65 bpm and another sits at 80 bpm, the second person’s heart beats 900 more times in an hour of rest. That adds up to 21,600 additional heartbeats over the course of a day — before any activity even starts.Regular training, stress management, and adequate hydration all help support a lower, more efficient resting heart rate.There’s natural variation in resting heart rates from person to person, but <a href="https://my.clevelandclinic.org/health/diagnostics/heart-rate" rel="noopener noreferrer" target="_blank">typical ranges look like this</a>:<ul><li>Healthy adults: 60-100 bpm</li><li>Recreational athletes: 50-60 bpm</li><li>Elite athletes: ~35-60 bpm</li></ul><h3>Dehydration and heart rate</h3>Blood is mostly water, so when you’re dehydrated, circulating blood volume drops. Blood vessels constrict, which makes it harder for blood to return to the heart between beats.The heart itself has two sides. Both sides of the heart depend on adequate blood volume to pump effectively. When that volume drops, the heart compensates by beating faster. If you don’t have enough volume in the system, there isn’t anything to be pumped around. "When you get dehydrated... your heart rate has to go up to make up for the decrease in volume because our tissues need a certain amount of oxygen and nutrients to survive," Dr. London explains.If you go into a workout mildly, moderately, or severely dehydrated, that lower blood volume can show up as:<ul><li>Reduced oxygen delivery to working tissues</li><li>A faster <a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC10650885/" rel="noopener noreferrer" target="_blank">heart rate</a>, even if you’re not working hard</li><li>Quicker fatigue, because your heart is working harder </li></ul>Even mild dehydration compromises performance: A study in <a href="https://pubmed.ncbi.nlm.nih.gov/24736771/" rel="noopener noreferrer" target="_blank">The Journal of Strength and Conditioning Research</a> found that for every 1% of body mass loss from dehydration, heart rate can increase by about 3 bpm during exercise, depending on the workload and conditions. The impact of that increase would be different for each person, according to Dr. London. “Since hydration is so easily addressed, it makes sense to control what is easily within your control,” he says.<h2>HRV & Hydration</h2>Heart rate variability (HRV) is a measurement of the tiny differences in time between each heartbeat, usually recorded in milliseconds. Instead of telling you how fast your heart is beating, HRV reflects how much the timing between beats changes from one to the next.HRV is a signal. When the body is well-recovered and stress is well-managed, there’s more variability between beats. When the system is under strain, that variability is reduced.Subtle fluctuations in heart rate timing are a powerful window into how your nervous system is functioning, particularly the balance between its two branches:<ul><li>The sympathetic nervous system, which governs stress and “fight or flight”</li><li>The parasympathetic nervous system, which supports rest, recovery, and repair</li></ul><h3>What HRV tells you</h3>A higher HRV means there’s more variability in timing between beats. This is usually good — it signals your parasympathetic system is strong, and your heart rate can shift smoothly between stress and recovery.A lower HRV suggests your body is in a sustained stress state. The sympathetic system is doing more of the driving, and recovery is lagging. Heart rate stays elevated instead of quickly downshifting back to baseline.Many athletes track HRV over time to look for patterns. Wearables typically measure HRV during sleep, when external influences like movement and mental stress are minimized."A consistently higher HRV score means you're doing a really good job with your rest and recovery and you have your body more in that rest and recovery side as opposed to the fight or flight side of sympathetic surging,” says Dr. London. A one-off increase in HRV may reflect your body allocating resources to recover from a recent stressor — whether that's a hard training session, poor sleep, or something else. You’ve likely seen this play out in real life. After poor sleep, a late night, a hard training block, or a stressful week, HRV often trends downward. Dehydration can push it in the same direction, especially when fluid losses stack up over multiple days.<h3>What happens to HRV when you’re dehydrated</h3>At rest, one of the simplest relationships in cardiovascular physiology is this: when heart rate goes up, HRV generally goes down.HRV is driven by your autonomic nervous system — the branch that governs stress and recovery. When heart rate rises and the sympathetic (stress) side takes over, beat-to-beat variability narrows. When the heart is forced to beat faster, that window narrows.Dehydration drives this process in a few ways. <ul><li>Reduced fluid volume means the heart has to work harder to maintain circulation, which pushes heart rate up. </li><li>That volume drop triggers a mild stress response to preserve blood flow to vital organs, shifting the nervous system toward greater sympathetic activity.</li></ul>“The higher your heart rate, the less time there is for variability between those beats, which is why your HRV goes down when your heart rate is up. Whereas when your heart rate comes down, there's more opportunity for variability between those beats and your HRV goes up,” says Dr. London.This isn’t just theoretical: A 2019 study in <a href="https://www.nature.com/articles/s41598-019-52775-5?utm_source=chatgpt.com" rel="noopener noreferrer" target="_blank">Scientific Reports</a> found that mild dehydration was associated with lower HRV and increased perceived effort and anxiety, suggesting that even small fluid deficits can shift autonomic balance unfavorably.Another <a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC10650885/" rel="noopener noreferrer" target="_blank">study</a> comparing hydration versus dehydration found that water intake before, during, and after exercise led to higher HRV values during recovery, showing better hydration before and during your workout leads to a faster re-balancing of your autonomic nervous system.Why this matters: Recovery is where adaptations happen, Dr. London says. “If you're not staying adequately hydrated and repleting your electrolyte imbalances, then your recovery is not optimized.”Proper hydration removes an unnecessary stressor from an already taxed system.

"“Try to avoid electrolyte supplements that are high in sugar, use artificial additives, and have a long list of ingredients. If you're looking for the cleanest form of electrolytes, opt for unflavored versions.” -Dr. Jeremy London, MD "

<h2>Electrolytes Matter, Too</h2>By now it’s clear that hydration helps maintain healthy blood volume so the heart doesn’t have to work as hard. But water alone isn’t always enough to do that job.<a href="https://science.drinklmnt.com/electrolytes/fluid-and-electrolyte-imbalance" rel="noopener noreferrer" target="_blank">Fluid balance</a> is governed by electrolytes — minerals that help determine where water goes and how long it stays there. Without enough electrolytes — particularly sodium — your body is less effective at keeping that water in circulation, especially during periods of high fluid loss.Sodium plays the starring role here. It helps drive water from the digestive tract into the <a href="https://science.drinklmnt.com/electrolytes/salt-and-high-blood-pressure" rel="noopener noreferrer" target="_blank">bloodstream</a> and supports fluid retention in circulation. When sodium intake is too low relative to fluid losses, blood volume can drop even if total water intake seems fine on paper.Other electrolytes play supporting roles:<ul><li>Potassium helps regulate fluid balance at the cellular level and is essential for proper electrical signaling in the heart and muscles.</li><li>Magnesium supports nerve conduction and muscle relaxation. Low levels can amplify stress responses, which indirectly affects autonomic balance and HRV.</li></ul>“When we rehydrate with the necessary electrolytes to keep a healthy blood volume, it makes our cardiovascular systems more efficient and our heart rates naturally come down,” says Dr. London. “This allows more space between the beats, which allows more variability, which is better for our overall cardiovascular efficiency.”<h2>Practical Hydration Advice for Athletes</h2>Being well-hydrated supports recovery. When blood volume is adequate, the heart doesn’t have to work as hard, heart rate comes down, and HRV has more room to rebound, Dr. London explains.“Our bodies strive to be in balance down to the cellular level — in homeostasis. So anything that you can do to help maintain that level of balance is really what we should be shooting for,” he adds.For athletes, this means having a solid hydration strategy to replenish your body <a href="https://science.drinklmnt.com/electrolytes/hydration-timing-exercise-performance" rel="noopener noreferrer" target="_blank">before, during, and after workouts</a>.Rather than following rigid rules, Dr. London recommends matching your water and electrolyte intake to your <a href="https://science.drinklmnt.com/did-you-know/sweat-rate" rel="noopener noreferrer" target="_blank">sweat level</a>. You can estimate your <a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/" rel="noopener noreferrer" target="_blank">individual sweat rate</a> by weighing yourself before and after a workout and adding in any fluid you drank, or look for external signs like seeing salt streaks on your shirt after training. For Dr. London, that might look like:<ul><li>Light activity, low-sweat days: Dr. London typically gets his baseline electrolytes from his diet.</li><li>Hard training days with high sweat levels (e.g., 2-hour bike ride): This is highly individual and should be customized to activity levels on any given day, but Dr. London typically mixes 1 LMNT stick pack into 24-32oz of water.</li><li>Post-sauna sessions with high sweat loss: "If I go and do a long sauna session, then I lose a ton of water and electrolytes in the sauna. And so I bump [it] up," says Dr. London. So consider increasing intake, replacing between <a href="https://www.nata.org/sites/default/files/2025-08/fluid_replacement_for_the_physically_active.pdf" rel="noopener noreferrer" target="_blank">100 and 150 percent</a> of what you lost through sweat (weigh yourself before and after to gauge).</li></ul>That said, it isn’t always obvious how much fluid you’ve lost. Dr. London points to several signs that hydration or electrolyte intake may be falling short:<ul><li>Feeling lightheaded when you stand</li><li>A heart rate that feels out of proportion to your activity level</li><li>Experiencing muscle spasms</li></ul>Also worth keeping in mind: You can’t “load” electrolytes like you can carbs. Your kidneys are an amazing filter and if you don’t need the electrolytes or fluid in the system, you just pee them out."We're not camels, we can't load volume into our body and hang onto it because our kidneys work so well,” Dr. London adds.To be crystal clear, hydration isn’t the only factor influencing your resting heart rate or HRV — sleep, stress, training load, nutrition, and alcohol all play a role. But it’s one of the easiest, most controllable levers you have, Dr. London says.“Get the easy stuff right… If you see that HRV continues to be marginal or low [on your wearable], hydration is always a great place to start because you have total control over it.”<h2>FAQ</h2>Q: How quickly can rehydrating affect my HRV score?A: HRV is a real-time indicator that responds relatively quickly to hydration changes. It can respond relatively quickly to improved hydration — sometimes within an hour — though the timing depends on factors like the degree of dehydration.Q: Should I drink extra electrolytes the day before a race to "load up"?A: No. Your kidneys are highly efficient filtration systems that eliminate excess electrolytes you don't immediately need. The strategy is consistent daily hydration leading up to the event, then replacing what you lose during and after the race itself.