The science of hydration timing for exercise performance

Written by Luis Villaseñor (opens in a new tab)

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

<h3>Key Points:</h3><ul><li>Even mild dehydration (1–2% bodyweight loss) can drop endurance by up to 10% and impair performance.</li><li>You can’t fully reverse dehydration mid-workout. Once performance drops it’s difficult to completely recover during the same session. Fluid and sodium intake can slow or partially mitigate further dehydration.</li><li>Timing water and electrolytes before, during, and after training keeps your heart, muscles, and nervous system firing at full capacity.</li></ul>Some workouts start strong and slowly deteriorate. Your muscles feel rubbery, your coordination falters, and your last set feels like you’re lifting under water. It’s tempting to blame general fatigue or “just an off day,” but the drop off may have happened hours before you touched a barbell.Even mild dehydration (losing just 1–2% of your body weight in fluid) can<a href="https://journals.lww.com/acsm-msse/fulltext/2016/03000/nutrition_and_athletic_performance.25.aspx" rel="noopener noreferrer" target="_blank"> meaningfully impair endurance</a> — especially during prolonged exercise or training in the heat —and may<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/" rel="noopener noreferrer" target="_blank"> blunt</a> strength and power. And guzzling water as you walk into the gym or between sets can’t undo that. To perform well, you need to time fluids and electrolytes around three key windows — before, during, and after training — to keep your heart pumping, your muscle fibers twitching, and your concentration focused. “If you show up dehydrated, you’re dehydrated even further by doing activity and then trying to rehydrate,” says Dr. Mike Israetel, Ph.D., a sports physiologist and co-founder of RP Strength<a href="https://rpstrength.com/pages/team/michael-israetel" rel="noopener noreferrer" target="_blank"> (Renaissance Periodization)</a>. “Dehydration takes minutes; rehydration takes 30 minutes, an hour, even more. If you don’t time hydration right, you’re kind of chasing a car that’s moving away faster than you.”Here’s how electrolytes fit into the hydration equation — and how to time your intake to make every session count. <h2>The Science of Hydration</h2>Proper hydration starts with drinking water, but you don’t feel all the benefits until the water is available to the tissues that support performance. This is where electrolytes come in.But before we get to that, let’s ground this in some basic physiology.<a href="https://www.ncbi.nlm.nih.gov/books/NBK541059/" rel="noopener noreferrer" target="_blank"> Most of the water</a> in your body lives inside your cells. The rest sits outside your cells in your blood and the spaces between tissues, which together make up extracellular fluid. Your body tightly controls how much water moves between these areas, since even small swings in<a href="https://courses.lumenlearning.com/suny-ap2/chapter/body-fluids-and-fluid-compartments-no-content/" rel="noopener noreferrer" target="_blank"> cell volume</a> can impair function.Dehydration causes fluid loss from multiple areas. When you’re dehydrated, blood plasma volume drops, and your body moves water from your cells and between tissues into the blood. When you work out, minor fluid loss can cause cell shrinkage within 30–60 minutes — often before you even feel thirsty. Shrunken cells have overly concentrated ions, enzymes, and other solutes, which slows muscle contraction, increases fatigue, and disrupts nerve signaling, making movements feel weaker and less coordinated.<a href="https://science.drinklmnt.com/electrolytes/what-are-electrolytes" rel="noopener noreferrer" target="_blank">Electrolytes</a> like sodium, potassium, and magnesium make this fluid regulation possible:<ul><li>Sodium helps keep water in your bloodstream and<a href="https://science.drinklmnt.com/electrolytes/sodium-potassium-pump" rel="noopener noreferrer" target="_blank"> sets the gradients</a> that let water follow into cells</li><li>Potassium helps keep water inside your cells</li><li>Magnesium helps these two minerals work together efficiently</li></ul>Here’s how each electrolyte optimizes hydration to keep you performing at your peak.<h3>Sodium: The Signal Starter</h3>Sodium is<a href="https://www.msdmanuals.com/professional/endocrine-and-metabolic-disorders/fluid-metabolism/water-and-sodium-balance" rel="noopener noreferrer" target="_blank"> the main regulator</a> of fluid balance outside cells — and the primary electrolyte lost through sweat. It helps you hold onto water in your bloodstream, which keeps<a href="https://www.ncbi.nlm.nih.gov/books/NBK482447/" rel="noopener noreferrer" target="_blank"> plasma volume high enough</a> to deliver oxygen and keep your body temperature and heart rate stable during training. When sodium drops,<a href="https://www.msdmanuals.com/professional/endocrine-and-metabolic-disorders/fluid-metabolism/water-and-sodium-balance" rel="noopener noreferrer" target="_blank"> plasma volume drops with it.</a> Your heart has to beat faster to move the same amount of blood, your core temperature climbs, and your muscles fatigue sooner.Sodium also plays a direct role in muscle activation. Each time your brain tells a muscle to move, sodium floods into nerve and muscle cells, sparking the electrical signal that makes the fiber contract. After that signal fires, your cells use a reset system called the<a href="https://science.drinklmnt.com/electrolytes/sodium-potassium-pump" rel="noopener noreferrer" target="_blank"> sodium-potassium pump</a> (Na+/K+) to push sodium back out of the cell and pull potassium in so the next contraction can happen. When sodium runs low, this signaling becomes less efficient. You may struggle to concentrate, muscles will start to cramp, fatigue and weakness will creep in.<h3><a href="https://science.drinklmnt.com/electrolytes/what-are-electrolytes" rel="noopener noreferrer" target="_blank">Potassium: The Muscle Conductor</a></h3>Potassium is the yin to sodium’s yang. While sodium manages fluid outside your cells, potassium helps water stay inside them — keeping electrical signals sharp to facilitate muscle contractions. Most of the potassium in your body lives inside your muscle and nerve cells. After a muscle fires, potassium is briefly pushed out of the cell so the muscle fiber can reset and fire again. This reset phase — called<a href="https://courses.lumenlearning.com/suny-ap1/chapter/the-action-potential" rel="noopener noreferrer" target="_blank"> repolarization</a> — is what allows muscles to contract rhythmically and with force.Potassium also plays a role in fluid balance. When it drops to clinically low levels, your kidneys struggle to hold onto water. But exercise typically doesn't deplete potassium enough to cause that problem. If you're chronically low on potassium from diet or health issues, that's a different story. But for exercise hydration? Sodium does most of the heavy lifting.<h3><a href="https://science.drinklmnt.com/electrolytes/magnesium-deficiency-symptoms" rel="noopener noreferrer" target="_blank">Magnesium</a>: The System’s Groundwire</h3>Magnesium’s biggest role in hydration is<a href="https://science.drinklmnt.com/electrolytes/electrolytes-and-energy" rel="noopener noreferrer" target="_blank"> activating adenosine triphosphate (ATP</a>) — your body’s main energy currency — to power the sodium-potassium pump. The pump only works when ATP is bound to magnesium, which means magnesium determines how efficiently your cells can generate and use energy during training.Without enough magnesium, the sodium-potassium pump slows down and more sodium enters the cell. Electrical signals become less reliable, and muscles can’t contract or reset efficiently. You may feel your muscles spasm when you reach this state. <h2>Pre Workout Hydration: Priming the System</h2>Pre-hydration is one of the most overlooked performance tools. More<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/" rel="noopener noreferrer" target="_blank"> than half of all athletes</a> — from high school competitors to seasoned lifters — start training already under-hydrated. Starting a workout dehydrated has three primary consequences:<h4>1. Reduces performance capacity</h4>Once you begin a hard workout dehydrated, it’s difficult to claw your way back. Your gut absorbs fluid within seconds but complete absorption into your bloodstream, muscles, and connective tissue takes<a href="https://pubmed.ncbi.nlm.nih.gov/21997675/" rel="noopener noreferrer" target="_blank"> minutes or even hours</a>.When you’re under-hydrated, blood plasma volume is already reduced. As exercise continues, fluid losses add up faster than your body can replace them. Strength, power, and endurance decline earlier in the session — often within 30–60 minutes — even if effort remains high.“If you start hard training, and you continue training hard but your hydration wasn't in touch beforehand, you're gonna start to see decrements of performance hit somewhere between the<a href="https://link.springer.com/article/10.1007/s40279-019-01223-5" rel="noopener noreferrer" target="_blank"> 30-minute and hour mark</a>,” Dr. Israetel explains. <h4>2. Compromises thermoregulation</h4><a href="https://pubmed.ncbi.nlm.nih.gov/17277604/" rel="noopener noreferrer" target="_blank">Starting a workout dehydrated</a> also compromises thermoregulation. When you’re under-hydrated, you can’t move heat as effectively, so your core temperature rises faster. Your sweat rate drops — which makes cooling even harder. Fatigue hits faster, weakness creeps in, and you might start to feel dizzy or nauseated. <h4>3. Increases risk of injury</h4>Pre-hydrating also protects you against injuries. Proper hydration can affect the amount of fluid in your<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC6536550/" rel="noopener noreferrer" target="_blank"> tendons and muscles</a>, while dehydration can make tissues<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC1421497/" rel="noopener noreferrer" target="_blank"> more susceptible</a> to tears and strains.<h3>The Formula: Drink water with electrolytes 2–4 hours before training.</h3>The goal is to get water and electrolytes into your system early enough that it's distributed where you need it.Standard approach: Drink .08–.15 oz of water per pound of body weight 2–4 hours before training. For a 150-lb person, that’s 1.5–2.5 cups of water (or 11.5–23 oz). Aim for ~500–1,000 mg sodium per hour during heavy sweating. This can come from commercial electrolyte mixes, salted fluids, or food sources, and will help water get absorbed and distributed. This gives your body time to raise plasma volume and hydrate tissues.Early morning workouts: If you don’t have hours of lead time, research suggests drinking about<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC8336541/" rel="noopener noreferrer" target="_blank"> 10 oz of electrolyte fluids 10–20 minutes before your workout</a>, and then another 7–10 oz of electrolyte fluids occasionally throughout the day. Heat or heavy sweating: Increase the quantities. I typically suggest adding 1 extra LMNT stick and between 20–40 oz of extra water. My average client uses 2 LMNTs daily and may add extra when competing in a competition or increasing activity levels. These are all guidelines, not rules. Hydration needs vary based on sweat rate, environment, training intensity, and individual physiology. Adjust your intake over time based on how you feel and perform.<h2><a href="https://science.drinklmnt.com/electrolytes/how-to-rehydrate-fast" rel="noopener noreferrer" target="_blank">Hydration During a Workout</a>: Keeping the Signal Strong</h2>As soon as you start training, you lose both water and electrolytes through sweat. In an hour of exercise, a moderately heavy sweater loses about<a href="https://pubmed.ncbi.nlm.nih.gov/26841436/" rel="noopener noreferrer" target="_blank"> 1–1.2 liters of fluid</a> and a few hundred to over 1,000 mg of electrolytes<a href="https://www.sciencedirect.com/science/article/abs/pii/S0022316623141836" rel="noopener noreferrer" target="_blank"> per hour</a>, depending on sweat rate, heat, acclimatization, and individual physiology.Drinking pure water can dilute electrolytes further, weakening electrical signals. Fewer muscle fibers activate and it takes longer for them to reset between contractions. Movements become slower, shakier, and less precise, and you risk slipping into junk volume where reps feel harder but fail to stimulate progress.During exercise, thirst isn’t a reliable signal that it’s time to drink. The sensation often<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/" rel="noopener noreferrer" target="_blank"> lags</a> behind your actual needs, especially when you’re going hard. Many folks <a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC6090881/" rel="noopener noreferrer" target="_blank">don’t feel parched</a> until beads of sweat are rolling. A better signal: your phone’s timer. Plan out when to drink, and how much, and stick to that schedule, even if you don’t feel thirsty. <h3>The Formula: </h3>For workouts under an hour or that don’t involve excessive sweating, plain water is enough for folks who regularly nail their hydration and sodium needs. Drink<a href="https://science.drinklmnt.com/fasting/fasting" rel="noopener noreferrer" target="_blank"> 7–10 ounces of fluid every 15–20 minutes. Add electrolytes (½ LMNT stick pack) if you’re training fasted. </a>For sessions that are over an hour, include intense intervals, or take place in the heat, aim for<a href="https://www.sciencedirect.com/science/article/abs/pii/S0022316623141836" rel="noopener noreferrer" target="_blank"> 16–20 oz</a> of water with ½ – 1 LMNT stick pack per hour to help offset higher fluid and sodium losses. For workouts lasting 60–90 minutes — recommendations are highly individual. Research shows wide variation in sweat and sodium losses, even among people doing the same workout in the same conditions. But a good rule of thumb is to drink 20–32 oz of water with ½–1 LMNT stick pack throughout your workout. For super-long workouts (think: over 2–3 hours) or endurance events, cap your use at 2–3 LMNT stick packs a day, and get additional electrolytes from food sources like pickles and olives, says Robb Wolf, former research biochemist, 2X New York Times/Wall Street Journal bestselling author and LMNT co-founder. As with pre-workout hydration, you might need to tweak these numbers based on the specifics of your workout and individual needs. For example, a higher fluid intake might make sense if you’re a heavy sweater, which you can figure out based on your<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/" rel="noopener noreferrer" target="_blank"> individual sweat rate</a>. To estimate it, weigh yourself before and after a workout, then account for what you drank (urine output during a single workout is usually negligible and can be ignored for practical purposes). The math is simple:Sweat loss = (pre-workout weight – post-workout weight) + fluid consumed Sweat rate = sweat loss ÷ hours trainedWhile there’s no official definition of a “heavy sweater,” experts often use 1.2 liters per hour as the minimum cut-off — but if you see salt streaks or residue on your shirt and skin after training, that's a pretty clear indicator that you're losing significant sodium.<h2>After Training: Restore the System</h2>After training, the purpose of hydration shifts to promoting recovery. You want to<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/" rel="noopener noreferrer" target="_blank"> replace the fluids lost during training</a> within a<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC8336541/" rel="noopener noreferrer" target="_blank"> few hours</a>. Restoring your blood plasma levels helps your body deliver nutrients, move oxygen, clear metabolic byproducts, and start repairing the tissues you just stressed.Electrolytes<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC12297025/" rel="noopener noreferrer" target="_blank"> enhance this process</a>, especially if you finish your workout in a<a href="https://pubmed.ncbi.nlm.nih.gov/17277604/" rel="noopener noreferrer" target="_blank"> fluid deficit.</a> Here’s how long it takes:<ul><li>Mild dehydration can be reversed within<a href="https://science.drinklmnt.com/electrolytes/how-to-rehydrate-fast" rel="noopener noreferrer" target="_blank"> 45–60 minutes</a>. </li><li>Moderate hydration: 1–2 hours</li><li>Severe dehydration: 3–6 hours or longer</li></ul>Sleep is the final piece. Your recovery<a href="https://science.drinklmnt.com/electrolytes/electrolytes-and-sleep" rel="noopener noreferrer" target="_blank"> depends on it</a>, and electrolyte status plays a role here too.<a href="https://science.drinklmnt.com/electrolytes/magnesium-for-sleep-and-anxiety" rel="noopener noreferrer" target="_blank"> Magnesium</a>, in particular, helps your brain power down, reduces nighttime leg cramps that interrupt rest, and supports circadian rhythm alignment — all factors that influence overnight recovery and next-day heart rate variability (HRV).<h3>The Formula</h3>If you have 12+ hours before your next workout: Simply replace what you lost. Drink 16 oz of water with electrolytes (1 LMNT stick pack) within 30 minutes of finishing, and continue hydrating normally throughout the day.If you have 4 hours or less before your next workout: You need to restore plasma volume faster.<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/" rel="noopener noreferrer" target="_blank"> Experts recommend</a> replacing more fluid than you lost exercising. If you want to get even more granular with the math: <ul><li>Weigh yourself before and after training</li><li>Each pound lost = ~<a href="https://pubmed.ncbi.nlm.nih.gov/17277604/" rel="noopener noreferrer" target="_blank">23 oz of fluid</a> lost</li><li>Multiply fluid loss by<a href="https://pmc.ncbi.nlm.nih.gov/articles/PMC5634236/?utm_source=chatgpt.com" rel="noopener noreferrer" target="_blank"> 1.5</a> to get your target</li><li>Drink that amount (with electrolytes) over the next 4 hours</li></ul>Example: Lost 1.5 lbs? That's 34.5 oz lost × 1.5 = ~52 oz to drink. Dissolve one LMNT stick pack in this amount and you should be golden.This approach is most relevant after long-duration sessions, high-intensity workouts, training in heat, or any workout that produces noticeable sweat loss. For shorter or lower-intensity sessions with minimal sweating, aggressive rehydration is usually unnecessary.The bottom line: If you're hydrating based on thirst, it’s likely you’ve already lost ground. Think ahead so you're not chasing performance when it comes time to grind.<h2>FAQ </h2><h3>Q: How do I know if I’m under-hydrated before a workout?</h3>Check your morning cues: dark yellow urine, a higher-than-usual resting heart rate, or feeling sluggish and unfocused are early signs. <h3>Q: During my workout can I just drink water and add salt?</h3>Table salt replaces sodium but not potassium or magnesium. A balanced mix keeps your muscles and nerves firing properly and prevents imbalances that can cause cramping or fatigue.<h3>Q: What if I drink too much water during a workout?</h3>Overhydration dilutes sodium in your blood — a condition called<a href="https://journals.lww.com/acsm-msse/fulltext/2016/03000/nutrition_and_athletic_performance.25.aspx" rel="noopener noreferrer" target="_blank"> hyponatremia</a>. It’s rare during typical gym workouts but can occur during prolonged endurance events when large volumes of low-sodium fluid are consumed. Symptoms include dizziness, nausea, and fatigue. Take small, consistent sips of electrolyte fluid instead of chugging fluids.<h3>Q: What other nutrients are important for workout performance and recovery?</h3>For athletes using carbohydrates for fuel, adding 30–60 grams per hour during longer workouts can help maintain blood sugar. Post-workout<a href="https://journals.lww.com/acsm-msse/fulltext/2016/03000/nutrition_and_athletic_performance.25.aspx" rel="noopener noreferrer" target="_blank"> carbs and protein</a> support glycogen replenishment and muscle repair.Fat-adapted athletes may not require intra-workout carbohydrates for lower-intensity or steady-state efforts. Focus on adequate protein (20–40g) after training and trust your fat metabolism to handle the energy demands. But high-intensity or glycolytic training may still benefit from carbohydrate availability.