Rehydration and electrolytes after exercise – How to get the balance right
- Maryke Louw
- Jul 22
- 6 min read
Updated: Jul 28
Just chugging lots of water to rehydrate after a workout won't replace the salt you lost and might even worsen the electrolyte imbalance in your body. Here's our guide to the optimal mix of sodium, carbohydrates, and protein in your post-exercise rehydration drink, based on the latest research.
In this article:
Water is not enough
Recent studies confirm that water alone is less effective for rehydration than drinks containing electrolytes and, in some cases, protein. This is because electrolytes and protein improves the absorption of fluids in your gut.
In fact, if you drink a lot of water quickly, your body naturally increases the rate at which it produces urine (you just pee most of it out again). You also run the risk of causing yourself harm if you only replace water and not salts and minerals. This can lead to cramps or hyponatremia (a condition that can be fatal).
However, many sports drinks have an electrolyte concentration that is too low and no protein at all.
What else is needed, and how much?
👉 For efficient fluid replacement, beverage composition – especially its electrolyte and carbohydrate content, and to a lesser extent the protein content – matters as much as volume.
The concept of osmolality is useful here. Osmolality is a way of measuring how concentrated a liquid is – specifically, how many dissolved particles (like salts and sugars) are in it. The more particles in the liquid, the higher its osmolality.
In the context of hydration drinks, osmolality affects how quickly the liquid moves from your gut into your bloodstream.
Drinks containing electrolytes and carbs with an osmolality close to that of your body’s natural fluids (called isotonic drinks) are generally absorbed faster and help with rehydration more effectively than drinks that are too concentrated (hypertonic) or too diluted (hypotonic).
How to check the osmolality of a sports drink
To assess osmolality from a sports drink label, you should look for the osmolality value in milliosmoles per kilogram (mOsm/kg), or check for clues based on carbohydrate and electrolyte content.
(Carbohydrate replenishment after exercise is another topic, but it is important here because the amount of carbs in a drink affects its osmolality and therefore how well your body will absorb the fluid and much-needed electrolytes.)
Bodily fluids have an osmolality of 275–295 mOsm/kg.
If the osmolality is listed directly:
Look for values in the 275–295 mOsm/kg range:
Isotonic = ideal (≈285 mOsm/kg)
Hypertonic = too high (>330 mOsm/kg) → slower absorption
Hypotonic = low (<250 mOsm/kg) → fast absorption, less electrolyte replenishment
If the osmolality is not listed:
Sodium content:
Isotonic drinks usually contain ~400–700 mg sodium per liter
Below 300 mg/L = less effective for rehydration
Over 800 mg/L = potentially hypertonic unless carb content is low
Carbohydrate content:
Isotonic drinks typically contain 4–8 g of carbs per 100 ml (4–8%)
>8% = likely hypertonic (e.g. undiluted fruit juice)
<4% = likely hypotonic (e.g. very diluted electrolyte drinks)
If, like me, you feel your 🧠 brain wants to go on ✈️ vacation when you look at all of these numbers, download our hydration cheat sheet here.
👉 Of course, you can also use these guidelines if you want to mix your own post-exercise rehydration drinks.
Below is an example of a ready-to-drink rehydration drink with a useful osmolality, as well as a powder for self-mixing your rehydration drink that will also have the right osmolality when used as directed.
How much fluid after exercise?
How much rehydration fluid to take after exercise should be determined by two factors.
1. How much fluid you have lost
You can get an idea of how much fluid you have lost during exercise by measuring your body weight before and after the race or activity. Learn more about how you can use your weight loss, thirst, and urine colour to determine whether you are dehydrated.
2. The time interval before your next exercise session
If you have more than 24 hours until your next session
Research has shown that athletes naturally take in enough food (that include salts and electrolytes) and fluids during a 24-hour period to fully replace all the fluids lost during exercise. You do not have to follow any special guidelines. Just eat a balanced diet and drink fluid according to thirst.
If you have to exercise within 6 hours of your previous session
In this case, you will have to ingest a combination of water and sodium in excess of your existing body weight deficit (more than the weight that you've lost). This will ensure that you replace all your fluids and allow for the amount you'll lose to the toilet.
The ACSM recommends that:
you aim to replace 125% to 150% of your decrease in body mass,
and divide the fluid into 500 ml portions,
and take it about 30 minutes apart.
Taking the fluid in portions is important, because drinking a large volume of fluid in one go stimulates urine production, so you'll pee out most of the stuff you've drunk.
💡 Example: If you found that you had lost 2 kg of body weight during exercise, you would aim to drink about 2.5 L to 3 L of fluid.
🚨 Get your doctor's advice if you're supposed to follow a low-salt diet.
Download the post-exercise hydration guide
👉 We’ve put the key advice from this article into a handy PDF you can download.
Other nutrients that are important after exercise
Research has shown that it is also important to restock on protein after exercise to ensure a full recovery. I have previously written about exactly how much protein you should eat and when for best recovery.
Research has also shown that milk-based drinks improve fluid retention more effectively than standard sports drinks or water alone. The osmolality of milk and milk-based drinks might be somewhat higher than drinks containing only electrolytes and carbs, which means that the fluid is absorbed somewhat slower, but the fluid retention is better.
👉 Because of the slower fluid absorption rate, using milk-based drinks during exercise is not recommended. It’s best to save them for afterwards.
The American College of Sports Medicine (ACSM) guidelines state that caffeine and low alcohol content drinks (such as beer) are OK to drink in moderation if you are aiming to rehydrate over 24 hours. It might be best to avoid them if you want to rehydrate quickly, since they have a mild diuretic action.
Strong alcohol (spirits) is not advised, because it acts as a strong diuretic (makes you lose a lot of fluid).
While sodium is the most important mineral to replace, other minerals such as potassium and magnesium might also play important roles in dehydration at a cellular level.
🍌 Natural sources of potassium and magnesium are asparagus, bananas, leafy greens such as spinach, Swiss chard, kale, white and sweet potatoes with skin on, citrus fruits, tomatoes, kiwi, papaya, squash, almonds, cashews, peanuts, and walnuts.
Bonus video: We interview former athlete Jesse Funk about how people have different sweat rates and the best way to go about replacing lost electrolytes.
How we can help
Need more help with your injury? You’re welcome to consult one of the team at SIP online via video call for an assessment of your injury and a tailored treatment plan.
We're all UK Chartered Physiotherapists with Master’s Degrees related to Sports & Exercise Medicine or at least 10 years' experience in the field. But at Sports Injury Physio we don't just value qualifications; all of us also have a wealth of experience working with athletes across a broad variety of sports, ranging from recreationally active people to professional athletes. You can meet the team here.
About the Author
Maryke Louw is a chartered physiotherapist with more than 20 years' experience and a Masters Degree in Sports Injury Management. Follow her on LinkedIn and ResearchGate.