Rehydration and electrolytes after exercise – How to get the balance right
- Maryke Louw
- Jul 22, 2025
- 6 min read
Updated: Jan 13
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 and carbohydrates in your post-exercise rehydration drink, based on the latest research.
In this article:
We've also made a video about this:
Water is not enough
Recent studies confirm that water alone is less effective for rehydration than drinks containing electrolytes. This is because electrolytes improves the absorption of fluids in your gut.
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 or too high.
What else is needed, and how much?
This will depend on your nutrition goal – are you only interested in replacing fluids or do you also want to replenish some of the carbohydrates and other nutrients you might need?
👉 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.
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 about 270–300 mOsm/kg.
If the osmolality is listed directly:
⭐ Isotonic = 270–300 mOsm/kg range
Best when you want to rehydrate and replenish electrolytes and carbohydrates at the same time
Fluid absorption is good
Carbohydrate and sodium intake are balanced
Hypertonic > 330 mOsm/kg
Slower fluid absorption
Better suited carbohydrate loading
Less effective for rapid rehydration, especially immediately after exercise
Hypotonic < 270 mOsm/kg
Fastest fluid absorption
Very effective for rapid rehydration, provided you've taken in enough sodium in other food or drinks in your day
Limited contribution to carbohydrate restoration
⚠️ Drinking very large quantities of hypotonic drinks without adequate sodium replacement can be harmful, as it may dilute blood sodium levels (hyponatraemia)
If the osmolality is not listed:
Sodium content:
Isotonic drinks usually contain ~400–700 mg sodium per litre, which works well for most people
Below 300 mg/L = less effective for fluid retention during heavy sweating
800-1000 mg/L = can still be isotonic if carb content is low, but high carb + high sodium increases hypertonicity risk
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)
💡 The wide sodium range exists because sweat sodium losses vary greatly between individuals, and drinks must balance fluid absorption, sodium retention, and carbohydrate content without becoming hypertonic.
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.