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What is the best way to replace fluids after exercise?

You may think that the answer to this question is rather obvious….drink lots of water. Well, it is and it isn’t. It is important to get the balance right when you rehydrate after exercise.


If you drink a lot of fluid 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 may be fatal).

In this article:

  • The importance of Salt (sports drinks are not enough)

  • How you rehydrate after exercise should be dictated by 2 factors

  • The time interval before your next exercise session

  • Other nutrients that are important after exercise


The importance of Salt (sports drinks are not enough)


You should always make sure that you replace the salts that you have lost during exercise at the same time as the fluids. If you drink too much fluid without replacing the salts, you may be at risk of hyponatreamia - a condition that causes swelling on the brain and that can be fatal.


The good news is that you can easily get the salt you need from regular food e.g. pretzels, soup bouillon (2.2 g/L sodium) or tomato juice (2.2 g/L sodium). Sports drinks usually only have a sodium concentration of around 0.46 g/L which is not enough (see next section).


While sodium-chloride is the most important to replace, other minerals such as potassium and magnesium may 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.


How you rehydrate after exercise should be dictated by 2 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 if you are dehydrated.


2. The time interval before your next exercise session


If you have 24 hours until you 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 combined with the equivalent of about 1 g/L to 2 g/L of sodium. They further recommend that you divide the fluid into 500ml portions and take it about 30 minutes apart. Drinking large quantities of fluid all at once stimulates urine production. This means that you'll lose most of the fluid you drink in the loo.


Example: If you found that you had lost 2kg of body weight during exercise, you would aim to drink a mixture of water and salt which equates to about 2.5L to 3L of water with around 5,7 g to 6,9 g of sodium (salt) in it.


Yes, this will likely taste worse than seawater so rather choose foods like tomato juice (2 g/L sodium) than salt tablets. You can have sports drinks, but you'll have to add some salt since they normally do not contain enough. Please make sure that you first ask your doctor's advice if you're supposed to follow a low salt diet.


Other nutrients that are important after exercise

Research has also shown that it is important to restock on protein and carbohydrates after exercise to ensure a full recovery. I have previously written about exactly how much protein you should eat and when, for best recovery.


The ACSM guidelines further state that caffeine and low alcohol content drinks (beer) are OK to drink in moderation if you are aiming to rehydrate over 24 hours. It may 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).


Let me know if you have any questions. Need more help with an injury? You can consult me online using Skype video calls.

Best wishes

Maryke

Sports Physiotherapist


References


Sawka M, Burke L, Eichner E, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and science in sports and exercise 2007;39(2):377.

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