Post-Race Rehydration: Calculate What You Actually Need

The Most Dehydrated of Buzzes

It feels weirdly brilliant crossing the finish line of a race you've spent months preparing for. Me, I feel the extremes all at the same time, I'm wrecked beyond belief, raisin shrivelled, and utterly relieved having finally completed the thing. The work isn't done though, this is the exact moment recovery starts. 

Whether you've just completed your first half marathon or have powered through setting a new personal best, your body has potentially sweated out litres of fluid. Many if it's also a hot and sunny day and you've been able to take on liquid during the route. Alongside water, you've also lost electrolytes, in particular physiologically significant amounts of sodium. Your body needs those salts to retain the water you're taking on post-race and to keep your nerves and muscles firing as they should. 

It can be tempting to grab the biggest bottle of water you can find and drown your post-completion endorphins with it. I mean like really chug. While well intentioned, this method isn't so efficient for boosting your blood volume and keeping that water inside your body (big chugs actually stimulate the kidneys to produce more urine due to short term excess - not ideal).

In this guide, we'll explore how to restore hydration safely and effectively, why electrolytes matter, how to estimate your individual fluid needs, and when more aggressive rehydration strategies may be appropriate.

Post-competition Rehydration is Critical to Recovery

During prolonged exercise your sweat rate tends to increase with exercise intensity. Stopping sweating despite maintaining exercise intensity is a big red flag for heat stroke and severe dehydration. If you recognise this happening to you during intense competition, get help from someone, anyone, as soon as you can. 

Even relatively small levels of dehydration can increase the level of cardiovascular strain experienced during competitive performance and intense exercise. Your physical performance will diminish compared to your euhydrated self. Delay rectifying the situation, and you'll feel fatigued long after your competition is completed.

Fluid losses of more than 2% of total body weight are associated with significantly reduced athletic performance and an increased risk of heatstroke and other heat-related illnesses. Severe dehydration also reduces the cerebrospinal fluid's ability to cushion the brain as it floats in the skull, creating an additional risk of brain trauma on impact. This is one of the reasons water cutting for combat sports is viewed as so incredibly dangerous without appropriate rehydration.

While water might be the star of the show, we can't hold the liquid we're taking in effectively without also replacing salts lost through sweat, in particular replacing sodium. Low blood sodium levels, also called hyponatremia, can be a life threatening situation that shuts down organ systems and disrupts nervous signalling.

Hyponatremia more often results from too much water being taken on too quickly rather than a lack of adequate sodium consumption, demonstrating why a pragmatic approach to post-race rehydration is a benefit. 

How much should you drink after a race or competition? 

A good rule of thumb is to replace around 1.5 litres of fluid for every kilogram of body mass lost during exercise. This accounts for the fact that some of the fluid you drink will inevitably be lost through urine as your body restores balance. 

If you have another competition or intense training within 24 hrs of finishing, aim to replace this lost fluid within the first 3 hours post-comp. 

How to calculate rehydration volume: 

1. After going to the toilet one last time, weigh yourself in minimal clothing immediately before commencing activities (warm up, matches, racing etc)
2. Once complete, weigh yourself in the same minimal clothing. Wring out clothing if needed for a more accurate reading

Target fluid replacement in litres = (Weight before in kg) - (Weight afterward in kg) x 1.5

For example: a 74 kg athlete weighed 71.3 kg after completing their race. They lost 2.7 kg in weight through sweating. They should aim to rehydrate with 4.05 l of fluid over the next 24hrs. 

How to calculate Sweat Rate: 

1. After going to the toilet one last time, weigh yourself in minimal clothing immediately before commencing activities (warm up, matches, racing etc)
2. Note the time you are starting exerting activities
3. Monitor your fluid intake all the way through your exertion period and note it down.
4. Once exertion is complete, weigh yourself in the same minimal clothing as soon as you can. Wring out clothing if needed for a more accurate reading.
5. Note the time that you weigh yourself, and use it to calculate the duration of your exertion period.

Sweat rate in L/hr  = [(Weight before in kg) - (Weight afterward in kg) + (fluid consumed in L)] / (duration of exertion in hrs)

For example: a 74kg athlete weighed 71.3kg after completing a 1.5 hr road race. They consumed 750ml of liquid during their activity. After accommodating for intra-activity drinking, the athlete sweated a total of 3.45 L during their event. If their exertion lasted 1.5 hrs, they sweated 2.3 L/hr.

Why bother besting my sweat rate? 

It's not always convenient or possible to weigh oneself immediately before and after races and competitions. 

Knowing what your body's typical sweat rate is for exertion levels comparable to an upcoming bout of exercise helps with anticipating and planning post-event rehydration. 

This is particularly useful for events where you're having to travel to participate. If you have to take all your required support with you, knowing what you need afterwards helps prevent you being caught short. If you're unsure what welfare will be available, especially at remote events like fell runs and ultras, having what you need waiting for you at the end can be a real blessing and gift to your future self. 

Post-Race Fluid Replacement by Weight Lost

I've made you a handy rehydration target table, because dehydrated brains are scientifically proven to be worse at maths than their euhydrated counterparts. FACT.

One of the most common mistakes is trying to drink everything immediately after finishing. Large volumes of fluid consumed quickly can leave you feeling bloated and uncomfortable, and they don't necessarily improve hydration. As mentioned above, big water boluses can actually promote urine production in the short term. 

Aim for the following to stay comfy and to rehydrate effectively:

• Begin drinking as soon as possible after finishing, ideally within 30 minutes.
• Sip little and often, 250 - 400 ml per hour is great, but don't stress about the precise rate (just don't chug!).
• Continue drinking consistently throughout the day.
• Include fluids alongside meals and snacks. 

Think of rehydration as something that takes you the next 24 hours, not something you rush tom complete before leaving the charity village.

When Should You Use Electrolytes?

Not every training session requires an electrolyte drink.

The truth is that unless you're sweating particularly heavily or exercising for a prolonged period, you probably don't need them. 

However, after longer bouts of intense exertion, they can be extremely useful for efficient recovery.

Consider including a sodium focused electrolyte in the following situations:

• your event lasted longer than about 90 minutes
• conditions were warm or humid
• you finished your event with visible salt marks on your clothing
• you're a particularly salty sweater
• you have a stronger than usual taste of salt when you lick your lips 
• you have another important event or intense session soon

The electrolyte of greatest importance is sodium.

Sodium helps replace what is lost in sweat, stimulates thirst and improves fluid retention, meaning more of what you drink stays within the body instead of simply being pulled into urine by your kidneys or excretion.

What Foods Help With Post-Race Rehydration?

Rehydration comes from food, too! Tons of foods contain a great deal of water, and can make a significant contribution towards your rehydration effort. This can be useful if drinking drinks is particularly unappealing, you struggle with acid reflux with larger volumes of fluid, or you've simply worked up a deep hunger. 

Including these as part of your recovery meals can contribute meaningfully to your overall hydration.

Good options include:

• Watermelon
• Strawberries
• Oranges
• Grapes (I love frozen grapes after competing. Cooling and refreshing.)
• Cucumber
• Tomatoes 
• Lettuce
• Soups and broths
• Greek yoghurt
• Smoothies
• Stews
• Homemade fruit salads

On top of boosting hydration, these foods also provide beneficial phytochemicals, antioxidants, vitamins, minerals and useful carbohydrates that support recovery beyond simple hydration alone. Having the additional bulk and carbohydrate load in your gut slows down gut transport, creating a longer opportune window for water absorption. Get amongst it!

Common Post-Comp Hydration Mistakes To Avoid

I've seen them all. I've fallen foul of a few myself. Let's all pledge to do our best to avoid these rookie errors. 

• Waiting until you're very thirsty before drinking.
• Turning down intra-race hydration stations because you're worried you'll need to pee.
• Drinking only plain water after heavy sweating.
• Ignoring sodium replacement after a long event.
• Ignoring food-based sources of hydration.
• Drinking huge amounts of liquid all at once.
• Celebrating with alcohol before refuelling or rehydrating.
• Assuming clear urine immediately after drinking means you're fully rehydrated. Pee can be a liar sometimes if you chugged a bunch of water. 

Rehydration Takeaways

Hydration after a race isn't simply about drinking more water. Effective rehydration combines fluid, sodium, carbohydrate and time.

The most useful strategy is often also the most simple:

• Estimate how much fluid you've lost.
• Replace around 1.5 times that amount gradually over the following 24 hours.
• Include sodium-containing foods or drinks.
• Pair fluids with carbohydrate and protein as part of your recovery meals.
• Continue checking in with your thirst, urine colour and overall energy levels.