Exercising? Why you must stay hydrated – and signs you’re not drinking enough

Ever felt particularly tired during a workout? It could be down to a lack of hydration when exercising and the experts explain why.

Found a workout particularly tough recently? Turns out your fitness levels might not be to blame, but your hydration levels instead.

Not only is drinking enough fluid vital to our overall health, but proper hydration when exercising is key to aspects of your performance, including your energy levels and your body’s ability to regulate your temperature. But how much fluid do we need to stay hydrated during exercise and how do we know if we’re getting enough? 

Man holding exercise mat and water bottle in the outdoors.Credit: Shutterstock/BLACKDAY

Current UK guidelines recommend that adults aim for six to eight glasses (equivalent to around 1.52 litres or 2.5-3.5 pints) of fluids per day as part of a healthy diet.

However, this doesn’t take into account any additional needs if you are doing strenuous exercise, and it’s worth noting your fluid needs can vary and depend on a variety of factors, including your age, lifestyle, body size and climate. 

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Why is it important to stay hydrated during exercise?

The importance of hydration during exercise extends to various bodily and mental functions.

Replaces fluids lost through sweat

“When it comes to exercise, hydration becomes even more important because we need to replace fluid lost from sweat to avoid becoming dehydrated,” says performance dietitian and personal trainer Kerri Major.

She points out that over half of the body (55% to 70%, depending on your sex and age) is made up of water, to emphasise the importance of staying hydrated to support numerous bodily functions. 

Dr Sunni Patel, a personal trainer, culinary medicine expert and nutritional therapist, explains: “Sweating is your body’s way of regulating temperature. But if you are not hydrated enough, your body may struggle to do this effectively, leading to an increased risk of overheating.” 

Maintains blood volume

Patel adds that adequate hydration during exercise maintains your blood volume (the volume of fluid in the circulatory system). If this drops, your heart must work harder to circulate oxygen and nutrients to your muscles, which can result in lower energy levels. 

Aids performance and recovery

Staying hydrated during exercise is also important because it supports all aspects of your workout from physical performance and endurance to mental focus and recovery.

Patel says being hydrated can help you maintain your “strength, power, speed and overall exercise capacity. Even mild dehydration can impair your ability to concentrate, make decisions, and react quickly, all of which are essential for technique, coordination and maintaining motivation when exercising.”

Major agrees: “Whether you’re a gym goer or training for a competition, if you’re dehydrated, you’re likely to make skill errors.” 

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Pre-and post-workout hydration is important too

It’s not just about the importance of hydration during exercise, but beforehand and afterwards too.

Starting your workout properly hydrated means your body is prepared for upcoming physical demands by ensuring your muscles are hydrated,” says Patel. “You also have a head start in compensating for fluid losses through sweat, reducing the chances of dehydration.” 

Meanwhile, post-exercise recovery requires proper rehydration because “it supports the restoration of energy, helps repair damaged tissues, and enhances the body’s ability to adapt and perform better in subsequent workouts.”

It’s important to be aware that you can drink too much and over-hydrate, but this is rare and most likely to affect high-performance athletes who need to consume larger amounts of fluid.  

How much fluid do I need when exercising?

There’s not a magic number for how much everyone should be drinking. Major says: “The amount of fluid that is needed during exercise is very individual and depends on various factors, such as how much sweat you lose as an individual, environmental temperatures and humidity.

“The goal is about trying to minimise dehydration, without over-drinking.” 

Person filling reusable bottle with water from the tap.Credit: Shutterstock/MargJohnsonVA

How much water to drink before a workout

Major says an issue for many is that they begin their workout dehydrated, so it’s best to first maintain your basic level of hydration. Following recommended daily guidelines is essential here.  

“Two to three hours before exercise, I’d recommend drinking around 500-600ml [just over a pint] of fluid,” says Major. “Drinking huge volumes before working out could make your stomach uncomfortable or you might need the toilet during your workout.” Patel notes that the amount you need depends on your body weight.  

Staying hydrated during exercise

“The amount of fluid you need during exercise is down to the intensity and duration,” says Major.

“Once you’ve made sure you start hydrated, top up regularly with fluids and electrolytes [more on these shortly] when it’s practical to do so.” 

During exercise, the NHS recommends sipping fluid every 10-15 minutes. 

However, Patel adds: “High-impact activities or exercises that last longer than an hour generally result in increased sweating and higher fluid requirements. Adjust your fluid intake accordingly based on the nature of your exercise.”

Examples of high-impact activities include HIIT workouts and some forms of cardio exercise such as running.

How to rehydrate after exercise

Replacing fluid losses after exercise is important. Monitoring your body weight before and after exercise can give you an estimate of your fluid loss.

For every kilogram (2lb 3oz) of weight lost during exercise, you should aim to consume approximately 1.2-1.5 litres (two to three pints) of fluid, according to the International Olympic Committee. 

Patel says: “Aim to start rehydration within 30 minutes to an hour after exercise and continue drinking fluids throughout the day to restore hydration levels and support recovery.” 

Best drink for hydration during exercise

Some of the best options include:

  • Water
  • Drinks containing electrolytes, such as coconut water
  • Herbal teas
  • Milk
  • Isotonic drinks (though they can be high in sugars – see below for a healthier homemade version)

 “Water is typically the best drink for hydration during exercise, as it is readily available, easily accessible and sufficient for most people,” says Patel.

“If the taste of plain water is unappealing, you can add slices of fruits like lemon, lime or cucumber for a hint of flavour.” 

Major also suggests cooled caffeine-free herbal teas are another good alternative.

There is some evidence that milk is more hydrating than water, so it can be a great post-exercise drink, especially as it also contains protein, which can contribute to your recovery from exercise. 

Drinks containing  electrolytes can also be beneficial. “Sweating during exercise not only causes fluid loss but also the loss of electrolytes, such as sodium and potassium, so rehydrating with electrolyte-rich fluids can replace these,” explains Patel.  

“Coconut water is a natural source of electrolytes, making it a suitable option for during exercise. If you’re watching your calorie intake, just be aware that coconut water contains calories.”

Glass of coconut water with pieces of coconut surrounding it.Credit: Shutterstock/Artem Oleshko

“Alternatively, you can buy electrolyte supplements in forms like gels and sports drinks,” adds Major, who advises consuming such drinks if the intensity or duration (approximately over an hour) of your workout is high.

Just be aware that the sugar content in sports drinks can be high, so consider no or low-added sugar options. 

Major notes that having some sugar in your drink to provide carbohydrates can be useful in certain instances if you need fuel for exercise in a quick digestible form, or if your appetite isn’t great. But in general, this isn’t needed.

She adds: “You can also make your own isotonic drink [a drink containing essential salts and minerals] by mixing 200mls (a quarter of a pint) of squash and 800ml (one and a half pints) of water with a small pinch of salt.” 

There is some evidence to suggest that certain non-alcoholic beers are as good as or better than water for rehydrating after exercise. Major says this is because “non-alcoholic beer usually contains some sugar along with electrolytes, in particular sodium, so it might partly help to accelerate rehydration better than plain water.

She adds that non-alcoholic beer is a better choice than alcoholic beer as the latter acts as a diuretic causing you to urinate more, which could lead to dehydration.

However, she says the evidence that it hydrates as well as or better than water is not convincing, and she would recommend drinking the fluids mentioned above instead.  

What happens if you don’t drink enough water?

Patel shares some common signs that you’re not getting enough fluid during your workout. 

  1. Increased thirst: Feeling excessively thirsty during your workout is a clear indicator that you may not be adequately hydrated. Thirst is your body’s way of signalling that it needs more fluids. 
  2. Dry mouth and lips: This can be a sign of dehydration. Inadequate fluid intake can reduce saliva production, leading to dryness in the mouth.
  3. Fatigue and weakness: If you find yourself feeling unusually tired and lacking energy, insufficient hydration could be a contributing factor. 
  4. Dizziness or feeling lightheaded: Dehydration can cause a drop in blood volume and blood pressure, leading to these symptoms. If you experience these during exercise, it’s crucial to stop and rehydrate.
  5. Muscle cramps: Inadequate fluid intake can disrupt the balance of electrolytes in your body, which are essential for proper muscle function, contributing to muscle cramps or spasms during exercise.
  6. Decreased performance: If you notice a decline in your endurance, strength, speed or overall exercise capacity, you might not be adequately hydrated.
  7. Dark-coloured urine: Checking the colour of your urine, using the NHS urine colour chart, can provide insights into your hydration status. Dark-coloured urine, such as amber or honey-coloured, can be a sign of dehydration. In a well-hydrated state, urine should be a pale-yellow colour. 

What to do if you’re struggling to drink enough

Woman sipping from hydration pack.Credit: Shutterstock/La Famiglia

Patel provides some tips for staying hydrated during exercise.  

  1. Make hydration convenient: Keep fluids readily available during your workout. Whether it’s carrying a reusable water bottle or using hydration packs for longer activities, it makes it easy to grab a drink when needed. 
  2. Set hydration goals: For example, aim to drink a certain amount of fluid every 10-20 minutes. Having a target can help you stay mindful of your fluid intake and ensure you’re staying hydrated (the QAZW Motivational Bottle, £19.99 from Amazon, comes recommended by a member of the Saga Exceptional team to help with this task).
  3. Use reminders: Set timers on your watch or phone to remind yourself to drink at regular intervals during your workout.
  4. Listen to your body: Pay attention to your body’s thirst cues and signs of dehydration. If you feel thirsty during your workout, take it as a signal that you need to drink fluids. 

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If you’re concerned about your hydration levels during exercise or want to make sure you’re getting enough fluid, speak to a health professional or registered sports dietitian. 

Expert bios

Dr Sunni Patel PhD, MBA, PGDip (Cul Med) is a culinary medicine and nutritional therapy expert. With more than 15 years of healthcare experience and 10 years working in senior corporate roles, Dr Patel has a passion and proven success for bringing wellness into everyday life. He is also founder of health coaching and food education platform Dish Dash Deets and has a PhD on the risk factors for type 2 diabetes and heart disease. 

Kerri Major is a registered dietitian with over 10 years of clinical experience working for NHS Scotland. She specialises in weight management, gastroenterology, coeliac disease, oncology and heart health, and, most recently, in surgery and critical care as well as stroke and elderly/neuro-rehabilitation. Major now works as a freelance sports dietitian and personal trainer. She is hugely passionate about helping individuals feel fit, healthy and strong through adopting an active lifestyle, as well as supporting athletes, both recreational and elite, to understand how to fuel themselves appropriately.

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Gemma Harris

Written by Gemma Harris she/her

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Gemma Harris has been a journalist for over seven years and is a self-confessed health and wellbeing enthusiast, which led her to specialise in health journalism. During her career, she has worked with top editors in the industry and taken on multiple high-discipline fitness challenges for certain outlets. She is particularly passionate about nutrition; after being diagnosed with irritable bowel syndrome in 2016, she discovered her fascination for gut health and founded thegutchoice.com – a dedicated space for providing a hopeful outcome for people with gut issues. Gemma’s core aim is to help others through her writing.

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