Health & Hydration: The Research

We always hear that drinking water is good for health and is especially recommended in hot weather and exercise. Here we examine why staying hydrated is essential and ways to ensure you remain hydrated throughout the day.

As adults, water makes up approximately 60% of our body weight. It has several vital functions, including regulating our core body temperature, maintaining our blood volume, helping transport nutrients around our bodies, and keeping our joints, eyes, and skin healthy. On a daily basis we lose water through our urine, faeces, sweat, and water vapours from our skin and breath.

How does dehydration affect our body?

As water constitutes a large proportion of our body weight, it’s unsurprising that a deficit can negatively impact how well we function in our everyday lives. From as little as a 2% loss in body water, which equates to a relatively mild state of dehydration, can negatively affect athletic performance [1], cognitive functioning, including attention, motor coordination, memory, executive function [2]. Alongside the cognitive decline, mild dehydration can lead to increased negative moods, tiredness, and sleepiness [3].

When exercising in hot and humid climates, as is common in Hong Kong, the proportion of body water lost due to sweat increases as the body works to regulate our body temperature [4]. Individuals engaging in vigorous physical activity are thus more likely to become dehydrated, which can lead to deficits in endurance, increased fatigue, and perceived effort [5]. Alongside water losses, excessive sweating can also result in losses of essential electrolytes, namely sodium (Na+), chloride (Cl−), and potassium (K+). A randomised trial found that as the intensity of exercise increased, sodium and chloride losses in sweat increased by approximately 150% [6]. As such, it’s crucial to replenish not only any lost fluids but also the electrolytes.

Signs you might be dehydrated

One of the most common signs you might not be drinking enough is the classic sensation of thirst. However, if you’re feeling thirsty, likely, you’re already dehydrated. Other warning signs of dehydration include dizziness, dry mouth, lips and eyes, tiredness, constipation, and reduced urine output. The easiest way is to examine the colour of your pee. If it’s dark and strong-smelling, it means you aren’t drinking enough.

How to remain hydrated

The amount of fluids each individual requires per day varies. On average, most adults should aim to consume between 6-8 cups [8]. This requirement will increase in high temperatures and during exercise. In cases of mild dehydration, water is the main recommended source to replenish the fluids we lose as it’s readily available and doesn’t contain any calories. However, other liquids, such as coffee, teas, juices, and smoothies, can also prevent dehydration [9].

‘But tea and coffee has caffeine, isn’t that dehydrating’?

Although a common myth, drinking caffeinated drinks in moderate amounts (<450mg, or about 4 cups a day) does not lead to dehydration, impact exercise performance in colder or hotter environments [10-12].  Alongside fluids, we can also rehydrate and replenish the lost electrolytes by consuming food, especially fruits and vegetables, due to the high-water content.


If you don’t particularly like water or struggle to drink throughout the day, here are some suggestions:

  1. You’re more likely to drink if you like the taste of something. Infusing water with fruits and vegetables can be a great option to add some flavour to your drink.
  2. Add a cup of hot or cold tea to your day. In addition to providing hydration, some teas, such as green tea, have anti-cancer properties [13].
  3. Keep a water bottle with you at all times, as this can help serve as a reminder to drink throughout the day.
  4. Downloading an app to monitor water intake can be a great way to assess whether you’re drinking enough throughout the day and can be used to set goals and reminders.
  5. Drink before, during and after exercise to ensure you replenish the fluids lost in sweat. For endurance exercises, adequate rest and water breaks are recommended

Incorporating these steps will help to keep you hydrated throughout the day, ensuring your body remains healthy and functions at its best. For cases of severe dehydration, which is a risk following sickness resulting in diarrhoea and vomiting in children, oral rehydration salts are recommended [14]. In some very severe cases, hospital admission may be required. Always seek advice from your doctors if in doubt.


    1. Murray, B., Hydration and Physical Performance. Journal of the American College of Nutrition, 2007. 26(sup5): p. 542S-548S.
    2. Wittbrodt, M.T. and M. Millard-Stafford, Dehydration Impairs Cognitive Performance: A Meta-analysis. Med Sci Sports Exerc, 2018. 50(11): p. 2360-2368.
    3. Pross, N., Effects of Dehydration on Brain Functioning: A Life-Span Perspective. Ann Nutr Metab, 2017. 70 Suppl 1: p. 30-36.
    4. Sawka, M.N. and S.J. Montain, Fluid and electrolyte supplementation for exercise heat stress. The American Journal of Clinical Nutrition, 2000. 72(2): p. 564S-572S.
    5. Cheuvront, S.N., R. Carter III, and M.N. Sawka, Fluid balance and endurance exercise performance. Current sports medicine reports, 2003. 2(4): p. 202-208.
    6. Baker, L.B., et al., Exercise intensity effects on total sweat electrolyte losses and regional vs. whole-body sweat [Na(+)], [Cl(-)], and [K(+)]. European journal of applied physiology, 2019. 119(2): p. 361-375.
    7. Inform, N. Hydration. 2020; Available from:
    8. GovHK. Healthy Eating. Food Safety & Nutrition 2019; Available from:
    9. Grandjean, A.C. and J.K. Bartram, Essential Nature of Water for Health: Water as Part of the Dietary Intake for Nutrients and the Role of Water in Hygiene, in Encyclopedia of Environmental Health (Second Edition), J. Nriagu, Editor. 2011, Elsevier: Oxford. p. 708-718.
    10. Armstrong, L.E., et al., Caffeine, Fluid-Electrolyte Balance, Temperature Regulation, and Exercise-Heat Tolerance. Exercise and Sport Sciences Reviews, 2007. 35(3): p. 135-140.
    11. Zhang, Y., et al., Caffeine and diuresis during rest and exercise: A meta-analysis. J Sci Med Sport, 2015. 18(5): p. 569-74.
    12. Maughan, R.J. and J. Griffin, Caffeine ingestion and fluid balance: a review. J Hum Nutr Diet, 2003. 16(6): p. 411-20.
    13. Yu, S., et al., Green tea consumption and risk of breast cancer: A systematic review and updated meta-analysis of case-control studies. Medicine (Baltimore), 2019. 98(27): p. e16147.
    14. World Health Organization. Fluid management in severly malnourished children under 5 years of age without shock. 2019; Available from:
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Mayara De Paula, Nutrition Student

Mayara is a Psychology graduate currently completing an MSc in Clinical Nutrition and Public Health at University College London. She has volunteered with a number of charitable organisations providing recipes and nutritional advice. Her interests include health inequalities, child health and prevention of non-communicable diseases.

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