The importance of hydration

An interview with Dr Stuart Galloway

The importance of hydration 1Dr Stuart Galloway is an Associate Professor in the Physiology, Exercise and Nutrition Research Group at the University of Stirling, UK.

Why is hydration important and which drinks help maintain a positive net fluid balance? The FrieslandCampina Institute spoke with Dr Stuart Galloway about his research at the University of Stirling, the United Kingdom about the hydrating potential of different drinks.


‘Hydration is a vast topic which Scientists have been researching for around 100 years.’ Early studies (Adolph, 1947) in the 1930’s and the 1940’s assessed the ability of soldiers to withstand the stress of physical activity in the Colorado desert, provided one of the first accounts of the physiological strain and associated decrease in exercise performance in the heat as a result of body water loss (1). More recent studies have explored real impact on physical markers of performance and provide the evidence base for practical advice (2).

Why is the maintenance of hydration important for athletes and sportspeople?

‘Generally research indicates that dehydration at a level of >2% loss in body mass (BM) will have some impact on performance (2, 4). However studies have shown that some individuals are sensitive to smaller changes, while others can perform very well with larger deficits, so it is important for hydration strategies to be individualised (3).

Key studies around skills sports have shown the impact of a decline in total body water on the ability to retain skills performance (4). In basketball players graded dehydration from 1-4% of BM showed a progressive decline in basketball skills (5). Our own research at University of Stirling has also shown that dehydrated cricket players experienced reductions in speed and accuracy of throwing and bowling (6).

The impact of dehydration on endurance performance has been researched more extensively (2). More recently there has been debate regarding potential  advantages for athletes from becoming lighter due to body water loss towards the end of a long race—such as in a marathon—or from mild dehydration prior to jumping events. The question is what level of dehydration is advantageous and the threshold when it becomes detrimental to performance. Further research is required in these areas.’

What does the science tell us about the hydrating potential of different drinks?

‘I have been involved in developing a tool called the Beverage Hydration Index (BHI). This index can be used to indicate which drinks are more effective for retaining the fluid consumed and keeping the body hydrated (Fig 1). We compared the effect on urine output of drinking 1 Litre of 13 commonly consumed drinks versus 1 Litre of plain water in euhydrated individuals (7).

Overall outcomes from our research found that the drinks with higher electrolyte content and/or energy density had a more positive BHI. The electrolyte content, in particular sodium, is important for fluid retention and a higher energy content most likely delays fluid excretion due to slowing down gastric emptying and delaying fluid delivery.’

Figure 1. The Beverage Hydration Index (BHI) of 13 drinks compared with water.

Milk: a good drink for after exercise?

‘According to our research, milk contains electrolytes to promote fluid retention but is also energy dense. This indicates it will likely slow fluid delivery through delayed gastric emptying. So, milk provides better fluid retention than water. Both full fat and skimmed milk performed equally well in our study, suggesting that fat content of milk was not playing a role. The oral rehydration solution (ORS) which is high in electrolytes—sodium in particular—resulted in the highest fluid retention. Orange juice which is higher in potassium and sugar, also fared quite well. These observations closely match what might be expected from the post-exercise rehydration literature which shows that the energy and electrolyte content of drinks are important for recovering fluid balance.

In terms of day-to-day living we did a follow-up study on people working in a call centre. These workers may restrict fluid intake to minimise toilet breaks and maximise the number of calls they take. Again we found they retained more fluid when milk was ingested compared with other drinks such as water, coffee and fruit juice. Our findings may also be helpful for other workers such as long distance lorry drivers, or workers in healthcare environment.’

Which drink? Are there any drinks which help maintain a positive net fluid balance?

‘The choice of drink will depend on the level of athlete and exercise goals. For example, is the priority to deliver fuel or mainly to provide fluid and electrolytes? During hard exercise, a drink providing carbohydrate and electrolytes is helpful for preventing a fluid deficit and providing additional fuel. However, for most recreational exercisers water is generally all they need, as they are not usually exercising for long enough to require additional fuel, and will likely eat something fairly soon after exercise.

In terms of post-exercise rehydration, full fat and skimmed milk perform well with fluid retention. As milk also provides protein and other nutrients, I would say it is a good option as a post-exercise recovery drink (8).’

Take home message: ‘Athletes and exercisers who are interested to understand more about their individual daily fluid needs should try keeping a record of their urine colour (and possibly also urine volume and number of voids per day), as well as body mass changes with exercise, and thirst response (3).’

 Looking ahead what questions would you like to address in your research?

 ‘I’m currently looking at the role of potassium in drinks and what happens with fluid and electrolyte balance. When the potassium dose is increased it may stimulate a greater excretion of sodium which will be detrimental to fluid retention. We are just completing a study on that topic and plan to go on to explore what is happening in the intracellular and extracellular fluid compartments using an old method based on chloride shift.

Our study is using 30-60 mmol per Litre potassium doses. This is modelled on levels that we have measured in commercially available coconut waters and natural coconuts. Coconut waters typically have a low sodium content of 10-15 mmol per Litre, although it is clear that some commercial coconut waters have added sodium. The role of sodium in drinks is well understood in terms of hydrating benefits, so it will be interesting to explore the physiological impact of drinks with a higher potassium content and provide a more detailed answer to whether coconut water is any better than water for hydration.

I’m also interested in the impact of hydration on the health of the wider population. Clinical markers of a low fluid intake have been associated with the development of metabolic disease, and increased frequency of urinary tract infections in older adults, so there are a number of interesting possibilities there for future work (9, 10, 11).’

“There is still much to learn in the field of hydration research, from effects on muscle function and performance, to impacts of adequate water intake on health and well-being in different population groups. It is certainly a very interesting and exciting area that I will continue to explore in my research.”

Read more:


  1. Adolph, E.F. Ed., 1947. Physiology of Man in the Desert. New York. NY: Interscience Publishers. In Sawka, M.N. and Coyle, E. 1999. Influence of body water and blood volume on thermoregulation and exercise performance in the heat. In: Hollozsy, J.O. (ed.) Exercise and sports science reviews. Philadelphia: Lippincott Williams & Wilkins. 27:167-218
  2. Sawka MN, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc 2007;39:377-90.
  3. Thomas DT, et al. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. J Acad Nutr Diet2016;116(3):501-28.
  4. Dougherty KA, et al. Two percent dehydration impairs and six percent carbohydrate drink improves boy’s basketball skills. Med Sci Sports Exerc 2006;38:1650-8.
  5. Baker LB, et al. Progressive dehydration causes a progressive decline in basketball skill performance. Med Sci Sports Exerc 2007;39:1114-23.
  6. Gamage J, et al. Effects of dehydration on cricket specific skill performance in hot and humid conditions. Int J Sport Nutr Exerc Metab 2016;26(6):531-41.
  7. Maughan RJ, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. Am J Clin Nutr 2016;103:717-23.
  8. James LJ, et al. Cow’s milk as a post-exercise recovery drink: implications for performance and health. Eur J Sport Sci 2019;19(1):40-8.
  9. Enhörning S, Melander O. The Vasopressin System in the Risk of Diabetes and Cardiorenal Disease, and Hydration as a Potential Lifestyle Intervention. Ann Nutr Metab 2018;72(suppl 2):21-7.
  10. Enhörning S, et al. Effects of hydration on plasma copeptin, glycemia and gluco-regulatory hormones: a water intervention in humans. Eur J Nutr 2017. Doi: 10.1007/s00394-017-1595-8.
  11. Hooton TM et al. Effect of increased daily water intake in premenopausal women with recurrent urinary tract infections: A randomized clinical trial. JAMA Intern Med. 2018;178(11):1509-15.