Minttu Hukka Blog: Fundamentals of Hydration

Minttu Hukka Blog: Fundamentals of Hydration

Dehydration impairs performance

Water is the largest component of the human body, representing about 45-75% of the body weight. Body’s water content is greatly determined by body composition, as adipose tissue only contains about 10% of water vs. 75% in the muscle tissue (1, 2). Humans will often dehydrate by 2-8% of their body weight during exercise-heat stress (3,4). Dehydration impairs athletic performance, and studies have found that even just a 2% decrease in body weight due to sweating has been found to negatively influence performance (5, 6), and a 5% loss of body weight can decrease the work capacity by about 30% (2). Other studies suggest that maximal oxygen consumption (VO2max) would be impaired by dehydration level that is equal to or more than 3% of body weight (2, 6), and one study found that cardiovascular and thermal drift in well-trained cyclists were linear to the extent of dehydration (7). That being said, dehydration makes maintaining effort more difficult, eventually forcing you to slow down if actions to offset it are not taken.


The physiology behind dehydration

The degree of how much performance is impaired by dehydration is highly task-dependent: prolonged, endurance exercise is likely more negatively impacted by dehydration compared to short-duration, anaerobic exercise (1). Individual sweat-rates, movement economy and acclimatization are just a few factors influencing the degree of dehydration. Dehydration starts to have a negative impact on aerobic performance when skin temperature exceeds 27 °C, and each additional 1 °C increase results in further 1.5 % impairment (8). Increasing body temperature does not only have a performance impairing effect, but can also be a risk factor for heat-related injury. The CORE sensor is effective in tracking the body's core temperature. Discover more about the CORE sensor here.


The primary reasons why dehydration impairs performance include reduction in blood volume, decreased skin blood flow, sweat rate and heat dissipation; increase core temperature and the rate of muscle glycogen use (9). Dehydration and the resulting performance decrements may be more obvious in warm climates and at altitude, but it is good to keep in mind that one can also become dehydrated in cold climates. Sometimes the risk of dehydration is actually greater in winter than in summer, as the cold weather suppresses the body's thirst response. Meanwhile, urine output increases (cold-induced diuresis) as a result of vasoconstriction, when the blood vessels are trying to direct blood flow towards the core rather than extremities, where the heat would be lost. The body is in a “survival mode”, and maintaining hydration balance is less relevant.


Clothing can also have a significant impact on hydration status: more layers help in staying warm, especially in winter, but the thicker and heavier winter clothing may also contribute to dehydration. More clothing may make you work harder (up to 10-40%) and produce more sweat than usual, which may not be as obvious as sweat will evaporate quicker in the cold air. You’ll also lose more fluids via increased respiratory fluid loss. Respiratory water loss is greater in cold temperatures because cold air is drier and needs to be warmed up and humidified in the respiratory tract.


Potential warning signs of dehydration 

Some potential warning signs of dehydration in both warm and cold conditions include:

  • Thirst
  • Headache and dizziness
  • Muscle cramps
  • Dry mouth and lips
  • Dry skin
  • Reduced urine output or dark urine


What, when, and how much?

The existing literature suggests that fluid replacement strategies should be personalized for each individual’s sweat rates and exercise conditions, with the goal of preventing the loss of 2% or more of body weight to maintain performance (10).


Water is often enough when the activity lasts <1h or is light-to-moderate in intensity. During prolonged (>1h) or more intense activities or when exercising in heat, extra calories and electrolytes may be beneficial to replace lost minerals and prevent cramping. Sodium and potassium are the primary electrolytes in sports drinks. Hydrating needs are greatly dependent on the individual sweat-rate, exercise duration and intensity, but for many a safe recommendation would be drinking ~500-750mL (i.e. one bottle) per hour during endurance exercise, and aim for 60-90g of carbohydrate per hour. To determine your hydration needs, weigh yourself before and after exercise. The Nix Hydration Biosensor is effective for assessing sweat composition. Find out more about the Nix sensor here.


It is recommended to prehydrate several hours prior to exercise (11). For each 1 kg body weight lost, consume ~1.5 L of fluid (12). If your workout clothes have visible salt coatings that may suggest you are a “salty sweater” and would benefit from extra electrolytes.


It’s worth noting that many sports drinks are high in calories and if those extra calories are not needed due to the short duration/lightness of the activity, the extra calories may contribute to excess energy intake (and thus, weight gain) over time.


How to choose a sports drink?

We often look for drinks that are palatable to us, which is definitely an important factor, but when choosing a sports drink you should consider at least a few more things: the before-mentioned personal needs (how long and hard is your training session, and whether you sweat a lot and if your sweat is salty) and the drink’s tonicity.


Tonicity is a measure of the relative concentration of carbohydrates and salts in the solution compared to that of in blood. Tonicity is the key-factor in determining how quickly the drink leaves your gut and the nutrients are being absorbed. This is called gastric emptying, and its rate is determined by the amount of fluids ingested and the concentration of the solution.


Ideally, sports drinks are either isotonic (the same concentration as blood) or hypotonic (lower concentration than blood), and have osmolality of less than 300 mOsm/L with 6-8% carbohydrates per 100 g.


Hypotonic drinks create a favorable osmotic gradient, so the water flows easily across the stomach walls into the blood vessels. Hypotonic drinks typically contain less than 6% of carbohydrate, and should be the preferred choice if the primary goal of the drink is to hydrate rather than deliver energy. E.g. Nosht Zero Calorie Sports Drink Tablet, Precision Hydration TubeSIS Hydro, Puhdistamo Electrolyte Powder, or Precision Electrolyte Capsules.


Traditional sports drinks are typically isotonic. They contain a reasonable amount of energy and leave the stomach relatively quickly. Isotonic drinks contain about 1.5-2x the amount of carbohydrates hypotonic drinks do, and are useful during short duration, high intensity exercise where quick carbohydrate delivery is the priority over hydration (13). However, isotonic drinks may cause more GI distress when consumed in large amounts, especially during endurance events in hotter conditions, where fluids are often consumed at a faster rate. E.g. Maurten Drink Mix 160Nosht Endurance Drink MixSIS Go ElectrolytePrecision Hydration Powder, Neversecond C30 Sports Drink, or Moonvalley Organic Sports Drink.


Hypertonic drinks have carbohydrate and salt contents higher than that of in blood (>300 mOsm/L). They typically contain large amounts of carbohydrate, and have been formulated to maximize energy delivery. As a result, consuming these drinks often leads to delayed gastric emptying (and possibly sloshing and side stitches if you haven’t trained your gut enough). Hypertonic drinks draw water into the gut to dilute the contents so that the nutrients and fluids can be absorbed. Therefore, hypertonic drinks technically have a slightly dehydrating effect and they often make you feel more thirsty and should only be used during times when maximal carbohydrate delivery is the top priority over hydration (13). E.g. Maurten Drink Mix 320Precision Fuel 60 Drink MixTailwind Endurance FuelNosht High Energy Sports DrinkNeversecond C90 Sports DrinkPrecision Fuel Carb Only Drink Mix, or SIS Beta Fuel.


When choosing a drink that contains carbohydrates, the composition matters. Your body can absorb ~60 g of the same kind of carbohydrate per hour, and you should always aim for a 60-70 g/hour intake of glucose (14). More than that leads to delayed gastric emptying and indigestion, and if the carbohydrate intake must be higher, you can add fructose or dextrose on top. Most commercial sports drinks are 2:1 or 1:0.8 glucose:fructose. 2:1 would likely be the optimal ratio for lower intakes <90 g/hour, and 1:0.8 in intakes >90 g/hour.

2:1 glucose:fructose


1:0.8 glucose:fructose


  1. Sawka, M.N. Physiological consequences of hypohydration: exercise performance and thermoregulation. Med Sci Sports Exerc. 1992 24(6): 657-670.

  2. Sawka, M.N. and Pandolf, K.B. Effects of body water loss on exercise performance and physiological functions. In: Perspectives in Exercise Science and Sports Medicine: Vol. 3. Fluid Homeostasis During Exercise. C.V. Gisolfi and D.R. Lamb (Eds.). Indianapolis: Benchmark Press, 1990. pp. 1-38.

  3. Sawka MN, Montain SJ. Fluid and electrolyte supplementation for exercise heat stress. Am J Clin Nutr. 2000 Aug;72(2 Suppl):564S-72S. doi: 10.1093/ajcn/72.2.564S. PMID: 10919961.

  4. Sawka, M.N. Body fluid responses and hypohydration during exercise-heat stress. In: Human Performance Physiology and Environmental Medicine in Terrestrial Extremes. K.B. Pandolf, M.N. Sawka, and R.R. Gonzalez (Ed.). Indianapolis: Benchmark Pres, 1988, pp. 227-266.

  5. Armstrong LE, Costill DL, Fink WJ. Influence of diuretic-induced dehydration on competitive running performance. Med Sci Sports Exerc. 1985 Aug;17(4):456-61. doi: 10.1249/00005768-198508000-00009.

  6. Craig, F.N, and Cummings, E.G. Dehydration and muscular work. J of Applied Physiology. 21:670-674, 1966.

  7. Montain SJ, Coyle EF: Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J Appl Physiol. 1992, 73: 1340-1350.

  8. Sawka M.N, Cheuvront S.N, and Kenefick RW. Hypohydration and human performance: impact of environment and physiological mechanisms. Sports Med. 2015 Nov;45 Suppl 1(Suppl 1):S51-60. doi: 10.1007/s40279-015-0395-7.

  9. Jeukendrup, A, and Gleeson, M. Dehydration and its effects on performance. In: Sport Nutrition 2nd Edition. Champaign, IL: Human Kinetics, 2009.

  10. McDermott BP, Anderson SA, Armstrong LE, Casa DJ, Cheuvront SN, Cooper L, Kenney WL, O'Connor FG, Roberts WO. National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active. J Athl Train. 2017 Sep;52(9):877-895. doi: 10.4085/1062-6050-52.9.02. PMID: 28985128; PMCID: PMC5634236.

  11. American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007 Feb;39(2):377-90. doi: 10.1249/mss.0b013e31802ca597. PMID: 17277604.

  12. Shirreffs, S. M., and R. J. Maughan. Volume repletion after exercise-induced volume depletion in humans: replacement of water and sodium losses. Am. J. Physiol. 274:F868-F875, 1998.

  13. Blow, A. (n.d.). Different types of sports drink and when to use them. Precision Fuel and Hydration. Retrieved on February 20 from

  14. Jeukendrup, A. (n.d.) The optimal ratio of carbohydrates. MySportScience. Retrieved on February 21, 2024 from


Minttu Hukka is a professional triathlete and an exercise physiologist, with experience in wearable technology research. Minttu currently works at Huawei Technologies Finland as a human performance laboratory specialist, while pursuing her professional triathlon career.

Are you unsure about how much hydaration you need or have questions about sports nutrition? Let our skilled sports nutritionists help you out. Secure a complimentary session on hydration with our sports nutrition coach for personalized advice. Schedule your session here.

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