When we are low on fluids, the brain triggers the body’s thirst mechanism and the kidneys will conserve water. As a decrease in body water from normal levels occurs (dehydration or hypohydration), changes appear with cardiovascular, thermoregulatory, metabolic and central nervous system functions (1). These changes are triggered due to changes in plasma osmolarity (increases in serum sodium). There is significant impairment in both cognitive function (alertness, concentration, short-term memory) and physical performance (endurance, sports skills) (2). Subjective effects of hypohydration are headaches, inability to concentrate and reduced alertness (3).
Indices for hydration status can be percent body weight change, urine colour, and urine specific gravity readings. Generally, well hydrated individuals will have + 1 to - 1% body weight change, urine will be 1 or 2 on urine colour chart and USG < 1.010. Significant dehydration will show a 1 to 3% reduction in body weight, urine colour 5 or 6, and 1.021 – 1.030 USG (4).
Generally, the amount of fluid to consume for healthy adults over 19 years is about 3 L daily from beverages for men, and 2.7 L daily from beverages for women (5). However when you exercise, you need to maintain hydration to offset sweat losses, to end up with less than 2% body weight loss. The guidelines are around 200 – 300 mL every 10 – 20 minutes (4, 6). Specific recommendations are calculated based on individual sweat rates (see calculator below). Consider your percentage body weight lost, and your sweat rate to determine how much fluid you need to increase during activity. For rapid rehydration after exercise, you can try to consume 150% of the body weight lost with sodium containing beverages (6). For example: if you lose about 1.5 kg in body weight at the end of exercise you should try to consume 2.25 L fluid over the next few hours. Therefore, monitor yourself before, during and after exercise to establish a hydration plan.
Plain water is perfect to consume for most usual daily activities, and sporting activities less than 1 hour. However, in some situations a carbohydrate electrolyte replacement drink may be more beneficial, to replace glycogen stores and lost fluids. Such as – an individual can easily become dehydrated through illness (i.e. vomiting, diarrhoea, sweating from fevers, inadequate fluid consumption, post-surgery, burns, eating disorders etc.) or through sporting situations (i.e. increased core temperature from exercise, inadequate fluids consumed, high sweat rates, high environmental temperatures etc). Carbohydrate electrolyte drinks should have a mixture of glucose and fructose so that multiple transportable carbohydrates are switched on for faster absorption, plus the water in the drink follows the carbohydrate across the intestinal wall into the blood, for hydration and energy (6).
In some sporting situations, additional sodium maybe required if the carbohydrate electrolyte drinks have insufficient sodium. If physical activity exceeds 4 hours, or during acclimatization periods in hot weather, adding modest amounts of salt (0.3 – 0.7 g/L) may offset electrolyte imbalance (4). The sodium is important for whole-body fluid retention. When an athlete is re-hydrating after exercise, we want the fluid to stay in their body, as opposed to losing it through urine. When plain water is consumed, it dilutes blood sodium concentration, which turns off the signal to the kidney to reabsorb water. This results in increased urine volume. By contrast, when a sodium-containing beverage is consumed, blood sodium concentration is increased, which in turn stimulates the re-absorption of water in the kidneys, so that less urine is formed. Fluid retention increases in direct proportion to beverage sodium concentration (7).
Take home message for athletes - Hydration protocols that consider sweat rate, sport dynamics (rest breaks and fluid access), environmental factors, acclimatization states, exercise duration, exercise intensity, and individual preferences (4), will help you prevent dehydration.
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1. Murray, B. (2007). Hydration and physical performance. Journal of the American College of Nutrition, 26(sup5), 542S-548S.
2. Jéquier, E., & Constant, F. (2010). Water as an essential nutrient: the physiological basis of hydration. European journal of clinical nutrition, 64(2), 115.
3. Shirreffs, S. M., Merson, S. J., Fraser, S. M., & Archer, D. T. (2004). The effects of fluid restriction on hydration status and subjective feelings in man. British Journal of Nutrition, 91(6), 951-958.
4. Casa, D. J., Armstrong, L. E., Hillman, S. K., Montain, S. J., Reiff, R. V., Rich, B. S., ... & Stone, J. A. (2000). National Athletic Trainers' Association position statement: fluid replacement for athletes. Journal of athletic training, 35(2), 212.
6. Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Medicine and science in sports and exercise, 48(3), 543-568.
Pre-exercise weight (g) = A 60 kg (= 6000 g)
Post-exercise weight (g) = B 58.5 kg (= 5850 g) (-2.5% weight loss)
Fluids ingested (mL) = C 700 mL
Urine output (mL) = D 100 mL
Exercise duration (min) = E 90 minutes
Sweat rate = ((A – B) + (C – D))/ E ((6000 – 5850) + (700 – 100))/ 90
= mL/min = 8.3 mL/min x 60
= mL/hour = 498 mL/hour