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THE SCIENCE OF HYDRATION
by Stacy Sims, PhD, CISSN
Hydration is a complex topic, with even more complex physiology. In this carb-centric
society, the emphasis in sports nutrition has been on carbohydrate availability in fluid form,
but this neglects the true meaning of “Hydration”.
1. The addition of water to a chemical molecule without hydrolysis.
2. The process of providing an adequate amount of liquid to bodily tissues.
Optimal hydration requires a balance of both fluids and electrolytes.
HYDRATION BENEFITS
Moistens tissues in eyes, nose, mouth. Is the medium for transportation (of nutrients,
Assists the body in thermoregulation via sweat. oxygen, waste products) of the blood and across cells.
Provides lubrication to the joints. Muscles are 75% water.
POSSIBLE INDICATORS OF UNDER-HYDRATION:
• Headache post-training, with high sweat rate or low
fluid intake pre and during training - hypohydration.
*Headaches post-training with high water intake
• Dizziness/light-headedness.
during long training sessions may be an indication of • Fatigue.
exercise associated hyponatraemia (EAH).
• Moodiness/irritability.
• Thirsty = drink.
Possible Causes of Under-Hydration:
• High volume of sweat,
intense workouts, long • Poor appetite and elevated
workouts. metabolism >1hour post
• Heavy workouts in cold exercise = dehydration.
weather with multiple layers. • Nausea.
• Hot & humid conditions. • Cramps – *May also be caused
by neuromuscular issues and/or
electrolyte depletion – research
still equivocal on one specific
• Heat intolerance during cause.
exercise-hypohydration.
• Dark, low volume of urine = dehydration.
• Decreased endurance performance.
*Exercise associated hyponatraemia - low sodium in the blood
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For more information visit Serious Hydration
BROUGHT TO YOU BY:
www.SportsRD.org. For more information visit
For Serious Athletes
Serious Hydration
www.SportsRD.org. NASA DEVELOPED For Serious Athletes
THE SCIENCE OF HYDRATION
DRINK TO THIRST OR ON A SCHEDULE?
DRINK TO THIRST DURING EXERCISE IF:
• The athlete has pre-hydrated, otherwise can be susceptible to injury (e.g. rhabdomyolysis, poor recovery, decreased motivation).
• The athlete is heat acclimated (for hot training and games/racing/events).
• The athlete is trained.
After significant time off with lower fitness levels, hypohydration and exercise stress can exacerbate thermal strain and
decrease performance metrics.
• If the athlete is a woman in the luteal phase of her menstrual cycle or on the progestin-only mini-pill (high estrogen and
progesterone decrease plasma volume and lower plasma osmolality, predisposing a woman to hyponatremia).
• If the athlete has a history of EAH or has Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH).
DRINK ON A SCHEDULE (NOT TO EXCEED 800ML/H IN A TEMPERATE ENVIRONMENT- SMALLER INDIVIDUALS
NEED LESS, LARGER NEED MORE; IN THE HEAT, MORE FLUID WITH SODIUM MAY BE NEEDED) IF THE ATHLETE:
• Is a junior athlete (e.g. has not gone through puberty).
• Has 2+ heavy training sessions/day (to avoid systemic dehydration).
• Is unacclimated and training at altitude.
• Has a history of heat illness.
• Is drinking plain water.
• Is hypohydrated, traveling, has low glycogen, or in a hot/humid environment.
HOW TO ASSESS HYDRATION?
In the Morning:
• Use WUT– Possible dehydration if 2 or more below markers are present:
Weight – Ensure maintaining stable body weight day-to-day within 1%.
Urine – Darkened first morning urine or reduced daily frequency.
Thirst – Dry mouth or the craving of fluids.
Multiple Practices in a Day or <24 Hours Between Practices:
• Pay attention to urine color and drinking something with salt and/or salted watery fruits or veggies.
• Pre/post-weight – check to assess fluid loss.
Ensure not just drinking plain water but added sodium.
PRE Training:
• Salted watery fruits and vegetables (e.g. salted tomatoes, apples, watermelon).
• Water with a dash of salt (1/16th tsp table salt per 20oz water).
• Use a specific hyperhydration beverage or high sodium broth/soup.
DURING Training:
• Drink appropriately (i.e. to thirst or on a schedule if the athlete meets the scheduling criteria) a beverage that contains
per 8 fluid ounces: Sugars (from glucose and sucrose): 7 – 9.5 grams (3-6% carbohydrate solution); Sodium: 150-
180mg; Potassium: 60-75mg.
POST Training/Acute Rehydration:
• Urine should be clear 2-3 hours post-training.
• Protein+carbohydrate-based recovery drink/smoothie.
• Low-carbohydrate electrolyte drink.
• Soups.
• Salted watery fruits/veggies (salted tomatoes, salted [water]melons).
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