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Do you have a strategic drinking plan for training and competing, or do you just drink to thirst? What do you drink and how does that impact your performance?
Hydration is a complex topic, with even more complex physiology. Plus, women and men differ in their hydration needs during prolonged exercise. In fact, women are three times more likely to develop exercise associated hyponatremia – low sodium in blood-than their male counterparts. Learn more about hydration and the science behind it below.
What is 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
How much and when do I hydrate?
There is a throw-down happening in the sport science world. In one corner there is the current dogma that “the goal of drinking during exercise is to prevent excessive dehydration and excessive changes in electrolyte balance to avert compromised performance”*. The other train of thought is to “drink to thirst, no matter how much weight you lose”**. Both groups are trying to determine what is appropriate for performance and health, but the answer might be somewhere in the middle.
Open any exercise physiology textbook and the first factor of importance to fatigue is a drop in blood volume with its associated hormonal responses, with the second, less impressive, factor being decreased carbohydrate availability.
Whereas you can fix low circulating carbohydrate pretty effectively by eating something and feeling the effects within minutes, a drop in blood volume is more complex, taking hours to rectify.
The boiled down explanation:
The complexity of the body extends well into the aspect of “hydration” and what it means to maintain body fluids for health and for performance. The myriad of intricate responses, from cellular mechanisms to sex differences, further complicates scientific research, as does the environment in which the research is conducted. The recommendations for drinking on a schedule or drinking to thirst should be individual, not generalized. However, simple categorizations of individuals may help guide hydration strategies:
Drink to thirst during exercise
- if the athlete has pre-hydrated prior to training session, drinking to thirst is warranted, otherwise, hypohydration can predispose the athlete to tissue injury, decreased motivation during the session, and poor recovery (adaptations, sleep, rehydration).
- if the athlete is heat acclimated (for hot training and games/racing/events)
- if the athlete is trained (e.g. after significant time off with lower fitness levels, hypohydration and exercise stress can exacerbate thermal strain and decrease performance metrics)
- if a woman is in the luteal phase of her menstrual cycle, which is the latter phase post ovulation and before your period, or on the progestin-only mini-pill
- if the athlete has a history of exercise-associated hyponatremia – sometimes called “water intoxication” – or has syndrome of inappropriate antidiuretic hormone secretion.
Drink on a Schedule
- Do not exceed 800ml per hour (of a low carbohydrate-sodium containing solution) in a temperate environment. Smaller individuals need less, larger need more. In the heat, more fluid may be needed.
- If the athlete is a junior athlete (e.g. has not gone through puberty).
- If the athlete has two or more heavy training sessions a day to avoid systemic dehydration.
- If the athlete is peri or post menopausal.
- If the athlete is unacclimated and training at altitude.
- If the athlete has a history of heat illness.
What should I drink?
Unfortunately, a single beverage suitable for all environmental and race conditions probably does not exist.
To maximise water absorption, consideration should be given to beverages formulated with:
a) 2-4% glucose and sucrose to enhance fluid uptake via co-transport mechanisms;
b) sodium, which helps water flow from the intestines into the blood.
The science: exercise-associated ‘overdrinking’ and gender differences
Sodium is critical to maintaining extracellular fluid volume. When sweating takes place without fluid replacement, total body water is reduced from each fluid compartment due to the free exchange of water between compartments with a loss of electrolytes, primarily sodium. Therefore, the balance of sodium in the body is crucial to maintain extracellular fluid volume. Research has shown that commercially available sports drinks typically contain less than adequate amounts of sodium for prolonged exercise or heat stress***.
Women are at greater risk for “water intoxication” or “overdrinking”
Sustained, excessive intake of water, sports drinks or other fluids can exceed the body’s ability to eliminate fluids in the form of sweat and urine. The excess fluid dilutes the body’s sodium level, interfering with normal regulatory processes.
Exercise-associated hyponatremia or “water intoxication” refers to reductions in the body’s sodium level during or up to 24 hours after physical activity; hyponatraemia occurs from a dilution of the extra-cellular fluid with or without an excess of body water volume.
Women are at greater risk for exercise-induced hyponatremia (low blood sodium concentration) and this risk has been attributed to their lower body weight and size, excess water ingestion and longer racing times relative to men.
A recent study compared men and women of equal fitness to determine fluid balance changes during ultra-endurance exercise (50km walking). The results were compelling: Men had greater body mass loss and a higher blood sodium level, with greater fluid intake; whereas women had greater body water expansion, thus diluting blood sodium, with less fluid intake per kilogram of body weight. While the above mentioned factors contribute to the greater incidence of hyponatremia in women, estrogen levels in the blood and tissues play a role in increasing the risk of hyponatremia in women.
The hormonal influences of the menstrual cycle affect fluid dynamics. The elevations in plasma progesterone concentrations during the latter phase of the menstrual cycle inhibit aldosterone-dependent sodium re-absorption at the kidneys due to progesterone competing with aldosterone for the mineralocorticoid receptor.
Layperson speak: more sodium is lost from the body with elevated progesterone.
With this hormone-induced shift in sodium retention, the body responds by inducing fluid shifts away from the plasma (resulting in a drop by about 8% of plasma volume) into different fluid compartments. Ergo, there is less total body sodium, with the same body fluid.
Result: With this set up, a woman who overdrinks plain water or a carbohydrate-electrolyte solution that impedes fluid and electrolyte absorption, runs the risk of becoming hyponatremic or in other words having a low blood sodium concentrate.
With these hormonal influences and physiologic perturbations in fluid balance, the signal for thirst is dampened. Thirst sensitivity is lessened as a physiological construct – otherwise women would go crazy with the drive to drink in the high hormone phase – yet they are closer to the clinical definition of hyponatremia in the latter phase of the menstrual cycle due to the aforementioned physiological changes.
Menopause and hydration
Independent of menopause, ageing in itself has important effects on fluid balance. Ageing is associated with a higher baseline plasma osmolality (which is a measurement of the body’s electrolyte water-balance), coupled with an age-related blunting of thirst sensation during exercise (and water deprivation); the usual thirst mechanism which occurs with dehydration. Older women are slower to rid themselves of water, compared to younger pre-menopausal women, increasing the risk of hyponatremia.
Moreover, rehydration is a slower process with ageing, primarily due to slower kidney function and hormonal response to sodium and water flux.
When a woman is prescribed estrogen-based hormone therapy, there is no change in the thirst and drinking mechanisms, but there is a reduction in urine output, resulting in a greater overall fluid retention. Interesting, however, is that this overall fluid retention is not due to increased free-water retention, but via increased sodium retention- the synthetic estrogens induce a reduction in sodium excretion; eliciting a slight reduction in the hyponatremic risk.
* Sawka et al. 2007
** Hew-Butler et al. 2015 Noakes 2012
*** Vrijens and Rehrer 1999
Dr. Stacy T. Sims, MSc, PhD, is a monthly columnist for Ella CyclingTips. Sims has contributed to the environmental exercise physiology and sports nutrition field for more than 15 years as both an athlete and a scientist. The former founder and scientist behind Osmo Hydration, Dr. Sims served an exercise physiologist and nutrition scientist in the human performance lab at Stanford University from 2007-2012 where she specialised in sex differences of environmental and nutritional considerations for recovery and performance. Her personal interest in sex differences and performance has been the precedence of her academic and consulting career, always looking at true physiology to apply innovative solutions in the sport nutrition world.