NEW YORK - How much fluid should a marathon runner
drink while racing? The answer depends on a variety of factors, but the best
approach is to replace fluids in response to thirst, according to experts from
the International Marathon Medical Directors Association (IMMDA).
In their latest position statement on the topic, they assert that there is no
"blanket advice" to give to people about how much fluids are needed while
exercising. Instead, they write, athletes should be encouraged "to explore,
understand and be flexible toward their own needs."
"Athletes should learn to trust the sensation of thirst, rather than adhere
to rigid guidelines that do not allow for the flexibility that is needed in a
dynamic race situation," co-author Dr. Tamara Hew-Butler, of the University of
Cape Town in South Africa, told Reuters Health.
Their statement, published in the Clinical Journal of Sports Medicine, was
issued in response to the existing controversy surrounding the optimal amount of
fluid that should be consumed by athletes. Some of the previously published
recommendations focused on what happens when athletes do not consume enough
fluids, i.e. the elevated body temperature and other signs of dehydration, while
other recommendations focused on the effects of overhydration.
Historically, the American College of Sports Medicine (ACSM) maintained that
thirst is not an accurate indicator of the body's need for fluid replacement,
and focused instead on bodyweight as a more accurate marker.
In light of this, some exercise science experts previously advised that
athletes consume enough fluids to replace what they lose through sweating. Yet,
research dating back to 1932 shows that voluntary water consumption replaces
just over half, and not all, of the sweat a person loses while exercising. Other
research shows that some athletes who try to replace all of the fluid lost
through sweating end up experiencing nausea, vomiting and abdominal distress as
a result.
What's more, a blanket guideline for fluid consumption, such as that
previously promoted by ACSM, may not be appropriate for the diverse population
of athletes participating in various sports, according to Hew-Butler and her
colleagues.
For example, ACSM guidelines state that athletes should consume 600 to 1200
milliliters of fluid per hour, but this research was conducted among elite male
athletes. Slower, less experienced women who followed this recommendation and
consumed too much fluid, albeit within the recommended range, developed
exercise-associated hyponatremia, a condition in which the body's sodium
concentration is diluted too much.
"'Blanket' guidelines and drinking beyond thirst have contributed to the
growth of this condition over the past decade," according to Hew-Butler, who
cited the hyponatremia-related deaths of four otherwise healthy female runners.
Further, since a variety of factors, including the athlete's bodyweight,
running speed, and the weather, determine how much fluid is lost during
exercise, it is almost impossible to determine a single range for optimal fluid
intake that would account for all of these factors, the IMMDA experts note.
Instead, they recommend that athletes drink when they feel thirsty, as the
best way to protect against consuming too much or too little fluids.
Athletes should also pay attention to their bodies so that they decrease
their fluid consumption if they start urinating more frequently than normal or
feel bloated. Also, they note, the body's thirst mechanism may take longer than
normal to be stimulated in older individuals, and those who exercise in cooler
temperatures.
"The shift in the current paradigm regarding the 'acceptance' of thirst as
the body's inherent guide governing fluid balance centers around the
acknowledgment of current scientific evidence supporting the body's regulation
of plasma osmolality (body sodium) over plasma volume instead of bodyweight
during exercise," Hew-Butler told Reuters Health.
"The take home message is that thirst is an adequate index of hydration
status during exercise," she concluded.
