Is it possible to drink too much water?

Have you heard of this advice – Drink as much water as you can because by the time you are thirsty, it is already too late? This has been the advice given to many sports participants to prevent symptoms of dehydration at least until 1985. following a scientific report entitled a paper entitled “Water intoxication: a possible complication of endurance exercise”.

Our body is made up of about 60% water, it is the medium in which cellular reactions takes place and nutrients and wastes are transported. During an exercise, the breakdown of energy providers raises our core body temperature. In order to cool the body, we produce and excrete sweat which causes the body to lose water. If this water is not replaced, the body will become dehydrated. Conversely, it is also possible to get overhydrated, resulting in exercise-associated hyponatraemia (EAH).

The diagram below describes what happens when the body gets dehydrated¹ and overhydrated².

Dehydration is indicated as a percent of weight loss (the percentage of body weight at the start of exercise that has lost at the end). For example, a weight loss of 2% if the loss of about 1.1kg for a 57kg person. Bearing this in mind, however, some loss of body weight is expected with prolonged physical activity due to the burning of glycogen and to a lesser extent body fats and proteins. For example, during a typical 42 km marathon a 1–2% decrease in body weight typically occurs without a change in total body water.

How can drinking too much water be bad?

Researchers have noted the prevalence of hyponatremia (low blood sodium) amongst participants of endurance sports such as ultra marathons and triathlons who had hydrated excessively. These runners developed exercise associated hyponatraemia due to an excess of total body water relative to the amount of sodium in the body. Contrary to the term hyponatraemia, EAH is not due to excessive sodium loss since research has shown that sodium loss is no greater in individuals who develop EAH than in individuals who do not.

Several factors have been linked to EAH which include:

Athlete-related

·         excessive drinking

·         weight gain during exercise

·         low body weight

·         female sex

·         slow running or performance pace

·         event inexperience

·         use of nonsteroidal anti-inflammatory agents and medical risk factors

 Event-related

·         high availability of drinking fluids

·         4 hours exercise duration

·         unusually hot environmental conditions

·         extreme cold temperature

How can we prevent EAH?

Since EAH is caused primarily by the consumption of fluid in excess of urinary and sweat losses, the Statement of the Second International Exercise-Associated Hyponatremia Consensus Development Conference in 2007²  recommends 1) avoiding excess fluid retention and 2) aim to lose up to two percent of body weight and never to gain weight during exercise. This includes drinking only when thirsty, and to and monitor body weight during exercise. Specifically, for an Ironman distance triathlon, cycle aid station placement every 20 km, and run stations every 2.5 km are recommended. In a standard marathon footrace, placement of aid stations every 5 km is associated with an absence of EAH.

Of interest is that ingestion of electrolyte-containing sports drinks (so called isotonic drinks) cannot prevent the development of EAH in athletes who drink to excess. This is due to two factors: 1) such drinks have sodium concentration less than 135 mmol/L and therefore will cause dilution of sodium if excess water is retained in the body during exercise and 2) it is well known that even administration of isotonic saline in hyponatremic patients with SIADH will not help because the infused sodium will be excreted in the urine rather than retained. What about taking salt tablets or capsules? There is conflicting evidence as to the effect of sodium supplementation, either by tablet or drink, on the incidence of EAH. Moreover, there may be some risks associated with excessive sodium supplementation in combination with overhydration.

So how much water should I take?

For regular daily hydration needs about 33-39 ml/kg body weight makes a more accurate standard than the eight glasses a day commonly recommended.

Prior to a workout or race::

1)       One liter of fluid (about 34 ounces) in the two hours prior to the start (about 17 ounces/500 milliliters per hour), ceasing consumption about 30 minutes before you begin the workout or race; or

2)       About 295-355 milliliters (10-12 ounces) of fluid each hour up to 30 minutes prior to the start (24-30 ounces total fluid intake).

During an exercise under most conditions, 500-750 ml/hr (about 17-25 oz/hr) will fulfill most athletes' hydration requirements³.

Keep in mind that these are general guidelines, you need to determine what works best for your system and adjust according to a particular race or training session.

REFERENCES

1.       Nutrition for Cyclists, Grandjean & Ruud, Clinics in Sports Med. Vol 13(1);235-246. Jan 1994

2.       Clin J Sport Med  Volume 18, Number 2, March 2008

3.       TOP 10 - The biggest mistakes endurance athletes make by Dr. Ian Rogers