This is a re-post of an old post I did on hydration.  It got lost when the hacker wiped my site. 

 

This is just a draft of a short note on hydration that I want to turn in to a longer note with more info.  Why?  Because when I do L’Etape and the Ironman it is likely to be quite a long day and may also be quite hot.  I don’t want to get dehydrated and I don’t want to get hyponatremia.

Dehydration

So how much water do you need to drink a day to prevent dehydration?

The first thing I have found is that there is a lot of rubbish on the web!  Also, the guidance seems to have changed a lot over the years.

A long time ago, people who ran marathons were discouraged from eating or drinking on the run.   Then it seems that there was a message to drink plenty, and drink before you feel thirsty.  Then people started getting ill during races because they drank too much water (they got hyponatremia).  Now the message seems to be “Drink when you are thirsty” or “Drink to maintain body weight, but not to increase weight”.

One of my favourite quotes comes from this article by Amby Burfoot:

In Dr. Noakes’s view, short-term dehydration isn’t an illness or health threat, but a condition of human existence. He reaches under the dinner table for his computer, and then plays a four-minute video clip that he watches with childlike delight. The video shows a bushman pursuing an antelope. The hunt starts at a slow walk-trot while a blazing Equatorial sun beats down on both man and beast. Before long, the bushman’s forehead and chest are covered with sweat, but he presses onward. This is slow, hot, arduous stuff–a life or death pursuit. Eventually, the bushman seizes the moment, breaks into a sprint, and buries his spear in the antelope’s chest. “It makes no sense to me that we have evolved without the ability to continue exercising while dehydrated,” says Dr. Noakes, a grin spreading across his face.

That article got me search for stuff about antelopes and I found the Heinrich hypothesis:

The fact that we, as savanna-adapted animals, have such a hypertrophied sweating response implies that if we are naturally so profligate with water, it can only be because of some very big advantage. The most likely advantage was that it permitted us to perform prolonged exercise in the heat. We don’t need a sweating response to outrun predators, because that requires relatively short, fast sprinting, where accumulating a heat load is, like a lactic acid load, acceptable. What we do need sweating for is to sustain running in the heat of the day – the time when most predators retire into the shade.

Dr Noakes then took this one stage further to create his own hypothesis:

Since “ade” stations were not provided every mile on the hot African savannah, so humans must also have evolved the capacity to resist any detrimental effects of moderate dehydration (Noakes’s hypothesis).

In terms of cycling, this was the advice from the Tour de France in 1960’s:

Avoid drinking when racing, especially in hot weather. Drink as little as possible, and with the liquid not too cold. It is only a question of will power. When you drink too much you will perspire, and you will lose your strength.

W. Fotheringham. Put me back on my bike (In Search of Tom Simpson), p.180, 2002.

Hyponatremia

So what is hyponatremia?

One site I saw said that hyponatremia involves not having enough sodium in the body fluids outside the cells. It then went on to say that sodium is the main cation (positive ion) that circulates in the body fluids outside the cells.   It is a critical component in blood pressure maintenance.   Sodium is also essential for the proper workings of nerves and muscles.

When sodium levels drop in the fluids outside the cells, water will seep into the cells in an attempt to balance the concentration of salt outside the cells. The cells will swell as a result of the excess water. While most cells can accommodate this swelling, brain cells cannot, because the skull confines them. Therefore, most symptoms of hyponatremia will result from brain swelling.  Common symptoms include: Loss of appetite Nausea Vomiting Headache Restlessness Fatigue Irritability Abnormal mental status Possible coma Hallucinations Consciousness, decreased Confusion Convulsions Muscle weakness Muscle spasms or cramps

In study on the 2002 Boston Marathon, Dr Chambers looked at the incidence of hyponatremia on marathon runners.  The study notes that excessive fluid intake was believed to be the primary risk factor for hyponatremia.  This was on the basis of observing marathon runners who have collapsed and studies of elite athletes. It also noted that other risk factors have also been suggested, including the composition of fluids consumed (e.g., plain water, rather than sports drinks that contain electrolytes), relatively low body-mass index, long racing time, lack of marathon experience, use of non-steroidal anti-inflammatory drugs (NSAIDs), and being female.

The study concludes that hyponatremia occurs in a substantial fraction of non-elite marathon runners and can be severe. Considerable weight gain while running, a long racing time, and body mass-index extremes were associated with hyponatremia.  The findings suggest that being female, composition of fluids ingested, and use NSAIDs were not.

Of the 488 who provided sufficient blood at the finish line, 13% had hyponatremia and 0.6% had critical hyponatremia.  That sounds like a lot to me!

Some quotes from the study are set out below:

The strongest single predictor of hyponatremia was considerable weight gain during the race, which correlated with excessive fluid intake

One relatively simple strategy to reduce the risk would be for runners to weigh themselves before and after training runs to gauge the effectiveness of their overall hydration strategy and adjust their fluid intake accordingly. This could be particularly useful during long training runs in which the distance and duration most closely approximate those of an actual marathon. Because runners vary considerably in size and in rates of perspiration, general recommendations regarding specific volumes of fluids and frequencies of intake are probably unsafe and have been superseded by recommendations favoring thirst or individual perspiration rates as a primary guide 

In another study on 18 participants in the 1996 Ironman New Zealand is interesting.    The conclusion of the study was:

Athletes lose 2.5 kg of weight during an ultra-distance triathlon, most likely from sources other than fluid loss. Fluid intakes during this event are more modest than that recommended for shorter duration exercise. Plasma volume increases during the ultradistance triathlon. Subjects who developed hyponatremia had evidence of fluid overload despite modest fluid intakes. So my first question was what is the source of this 2.5kg weight loss?  The study estimates that a typical 12 hour finisher would use 40,000kJ of energy.  Therefore, the weight loss comes from fat (260g), carbohydrate (535g) and water stored with the glycogen (1,444g). 

The study found that the athletes drank on average 889ml per hour on the bike (compared with a fluid loss of 808ml per hour) and 632ml per hour on the run (compared with 1,021ml fluid loss per hour). 

The study also noted that: 

It is interesting that subjects in this study lost weight during the swim and the run sections but gained weight during the cycle. This would suggest that athletes may be drinking in excess of their fluid needs during the cycling section of an ultradistance triathlon. Indeed calculated fluid losses during the cycle were lower than during the run; however, the hourly intake on the cycle was significantly higher on the bike than during the run. It is not clear from our data whether an increased fluid intake during the cycle section is beneficial or detrimental. The athletes who developed hyponatremia in this study had a relatively high fluid intake on the cycle section, suggesting that this may have contributed to the development of their hyponatremia. Conversely, a higher fluid intake on the bike section may protect an athlete from subsequently becoming dehydrated on the run. The most likely explanation for the higher rates of fluid intake during cycling is the ready availability of fluids during the cycle (every 12 km), and the ease with which an athlete tolerates drinking when cycling as compared with running.  The conclusion of the report said: Our data demonstrate that athletes lose approximately 2.5 kg of weight during an ultradistance triathlon, most likely from sources other than fluid loss (loss of fat, glycogen, and water stored with glycogen). Typical fluid intakes during an ultradistance triathlon are in the range of 400-1,000 ml/h, which is more modest than that recommended for exercise of shorter duration. The cycle section of the race is associated with the highest fluid intake and with weight gain, suggesting that athletes may drink too much on the cycle section, in part because they sweat less during cycling probably as a result of increased convective heat losses, yet are able to drink more. Plasma volume increases by approximately 10% during an ultradistance triathlon, the mechanism of which has yet to be determined. Lastly, the subjects who developed hyponatremia had evidence of fluid overload despite modest fluid intakes, suggesting both a failure of excretion of the fluid and that the fluid needs in some ultradistance athletes may be less than the current recommendations.