The Dunedin Marathon is almost upon us, and most of those competing in this year's events, be they competitive athletes or social runners, are doubtless giving considerable thought to the amount of fluid they need to consume before or during the race.
The need to drink substantial quantities of fluid within minutes of starting exercise is a belief promoted so enthusiastically by prominent sportsmen and women, sports "experts" and sports bodies that drinking has for many become an obsession.
Any poor performance in a distance event is highly likely to be attributed to not drinking enough, despite no scientific evidence to support it. "Pre-loading" is encouraged, which means many athletes start major races with the discomfort of not only a distended stomach but a rapidly filling bladder.
Even small children playing Saturday morning sport can be seen running from the field after barely 10 minutes of mild to moderate exercise and demanding a drink. Such behaviour is reinforced by television role models, anxious parents and even more by the manufacturers of sports drinks.
It is this hugely profitable and almost entirely unnecessary industry that has systematically promoted the myth that thirst is unreliable as an indicator of dehydration which occurs without warning unless copious fluid is regularly and diligently ingested. (Readers with further interest should read Deborah Cohen's "The Truth about Sports Drinks" in the British Medical Journal from July of this year: BMJ 2012;345:e4737)Yet in the 1970s, drinking during events such as the New York Marathon was virtually unheard of. Runners had learned that a significant volume of fluid in the stomach caused discomfort at the very least and often nausea or vomiting. Interestingly, collapse during or after marathons was a rare event.
Since the early 1980s, however, the problem of collapse has reached epidemic proportions, and it has become the norm to treat collapse or any sort of dizziness with intravenous fluids on the presumption that the cause is "dehydration" or "heat stroke".
Vast tents began to appear at events and armies of paramedics, nurses and doctors stood ready to seize upon any runner who so much as laid down or looked slightly dizzy on crossing the finish line.
However, it was clear to a small group of sports doctors and physiologists that the explanation being touted for this problem was far too simplistic.
Laboratory results were also at odds with the dehydration hypothesis, because the concentration of sodium in the blood (or plasma) should rise in the case of excess water loss.
What they found was that many of these athletes actually showed a fall in plasma sodium (hyponatraemia). This can only occur with either water overload or massive depletion of sodium in excess of water. It appeared to them that in fact water intoxication had become the new threat.
One study in a major marathon of more than 700 randomly selected runners showed a 13% incidence of hyponatraemia by race end, and this was strongly associated with weight gain, demonstrating excessive water consumption.
Prospective studies in ironman events have revealed total water intakes of up to 16 litres recorded with some athletes actually gaining weight (up to 7kg) during the race. Consumption of water to this degree leads inevitably to hyponatraemia, and with it an increasing risk of rapid movement of water into the brain causing brain swelling.
Symptoms of this condition, now called Exercise Associated Hyponatraemia (EAH ), include headache, vomiting, decreased consciousness, seizures and death.
A dramatic case of the death of a young woman in the Boston Marathon in 2002 caused widespread alarm but despite the finding that she had hyponatraemia, the advice to drink more was not altered.
In the medical literature to date, at least 13 deaths have been reported due to hyponatraemia.
Dr Ian Rogers, of the University of Western Australia, one of the leading researchers in the field, has a particular interest in true endurance events (lasting longer than four hours) and has shown that water intake should be governed by thirst rather than any premeditated plan, along with solid food supplements that the athlete finds palatable.
Shorter events which include marathons require virtually no fluid intake in most instances, with the body able to compensate by volume contraction without compromising blood volume, circulation or cellular function.
Most people will gradually develop thirst after they cease exercising and should drink then.
It is normal to lose weight during endurance exercise (exceeding four hours in duration), but this is largely due to the loss of glycogen, and, to a lesser extent, fat. Glycogen is the principal storage form of carbohydrate in muscle and the liver. It is not stored "dry" but rather as the "hydrated" form (associated with water molecules). It is used up in the first hour or two of significant exercise. As it is used, the water molecules bound with it are released. This constitutes a form of internal water reserve.
Most athletes who simply respond to thirst drink enough to keep the osmolality (concentration) of the plasma constant, even though they are losing weight, indicating that weight loss is not primarily due to water loss.
Sodium (salt) loss is another common myth. Real losses fall far short of those commonly believed. In events of less than four hours duration, salt depletion from sweating is categorically not the cause of EAH and there is no evidence that sodium-containing sports drinks do anything to alter the incidence. They are still lower in sodium than the plasma and will inevitably cause dilution if sufficient quantities are ingested.
The important message is that over-consumption of fluid is the problem, not excessive loss or under-consumption of salt.
Fortunately, the organisers of a number of marathons have spaced out the drinking stations to a minimum of 5km apart (previously 1-2km) and in these events the problem has been virtually eliminated.
So what should the average runner do?
It is difficult to give universal advice, as some people retain more fluid than others, there is considerable variation in rates of sweating (and these are usually over-estimated) and those with small body mass are at much higher risk of hyponatraemia. For races from one to four hours in length, a maximum of 500ml per hour is probably reasonable for most, but drinking according to thirst is safest.
Finally, fainting or dizziness are common symptoms at the end of a race like the marathon, usually when runners stop suddenly. This is almost always due to a drop in blood pressure due to the sudden cessation of the active calf muscle pump returning blood to the heart as well as a dilated arterial circulation. It is harmless and best treated by lying down with the legs raised and waiting for a few minutes. If recovery is not reasonably rapid, a medical check is warranted and body temperature should be measured, but treatment is seldom required.
In those who exhibit changes in mental status or persistent poor balance, measuring the sodium level in the blood becomes urgent, and they may need transport to hospital and emergency treatment with intravenous hypertonic saline (3%). This should be a very rare event.
• Dr Mike Hunter is the clinical leader of the Intensive Care Unit at Dunedin Hospital.
