A large number of endurance athletes experience symptoms such as light headedness, drop in performance level and weakness towards the end of events such as the Ironman. A significant proportion go on to the medical tent where they need an intravenous injection (IV) of saline. These problems occur more often when the temperatures climb and when the humidity is high. What is causing this and is there a solution? (Excuse the pun)
Low blood sodium - a common problem
Over the last 15 years it has become clear that a lot of athletes in long distance endurance races in the heat are developing low blood sodium levels. This is termed hyponatraemia. In some events up to half the field can be affected. Both the duration of the event, nature of the event and the heat and humidity determine how many are affected. In extreme cases even an hour of exercise may lead to hyponatraemia, but this is rare. Normally it is events of 4+ hours duration that cause significant amounts of hyponatraemia to occur.
Normal blood sodium levels are typically between 135-145 mmol/L (3,100-3,330mg/L). Note that to convert between the two measures you multiply by the molecular weight of sodium (23). This helps you relate the typical values you'll be quoted after a hospital test with the amounts in your drinks. The intravenous (IV) drips that are administered in the medical tent at the end of endurance events normally contain 3,500mg/L, unsurprisingly close to your normal blood levels of sodium
Sweat contains sodium, but the amount contained varies widely between individuals. Ranges of between 230-2,553mg/L have been found1. This shows us that it is more dilute than blood, containing typically 1/3 as much salt in a given volume (c.950mg/L). Another way of looking at this is that 0.1% of your sweat is sodium.
Sports drinks are normally more dilute than typical sweat. If you make them up yourself the stronger they are the more salt they will contain. Coca cola contains no sodium, Lucozade about 500mg/l and Powerbar Endurance about 1000mg/l. A list of the strengths of most common sports drinks is maintained by DrDobbin Nutrition.
What evidence is there for hyponatraemia during Ironman events?
Reseach done on 18 Triathletes in Ironman New Zealand over 10 years ago by Speedy, Noakes et al.2 showed up some surprising facts about Ironman Nutrition. The main results of the research are summarised below:
- Sodium in the blood went down marginally for most, although 5 people (over 20%) of the sample were technically suffering from hyponatraemia (low blood sodium) at the end of the race.
- Fluid intakes were modest during the race for most, including those with hyponatraemia. Typically 715ml/hr, which is one medium large water bottle.
- The median weight losses (middle measure of weight loss) for the group were swim: -1kg, bike: +0.5kg and run: -2kg) showing that weight is lost in the swim, gained in the bike and lost again during the run.
- The 5 people who had the lowest blood sodium (hyponatraemia) were those that lost the least weight or indeed gained weight.
- Plasma volumes went up 11% - that is the blood retained its volume. Perhaps surprising when you consider that most experts used to consider dehydration as the greatest threat during endurance exercise.
- Measures of oxygenation of the blood (haematocrit -2.5%; haemoglobin -7g/L) went down. This partly explains the slow down in pace towards the end of the race.
What is all this telling us?
While there are a number of reasons for not completing a long distance endurance race in the heat probably, one of the major factors is a shortage of salt in the blood (hyponatraemia), and gives rise to symptoms such as nausea, vomiting, light headedness, confusion, weakness and muscle cramps. The New Zealand Ironman study above highlighted the fact that overdrinking could be a factor. Another factor could be excessive loss of salt in the sweat.
Overdrinking can occur even when salt concentration falls3 and appears to reflect the fact that our thirst mechanism is based more on maintaining fluid volume in our body than salt concentration. Typically smaller framed athletes and women have been most at risk of over drinking.
Recent research has shown that the amount of salt contained in sweat is individually determined at birth. In other words whether you lose a lot of salt in your sweat is down to your genes. The saltiness of your sweat increases slightly up until the age of 12 and then is pretty much static4. What is surprising perhaps is that the amount of salt in your sweat varies by a factor of as much as 8 times between different people5. Just because you are a heavy sweater does not neccessarily mean that you lose a lot of salt, as your sweat may be dilute.
How can I find out how salty my sweat is?
There are two standard ways of finding out how salty your sweat is. One is to have a test in a climate controlled chamber and have what is termed a whole body washdown (wbw). The other is to use patches attached at one or more sites on the body to collect sweat. While wbw is more accurate, the patch tests if done properly give a good indication of how salty your sweat is. A patch test is available from PrecisionHydration in the UK that can be arranged through DrDobbin nutrition for groups of athletes at a discounted rate at a venue near the DrDobbin clinic. For more information on this see the sweat testing page here.
What should I do if I have salty sweat?
When exercising in the heat it would be advisable to drink in a way that minimises the risk of hyponatramia occurring. Some researchers have emphasized the need to limit the amount that is drunk, while others emphasize the importance of consuming drinks that contain enough sodium. The argument is that people with salty sweat should drink stronger solutions and that if you find your stomach can tolerate these solutions then drinking them should give you a much better chance of finishing your Ironman events without nausea, light-headedness, cramps and other symptoms of hyponatraemia.
Isn't too much salt bad for me?
That probably depends on who you are. While there are studies that relate excess salt in large populations with increase in blood pressure there is a lack of specific proof. Don't forget that associating two things in a large population study (epidemiology) is fraught with problems. Just consider that a high salt intake may be associated also with eating more processed food. It could be that other factors related to the lifestyles of people eating this type of food such as more TV time spent munching through packets of crisps is the key reason for higher blood pressure. Equally there could be other components of the processed food as well that could be causing the relationship e.g. trans fats and other additives.
Those that have blood pressure levels that are affected by salt are termed salt-sensitive. In fact most people are able to control the amount of salt their body contains through the hormones released by the heart and adrenal glands and the action of the kidneys. While I have not found any studies relating high sodium sweaters with reduced salt sensitivity, it seems highly likely that if you are a high salt sweater, consuming more salt before, during and after performing hard exercise in the heat, would be sensible.