Dr Lindsay Weight
Posted Mon, 13 Jun 2005
The three things that appear to wreck ultra runners days are nausea, cramp and gastrointestinal disturbances, appropriately termed ‘the trots’. Let’s consider the common problem of nausea, identifying causes and possible solutions.
Nausea
There cannot be a distance runner who has not had this unpleasant experience at some stage in their running career.
However, because nausea is a symptom of so many conditions it is difficult to pinpoint a precise and single cause. For example, nausea can be the result of too little blood sugar (hypoglycaemia) and too much (hyperglycaemia), a drop in blood volume (dehydration) or water retention (overhydration), salt loss and too much salt…
Lets consider each option more carefully.
Too little glucose:
a decrease in the amount of glucose (sugar) in the blood stream is the most commonly ascribed cause of nausea. Hypoglycaemia is bound to occur in any sports event of longer than 2 hours duration if the athlete has not ingested any carbohydrate during the event. You can only store sufficient carbohydrate (from whence comes your glucose) for about 2 hours of running at race pace. Once your stores are depleted your glucose levels will decline and as glucose is the only nutrient the brain can use as fuel, it is not going to work so well as the supply dwindles. Hence the feeling of lightheadedness and inability to concentrate as well as a rapid, almost instantaneous decline in running speed (hitting the so called ‘wall’) – and nausea.
The solution: eat a light carbohydrate meal 90-120mins before the race (porridge, bread, biscuits) and drink carbohydrate fluids (coke or PowerAde) from the start of the race at the rate of 150-200ml every 15 minutes (basically at every aid station).
Too much glucose:
well to be more specific, ingesting carbohydrate-containing fluids at a rate faster than the stomach can cope with. The stomach is quite a small organ – it can contain the equivalent of a can of soft drink! So, if the rate at which it is emptying (at the bottom end) is slower than the rate you are pouring fluid in (at the top end), where is the fluid going to go ? Straight back up!
The solution: several factors limit the rate of gastric emptying: the higher the carbohydrate concentration of the fluid, the slower the rate of absorption. Studies have shown that the optimal concentration for gastric emptying in a moderate to warm environment is a 7-8% solution. PowerAde and Energade fall in this range, coke is a 10-11% solution. Also, cold fluids are absorbed quicker than warm fluids. And importantly, the more full the stomach is the quicker it will empty. Many athletes make the mistake of not drinking for the first hour of a race or drinking intermittently. This fluid then tends to stay in the stomach instead of exiting because there is no pressure, as such, to do so. Put more fluid in and the first lot will hurry on out. So you have to keep the stomach close to full up – this takes practice and diligent drinking during the race.
Related to this tendency to drink to little too seldom is that the runner is going to start feeling a wobbly as glucose levels drop. So in panic they ingest whatever they can find that has sugar in it. And then wonder after this sweet assault why they feel ill! This is another reason why coke is not the ideal replacement fluid over very long duration – the high glucose content, acidity and carbonation all irritate the gastric lining. So while coke used alternately with other drinks provides variety and carbohydrate, it is best not used as the sole source of glucose.
Too little fluid:
a mild degree of dehydration in a running event of 42 km or further is almost inevitable unless you have been running very slowly and drinking substantial amounts. Sweat loss amounts to 700-1500ml an hour, depending on gender, body size, running intensity and environmental conditions. Drinking more than 800ml and hour is very difficult if running at even moderate intensity, so you are unlikely to match rates of loss with rates of replacement. Losing 2-3% of your body mass (1.5-2.0L in a 70kg person) has not real detrimental effect on running performance. Losing between 4-5% of body mass constitutes dehydration and will lead to a decrease in performance and possibly nausea. A greater than a 6% body weight loss (4-5L in the 70kg person) is entering danger zone where the ability to keep exercising is severely compromised and the risk of heat injury high. Using the hypothetical 70kg runner, sweating 1.0L/hr but drinking only 500ml, this point is going to occur after about 8-9 hours.
The solution: Drink! As above, 150-200ml every 15mins.
Too much fluid:
this is a relatively rare situation. It is more likely to occur in women runners, as they sweat less but conscientiously attempt to drink the same if not more, than the hefty fellow who is sweating at twice the race. It also happens more often among the backmarkers who are doing more walking than running (and hence sweating less) but who are still drinking to match running sweat loss rates. If you are only sweating 500ml an hour and drinking 800ml, then over the same 8-9 hour period quoted above, you are going to have 2.4-2.7L or more fluid that you started out with! You also do not excrete this water as you might under non-exercising conditions as the kidneys are in a go slow during prolonged exercise. Associated with this water overload is a decrease in the concentration of sodium in the blood – and this is the critical factor. Low sodium has a host of potentially dangerous consequences, but the one thing you are going to feel is nauseous! (You may also experience confusion, will stop sweating and urinating, your watch or rings may feel tight and you are unable to continue running).
The solution: try and get some idea of your sweat rates by weighing yourself before and after long training runs (ideally in environmental conditions similar to what you would expect in the race) and use this as a guideline for your own fluid replacement needs.
Too little salt:
If you are sweating 1.0-1.5L an hour you will also be losing about 3g of sodium chloride (salt) in that time. You cannot replace this sodium loss during the race even by drinking replacement fluids such as PowerAde which has a reasonable amount of sodium in it. (Coke has almost no sodium). Also, if you tried to drink a litre of fluid with the equivalent of 3g of salt in it it would not end up where you need it, but on the side of the road. So herein lies a big problem and a possible major cause of nausea: salt loss.
The solution: if you are running slowly enough and getting hungry eat food like potatoes with some salt or a sandwich with marmite (both are provided in the Comrades marathon). You can also try using salt tablets (Hemapep for example) at the rate of 1 an hour after the first 2 hours but please try this out in a training run first!
Too little blood supply to the gut:
even if you remain well hydrated, the amount of blood going to the gut and kidneys is compromised during prolonged running as their supply is diverted to the leg muscle (you only have so much to go around). After hours and hours of reduced oxygen and nutrient supply as a result of this blood flow diversion the gut gets understandably unhappy and irritable. It also does not take kindly to hours and hours of fluid ingestion with no solids (think about it – in the course of an ordinary day you would not only drink, you would hopefully eat something and the stomach does not react well to a change in routine). So, much of the nausea can be simply a case of stomach abuse.
The solution: none other than slowing down – and staying as hydrated a possible.
Too much movement:
Tim Noakes has likened the nausea to a form of motion sickness – the result both of the swaying motion of running and the rock and roll your insides undergo.
The solution: Noakes suggests taking Valoid or Stemmetil (both anti-emetics), but no more than 2 over a 6 hour period. Again, try this on a training run first!
Sheer effort:
Running puts the body under metabolic stress. The higher the intensity or the longer the duration the more stress and the more likely you are to feel nauseous as your system is overwhelmed by the demands placed on it.
The conclusion Nausea, like pain, is one of those occupational hazards of distance running. While you can do your best to minimize it, its also something that you just have to deal with.
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