Ultra-endurance athletes: What you can do to avoid hyponatraemia and MSC during ultra-endurance events?

Ultra-endurance athletes compete in events which commonly last for six to 30 hours, and some competitions even stretch over several days. Such ultra-endurance efforts include triathlons, road-and-trail foot races, and cycling competitions like the Tour de France.

Strangely enough, there has been very little research into optimal nutritional practices during such events. The few investigations that have been conducted have usually been case studies, in which just one or two athletes were monitored over six to 24 hours of exertion. There have been no carefully controlled studies linking specific nutritional strategies with higher performances, nor have there been comprehensive nutritional analyses of the intake patterns of ultra-endurance athletes who manage to avoid the health difficulties (gastrointestinal distress and mental disorientation) commonly associated with ultra-distance efforts, compared with the nutrient and fluid intakes of ultra-athletes who fall prey to these problems.

This is an unfortunate situation, because studies reveal that as many as 43% of runners competing in an eight- to 10-hour competition experience significant gastrointestinal distress (1). Bowel problems are reported to be the most common cause of discontinuing an ultra-endurance race, and the intestinal difficulties may also be associated with more severe problems, such as declines in body and plasma mass, as well as dehydration. Little is known about how to minimise such problems during ultra-long exertion, although there is some indication that the consumption of hypertonic drinks and/or fibre-rich foods increases the likelihood of gastrointestinal upset, as one might expect. There is also a suggestion that eating just before an ultra-endurance event predisposes an athlete to troubling stirrings in the gut (2).

A drop in sodium
Somewhat surprisingly, hyponatraemia (abnormally low levels of sodium in the blood) is a common – and potentially very serious – malady in ultra-endurance competitions. We use the phrase 'somewhat surprisingly' because many ultra-endurance athletes, sports-medicine doctors, and EMT personnel are more used to thinking about dehydration as a negative consequence of ultra-endurance exertion. That is logical thinking, because of the potentially huge sweat losses associated with 12 to 30 hours of continuous exercise – and the potential inability of athletes to match the water lost in sweat with water imbibed during the competition. Of course, dehydration is often associated with higher electrolyte (including sodium) concentrations, which makes the common appearance of hyponatraemia seem surprising.

Nonetheless, some research has indicated that up to 29% of individuals competing in an ultra-endurance event may develop hyponatraemia over the course of the competition(3). Hyponatraemia can be extremely serious, with symptoms ranging from nausea, confusion, uncoordination, and weakness all the way to grand-mal seizures and life-threatening comas (4).

The reason why
Why is hyponatraemia so likely to occur during ultra-endurance competition? It is true that sweat itself is a hypotonic solution, meaning that it is rather low in sodium. However, although the amount of sodium in any unit volume of sweat is quite modest, the hyponatraemic problem arises because sweat rates can be high, and sweating is of course continuous over many hours of exertion. This leads to a cumulative loss of sodium which can be quite significant. If you have trouble conceptualising this, merely think for a moment about a mountain stream. If you fill a bottle with water from the stream, the amount of sodium in your bottle will generally be small, since the sodium content of the water is slight. However, if you keep track of all the sodium which moves past your sampling point in the stream over a period of 12 to 24 hours, you will end up with a rather impressive number.

The other part of the hyponatraemic equation is that ultra-endurance athletes often gulp down sports drinks during their events; by design, such drinks are low in sodium. Ultra-athletes also tend to take in a fair amount of water during competition, believing that such drinking is a great way to ward off dehydration. Of course, the net result is that a lot of sodium can be lost from the body through the sweat glands, while little may be entering the body at the mouth.

New York research
To get a better handle on what nutritional practices limit the risk of gastrointestinal distress, mental confusion, and hyponatraemia during an ultra-endurance competition, scientists from the Nicholas Institute of Sports Medicine and Athletic Trauma at Lenox Hill Hospital in New York City recently studied the nutritional practices of 19 athletes (18 males and one female) before and during a 160-kilometre race (5). All of the athletes had completed at least one competition of 50 miles or more during the 12 months preceding the study, and all had an anticipated finishing time of 23 hours or less in the 160-kilometre race. Typical training distance during the three months preceding the race was 98.3 kilometres per week, and the longest training run during the same period averaged 88 kilometres.

Pre-race diet records were provided, and runners recorded all food items consumed during the 12 hours leading up to the race start; these food records were collected by the researchers just prior to the 4am beginning of the competition. The scientists positioned themselves at 12 of the 37 food stations on the race course and briefly interviewed the runners about their mental and physical conditions, as well as nutritional intakes during the race, at roughly 13-kilometre intervals. Each time a runner was interviewed, he/she reported exactly what had been eaten or imbibed since the prior observation point. Over the course of the race, the runners lost an average of 1.6 kilograms (3.5 pounds) of body mass, with body fat accounting for approximately 1.13 kilograms (2.5 pounds or 71%) of the loss.

Eighteen litres of fluid
During the 12 hours before the race, the athletes consumed an average of four litres of water and a total of 2642 kilocalories, with 67% of the calories coming from carbohydrate, 21% from fat, 10 % from protein, and 2% from alcohol. Fourteen of the 19 competitors were able to finish the race (mean finishing time was 24.3 hours), and these finishers consumed an average of 6047 kilocalories within the race itself (very similar to the daily consumption pattern reported for athletes participating in the Tour de France), with carbohydrates accounting for 82% of this intake. An incredible 18 litres of fluid were ingested during the competition (but water in food accounted for 30% – 5.4 litres – of this total). Meanwhile, total sodium intake averaged a rather miserly .5 grams per hour – or about 12.5 grams for the whole event.

Nine athletes experienced gastrointestinal problems during the race – one runner experienced nausea during the first half of the race, and eight suffered from nausea or vomiting during the second half of the 160-kilometre competition. However, there was no link between food- or liquid-intake patterns and stomach/intestinal troubles. That is, individuals who developed significant gastrointestinal distress during the race did not take in significantly more or fewer kilocalories of food, compared with athletes who had no distress, nor did those with tummy troubles ingest a larger or smaller amount of fluid. There was a strong trend for the use of non-steroidal anti-inflammatory medications to be related to gastrointestinal maladies during the race, however.

In addition, runners who suffered from stomach/intestinal upsets tended to have trained less than trouble-free athletes (85 vs. 103 kilometres per week) and also tended to have carried out more abbreviated long runs (76 kilometres against 93 kilometres for the runners with symptom-safe guts).

Feeling 'out of it'
The frequency of mental status changes (MSC) was also very high during the race, with 10 runners reporting significant symptoms of MSC during the competition. Most of this MSC was reported after 88 kilometres of running and was described as lightheadedness, disorientation, dizziness, or bonking (being confused and unable to concentrate and feeling 'out of it'). Interestingly enough, runners who experienced MSC during the race consumed nearly twice as many kilocalories during the 12 hours before the race, compared to athletes with no MSC (3413 versus 1776 kilocalories). Runners with MSC also knocked down about twice as many carbohydrates pre-race (592 versus 290 grams) and approximately twice as much fat (83 versus 39 grams), compared with MSC-free runners.

Intriguingly, runners experiencing MSC also took in more kilocalories, carbohydrate, and fluid during the race, compared with the no-MSC competitors, and they also tended to eat larger quantities of protein and fat. Basically, athletes who struggled with MSC ingested about 45 to 60 kilocalories of food per kilometre of running over the last three quarters of the race, while no-MSC athletes were more moderate at just 20 to 35 kilocalories per kilometre. Similarly, MSC athletes drank a whopping 140 to 170 millilitres of fluid per kilometre over the last 120 kilometres, while the no-MSC group settled for about 100 to 120 millilitres per kilometre.

Excessive fluid intake?
Why were high food intakes before the 160-kilometre race and high fluid-and-food intakes during the race associated with mental-status changes? The answer to this question is not completely clear, but it is possible that the enhanced food intake is a red herring and that the real culprit was the excessive fluid intake by MSC athletes during the race. In effect, the excessive drinking of the MSC runners may have made them at least mildly hyponatraemic and produced their symptoms (disorientation, confusion, and dizziness), which are very characteristic of hyponatraemia. The copious drinking may have diluted the MSC runners' plasma dramatically, a situation made worse by the fact that sodium intakes were very low during the competition, averaging just .5 grams per hour, well below the recommended one to two grams per hour. In fact, one MSC runner's intake was found to be just .25 grams of sodium per hour. In short, the incomplete restoration of sodium, combined with over-consumption of fluid, probably induced at least some degree of hyponatraemia in the MSC runners and produced the performance-hindering symptoms they developed.

Because of the potential risk of dehydration, it would be hard for us to recommend abbreviated fluid intakes during ultra-endurance events, so how can one reduce the risk of hyponatraemia by ingesting at least one gram of sodium per hour during such efforts? Remember that typical 'sports foods' and supplements, including gels, bars, and sports drinks, tend to be quite low in salt. One might use 'Gatorlytes' (small sachets of sodium and other minerals which can be added to a sports drink during competition), but it is also easy to get the gram of sodium from easily digestible foods available at aid stations. For example, a cup of instant chocolate pudding has a whopping 820 mg of sodium (.82 grams), a cup of chicken-noodle soup (prepared from a tin) offers 1.1 grams of sodium, 'Saltine' crackers provide about 40 mg of sodium (.04 gm) per cracker, whole-wheat bread furnishes .18 grams of sodium per slice, English muffins offer a rather lofty .38 grams per muffin, a cup of commercial breakfast cereal ordinarily gives at least .25 grams of sodium (and often more), and three tablespoons of peanut butter yield .225 grams of sodium (thus a peanut-butter sandwich made with two slices of whole-wheat bread would get you well over half-way toward your goal of one gram of sodium per hour).

Boiled, salted potatoes
If all else fails, remember that a half-teaspoon of salt contains one gram of sodium, and thus you may sprinkle this quantity of salt on your preferred low-sodium foods as you move along. Increasingly, food stations at ultra-distance events are offering boiled, salted potatoes to competitors, and this is an excellent choice of provisions: the salt should help reduce the risk of hyponatraemia and MSC, and the potatoes have a high glycaemic index, meaning that their carbohydrates will make it to the muscles more quickly, compared with many other foods with lower glycaemic indices. In general, it is reasonable to choose foods with a high glycaemic index during ultra-events – things like potatoes, breads, jellies, cooked carrots, and of course sports drinks.

How do you stay away from gastrointestinal troubles during ultra-endurance events? Stay well hydrated for one thing, as the athletes in the New York study did (average rate of fluid intake over the entire race was more than 100 ml of liquid per kilometre).

Beyond that, training appears to be a paramount factor. In the New York research, runners who ran more kilometres per week and conducted longer long runs tended to be much more resistant to stomach and intestinal upsets. Other research also supports the idea that appropriate, race-specific training decreases the risk of nausea, vomiting, and diarrhoea during prolonged competition (6).

Jim Bledsoe

References

(1) 'Physiological Changes and Gastrointestinal Symptoms as a Result of Ultraendurance Running', European Journal of Applied Physiology, Vol. 64, pp. 1-8, 1992
(2) 'Fluid Intake and Gastrointestinal Problems in Runners Competing in a 25-Km Race and Marathon', International Journal of Sports Medicine, Vol. 10, pp. 22-25, 1989
(3) 'Fluid and Electrolyte Status in Athletes Receiving Medical Care at an Ultradistance Triathlon', Clinical Journal of Sports Medicine, Vol. 5, pp. 116-122, 1995
(4) 'Acute Hyponatremia in Ultra-Endurance Athletes', American Journal of Emergency Medicine, Vol. 12, pp. 441-444, 1994
(5) 'Food and Fluid Intake and Disturbances in Gastrointestinal and Mental Function during an Ultramarathon', International Journal of Sport Nutrition and Exercise Metabolism, Vol. 12(4), pp. 414-427, 2002
(6) 'Is the Gut an Athletic Organ? Digestion, Absorption, and Exercise', Sports Medicine, Vol. 15, pp. 242-257, 1993