In light of the recent tragedy at Ironman Frankfurt, where a 30-year old athlete collapsed at the finish and died in hospital a few days later, there has been broad speculation that the significant heat during race day and the athlete's hydration conduct caused irreversible brain swelling.
Was the death caused by an insufficient attention to electrolyte ingestion, or drinking to a schedule rather than to thirst? "Swelling of the brain is the cause of death," said Professor Leo Latasch, head of the medical emergency team of the Gesundheitsamt in Frankfurt, and it was likely caused by insufficient salt intake while racing in unusually hot conditions. According to Professor Latasch, the athlete drank mostly water during the race and did not take in enough minerals.
Meanwhile, Dr. Tim Noakes tweeted, "Another avoidable death from hyponatraemia. Can't happen if athletes advised to drink to thirst, not to a schedule."
Not finding a comprehensive mathematical model that illustrated the full scope of struggle between consumption and replacement during the course of an endurance event or long training day, I published one here in 2009. It was not specific to any case, just a mathematical and scientific model.
Both fluid and electrolyte intake are necessary during longer training and racing, depending on the athlete and conditions. For shorter efforts of two hours or less, most athletes do not need any form of fluid or electrolyte supplementation. Dehydration will set in and your electrolyte levels will rise, but in most cases will remain within a normal range. For efforts longer than about 2 hours, evidence would suggest that an athlete engaging in reasonable ingestion of fluid and electrolytes would attenuate the loss to a point where neither dehydration nor over-hydration would take place, and that electrolyte levels would remain within a normal range.
"Reasonable ingestion" will depend on the losses specific to the athlete under the particular conditions.
With a nod to Dr. Noakes and his extensive research on the issue of over-hydration (summarized in his book Waterlogged), it was reported that "Athletes should drink to thirst during the race and not more than 800 mL/h," in the context of marathon running. On the subject of sodium intake, it was reported that during very prolonged exercise (such as an Ironman) that an acute sodium deficit may contribute to exertional hyponatremia.
Our opinions differ on this point. As I illustrated in the 2009 model, deficit can occur as soon as just a few hours, as well as by significant personal experience of sodium loss. Our bodies have finite reserves of sodium. A 70 kg (154 lb) human male contains a total of 100 g sodium (Human Body Composition, Volume 918, Steven Heymsfield) of which only 68 g is present in soft tissue (slow, but achievable exchange with plasma), including 10 g in blood.
What does this mean for an athlete losing 0.92 g of sodium in 1 L of sweat? It means that a meaningful deficit will occur with sufficient loss over time and without replacement.
Dr. Noakes brings up a considerable body of evidence of over-drinking to support the position that most hyponatremia is induced by consuming more fluid than necessary. No disagreement. The 2009 model supports this as well: After 4 hours with 100% water replacement, hyponatremia is demonstrated. The model also supports the need for "reasonable intake" of fluid and electrolytes to slow the loss of both and to provide the athlete with a longer time window in which to sustain a given effort.
Dr. Noakes and I disagree on the need for any sodium replacement, and his book and papers go to great lengths to implicate the sports drink industry, and what he considers its questionable research and conclusions. If Dr. Noakes's criticism of the industry extends to supplements, I also have a product to sell (SaltStick). He brings up many relevant points.
Often-quoted is the excessive daily salt intake (10-15 g) of the "western diet". My diet? I've calculated my daily salt consumption at under 2 g per day through food alone. You do the math if I'm out training for 2, 4 or 6 hours a day.
What about the change in loss of sodium in sweat over time? That's been documented, for sure. "In response to a low-salt diet and daily exercise in the heat, the kidneys begin to reduce their salt output within 1 to 2 hours, whereas salt excretion in sweat begin to fall only after 8 to 24 hours and requires several days to achieve a maximum effect (Robinson, Nicholas et al. 1955, as reported in Waterlogged). So reduce your daily intake to allow your body to adjust, but if you have a higher intake, you'll need to maintain a higher intake during exercise to provide the support to which your body is habituated.
My advice? Strive for a lower-sodium diet, and supplement according to the needs of your body. Similar to the thirst mechanism to seek hydration, the body's mechanism to seek sodium (salty foods, etc...) especially during exercise, indicates a need for added sodium intake. On the other hand, weight gain indicates too much intake of fluid and possibly sodium, and both should be restricted until body weight returns to normal or below normal.
Two competing factors, dehydration and hyponatremia, necessitate the balance between proper fluid intake and electrolyte intake. The key term here, "balance", means that there is an ideal athlete- and condition-specific plan to optimize performance and minimize health risks. Whether the supplementation is a solid electrolyte and water or a sports drink, long-distance athletes ignoring one or both of these aspects do so with the peril at best of limited performance, or at worst, at a danger to their health.