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13% of Boston finishers were hyponatremic: how much to drink

I race 100 km, and the aid station is where I make my worst decisions. Drink because the table is there. Take the salt tablet because someone held one out. Christopher Almond's team drew blood from 488 finishers of the 2002 Boston Marathon and found 13% with a serum sodium concentration of 135 mmol/L or below, and 0.6% at or under 120, the range where people seize 1. The runners in trouble had gained weight over 42 km.

What the research shows.

Almond's risk factors read like a list of good intentions: weight gain during the race, more than 3 litres of fluid swallowed, drinking at every mile marker, a finishing time past 4 hours, a low body mass index 1. Slower runners have more hours and more tables. The association ran in the direction of weight gained.

Martin Hoffman and Kristin Stuempfle followed that thread to the Western States Endurance Run, 161 km in mountain heat. Sodium supplements were used by 93.9% of the runners in their sample, and 6.6% of finishers still came in hyponatremic. Overhydration was the characteristic that separated them, and the authors concluded that low sodium intake carries minimal responsibility for hyponatremia in exercise lasting up to 30 hours 2. In a second study at the same race, in air that reached 39°C, 20 finishers logged every gram they ate and drank. Total sodium intake averaged 13,651 mg, with a range from 2,541 to 38,338 mg across the field. Muscle cramping, nausea, vomiting, dehydration and post-race plasma sodium were all unrelated to that number 3.

Seventeen experts met in Carlsbad in 2015 to update the consensus on exercise-associated hyponatremia. Their fluid recommendation runs to one sentence: use thirst to guide how much you drink, which limits overdrinking while covering the losses that matter 4.

The performance literature points the same way. Hassane Zouhal weighed 643 finishers of the 2009 Mont Saint Michel marathon before and after the race. Runners under 3 hours had lost 3.1 ± 1.9% of body mass. Between 3 and 4 hours, 2.5 ± 2.1%. Past 4 hours, 1.8 ± 2.4% 5. The bands run in one direction: the shorter the finishing time, the larger the deficit. Eric Goulet pooled 5 cycling time-trial studies covering 39 subjects and 13 effect estimates, and found no performance penalty until the fluid deficit passed roughly 4% of body mass 6.

The lab evidence splits at 3%.

Brad Wall's group dehydrated trained cyclists intravenously to 0%, 2% and 3% of body mass, then sent them into a 25 km time trial in the heat without telling them which condition they were in. Times came back the same across all three 7. Mark Funnell's group ran a similar blinding design with 14 trained cyclists at 31°C, using a 120-minute ride to produce a deficit near 3% of body mass before a time trial of about 15 minutes. Blinded riders went from 903 to 1,008 seconds, 11.4% slower. Unblinded riders went from 874 to 967 seconds, 10.1% slower 8. The deficit cost both groups about the same, whether or not they knew they were carrying it.

The methods explain part of the gap between the two results. Wall moved fluid out with a diuretic and back in through a vein, so the mouth and the gut never registered a drink. Funnell dehydrated through exercise, the way a race does. Neither study supports drinking past thirst to defend a number on a scale, and both leave 2% looking harmless.

Thomas Dion tested the practical version on runners. Ten trained distance runners covered 21.1 km twice on a treadmill at 30°C and 42% humidity, once drinking to thirst and once on a schedule engineered to hold losses under the 2% figure the American College of Sports Medicine recommends. The schedule bought no time 9.

Your sweat rate is not the table's.

Lindsay Baker compiled patch-method data from 1,303 athletes tested between 2000 and 2017. Whole-body sweating rate averaged 1.13 ± 0.58 L/h, with a range from 0.16 to 5.73 L/h. Endurance athletes sat higher at 1.28 ± 0.57 L/h and lost sodium at 51.7 ± 27.8 mmol/h 10. Roughly a factor of 35 separates the slowest sweater in that sample from the fastest, and one hourly drinking target cannot serve both ends of that range.

The protocol.

  1. Measure your sweat rate on two runs. Weigh yourself naked before an hour at race effort and naked after, towel dry. Add the fluid you drank, counting 1 litre as 1 kg. Sweat rate in L/h equals kilograms lost plus litres drunk, divided by hours. Do it once in the cold and once above 25°C, and write the temperature next to each number.
  2. Treat that number as your ceiling, and thirst as your floor. Drink when thirsty, and stop short of your measured hourly rate. Both blinded studies and the thirst trial leave room under it 789.
  3. Aim to lose 2% of body mass over a race, and tolerate 3%. For a 70 kg runner that is 1.4 to 2.1 kg gone at the finish. Zouhal's sub-3-hour group averaged 3.1% and beat everyone else there 5.
  4. Never finish heavier than you started. Weight gain is the risk factor that ran through both the Boston cohort and the Western States cohort 12. If your rings and your watch strap feel tight at 25 km, stop drinking until you are thirsty again.
  5. Stop treating salt tablets as insurance. Sodium did not protect the 93.9% who took it at Western States, and total intake across a 15-fold range predicted nothing 23. Take sodium with fluid if it keeps food down or makes the drink palatable, never as permission to drink more.
  6. Raise your guard on the profile that fills the medical tent. Finishing past 4 hours, a small frame, a cool day that lowers sweat losses without lowering the urge to drink 1. Under those conditions, cut your planned volume and check in with thirst every aid station.
  7. Practise the plan in training at race intensity. Gut tolerance for fluid is trainable, and race morning is a poor place to discover your ceiling.

Weigh yourself before and after your next long run and write the two numbers in your log. Four extra minutes, and the number holds for years.

Sources.

  1. Almond, C. S. D., Shin, A. Y., Fortescue, E. B., Mannix, R. C., Wypij, D., Binstadt, B. A., et al. (2005). Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 352(15), 1550-1556. doi.org/10.1056/NEJMoa043901
  2. Hoffman, M. D., & Stuempfle, K. J. (2015). Sodium supplementation and exercise-associated hyponatremia during prolonged exercise. Medicine & Science in Sports & Exercise, 47(9), 1781-1787. doi.org/10.1249/MSS.0000000000000599
  3. Hoffman, M. D., & Stuempfle, K. J. (2015). Sodium intake during an ultramarathon does not prevent muscle cramping, dehydration, hyponatremia, or nausea. Sports Medicine - Open, 1(1). doi.org/10.1186/s40798-015-0040-x
  4. Hew-Butler, T., Rosner, M. H., Fowkes-Godek, S., Dugas, J. P., Hoffman, M. D., Lewis, D. P., et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303-320. journals.lww.com
  5. Zouhal, H., Groussard, C., Minter, G., Vincent, S., Cretual, A., Gratas-Delamarche, A., et al. (2011). Inverse relationship between percentage body weight change and finishing time in 643 forty-two-kilometre marathon runners. British Journal of Sports Medicine, 45(14), 1101-1105. doi.org/10.1136/bjsm.2010.074641
  6. Goulet, E. D. B. (2011). Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis. British Journal of Sports Medicine, 45(14), 1149-1156. doi.org/10.1136/bjsm.2010.077966
  7. Wall, B. A., Watson, G., Peiffer, J. J., Abbiss, C. R., Siegel, R., & Laursen, P. B. (2015). Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat. British Journal of Sports Medicine, 49(16), 1077-1083. doi.org/10.1136/bjsports-2013-092417
  8. Funnell, M. P., Mears, S. A., Bergin-Taylor, K., & James, L. J. (2019). Blinded and unblinded hypohydration similarly impair cycling time trial performance in the heat in trained cyclists. Journal of Applied Physiology, 126(4), 870-879. doi.org/10.1152/japplphysiol.01026.2018
  9. Dion, T., Savoie, F. A., Asselin, A., Gariepy, C., & Goulet, E. D. B. (2013). Half-marathon running performance is not improved by a rate of fluid intake above that dictated by thirst sensation in trained distance runners. European Journal of Applied Physiology, 113(12), 3011-3020. doi.org/10.1007/s00421-013-2730-8
  10. Baker, L. B., Barnes, K. A., Anderson, M. L., Passe, D. H., & Stofan, J. R. (2019). Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: an update and analysis by sport. Journal of Sports Sciences, 37(20), 2356-2366. doi.org/10.1080/02640414.2019.1633159

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