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Sodium and Electrolyte Losses During Prolonged Exertion

This page is a followup to my comments on the 2019 Joshua Tree Double Century. It is an attempt to understand what I term “drop outs” of power and focus during a double century.

Franklin K writes:

I have a few suggestion tracks regarding your double centuries. I’m not very familiar with sport medicine, - you know I’m a nephrologist and having worked very long in ICU - I feel involved in understanding precise and subtle mechanisms in physiology.

I point first you didn’t drink for the first two hours. OK, but what about sweat losses, I mean H2O and above all Na losses. You noticed temperature was as low as 34°F, but it won’t keep you to loose more than a liter per hour, which means more than 9g of NaCl per sweat loss, roughly 3.5g of sodium (Na). 66 miles, which means 2,5-3 hours without drinking, mean nearly 3 liters of H2O and 10.6 g sodium !!

And when you’re late about H2O and Na, as you quote, it is a long time to catch up. About this point , and for a better understanding go and read Na+/K+-ATPase OK I noticed you had four 1-liter bottles but as I told you, in this era when you’re late.

Our life system is organized to maintain constant body composition and the only organs to regulate this are the kidneys, yes… BUT… if you’re Na/H2O short, the kidneys are not able to manage this regulation. Do you know that the 2 kidneys are « using » 1,2l of blood per minute ? You understand what it means in terms of hypovolemia. By the end, I propose an explanation - I wrote « propose » ?, that your « slack » was H2O/Na dependant.

WIND: first a set-aside: given my relative lack of fatigue and very rapid recovery, it seems clear-cut that the issues I experienced were not a fitness issue. Indeed, my heart rate and breathing rate were relatively low. It surely has to be body statis of some kind, so I take the sodium (electrolyte) theory seriously as a key point to research.

This was about my 45th double on top of 90K miles of riding over the past decade and I’ve yet to crack this nut. Maybe it all comes down to some numbers game of increasing sodium+electrolytes until the problem goes away? Solving it would vault me into the top tier of riders consistently (maintaining 210 watts vs 180 for 10 to 12 hours is a HUGE difference), so I have a lot of incentive.

While I suspect it is a brain/neurological effect and not muscular/kidneys one, electrolytes could still be involved.

Quantifying sodium and electrolytes and energy

During the 2019 Joshua Tree Double Century, I took in about 2500 calories total (15 scoops Tailwind, 3 100-calorie Panda black licorice, 7 servings GU). That's plenty (36% of burn) for an event burning 6900 calories—much more than I typically consume by as much as double (usually it is 20% to 25% of total caloric expenditure). So I am going to rule out energy intake as a factor.

I use Tailwind for energy and electrolytes, 2.5 scoops per 1L bottle (see nutritional info for Tailwind). A one-liter bottle thus contains:

760mg Na + 220mg K + 65mg Ca + 35mg Mg + 250 calories

Since I consumed 6 such bottles during Joshua Tree, I took in from Tailwind alone:

4560mg Na + 1320g K + 390g Ca + 210mg Mg + 1500 calories

PLUS another bottle of some unknown drink (low in whatever it had) and 3 Panda 100-calorie black licorice sticks.. Plus about 7 servings of GU energy gel containing:

875mg Na + 280mg K + n/a Ca

= absolute minimum intake (not counting the licorice and that other unknown drink):

5430g Na + 1600g K + 390g Ca + 210mg Mg <=== accurate minimum taken in during JT

When done, my skin is salty, but not overly so unless it is very hot (in which case a lot drips off and fluid loss doubles). After Joshua Tree, I did not crave salt or anything like that, though I have done so after very hot double centuries.

If I were losing 3500g of sodium (Na) per liter of sweat, then my total intake for the day would replace only the first 1.5 hours worth, which seems dubious. Still, if the losses are 3500 mg/hour then the total loss is 35000mg or so, for a deficit of about 30000 mg. That does not seem believable: if it affected me at hour 5, then why did I feel no worse at hour 11?

Thus I remain a bit skeptical that I am losing 3500 mg/hour of sodium.

How to test it

Update: see my simple test of increasing sodium content.

I intend to change my dilution from 2.5 scoops of Tailwind per liter to 4 scoops in order to test that theory in the next double century. That matches up better for my cool weather (70°F or cooler) fluid intake needs balanced with caloric needs. In hot weather, I’d need more fluid and that concentration would need to be cut it back.

What science says

Science seems to have few good answers. Sodium and electrolyte losses can be high, yet one study of ironman participants concludes that no supplementation is needed!

Franklin K’s comments on 9000mg of sodium (Na) loss per liter of fluid are in line with the high end of what is suggested in studies. However, it is also true the acclimatized individuals might lose much less than that. I have no way of knowing my own sodium and electrolyte losses BUT I can say that in cool weather my skin is not particularly salty‚ nor does my clothing stain white under those conditions. During very hot weather my skin does get very salty and my clothing stiff with salts, but the sweating rate is at least double.

NIH: Sweat rate and sodium loss during work in the heat suggests 480 to 600mg sodium loss per hour for workers under hot conditions for workers, depending on acclimatization.

People working in moderately hot conditions for 10 hours on average will lose between 4.8 and 6 g of sodium (Na) equivalent to 12–15 g of salt (NaCl) depending on acclimatisation.

However due to the substantial interindividual variation in sweat rate and sodium concentration individual losses may be much higher. This essential electrolyte must be replaced in order to avoid fluid imbalances, thus eating during the shift is a must.

One work session in the heat, for an acclimatised person is sufficient to activate sodium-conserving mechanisms. However in the unacclimatised worker longer exposure is required. A worker starting work in harsh conditions should be given 10 days or more to acclimatise before performing heavy manual work in the heat.

Since I am highly trained and highly acclimatized, I take the 480mg/hour figure as most likely. However, riding at ~200 wattsfor 10+ hours continuallu is surely far more energy intensive than virtually all workers. Thus that should be considered a bare minimum.

NIH: Sweat Rates, Sweat Sodium Concentrations, and Sodium Losses in 3 Groups of Professional Football Players notes that overconsumption of fluid can be not only uncomfortable but dangerous—my rule of thumb has always been 1L/hour, as that is about all the stomach can handle, regardless of conditions.

Ironman study of sodium

An ironman effort is pretty darn close to a double century effort, both taking 10+ hour.

Sodium supplementation is not required to maintain serum sodium concentrations during an Ironman triathlon

Main outcome measures

Sodium supplementation was not necessary to maintain serum sodium concentrations in athletes completing an Ironman triathlon nor required to prevent hyponatraemia from occurring in athletes who did not ingest supplemental sodium during the race.


Subjects in the sodium supplementation group ingested an additional 3.6 (2.0) g (156 (88) mmol) sodium during the race (all values are mean (SD)). There were no significant differences between the sodium, placebo, and no supplementation groups with regard to age, finishing time, serum sodium concentration before and after the race, weight before the race, weight change during the race, and rectal temperature, systolic and diastolic blood pressure after the race. The sodium supplementation group consumed 14.7 (8.3) tablets, and the placebo group took 15.8 (10.1) tablets (p  =  0.55; NS).


Ad libitum sodium supplementation was not necessary to preserve serum sodium concentrations in athletes competing for about 12 hours in an Ironman triathlon. The Institute of Medicine's recommended daily adequate intake of sodium (1.5 g/65 mmol) seems sufficient for a healthy person without further need to supplement during athletic activity.

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