Another way to bonk during a marathon — it’s not just about carbo-loading
I am going to go out on a limb and propose an alternative explanation for why some runners (e.g. me) crash during a marathon.
Normally, “hitting the wall” in a marathon is attributed to the point at which the runner’s body runs out of energy and they can no longer maintain their pace.
To avoid this fate, runners top off their body’s glycogen stores by carbo loading during the few days leading up to the race. Also, since even a full tank of glycogen will only last for 90 minutes or so, they will consume carbohydrates throughout the race to make up the difference.
I always carbo load voraciously and eat plenty of carbs during the race yet it is rare that I don’t encounter a crash at 17 miles or beyond. Of the 22 marathons I have run in the last four years, I hit the wall in all but 5 of them. Given how meticulously I plan my carbo loading, it is hard for me to believe that I have been depriving my body of sufficient fuel.
I believe that a more likely explanation for me is electrolyte depletion, or more specifically, an inadequate supply of sodium. I suspect that others may be suffering from this scenario.
On the surface, this is not earth-shattering. The requirement to replenish electrolytes during a long run is well known.
The part that I find revalatory, is the magnitude of the issue. Yes, it is important to replace the electrolytes one loses from sweating. But how much is enough? There are rules of thumb but these won’t apply to everyone. For example, I have Parkinson’s disease (PD). Does PD influence this issue?
PD can make it worse. One of the many non-motor symptoms of PD is the tendency for poor temperature regulation of the body. People living with PD often sweat much more than others when exercising and the additional perspiration further decreases the supply of electrolytes.
For now, let’s focus on sodium as it is the most significant of the electrolytes found in the fluid surrounding cells, the fluid that eventually makes its way into sweat. Sodium is essential for muscle contraction. Without sodium, muscles don’t work right.
Hyponatremia is the condition whereby sodium levels drop too low. Symptoms include fatigue, nausea, and muscle cramping. Drinking plain water further dilutes the extra-cellular fluid thus worsening the effect. Hyponatremia forces water back into the cell, which puts pressure on neighboring cells. This situation is most severe in the brain. It can cause encephalopathy and even death.
Hyponatremia is common. One study found that 13% of the runners in the 2002 Boston Marathon suffered from it.
(FUN FACT: Natrium is a latin-based word for sodium and is where we get Na as the chemical symbol for this element. It shows up in the terms “hyponatremia” (low sodium concentration in blood) and “hypernatremia” (excess sodium in blood). Similar wordplay goes on with kalium, the latin-based word for potassium, its chemical symbol K, hypokalemia, and hyperkalemia.)
I get those symptoms — fatigue, muscle cramps, and nausea — in most races and some of my longer training runs. It’s worse on hotter days. I have managed to corral muscle cramps somewhat, most notably by popping SaltStick Electrolyte capsules throughout the race. But a strategy for completely avoiding them has eluded me.
I have always suspected that I am a heavy sweater and, given my propensity for cramping, most likely a salty one. Now I have a means to quantify my sweating and the composition thereof.
My new toy is the hDrop Hydration Sensor that I have been wearing during exercise for the past two months. At the end of a workout, the hDrop app provides data regarding sweat rate, sweat composition in terms of sodium and potassium, and body temperature.
When I run, I sweat at a rate of about one liter per hour. That’s slightly above average for the general population. (I have averaged my data over 27 workouts including runs from 4 to 22 miles and temperatures from 26 to 65 degrees F).
But here’s the kicker: the sodium content of my sweat averages to 1455 mg/L. hDrop says that this is a very high sodium concentration compared to the average person. It works out to 263 mg of sodium lost for every mile I run.
That astonished me. To give some context, my practice has been to take one SaltStick capsule every 3 miles in a marathon. Each tablet has 215 mg of sodium in it. If all these numbers are correct, I have been replenishing less than one-third of the sodium I lose during a run! No wonder I crash!
Sometimes after a long hard workout I feel listless and nauseated. In such cases I have noticed that I feel much better after consuming up to 10 or more Nuun tablets dissolved in water. Each tablet contains 300 mg of sodium. With the data from the hDrop sensor, it all makes sense now.
And the data suggests a more aggressive replenishment strategy for my upcoming Tokyo marathon. I will at least double, and perhaps triple, the rate at which I pop SaltStick capsules and make certain that I chase these with the hDrop recommended water volume.
This strategy ought to help. I already proved it out on a 22 mile training run last week. But it won’t be sufficient if it is a hot day (65 F or greater). At such temperatures, the body prioritizes keeping itself cool and diverts blood away from muscles. Not much one can do in that case except help the cooling effort by slowing down and dousing oneself with water.
PLEASE NOTE: The author receives no compensation whatsoever for the products mentioned in this post.
