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Electrolytes and Muscle Cramps: Separating Science from Anectodes

Electrolytes and Muscle Cramps: Separating Science from Anectodes

Pvt. Joe Bowers: “What are these electrolytes? Do you even know?”

Secretary of State: “They’re… what they use to make Brawndo!”

Pvt. Joe Bowers: “But why do they use them to make Brawndo?”

Secretary of Defense: [raises hand after a pause] “Because Brawndo’s got electrolytes.”

(Idiocracy, 2006 film, dir. Mike Judge):

Among the many lessons passed down through the generations of endurance athletes and their coaches is the necessity of electrolyte supplementation in preventing exercise-associated muscle cramps (EAMC). However, like many such teachings, it appears to be based primarily on anecdotal observations and personal accounts. Very little, if any, scientific data has supported this claim (1, 5), and there does not seem to be a plausible physiological reason to believe it would be true.

When this has been studied prospectively, both in marathon (2,3) and in ironman competitors (4), no association was found between muscle cramping and serum electrolyte abnormalities.

More importantly, though, is why would this be thought to be true in the first place?

Let’s begin with what we actually do know about muscle cramps.

Muscle cramps are ultimately a failure of the normal regulatory mechanisms your body uses to coordinate movement of its muscles. Your body has an incredibly complex system of sensors and feedback mechanisms to ensure smooth and appropriate muscle movement at all times. Included in this system is a sensory receptor within your tendons called the golgi tendon organ. When stretched, this receptor sends inhibitory signals to your spine preventing what are known as “alpha” motor neurons from stimulating the muscle to contract.

As your muscles fatigue, though, this normal inhibitory mechanism somehow becomes weakened. The alpha motor neurons then receive less inhibition, and/or more stimulation, and this ultimately leads to muscle cramping.

Why this dysregulation occurs in fatiguing muscles is where our knowledge ends and our speculation begins. But is there a reason to suspect that electrolyte concentrations are involved?

Electrolytes flow freely from within the intravascular compartment (i.e. within your blood vessels) to the interstitium (the area that is outside of both your blood vessels and the other cells of your body’s tissues). The exception to this is in the vessels of your brain, where a much tighter barrier exists around your capillaries. So, ultimately what the cells in your muscles “see” in terms of electrolytes is essentially the same as the concentration of electrolytes in the bloodstream. In order to make the case that loss of electrolytes leads to muscle cramping, you must first demonstrate that there are lower levels in the blood stream in those individuals having cramps. Because it is not the total amounts of electrolytes in the blood stream that is relevant, it is the concentration. And this is where the “electrolyte loss” theory begins to break down.

When you sweat, you lose electrolytes. But what you are losing more of is water. The concentration of electrolytes (predominantly sodium and chloride) in your sweat is much less than what is in your bloodstream. So as you sweat, you lose more water than electrolytes, and the electrolyte concentrations in your blood go up, not down.

This is demonstrated when you look at sodium levels in endurance competition (6, 7). The predominant abnormality is high sodium levels, not low. In fact, those competitors that have low sodium levels are more likely those who have taken excessive water (7), beyond what their kidneys can excrete. But ultimately that is a problem with water, not sodium.

To understand why, first understand that your body regulates the balance of sodium and other electrolytes in your blood very tightly. When you lose water and electrolytes in your sweat, these regulatory mechanisms maintain a relatively constant concentration of such electrolytes in your bloodstream. It does so through several ways. Your kidneys either excrete higher or lower concentrations of such electrolytes depending on the changes in the bloodstream. And your body taps into its massive stores of such electrolytes, that are normally inactive, to maintain the concentrations in the blood stream.

But ultimately, though, what your muscles are seeing is a relatively constant concentration of electrolytes throughout endurance competition. Supplementing such electrolytes should not, in theory, have any real effect on that, since your body will just sense the new, higher, concentrations it sees and then bring the concentrations back to their normal levels.

As always, though, whenever there is a clinical controversy, the next step is to look to the evidence.

Let’s begin with magnesium, one of the more commonly promoted causes of EAMC. A systematic review by the Cochrane Collaboration identified seven separate studies, and found no evidence to support a role of magnesium supplementation for preventing muscle cramps (9). In fact, in at least one prospective study, in ultra-marathon runners, the serum magnesium levels were significantly higher in the group who experienced muscle cramps (2). Despite anecdotal reports, there is simply no evidence or reason to support a role for magnesium supplementation in preventing cramps.

How about potassium? The evidence there is lacking also. When looked at prospectively in both ultra-marathon runners (2) and in ironman competitors (4), no association was found between pre- or post-race potassium levels and EAMC. And there are significant safety concerns with excessive potassium intake in the setting of dehydration.

Same goes for calcium. No significant differences in serum calcium, either before or after endurance competition, have been found for those who develop cramps and those who don’t (2).

Sodium, however, may be a different story. Despite research indicating that the rates of serum sodium abnormalities was no different in those among distance runners who experienced EAMC than those who did not (2), those runners did have significantly lower sodium levels. It was just that their serum sodium was still within what is considered to be normal range. Could it be that being on the lower range of normal for serum sodium concentration was what made the difference?

Additional evidence comes from the medical world. Using a technique called “variable sodium profiling”, muscle cramps during hemodialysis have been demonstrated to be reduced (13). How much of this is related to sodium levels in the bloodstream versus prevention of hemodialysis-associated hypotension is not clear, but certainly it appears that sodium has a role in the process.

If there is a difference, though, precisely how much sodium supplementation is necessary has yet to be determined. The amount of sodium lost in sweat varies considerably from competitor to competitor (12). And research has shown that higher levels of sodium supplementation do not affect serum sodium concentration or plasma volume when compared to lower levels (11). In fact, some evidence exists that no sodium supplementation at all is necessary (8), although this particular study did not address effects on performance or on cramping.

And again, how much of this is a water issue, rather than a sodium issue, is up for debate. You lose much more water than sodium in your sweat. Hyponatremia in endurance competition may primarily be a result of replacing sweat losses (which contain sodium) with pure water, to the point where you have exceeded your kidneys’ capacity to correct the abnormality. This is demonstrated by the inverse relationship between weight gain and serum sodium levels following endurance activity (7). This is why recommendations are to just drink to what your thirst requires. Could it be that the problem is not lack of sodium supplementation but instead just too much water?

So where does this leave us?

– There is no science or evidence based role for supplementation of magnesium, potassium, or calcium in preventing exercise-associated muscle cramps
– There may possibly be a role for sodium supplementation, although the amount necessary has not been well determined

Exercise-associated muscle cramps remain a serious problem in endurance competition. Cramps can be painful, debilitating, and may even knock you out of the competition altogether. But do not let desperation for a solution force you into poor choices. As always, rely on science, not anecdote, to guide you. It’s not a decision to take lyte’ly!

1. Schwellnus MP. Muscle cramping in the marathon : aetiology and risk factors. Sports Med. 2007;37(4-5):364-7.
2. Schwellnus MP, Nicol J, Laubscher R, et al. Serum electrolyte concentrations and hydration status are not associated with exercise associated muscle cramping (EAMC) in distance runners. Br J Sports Med 2004; 38: 488-92
3. Maughan RJ. Exercise-induced muscle cramp: a prospective biochemical study in marathon runners. J Sports Sci 1986; 4: 31-4
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12. Lara B, Gallo-Salazar C, Puente C, Areces F, Salinero JJ, Del Coso J. Interindividual variability in sweat electrolyte concentration in marathoners. J Int Soc Sports Nutr. 2016 Jul 29;13:31.
13. Meira FS, Poli de Figueiredo CE, Figueiredo AE. Influence of sodium profile in preventing complications during hemodialysis. Hemodial Int. 2007 Oct;11 Suppl 3:S29-32.
14. Cosgrove SD, Black KE. Sodium supplementation has no effect on endurance performance during a cycling time-trial in cool conditions: a randomised cross-over trial. J Int Soc Sports Nutr. 2013 Jun 3;10:30.