14

According to Ace Fitness, it is a myth that potassium deficiency causes cramps, since there isn't enough potassium in sweat to deplete potassium. Sodium deficiency is blamed instead. Is this true?

One commonly held myth is that muscle cramping in active individuals is due to the loss of potassium; however, the amount of potassium in sweat is likely too low for this to be the culprit. Muscle cramping due to electrolyte imbalance is more likely associated with the loss of high amounts of sodium through sweat.

5

The ratio of intracellular to extracellular potassium is important in determining the cellular membrane potential. Small changes in the extracellular potassium level can have profound effects on the function of the cardiovascular and neuromuscular systems.

Low potassium (hypokalemia) refers to a lower than normal level of potassium in your bloodstream. Potassium is a chemical (electrolyte) that is critical to the proper functioning of nerve and muscles cells, particularly heart muscle cells. Hypokalemia is generally defined as a serum potassium level of less than 3.5 mEq/L (3.5 mmol/L). Severe hypokalemia is a level of less than 2.5 mEq/L.

Most cases of hypokalemia occur in the setting of specific disease states, and most common cause of low potassium is excessive potassium loss in urine or from the digestive tract, transcellular shift (movement of potassium from serum into cells), or the direct effect of medications. Only rarely is low potassium due to sweating or not getting enough potassium in your diet.

Patients are often asymptomatic, particularly those with mild hypokalemia. Symptoms that are present are often from the underlying cause of the hypokalemia rather than the hypokalemia itself. The symptoms of hypokalemia are nonspecific and predominantly are related to muscular or cardiac function.

Those symptoms include: weakness and fatigue (most common), palpitations, psychological symptoms (eg, psychosis, delirium, hallucinations, depression). Muscle cramps and pain are only present in severe cases, as Troponin's (a regulatory protein complex that is integral to muscle contraction in skeletal and cardiac muscle) becomes impaired. Therefore, hypokalemia can hyperpolarize skeletal muscle cells, impairing their ability to develop the depolarization necessary for muscle contraction. It can also reduce blood flow to skeletal muscles. The reduced blood flow can predispose patients to Rhabdomyolysis (damaged skeletal muscle tissue breaks down its breakdown products are released into the bloodstream) especially when vigorous exercise is combined with impaired blood-flow regulation. The combination of these effects frequently leads to muscle weakness, easy fatigability, cramping, and myalgias. Paralysis, although uncommon, can occur in cases of profound potassium deficiency.

References:

Assadi F. Diagnosis of Hypokalemia: a Problem-solving Approach to Clinical Cases. Iran J Kidney Dis. Jul 2008;2(3):115-22.

Gennari FJ. Hypokalemia. N Engl J Med. Aug 13 1998;339(7):451-8.

Zull DN. Disorders of Potassium Metabolism. Emerg Med Clin North Am. Nov 1989;7(4):771-94.

Ingram TC, Olsson JM. In Brief: Hypokalemia. Pediatr Rev. Sep 2008;29(9):e50-1.

  • Rhabdomyolysis causes the release of various substances from the muscle tissue involved - including potassium - is this at least in part an homeostatic process? On the face of it, that seems possible. – Duckisaduckisaduck Apr 22 '14 at 13:57
  • The part I would be worried about regarding Rhabdomyolysis is the release of Myoglobin (one of the main proteins that form muscle cells) into the bloodstream, that being a macro-molecule produces permanent Kidney damage and is a life threatening situation. The most common cause of Rhabdomyolysis is crushing trauma, that damages the cellular wall of type II muscle fibers. I wouldn't be much concerned about Potassium release in that scenario. – Carlos Teran Apr 22 '14 at 23:21
  • awesome! another example of the conglomerate fruit companies lying to get us buy their crap. – Keng Apr 23 '14 at 20:20
4

My claims:

a) Short-term exercise, including marathon running (~4 hours), does not result in low blood potassium levels and therefore does not likely cause muscle cramps. NAP.edu

b) Long-term exercise (hard physical work, several days in a hot environment) can result in mildly lowered blood potassium levels but probably not severe enough to cause cramps. NAP.edu

c) Hyponatremia can cause cramps, but fitness-level exercise does not likely cause hyponatremia. Hyponatremia is rare even in marathon runners, and is not caused solely by sodium loss through sweat, but by a combination of sodium loss and excessive water drinking. PubMed Central

Some facts: - Mildly lowered blood potassium levels usually do not cause any symptoms, but moderately lowered levels can cause cramps. You can lose about 500 mg of potassium per liter of sweat, but this is not enough to cause hypokalemia. Potassium that moves from blood to sweat is quickly replaced by potassium that moves from cells to blood. Nutrientsreview

Someone who has moderate hypokalemia can get cramps without any exercise, even during sleep.

The exact mechanism of exercise-related cramps is still not known. Lactic acid buildup and muscle breakdown (rhabdomyolysis) are two possible causes. Hyponatremia is very unlikely to develop in exercise lasting less than 2 hours and it occurs only in combination with excessive water drinking.

Conclusion: to have potassium-related muscle cramps, you would need to have potassium deficiency, which can develops within weeks, not within few hours of exercise.

  • You might want to better define in your answer what "short-term exercise" means since the median marathon time to finish a marathon is 4:16 for men and 4:41 for women. – rjzii Jul 21 '14 at 17:50
  • Yes, "4 hours." – Jan Jul 21 '14 at 18:33

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .