What they are, what they do, and why you need all five.

by Jeff Rocco, MD


The science of cramp prevention is both complex and surprisingly simple. The causes and cellular chemistry involved – that’s the complex part. The solution, fortunately, is the easy part: take all five electrolytes before, during, and after exercise in order to maintain balanced electrolyte levels. Overloading one or two – like, say, by relying exclusively on pickle juice – can actually throw off your internal balance and will only contribute to symptoms like brain fog, fatigue, muscle weakness, and – of course – cramping.

In this blogpost, Dr. Jeff Rocco breaks down each of the five electrolytes, summarizing what they do, how they’re lost, and what happens when you’re deficient in them. Future blog posts will go into more detail about how these electrolytes earn their salt, but for now, let’s meet the players.


You may recall from high school science class that electrolytes are dissolved mineral salts contained in our body fluids – both inside and outside our cells. Proper hydration and a balance of all five electrolytes allow neurons to fire appropriately so  muscles can contract forcefully and predictably.

For athletes, electrolyte losses occur primarily through sweating, which we do a lot, making dehydration an obvious and significant problem. Fluid losses as little as 1-2% of body weight can cause a drop in performance, while losses exceeding 4% of body weight can cause critical failure.

Realizing this, athletes drink, but things get worse for athletes who drank primarily or exclusively water, because consuming water without electrolytes replaces fluid losses, but dilutes electrolytes by throwing off those critical concentration levels. For example, in IRONMAN medical tents, electrolyte deficiencies are found more commonly than dehydration. Hydration before and during longer events is good, but it should always contain electrolytes.


It is not our focus here to review in depth all of the cellular biology related to electrolyte homeostasis; instead, we’re identifying where electrolytes go when they are lost, what the symptoms are when electrolytes are deficient, and how to maintain electrolyte levels to achieve optimum performance.

The five key electrolytes for endurance athletes are sodium (Na+), potassium (K+), chloride (Cl-), calcium (Ca++), and magnesium (Mg++). In our bodies, hydration is linked intimately with the concentrations of those electrolytes, both individually and in relation to each other. As those electrolytes are lost, those concentrations change, and things start to go haywire.


Sodium is the most abundant and perhaps the most important of the electrolytes. Many foods contain sodium, and if not, we’ll often add it in the form of Sodium Chloride (NaCl – or, simply, table salt).

Na+ is found in higher concentrations outside of cells in our bodies, and all cells depend on it (and potassium) to bring nutrients inside the cell and to remove waste. Nerve conduction – a process important for thinking and for activation of muscles – is also heavily dependent on sodium and potassium.

Deficiency of sodium is called hyponatremia, and it’s the most common electrolyte disorder in the US. Hyponatremia in athletes is usually due to sodium lost in sweat. Other disease processes may cause hyponatremia, but the symptoms of hyponatremia are the same for athletes and non-athletes, regardless of cause. These include fatigue, muscle weakness, muscle spasms or cramps, convulsions, nausea, vomiting, confusion, or decreased consciousness. Vomiting due hyponatremia can cause Na+ levels to drop even further – a compounding problem that’s common among athletes pushing the extreme limits of endurance.

I’m lumping chloride in here because it’s a negatively charged ion which binds readily to sodium and potassium – see the table salt example above. When sodium or potassium is consumed, chloride is usually present. It plays a role in electrolyte balance, but don’t look for supplements containing chloride by itself – they don’t exist!


Potassium is the primary electrolyte found inside of cells. It works closely with sodium and chloride maintaining fluid balance and cellular homeostasis and conducting nerve impulses. K+ is lost from contracting muscles with consumption of muscle glycogen during exercise, and it’s also excreted in urine or sweat.

Low potassium is called hypokalemia. Symptoms of hypokalemia are often similar to some symptoms of hyponatremia – muscle fatigue, weakness, drowsiness, confusion, or irregular heartbeat.


Bones are the largest reservoir of Ca++ in the body; however, soluble calcium in body fluid is also necessary for neuromuscular conduction, muscular contraction, and inter- and intracellular messaging, and it plays a key regulatory role in glycogen metabolism – extremely critical for endurance athletes.

Significant quantities of Ca++ can be lost in sweat. Studies in college basketball players have found that decreased bone mineral densities and stress fractures correlate with calcium losses in sweat – a risk multiplied by long-distance endurance training.

While chronic calcium deficiency may lead to depletion of bone mineral density, acute symptoms of hypocalcemia during exercise may manifest as muscle spasms or cramps, intestinal distress, confusion, and tingling in fingers and toes. Treatment with hydration mixes and supplements rich in Ca++ address those acute issues, but it also leads to increased bone densities (Klesges 1996). 


Magnesium is perhaps the most underappreciated electrolyte. Many athletes recognize the importance of sodium and potassium supplementation, and that becomes the priority, but low Mg+ is often the reason for sub-optimal performance.

Mg+ is important for proper transmission of nerve impulses, muscular contraction, and energy production associated with ATP – overall, more than 300 enzymatic reactions in your body depend on magnesium as a cofactor. That includes its fellow electrolytes; magnesium can have direct effects on sodium, potassium, and calcium channels located in cell walls.

Longer and more intense exercise depletes magnesium levels, and it’s excreted in sweat and urine. Symptoms of magnesium depletion include weakness, muscle cramps, confusion and irregular heartbeat.


Weakness, confusion, muscle cramps, and nausea do not make for a good day as an endurance athlete. By now you may begin to recognize a theme: Depletion of any of these five essential electrolytes will likely result in similar symptoms.

It’s difficult to identify an isolated electrolyte deficiency without a laboratory analysis, and we know that all five of these electrolytes are lost in sweat and depleted with exercise, so an athlete’s best course of action is to consume all five electrolytes before, during and after exercise.

Nutrition strategies that ignore some of these electrolytes will fail in more extreme circumstances. Hotter temperatures, longer or more intense exercise results in greater electrolyte losses, which need to be replaced to train and race effectively. And again, simply supplementing sodium or popping potassium isn’t going to cut it, and may actually create an electrolyte imbalance that throws off your internal chemistry, exacerbating rather than alleviating cramping.

June 05, 2024 — First Endurance

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