In 2016 I was fortunate to walk into a blank-slate PhD program – I could literally choose any topic I wanted to study. After thinking hard for about three seconds, I chose to study sodium replacement for endurance athletes. I’d never really had a strong passion for the topic, but my motivation was clear: of the three main things we use or lose during exercise, there was already quite specific guidelines for carbohydrates, and for fluids and hydration. Those guidelines weren’t, and still aren’t, perfect, but they’re a starting point. But when it came to electrolytes generally, or sodium specifically, there was quite literally nothing other than “if you lose a lot, you should consider replacing some of it”.
This isn’t a criticism of those writing such guidelines, but instead reflects the lack of research available to draw on at the time.
In the absence of official advice or guidelines around electrolyte replacement for athletes, something was inevitably going to fill the knowledge vacuum. Athletes were already getting their sweat sodium losses measured, so someone was having to tell these athletes what to do with the results. But that advice can’t have been based on science because there wasn’t any. Instead, it was likely a mixture of guesswork, personal opinion, and marketing-led messages designed to promote the use of electrolyte products.
What are electrolytes?
From a pure chemistry perspective, an electrolyte is a substance that conducts an electric current through the movement of ions (positive and negatively charged particles). If you Google electrolytes, you’ll find references to human physiology, and battery technology.
In humans these electrolytes are mostly minerals, often (but not always) found in food as salts (e.g. sodium chloride, or table salt), which split into their individual ions when dissolved in water. The main ones we think about are sodium, chloride, potassium, magnesium, calcium, phosphate, and bicarbonates. These electrolytes have a variety of roles in the body, from providing the minerals that give our bones their structural rigidity (mostly calcium and phosphate), to buffering changes in acid-base balance in the body (bicarbonates), and electrical signalling along and between nerves, and from nerves to muscles (multiple electrolytes).
They have other roles in the body as well, but a big one is controlling the amount and movement of water throughout the body. For this, sodium and chloride are the main players outside of cells, and potassium, phosphate, and magnesium inside the cells.
Which electrolytes are important for athletes?
Athletes tend to focus on electrolyte replacement in the context of sweat losses during exercise. When you look at the typical electrolyte composition of sweat, two electrolytes predominate – sodium and chloride. This is because sweat forms from the interstitial fluid, which surrounds the cells of the sweat glands, and here you’ll find heaps of sodium and chloride, and very little of the other electrolytes. It’s no surprise then, that our sweat glands have a mechanism to recapture some of the sodium and chloride, but none of the other electrolytes.
So while the other electrolytes are still important for good health (and some like calcium and magnesium can be significantly lacking in some people’s day-to-day diets), from a pure loss-and-replacement perspective during exercise, it’s sodium and chloride that we mostly focus on. Because they mostly come together in food, and our bodies for the most part regulate their losses through sweat and urine together, we need only worry about sodium, as chloride will tag along for the ride.
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