Ask whether you need electrolytes for your gym session and a $46 billion market says essential. Reddit says save your money. Neither side cites a study — and when we pulled the largest review, the first surprise was not what the evidence found, but who it tested.
When Jardine and colleagues pulled together the largest review of pre-exercise fluid loading, they found 38 studies across 403 participants. The review covered glycerol, sodium, combination protocols, and water with varying salt concentrations. It tracked exercise capacity, heart rate, core temperature, and gut symptoms across decades of research.
It never once tested your gym session.
Every study recruited endurance athletes — cyclists, runners, triathletes. Every protocol ran for at least an hour. Every environment was warm to hot, from 21 to 42 degrees. Not a single study tested a normal indoor session in an air-conditioned gym.
This is not a small gap. The research question was about prolonged heat exposure and cardiovascular strain. Your 45-minute mix of weights and cardio at 21 degrees was never part of the experiment — not because it was tested and dropped, but because nobody thought to ask.
The Better the Test, the Weaker the Result
Even in the population that was studied, the results do not settle cleanly.
Five of seven studies found athletes could keep going 14 to 26 percent longer before quitting when they loaded beforehand. That is the number every supplement brand reaches for.
But that test has a reliability problem. Day-to-day variability runs above 10 percent, meaning the same person on the same test can score significantly differently just by showing up on a different morning.
So researchers ran a better version. Instead of measuring how long before quitting, they measured how fast athletes could cover a set distance. This test wobbles less than 5 percent — far more trustworthy.
The result: seven of nine studies found no improvement. Even in endurance athletes, even in the heat, the more reliable test mostly said nothing happened.
The two studies that did find something both required extreme heat above 30 degrees. Take away the hottest conditions and the benefit disappears from the reliable test entirely.
The Mechanism That Needs Heat
There is a clean reason the benefit stays in the heat and does not follow you indoors.
Extra fluid before exercise expands your blood volume, which lets your heart maintain flow to both your muscles and your skin simultaneously. Heart rate stays lower, core temperature drops. When heat is the thing limiting performance, that cardiovascular support matters.
In an air-conditioned gym at 21 degrees, heat is not limiting your performance. The mechanism has nothing to work against. The extra fluid sits in your system without a job to do.
Even in the heat, the expected heart rate drop appeared in only nine of sixteen studies. Seven found nothing. The pathway does not reliably engage even under the conditions it was designed for — which makes the case for it working in your temperate gym even weaker.
What It Actually Costs
If you are loading electrolytes before indoor sessions, two costs are already running.
Twenty-six of 38 studies reported gut symptoms from the loading itself — bloating, nausea, diarrhoea, stomach fullness that lasted into the first 15 to 20 minutes of exercise. Three participants withdrew because the discomfort was too severe. The symptoms tracked with how much was consumed and how fast, not with which brand was used.
Then there is the money. A single packet runs one to two dollars. Five sessions a week, 52 weeks a year. You are spending $260 to $520 annually on a product that has never been tested for your workout.
A year of tap water for the same sessions costs nothing.
Based on everything in this evidence base, the answer for your situation is clear. Save the packet money. Drink water when you feel thirsty. If you eat balanced meals, your food already provides the sodium, potassium, and magnesium the packet promises — the supplement is solving a problem your plate already solved.
The answer changes if the conditions change. If you are running outdoors for over an hour in the heat, the evidence has something to say — though even there, the reliable test found no performance boost in most studies, and two-thirds reported gut problems. Electrolyte loading has evidence only in conditions most gym-goers never encounter.
Almost all of this evidence was collected on men — 89.6 percent of every participant across all 38 studies. One trial tested women exclusively and found one of the largest improvements in the entire review. It has never been replicated. What electrolyte loading does for women remains an open question in the research we examined.
And if you are leaving the gym without loading anything, a different question opens up. Twenty-eight studies examined what mild dehydration actually costs during resistance training — whether skipping the extra fluid before your session leaves a measurable dent in what you can lift. The answer splits by what you are doing, and it is not the story the internet tells.
A single electrolyte packet costs between one and two dollars. Used five days a week, that adds up to $260 to $520 per year — spent on a product tested exclusively on endurance athletes in heat, never on someone doing what you do. A year of tap water for the same indoor gym sessions costs nothing. The research that tested loading protocols found the gut symptoms tracked with dose and speed — not with brand or price point.