What actually keeps fluid in: the sports drink does not

Every fighter has a rehydration drink he trusts, and for most of them it is a commercial sports drink.

There is a measurement of how well different drinks are actually retained, and the sports drink does not appear where people assume.

The study

Seventy-two healthy men, each fasted and normally hydrated, drank 1 litre of a test beverage over 30 minutes. Urine output was then collected for 4 hours.

Thirteen drinks were tested against still water, and the result was expressed as a beverage hydration index: water is set at 1.0, and anything above that retained more fluid.

Four drinks beat water. Cola, diet cola, hot tea, iced tea, coffee, lager, sparkling water and a commercial sports drink all produced urine output that was not different from water.

The sports drink performed the same as water.

What separates the ones that won

Two mechanisms, and both are about slowing things down rather than adding anything.

Sodium is the main one. Sodium in the fluid reduces the drive to excrete it, and it assists absorption from the intestine through sodium-glucose cotransport. Later work on the same index found that electrolyte content contributed most to retention.

The second is what milk brings and a sports drink does not. Protein and fat slow gastric emptying, delaying the arrival of fluid into circulation. That sounds like a disadvantage and is not: a slower arrival preserves plasma osmolality, produces less diuresis, and leaves more of the fluid in you.

Which explains why milk sat level with a medical rehydration product. It is doing the same job by a different route.

And why the sports drink fails here

Commercial sports drinks are formulated for use during exercise, where the requirements are different.

During a session you want fluid absorbed quickly, carbohydrate delivered steadily, and a concentration low enough that it does not sit in the stomach. That is a genuine design brief and sports drinks meet it.

The sodium concentration that serves the exercise brief is well below what improves retention at rest.

So the drink is the correct product for the wrong moment, and a fighter rehydrating after a weigh-in is in the wrong moment.

The carbohydrate is not optional

Sodium alone is not the whole mechanism, and this is where homemade solutions frequently go wrong.

Water absorption across the intestinal wall runs partly through sodium-glucose cotransport, which requires a small amount of carbohydrate present alongside the sodium to work at full capacity. Salt in water without any carbohydrate is doing less than it could.

For a fighter that is convenient, because carbohydrate is wanted in that window anyway. The same drink that carries sodium for retention carries glucose for glycogen, and the two requirements point in the same direction rather than competing.

It also explains the orange juice result. Sugar and potassium with a little sodium is closer to a rehydration solution in structure than a sports drink is.

And the mistake that comes from overcorrecting

Having learned that sodium and carbohydrate matter, the obvious next move is to add more of both. That produces a specific problem.

The limitation, stated properly

Those subjects were normally hydrated and at rest. A fighter after a cut is neither.

Hypohydration changes the hormonal picture: aldosterone and vasopressin are both elevated, sodium conservation is already switched on, and the kidneys are holding onto fluid harder than at baseline.

Which means the numbers above describe a related situation rather than the exact one. The direction is well supported and the magnitudes may differ.

What does not change is the mechanism. Sodium concentration governs retention, and a drink with too little of it will be excreted faster regardless of the starting state.

The paradox with plain water

There is a specific failure worth naming because it happens in exactly the window a fighter cares about.

Drinking a large volume of plain water quickly, into a system that is depleted, can dilute plasma sodium enough to trigger diuresis. The body reads the dilution and excretes.

So a fighter who drinks 2 litres of water in the first hour after a weigh-in can end up in net negative fluid balance, having drunk more and retained less than somebody who drank less with sodium in it.

That is the mechanical reason the rehydration protocol specifies sodium rather than volume, and the reason the answer to falling short is not simply to drink more.

What this means in practice

For the hours after a weigh-in, the drinks that work are the ones with meaningful sodium and some carbohydrate.

An oral rehydration solution is the reference product, designed for exactly this and available in any pharmacy. It tastes medicinal, which is a real obstacle when tolerance is already poor.

Milk is the surprise, and it performed level with the ORS. For a fighter who tolerates dairy, it is cheap, palatable, and delivers protein alongside the fluid.

A homemade solution of water, salt and some carbohydrate reaches the same place, with the concentration caution above attached. The specific amounts belong in the protocol rather than on this page.

What does not work is a litre of plain water, and what works less well than people think is the sports drink they have always used.

Coconut water, since it comes up

Coconut water is marketed as natural rehydration and sits closer to a sports drink than to an oral rehydration solution.

It carries useful potassium and relatively little sodium, which is the wrong way round for the retention question. Sodium governs retention, and potassium is not a substitute for it.

It was not among the drinks in that trial, and comparisons since have generally placed it around the level of a sports drink rather than above it.

As a pleasant drink in heat, fine. As the rehydration strategy after a weigh-in, it is missing the ingredient that does the work.

The tablets and powders

The category has expanded enormously and the products vary more than the marketing suggests.

The number to look for is sodium per serving, and it is frequently lower than a fighter would assume. Many products are formulated for general daily use rather than for rehydration from a deficit.

Read the figure rather than the front of the sachet. A product delivering 200 milligrams of sodium per serving and one delivering 1000 are doing different jobs, and both are sold as electrolyte drinks.

For anybody under anti-doping rules the standard caution applies. These are supplement products and belong to that category with the batch-testing requirement attached. Salt added to water and juice is food.

What about plain water in camp

Everything above concerns one specific window, and it does not generalise.

For ordinary training, water is the correct default. A fighter drinking water through a normal session is doing the right thing, and adding electrolyte products to a Tuesday afternoon costs money and delivers nothing.

The situations where more than water is warranted are narrow: long sessions in heat, multiple sessions in a day, and the rehydration window after a weigh-in.

Outside those, the useful measure is whether you drink at all, more than what. Turning up to sessions already short of fluid is a considerably more common problem than choosing the wrong beverage, and it is the one that costs training quality across a camp rather than in a single afternoon.

Carbonation and temperature

Sparkling water performed the same as still water in the trial, which answers the question people usually have. No retention penalty and no advantage.

Two practical points specific to this sport. Carbonated drinks produce gas in the stomach, unwelcome in the final days when abdominal volume is being managed. And they are frequently drunk more slowly, which matters in a window where you are trying to get a quantity in.

On temperature: cold drinks empty from the stomach slightly faster than warm ones and are generally consumed in larger volumes, which for a dehydrated athlete is an advantage rather than a technicality.

Palatability drives intake more than most people credit. The best-formulated solution in the world delivers nothing if two thirds of it stays in the bottle.

Which is a practical argument for testing whatever you plan to use before the week that matters. That applies to oral rehydration solutions in particular: a fighter meeting that taste for the first time an hour after a weigh-in is meeting a problem he could have solved in week four.

Where it sits by phase

Check the number on the bottle

If you take one thing from this page, it is that the product most fighters reach for after a weigh-in performed no better than water in the trial that measured it.

Sodium is what keeps fluid in, with a little carbohydrate to help it across. Check the number on whatever you are using, and if it is low, you are drinking something formulated for a different problem.