Are Isotonic (6%) Drink Mixes Hydrating?

Hey guys,

Looking for a little science to back this up. If I mix in 60g of carbs into a 1 litre bottle does that diminish the hydrating effects of the drink? I’ve read that Isotonic (6-8%) don’t hydrate. Which kind of surprised me.

Thanks in advance.

A bit confused.

  1. Are you adding additional sugar into a isotonic drink mix?
    glucose is hypertonic, so if you add it to a 6% drink mix you’re ending up probably over 10%. So very much not hydrating

  2. Your drink mix contains 60g of carbs?
    I only have some SIS hydration drink mix, and that has only 36g of carbs per 500ml for 6% tonicity. So you’d probably be looking at similar figures, ie. drink your 60g of carbs with 1000ml of water and some salt.

From this podcast and some math:

No. 1 litre water + sodium + 40g maltodextrin + 20g fructose = 6%. = isotonic. Question is will only drinking that mixture suffice or will I get dehydrated ( when compared to drinking the Same volume sans carbs.

Isn’t that really dependant on how warm it it, how much you sweat, how long the ride is etc?

Just to preface this, I’m not an expert on this either. This is just my understanding:

You can be rehydrating from isotonic or hypotonic drinks. What makes something isotonic is that it has a similar osmolarity to blood plasma. The standard range is between 275 and 295.

No. 1 litre water + sodium + 40g maltodextrin + 20g fructose = 6%. = isotonic.

You should probably use a drink mix calculator and plug in the actual amount of sodium and the specific kind of carb you’re using. These numbers alone doesn’t actually make it isotonic. Try this: DIY Sports Drink Tonicity Calculator – PowerPacing.Run

Anyways, assuming the sodium level stays constant, removing the sugars will turn your drink into a hypotonic mix, which is hydrating.

Once you add back the carbs. the osmolarity goes up, first into the isotonic range, and then hypertonic - which is when it stops being hydrating. However, note that glucose does aid in the absorption of salt so having some carbs is going to be more hydrating than just a pure saline drink.

Finally, hydration isn’t purely about the tonicity of the drink. The idea is to replace what was lost, so if a lot of sodium was lost via sweating, then that should be top priority.

Comparing hydration impact when drinking the same quantities in the same conditions. one with 6% carbs one with none.


I add a ton of salt as I tend to be a salty sweater. Right now I have the mix right with just electrolytes and water, I just want to make sure adding the carbs won’t adversely effect my hydration. I used the calculator and the result was;

Osmolarity (mOsmol/L) 155 (<295) Hypotonic

But I’m not sure if lower means more or less hydrating. Yeah, I’m not that bright.

Yup it’s still hydrating. As long as the osmolarity is below 295, it’s still hydrating. Sorry it’s so complicated, but I think the details are quite useful because often we want as much carbs as possible as fuel while still keeping it’s ability to hydrate.

if lower means more or less hydrating

It’s sort of a hard cutoff. Below 295, it is hydrating, above it, it’s going to cause water to come out of your cells.

A lower number doesn’t mean it’s more hydrating than a higher number. Pure water is 0, and it’s not the most hydrating drink. In the podcast I linked above, Peter mentions that the most hydrating drink contains a 2:1 ratio of glucose to sodium (in millimole concentration).

So using your 40g maltodextrin + 20g fructose formula, that’s around 1200mg of sodium. But that’s just a number, and if you lose a lot of salt by sweating and you’re only carrying two bottles, then it’s probably more important to replenish the salt than creating the most optimum drink.

First off thanks so much for taking the time to help me understand! You’ve been so helpful. In regards to sodium I typically use 1000mg total across four 1 litre bottles on a 4 hour ride. I’ve just started playing with the carb in bottle thing. Mainly to cut costs on gels and to try and up my carb intake. How did you come up with the 1200mg number? Or will it become clear when I listen to the podcast?

No worries, was just using the calculator and making the carbs, which are circled in red, twice as much as the value in blue. This is using sodium citrate.

Awesome! Which site is that? I’d love to check it out. Also, I’ve heard jonathan talking about sodium citrate, what’s the benefit of difference over table salt? Thanks again! Oh and I need to find some lemon crystals. :joy:

That one

Sodium citrate is salty and acidic, and with lemon flavoring, it tastes like lemonade. Check out the make your on sis hydrating drink thread on this forum, lots of flavoring options

Awesome! Thanks so much. And yes, I saw that thread but got a little lost. lol