If you make up your own batch of fuel drink, is there a calculator to see if it’s hypo/hyper/isotonic? Say for example I use 20 grams of table sugar, 40 grams of maltodextrin, how much water does that require to be isotonic? And how much more water do I need to add for 300mg of sodium?
If you increase the carbs past a certain point, then sodium has to go down… but how do you calculate where that point is?
You can get fancy with the math if you want, could check out any scientific osm calculator online. Lab nerds might google this as required. However, if memory serves, 5% dextrose (D5W) is very close to isotonic, and 0.9% saline is isotonic (isomolar to human plasma).
If you dissolve 5g of dextrose in water to 100 ml total volume, (~97ml water or so, since it dissolves) you will be roughly isotonic. Put this in Bottle A. If you dissolve 0.9g of salt in 100 mL, you should get near isotonic. Put this in bottle B. If you mix any percentage of A with any percentage of bottle B (ie to taste) you should remain isotonic. Maltodextrin 30g/100mL. Someone check my math.
Wether or not you would want to drink this is a different story.
Edit2: Oops!!! Maltodextrin error. Maltodextrin is 150g/500ml. https://fellrnr.com/...ience_of_Energy_Gels
300/L
30g/100mL in “bottle 3.”
Edit 3: Maybe I shouldn’t do these anymore. 
Sorry messed up the % amounts too. 9g/1000ml is 0.9%, so 0.9g in 100ml previously was off by a factor. Corrected above.
Ok, let me see if I have some of the numbers right.
50g of simple sugars per liter is isotonic. Dextrose, but also sucrose or fructose? I know the whole bit about dextrose is the left molecule of glucose and sucrose is really fructose and glucose and all that jazz, my question is it the same for all simple sugars or do the different ones have different mole values (or whatever it’s called?)
Anyway, that’s about 200 calories per liter.
300g of maltodextrin is isotonic. At 4 calories per gram, that’s 1200 calories? That’s huge!
9% saline is isotonic, but is that “saline” 9g of sodium or 9g of salt?
Sorry I could have done a much better job above and also not include an error before posting. it looked like a fun exercise but chemistry 101 was a long time ago. I believe it is all correct above now. Long story short, compound solution using stock solutions and then adding is usually an easy way to do this and that how my brain works with the math. If you have each solution at the target osmolarity it will have the same osmolarity. The link above has some math too put in a different way.
Osmolarity/Tonicity is measured as osmolarity. Roughly, 300 mosm/Kg is “isotonic.” For IV fluids stuff around 250-350 are considered “close enough.” Saline is 308 mOsm/L, and it contains 154mEq/L of sodium and 154mEq/L chloride. so, 9g of Sodium chloride dissolved into 1 liter of water, is 0.9% weight/volume (w/v) and would yield a isotonic solution of a total 308 mOsm/L, of which roughly half of the osmolarity is contributed from the sodium and the other half is chloride. So, you just count NaCl as “one item” here.
Per the link above, Table sugar for nutritional purposes is best considered as separate constitutive components as it is absorbed that way, so I think it is koser to consider table sugar (which as you said is one fructose and one glucose, and nearer-makes-no-difference considered the same as a glucose solution. A 5% solution of glucose is considered to be near-enough to an isotonic solution at 252 mOsmL. A 2.5% solution of fructose + a 2.5% solution of glucose would be similar, which is basically the outcome of table sugar in water solution. If you disolve 50g of table sugar in 1 L water, you will indeed have a near-isotonic solution and roughly 200 kilocalories (4 kilocalories/gram).
On to maltodextrin. Being a large molecule “glucose polymer,” and as such it contributes less “osmotic effect” per molecule, and you can cram a lot more calories per “unit osmolarity,” compared to free sugar. Apparently, 300g/1L is considered to be isotonic (note: this is taken from nutritional websites, maltodextrin is not used in basic IV solutions and thus isn’t on brain speed-dial…) so, in other words, you can pack 6 times the grams into a solution with maltodextrin to get a isotonic solution.
For your solution:
what added final volume do we want to have a ~300 mosm solution containing 20g of table sugar (sucrose) 40g of maltodextrin and 300mg (0.3g) of salt.
Sugar first:
need 5% solution to contain 20g, so 400mL (@ 50g/L)
Maltodextrin:
need to contain 40g at 300g/L, so, 130mL
Salt:
need contain 0.3g, at 9g/L(0.9%), that would be 33mL
Dissolve 20g sugar + 40g maltodextrin + 300mg salt in a end-volume of 563ml (ish), and you would have a isomolar concoction with your ingredients.
Or, 20g sugar + 40g maltodextrin in 530ml and you would have a non-salt isotonic solution.
Somebody check my math before trying to ingest that stuff…
Well as a sanity check, SIS gel claims to be isotonic and is 60ml with 22g of maltodextrin (so ~36g per 100ml).