I make up a litre of drink with 3 or 4 teaspoons of fructose, 1/3 tsp salt, water and some flavour (eg a tea bag, lemon juice,etc)
But I am not very professional. I think the main thing is some kind of sugar and salt. The rest is fine-tuning (funny minerals and salts they have found in sweat by taping plastic bags under athletes arm-pits - yukkity yukk)