Work out your sweat rate
How much you sweat is your own, and it varies a lot from one person to the next. Weigh yourself before and after one session and you know your number.
Weigh yourself naked just before a session and again just after, towelled dry. Note what you drank. Go to the toilet before the first weighing, not between the two.
Sweat per hour
1 L
Sweat in total
1.5 L
Weight lost
1.3%
Under the 2% where performance starts to suffer.
That sweat carried somewhere between 0.15 and 0.75 teaspoons of salt, or more. How salty sweat is varies a great deal from person to person. White marks on your clothes after a long, hot session mean you are at the salty end.
Next time
Mix the drink for your next long session
Mix 3 L
- Water3 L
- Sugar, about 15 tablespoons180 g
- Fine salt, about 0.75 teaspoon4.5 g
- Lemon juice1.5 lemons
That covers your sweat for the session. Drink to thirst rather than to the bottle: sweat rates change with the heat, and nobody has to finish it. Why these amounts is in the homemade sports drink recipe. Then pack the rest of the bag.
Where the numbers come from
A litre of sweat weighs about a kilogram, so the weight you lose plus what you drank is what you sweated. Do it on a few different days: a hot one will read higher than a cool one.
A litre of sweat carries anything from about a tenth of a teaspoon of salt to more than half. (Baker 2017) Losing more than about 2% of your body weight to sweat is where endurance performance starts to drop. (Sawka et al. 2007)
The thinking behind this is in the book: Homemade sports drink.
The app plans each day around the challenge you are training for, the long days included, and coaches them on your Apple Watch.