You breathe and sweat all night without drinking anything, so most people wake up already slightly short on fluid. Then the day starts, and fluid is rarely the thing you think about until something feels off.
The problem is the signal. Thirst doesn’t warn you that you’re heading for a deficit, it tells you that you’re already in one.
“Coffee isn’t dehydrating you. Almost nothing you drink is. That was never the problem.”
Your coffee isn’t the problem
Researchers measured how well 13 common drinks hold fluid in the body, each against the same volume of still water. Coffee, tea and lager came out statistically indistinguishable from water. Nothing tested came out below it.
Only three drinks beat water: a rehydration solution, skimmed milk and full-fat milk, each holding roughly half again as much fluid. Sodium does the work in the rehydration solution; in milk it is the protein and fat, which slow the emptying of the stomach so the fluid arrives gradually instead of passing straight through. But hydration and nutrition are different questions, and this chart only answers the first: the three winners bring calories, sugar or fat along with the fluid, so the ranking isn’t a shopping list. It’s a reason to stop discounting the tea and coffee you already have.
Beverage Hydration Index
Ranked by how much fluid the body held on to. Water is the reference.
Retained more fluid than water
A statistically significant difference at two hours.
Rehydration solution
Skimmed milk
Full-fat milk
The reference
Water
No significant difference from water
Every remaining drink. Including the three you were told dehydrate you.
Sparkling water
Cola
Diet cola
Orange juice
Sports drink
Tea
Iced tea
Coffee
Lager (4%)
Maughan et al., Am J Clin Nutr 2016. Fluid value only. Read it as a hydration ranking and nothing else.
Where the cost becomes real: training
The threshold is lower than most people expect. Lose 2% of your body weight in fluid, about 1.4kg if you weigh 70kg, and aerobic performance measurably drops. It doesn’t feel like dehydration. It feels like a session that’s harder than it should be, for no obvious reason.
WHAT MILD DEHYDRATION COSTS YOU
2%
Body weight lost in fluid — aerobic performance drops
3%
Strength starts to suffer too
37–46%
of regular exercisers start their session already mildly dehydrated
Thirst arrives after the cost, not before it.
The two-second check that beats any formula
You don’t need a tracker or eight glasses a day. Just look before you flush. Pale straw to light yellow means you’re fine, darker means drink now rather than when you feel like it. It accounts for your size, your climate, your sweat rate and your training load, which is everything a bodyweight formula misses.
The colour guide
1
2
3
4
5
6
7
8
1–3
Well hydrated
4–5
Drink more soon
6–8
Drink now
Your first of the morning is always darker, that’s the overnight deficit, not a problem. High-dose B vitamins and vitamin C can brighten the colour, so factor that in.
The window that matters most
The habit worth building isn’t a daily total, it’s the two hours before you train. Drink then and your body has time to absorb what it needs and clear what it doesn’t.
Around a session
2 hours before
400–600ml. Water is fine, and the timing lets you absorb it and clear the excess before you start.
During, past 60 minutes
150–300ml every 15–20 minutes. Add electrolytes when it’s long, hard or hot.
After
Replace about 1.25× what you lost. Each kilo down on the scale is roughly a litre to drink back.
TRY THIS TODAY
Check the colour once today. If it’s darker than pale straw, drink 400–500ml now, before you feel thirsty.
The science
Maughan et al. (2016), American Journal of Clinical Nutrition, for the Beverage Hydration Index. Thresholds and the urine-colour method from Helms, Morgan & Valdez, The Muscle & Strength Pyramid: Nutrition v3.1.