Running & Endurance
The Best Hydration Formula for a Half Marathon
There is no single best drink for a half marathon, because the right amount depends on your sweat rate, the weather and your pace. But there is a sensible starting range, and a way to make it yours. Here is how to build it.
How much do you actually need over a half?
For a lot of runners finishing under about 1:45 in cool weather, the honest answer is: not much. The half marathon sits in an awkward middle ground. Your sweat rate is only slightly higher than in a marathon, but the race is half the length, so your total fluid need is much lower. In practice you can roughly halve the drinking rate you would use for a marathon.
The classic mistake is overdrinking. Taking on more fluid than you lose does not make you faster, and in extreme cases it dilutes your blood sodium to dangerous levels, a condition called hyponatraemia. Slower finishers in the heat are actually more at risk of drinking too much than too little, because they spend more time on course passing more aid stations. The goal is not to drink as much as possible, but to replace enough of what you lose to keep performing, and no more.
The starting formula
A range that works for most runners in most conditions looks like this. For fluid, 400 to 800 millilitres per hour, leaning low when it is cool and you are running under 1:45, and higher in heat or when you are out there longer. For sodium, 300 to 600 milligrams per hour, which is roughly 400 milligrams in every 500 millilitres of drink. And for carbohydrate, 30 to 60 grams per hour, sipped from around the 45-minute mark.
For a sub-90-minute half you can sit near the bottom of the carbohydrate range, or skip it entirely if you fuelled well beforehand. For a two-hour-plus effort the carbohydrate genuinely helps. Whatever the amount, spread it out: rather than gulping a full cup at one station, take smaller amounts every fifteen to twenty minutes so your gut absorbs it steadily and you avoid the sloshing feeling.
Why sodium is the ingredient people forget
If you change only one thing about your hydration, add sodium. Plain water on a hot day can make things worse, because it dilutes what sodium you have left, which is exactly the wrong direction. Most sports drinks are on the light side, often around 200 to 300 milligrams of sodium per 500 millilitres, when many runners would do better nearer 400 to 500.
If you change only one thing about your hydration, add sodium.
You lose more sodium than you would guess. If you finish a run with white salt crusts on your face or a sting in your eyes, you are a salty sweater and belong at the higher end of the range. If you never see salt streaks, you can go lighter.
Measure your own sweat rate
The generic range gets you close, but your body is not generic, and fluid loss varies enormously between runners and conditions. The most valuable twenty minutes you can spend is measuring your own sweat rate.
The method is simple. Weigh yourself with no clothes before a one-hour run in conditions like your race, run without drinking or track exactly what you drink, then weigh yourself again afterwards. Each kilogram lost is roughly one litre of sweat. A sweat rate calculator turns that into an hourly drinking target you can plan around. Do the test once in cool weather and once in the heat, and you will understand your own range far better than any chart can tell you.
A DIY bottle recipe and race-day plan
You do not need a lab-formulated product. A good homemade mix, per 500 millilitres of water, is 20 to 30 grams of sugar or maltodextrin for the carbohydrate, a quarter-teaspoon of table salt for around 400 to 500 milligrams of sodium, and a squeeze of lemon for taste. That gives roughly a 5 to 6 percent carbohydrate drink, which is the concentration that empties from the stomach well during exercise. Go much beyond about 8 percent and absorption slows and your gut complains.
On race day, arrive well hydrated with pale urine and 400 to 500 millilitres in the two hours before the start. Begin drinking early, around 20 to 30 minutes in, rather than waiting until you are thirsty and already behind. Take small amounts often, aiming at your personal sweat-rate number, and get sodium in, especially if it is warm. Add a gel or two if you are running longer than about 90 minutes. Above all, practise the exact plan on a long run first: race day is not the moment to try a new drink, gel or bottle.