Water or electrolyte. The aid-station decision Indian marathoners make twenty times in a race and rehearse zero times in training. The honest answer is not one or the other. It is both, in a sequence, in proportions you have tested. Here is the science, the field-tested rule, and the protocol that works on a humid Mumbai morning.
The physiology, in one paragraph
During a marathon, you lose fluid through sweat. That fluid carries sodium and, in smaller quantities, potassium, magnesium, calcium. Drink only water, and you dilute the sodium remaining in your blood — at the extreme end, hyponatremia, which has been documented to be fatal in marathoners. Drink only concentrated electrolyte, and you risk GI distress and slowed gastric emptying. The literature has converged on a middle path: enough sodium to maintain plasma concentration, enough water to maintain hydration, paced to thirst. The IOC consensus statement on nutrition for athletes restated this in 2018 and the messaging has not materially changed.
Why thirst is more useful than a schedule
The 2007 review by Noakes in the BMJ overturned an older orthodoxy that runners should drink at fixed intervals regardless of thirst. The evidence is now clear: drinking to thirst is broadly safe for most marathoners; drinking ahead of thirst is the more common cause of hyponatremia. Trust the signal. Calibrate the dose. Do not pour water down on a metronome.
The Indian marathon, specifically
The published hydration literature was largely built around European and North American marathon conditions. The Tata Mumbai Marathon starts in the high teens Celsius and warms steadily through the morning. By the back half of the race, mid-pack runners are often in 24 to 28 degree heat with high humidity. Sweat rates climb. Sodium losses climb. Drinking only water becomes more dangerous than in colder races.
The practical implication: Indian race-day hydration is closer to a tropical-marathon protocol than a temperate-marathon protocol. You need more sodium per hour than the European literature implies. You also need to manage fluid volume against a stomach that is being slowed by heat.
How much sodium per hour, actually
The IOC consensus and follow-up reviews recommend 300 to 700 mg of sodium per hour during prolonged endurance exercise in heat, scaled to individual sweat-sodium concentration. Most Indian recreational marathoners running between four and five-and-a-half hours are well-served by 400 to 600 mg of sodium per hour from race fluids and gels combined.
You do not need to test sweat sodium in a lab. You need to test your race fluid plan in training. A heavy salt-sweater — visible white residue on a black shirt, persistent post-run cramping — needs the upper end. A clean-sweater needs the lower end.
A protocol for race day
This protocol has worked for me, my running friends and the runners I have coached through their first Indian marathon. It is not the only answer. It is a defensible starting point.
First 10 km
Sip water at every other aid station — roughly every 5 km. Skip the electrolyte drink at this stage. Your fluid losses are still modest, your stomach is fresh, and most runners have started with a reasonable hydration baseline if breakfast was sensible.
The exception: if you started the race already feeling thirsty or if the temperature at start is over 22 degrees, begin electrolyte at km 5.
10 km to 30 km
This is the long middle. Alternate water and electrolyte at successive stations. Take a gel every 45 minutes to an hour, washed down with a small volume of water from the same station. See our fuel primer for tested gel options that work in Indian heat.
The single biggest mistake at this stage is taking only water for too long. By km 25 in 26-degree humid weather, you have lost enough sodium that water alone risks dilution. Switch to alternating early.
30 km onward
From km 30 onward, electrolyte becomes more important than water. Two reasons: cumulative sodium loss is now significant, and gastric emptying is slowing. Concentrated electrolyte in small volumes is absorbed better than large volumes of plain water at this stage.
Practical rule: take electrolyte at every aid station from km 30, water only if you specifically want to wash down a gel. Sip, do not chug. Walk through aid stations if you must — a five-second walk for proper hydration buys five minutes of stable pace in the final ten kilometres.
The mistakes I see at every Indian marathon
I run two or three Indian marathons a year and crew at others. The hydration mistakes are predictable and avoidable.
Skipping aid stations to chase pace
The five seconds saved by running through a station compounds into thirty seconds lost per kilometre after km 30 when you cramp or your stomach revolts. Every coach I respect walks through aid stations in the back half of a marathon. Walk, sip, swallow, jog.
Untested gels and untested salt
Race day is not the day to discover that a particular sodium tablet makes you nauseous, or that the on-course electrolyte at TMM is too sweet for your stomach. Run at least three of your long runs with the exact fuelling plan you intend to use on race morning. Use our calculators to estimate fluid loss per hour at your goal pace and rehearse accordingly.
Overdrinking 'just to be safe'
The 2007 hyponatremia incidents in international marathons that prompted the Noakes review were almost universally in slower runners who drank ahead of thirst. Do not pre-empt. Trust the signal. Hydration is a feedback loop, not a calendar.
How to train your fuelling
Race-day fuelling is a trainable skill. The stomach adapts to volume and concentration with repeated exposure. A 2017 paper by Costa and colleagues in Sports Medicine reviewed gastrointestinal training in athletes and concluded that progressive carbohydrate and fluid loading during long runs improves tolerance and absorption.
The practical version: in the final eight weeks of marathon training, every long run should rehearse race-day fluid and electrolyte volumes at race-day intensity. Carry the same brand of gels, use the same bottles, drink the same volumes. See our nutrition hub for the foundations.
What to do if it goes wrong mid-race
You will sometimes get it wrong. Most cramps in the back half of an Indian marathon are dehydration-and-sodium driven, not muscular. If you cramp around km 32, slow down, walk to the next station, take two cups of electrolyte and a salt tab if you carry one. Wait two minutes. Most cramps will release. Resume at a slower pace.
If you are dizzy, nauseous or stop sweating, that is a medical situation. Walk to the next aid station and ask for help. The day is over. Your next marathon is not.
Your next step
Pick a fuelling plan. Test it three times before race day. Use our plan generator to build a final block that includes race-pace long runs at race-morning timing, and read more across Running Lab for the rest of the picture. The aid station decision is not made at the aid station. It is made eight weeks before, in training.