These basics will get you down the road. To run at your best, and to feel your best, you’ll need to know a bit more. First, some background. With every step, with every breath, you lose moisture to the air around you. You lose some water through respiration, with each exhale. You lose some through passive perspiration, which is just water evaporating from your skin, maintaining a moist microclimate. You lose by far the most through active perspiration, which is how your body uses evaporative heat loss to cool itself.

The hotter the weather, the drier the air, and the harder you run, the faster you lose water. When your body’s hydration level drops just a few percent below normal, your performance will suffer. A few percent farther, and you’ll start to experience dry mouth, fatigue, and reduced motivation. Your mood can drop to the point where people have compared moderate dehydration with symptoms of clinical depression. When hydration levels drop farther still, you may experience dizziness, confusion, and even collapse-at which point it’s considered a medical emergency.

Notice that we didn’t list thirst as a symptom. You may experience severe thirst. Or just moderate thirst. Or none at all. It’s an unreliable indicator, which is why you must actively think about water consumption.

Ok, so you need water. But how much, and when? It’s not such a simple answer, because water can only drain from your stomach at a finite rate. If you overload, you risk stomach upset and possibly abdominal cramps. Also, you’re not a camel; your body can only store a modest amount of surplus water. If you try to drink a marathon’s worth at the start line, you’ll spend as much time in the first hour peeing as running-after which, you’ll be no better hydrated than when you started.

You should drink around a pint of water during the hour before the race. Don’t down it all right before the start, or you’ll overwhelm your stomach. Drink small portions and give yourself time to absorb it. After you’ve been running a half hour or so, you’ll have to start drinking on the go. The average stomach can absorb around 3/4 quart in an hour. This translates into around 6oz every 15 minutes-a pace that will work for most people and keep you about as hydrated as possible.

You’ll have to test this during training. Everyone is going to be a little different. You may be able to tolerate more water, or larger, more infrequent drinks, or you may find the opposite. Please don’t do your experimenting during the big race.

In a 5K, your pre-race water may be enough to get you through. In a marathon, you’re going to have to drink from start to finish.

Longer races will also require electrolyte replenishment, and carbohydrates for fuel. This generally means a sports drink or energy gels. These fuel sources complicate matters, because they further slow emptying of your stomach. It’s especially critical that you experiment with these during training. It can take a fair amount of trial and error, and even multiple brands, before you find a regimen that satisfies your caloric needs without giving you run-stopping bellyache.

Here are some tips: most sports drinks, especially the ones sold at the supermarket, are too concentrated and too sweet. You’ll probably have to dilute them at least 50/50 with water create a concentration amenable to your stomach. Gels need to be hydrated as you run, which means: slurp some gel, and then follow with several gulps of pure water (not energy drink). Never try to swallow gels if you’re out of water-it will be counter-productive. Your body will have to divert water to your stomach in order to dilute the gel and pass it to your intestines. This means the gel will take a long time to reach your blood stream, your hydration level will temporarily decline, and your stomach will hurt. Find some water, then enjoy the gel.

If you visit online discussions among endurance athletes, you’ll find much of the talk surrounding gels and energy drinks is about what brands sit best in the stomach. And you’ll see a wide variety of opinions, which means this is quite personal. All the more reason to do your experimenting during training. Don’t go looking for unpleasant surprises on race day.

Be aware that during longer events, especially ultra-length, you’re not going to be able to keep up with your body’s caloric needs. The more fit you are, and the faster you run, more your muscles’ appetite for glucose is going to outpace your stomach’s ability to deliver it. So you’re going to be relying quite a bit on your body’s limited stored carbohydrates, and also on body fat. And during hot weather, you probably won’t be able to keep pace with your body’s water loss. Which means that even if you do everything right, you may still finish the race a bit dehydrated.

So, rehydration needs to be part of your post-race recovery. Once again, don’t try to do it one epic gulp. Keep drinking steadily over the hours after the race. A cold one at the pub counts, but be sure to also drink water, energy drinks, and if possible, a smoothie with some protein and electrolytes.

Take time to check out the various hydration solutions available to runners, including hip packs, hand-held flasks, and even hydration vests. The best solution for you will depend on the length of your runs and the distance between watering opportunities. 

A final word of warning: despite all that we’ve said, it’s actually possible to drink too much. The dangers are surprisingly severe. If you drink so much water that your body’s electrolyte balance gets thrown out of whack, you develop a condition called hyponatremia, which comes on quickly and is far more dangerous than dehydration. A handful of runners die every year from this condition.

Hyponatremia almost always afflicts amateur runners in ultra-distance events. Here’s why: they are often running slowly enough that they’re not losing water quickly. And because they’ve been indoctrinated with the importance of hydration, they often stop at every rest stop and drink as much as they possibly can. Eventually, they become over-hydrated, and their blood electrolytes that regulate critical body processes become diluted, sweated out, or lost along with copious amounts of pee. Collapse comes on quickly.

Here’s how to spare yourself such an unpleasant defeat:

- Don’t try to drink more than 3/4 quart an hour.

- If your pace is very slow, don’t assume you have to drink a full 3/4 quart every hour. When you work at a lower intensity, you sweat at a lower rate. Especially if the weather is moderate.

- If the rest areas are stocked with pure water, fuel up on gels that include vital electrolytes. Or bring electrolyte supplements. As always, let your training experience guide you in using these.