I’m train­ing for a long-​distance race this sum­mer. After watch­ing London mara­thon run­ners suf­fer­ing from dehyd­ra­tion on tele­vi­sion, I’m con­cerned that I’ll put my health at risk while exer­cising when it’s hot. What can I do to make sure this doesn’t hap­pen?

Don’t be alarmed. More than 35,500 run­ners suc­cess­fully com­plete the London mara­thon with no abnor­mal after-​effects. As well as being pretty resi­li­ent, our bod­ies are remark­ably adapt­able, provid­ing we encour­age them in the right kind of way.

Above all, this means sound pre­par­a­tion. Hopefully you are fol­low­ing a train­ing sched­ule that gradu­ally increases your run­ning dis­tances. But it is a good idea, also, to do some strength and speed train­ing so that your car­di­ovas­cu­lar sys­tem and muscles are work­ing at top effi­ciency. The more well-​tuned your body, the less likely you are to over­heat dur­ing the race. Sound pre­par­a­tion also means good nutri­tion, good sleep and rest days built in to your sched­ule.

Many long-​distance races are run at cooler times of the day. But if you know you are going to be run­ning in high heat or – equally import­ant – high humid­ity, you should try to do some train­ing in sim­ilar con­di­tions. Sitting in very hot baths or jog­ging in a wet suit are poor sub­sti­tutes!

Perhaps the most con­fus­ing issue is how to stay prop­erly hydrated dur­ing your race. The “right amount” of fluid depends on many vari­ables, includ­ing your weight, run­ning speed, sweat rate, fit­ness level, heat and humid­ity on the day, how much breeze there is and so on. Official advice says you should drink as much as you can com­fort­ably tol­er­ate. That means don’t upset your stom­ach: if you develop symp­toms such as nausea, stom­ach cramps or diarrhea dur­ing the race, do not ignore them.

If your race lasts one hour or less and you are not run­ning at élite pace, the chances of get­ting ser­i­ously dehyd­rated are min­imal. Just ensure you are fully hydrated before­hand, and take reg­u­lar sips of water at the drinks sta­tions. For longer races, the guidelines sug­gest you drink just over half– to just over one litre (600 to 1,200ml) per hour. And you should def­in­itely include some car­bo­hydrate and nutri­ent replace­ments in your drink. An “iso­tonic” sports drink is very use­ful for this.

If you want to be a bit more sci­entific in your approach, weigh your­self imme­di­ately before and after a long run. The dif­fer­ence in your body weight will be fluid lost through sweat: if you lose 0.5kg dur­ing a one-​hour run, this equates to half a litre. So that’s how much you’ll need to drink per hour.

With good pre­par­a­tion, you have noth­ing to fear. Remember people run ultra-​marathons in the Sahara and sur­vive just fine! The bot­tom line is, listen to your body on the day, and don’t be too proud to slow down or quit if some­thing feels wrong. There’s always another race to enter on another day…