Hydration is easy to ignore on a trail run until it becomes a problem. By the time you feel seriously thirsty, you are already dehydrated, and your pace, focus and enjoyment have all suffered. Long trail runs, especially in heat or at altitude, demand a hydration plan, not just a bottle grabbed on the way out the door.

Sweat rates vary enormously between runners, from half a litre per hour to over two litres in hot conditions. A useful starting point is to drink to thirst during shorter runs, and to plan more carefully for runs over ninety minutes. Weighing yourself before and after a run is a simple way to learn your own sweat rate: each kilogram lost is roughly a litre of fluid.

A common mistake is drinking far too much water, which can dilute blood sodium and cause hyponatremia, a serious condition. The safest approach is to drink regularly in small amounts rather than gulping large volumes at once, and to include electrolytes on longer runs. Your urine colour is a rough guide: pale yellow is about right, dark yellow means drink more, and completely clear may mean you are overdoing it.

For runs under an hour, you may not need to carry anything, but for longer trail runs, a hydration vest or belt is essential. Soft flasks in a vest distribute weight evenly and let you sip while moving. A handheld bottle works for shorter runs but can become tiring over several hours.

Plan your route around water sources where possible. Many trails pass streams, fountains or villages where you can refill, but always carry a lightweight filter or purification tablets if you plan to drink from natural sources. Know your route's water points before you leave, and carry more than you think you need on hot days or unfamiliar trails.

Water alone is not enough on long runs, because sweat contains sodium and other minerals that need replacing. Electrolyte tablets or sports drinks help maintain the balance, and most runners need them on runs longer than two hours, or sooner in hot weather. Salt capsules are another option, but start with a small dose to see how your stomach handles them.

Food and hydration work together: eating small amounts of carbohydrate regularly helps your body absorb fluid and maintain energy. In hot weather, slow down, seek shade where possible, and consider a hat and light-coloured clothing. If you develop a headache, nausea, confusion or stop sweating, stop running and seek help, because those are signs of serious heat illness.

It is also smart to practise your hydration strategy in training rather than trying something new on race day. Test your vest or belt on a long run, learn how much you actually drink per hour, and find out which electrolyte products agree with your stomach. Some runners discover that a particular flavour or concentration causes nausea, and it is far better to learn that on a training run near home than in the middle of a race. Keep a simple note of what worked after each long run, including the weather, the distance, and how much you drank, because that record becomes your personal hydration plan. Over time, you will know exactly what your body needs in different conditions.

Staying hydrated on long trail runs means planning ahead: learn your sweat rate, carry enough water and electrolytes, drink in small regular sips, and plan your route around refill points. Your body will thank you at kilometre thirty.

Practice your hydration plan on training runs, never on race day. A strategy that works for two hours may fail at four, when salt losses and stomach tolerance both change. Try different drink mixes and sip schedules in training so race day holds no surprises. Your gut, like your legs, needs training, and a rehearsed plan is one less worry.