For decades the advice was simple and loud: drink before you are thirsty, drink as much as you can, a clear urine is the goal. It sounded prudent, and it has sent people to hospital. Since the late 1990s sports medicine has documented a steady stream of endurance athletes and hikers with dangerously low blood sodium, a condition called exercise-associated hyponatraemia, caused not by too little water but by too much. The modern position is more nuanced and, happily, easier to follow. This article sets out the numbers behind it.
How much a hiker actually sweats
Sweat rate is the starting point, and it varies more between people than most guidance admits. Laboratory and field studies of walkers and runners report rates from about 0.3 L per hour on a cool day at an easy pace to 2 L per hour or more for a large person climbing hard in heat. A typical hiker on a warm summer day with a pack sits somewhere around 0.7 to 1.2 L per hour. Altitude, humidity, clothing, fitness and simple body size all move the number.
The useful thing is that your own rate is easy to measure. Weigh yourself without clothes before and after a two-hour walk on which you drink a known amount and do not urinate. One kilogram of weight lost equals one litre of sweat. Add back what you drank, divide by the hours, and you have your rate for those conditions. Most people who do this are surprised, in one direction or the other, and the surprise is worth knowing before a long day.
What dehydration does, and does not do
Losing water reduces blood volume, raises heart rate for the same effort, and makes the body worse at shedding heat. The research consensus is that performance in the heat begins to decline measurably at a fluid deficit of about 2 percent of body mass, which is 1.4 L for a 70 kg (154 lb) person. Below that threshold the effects are small and, for a walk rather than a race, largely irrelevant. Mild dehydration is not dangerous in itself; the real risks are heat illness, of which dehydration is one contributor, and the poor judgement that comes with a headache at hour six.
What dehydration does not do is cause muscle cramps on its own, despite the folk belief. The evidence points more strongly to muscle fatigue as the primary cause of exercise cramp, with salt loss a contributing factor in some heavy sweaters. Drinking more plain water does not fix cramp and, past a certain point, makes the sodium problem worse.
Sodium: the part of sweat that matters
Sweat is not pure water. Its sodium concentration ranges from roughly 200 to 2,000 mg per litre, with an average close to 900 to 1,000 mg. The “salty sweater”, whose shirt dries with white lines, is at the top of that range and loses several grams of sodium over a long hot day. Replacing lost water without replacing lost sodium dilutes the blood. The kidneys can excrete only about 0.8 to 1 L of water an hour at best, and less during exercise because the hormones that govern fluid retention are switched on by effort. Drink faster than that for several hours and the surplus stays in the body, blood sodium falls, and cells, including brain cells, begin to swell.
The early symptoms of hyponatraemia, nausea, headache, bloating, confusion, are almost identical to the symptoms of dehydration, which is precisely why the “drink more” reflex is dangerous. A hiker who has been drinking steadily all day, has gained weight or not lost any, and feels unwell, does not need more water. In severe cases the condition causes seizures and death, and it has done so in walkers on ordinary summer trails.
The rule that actually works: drink to thirst
Position statements from sports medicine bodies now recommend drinking according to thirst for most endurance activity, and this is the guidance that suits hiking best. Thirst is a well-tuned signal that starts to register at a deficit of around 1 to 2 percent, which is exactly where the performance research says it starts to matter. Drinking to thirst keeps most people in a comfortable band without any risk of overdoing it. The exceptions are the very old, whose thirst is blunted, and those on very long days in extreme heat, who can benefit from a modest planned schedule. For everyone else the instruction is to carry water where it can be reached without stopping, drink when thirsty, and stop when not.
- Carry: 0.5 to 1 L per hour between refills, adjusted to your measured sweat rate and the heat.
- Drink: to thirst, in small amounts often, not a litre at each break.
- Salt: from food on any day over three hours, or from an electrolyte mix in one bottle on hot days.
- Check: pale-yellow urine is fine; clear urine all day suggests too much, dark amber too little.
Cold, altitude and the disappearing thirst
Two conditions defeat thirst. Cold air is dry, and every breath loses water vapour; cold also suppresses the thirst signal while the body sheds fluid through increased urination. Winter hikers routinely finish the day well over 2 percent down without ever having felt thirsty. Altitude compounds the problem: breathing is faster, air is drier, and appetite and thirst are both blunted above 2,500 m (8,200 ft). In both cases a light schedule replaces thirst as the guide: a few mouthfuls every half hour, warm if possible in winter, and a check of urine colour at breaks.
Weight, water and the planning arithmetic
Water is the heaviest thing most hikers carry: 1 L weighs 1 kg (2.2 lb). Carrying six litres for a dry ridge is a serious load; carrying six when the map shows a stream every hour is wasted effort. Planning hydration therefore means planning refills. Read the map for reliable water, remember that many streams marked in blue are dry by late summer in southern Europe and the western United States, and carry a treatment method of some kind so that a small stream is a usable resource rather than a gamble. Then the bottle can be sized for the gap between sources, not for the whole day.
The summary is calmer than the old slogan. Sweat rates are personal and measurable. Two percent down is where it starts to matter, and thirst gets you there. Salt matters on long hot days, and too much water is a real danger, not a theoretical one. Drink when you are thirsty, eat something salty, and keep the second bottle for the next dry stretch.