I thought a couple of notes transcribed from the Oxford Handbook of Expedition and Wilderness Medicine 2008 may be of interest.
(It should be understood that these are guidelines for expeditions, which some of us undertake, but I feel the principles apply equally to day or overnight hikes or trips, hence the post.)
P105 (Dietary Requirements - Water and Electrolytes)
While energy imbalances can be tolerated for days or weeks, water and electrolyte imbalances are poorly tolerated. Drinking large volumes of plain water for even a single day in a high humidity/high exercise situation can result in catastrophic hyponatraemia with muscle cramps, confusion, coma, and even death. In hot climates, additional electrolytes should be added to water to optimize rehydration (see Chapter 23).
Thirst is a poor indicator of hydration; typically thirst indicates more than 2% loss of body fluids and this corresponds to a 10% loss of exercise performance.
[My highlighting.]
Raised resting heart rate, or a higher than normal heart rate for a given exercise load, can indicate dehydration.
In cold dry climates dehydration can develop in the absence of obvious sweating or thirst, and manifests itself when returning to a warmed environment at the end of a day (see Chapter 19).
Fluid should be consumed before exercise (approximately 0.5 l), during (from 20 min into load and at a rate of 500-750ml/h), and after (until passing clear urine).
P106 (Water Purification)
Adults in temperate conditions need to drink about 3 litres a day, but intake can rise to as much as 15 litres per day in hot climates; a further 4 litres of clean water per person per day will be needed for cooking and washing up.
P586 (Cold Climates - Bases and Campsites)
Glacier outwash streams contain fine highly abrasive rock dust in suspension; this is a powerful laxative. If in doubt, filter water and then boil or sterilize it (p. 106).
P696 (Hot environments - deserts and tropical forests)
Fluids must be drunk before, during, and after exercise in a hot environment. Dehydration by as little as 1% affects heat toleance and thermoregulation. Acute mild dehydration (2-3% of body weight) significantly impairs exercise tolerance (overcoming any advantage conferred by acclimatisation), but does not initiate thirst. As dehydration progresses, cognitive function deteriorates and both thermoregulation and physical capacity become seriously compromised. A level of 6% dehydration is incompatible with furthe functioning in a hot environment. Even when a person is significantly dehydrated, urine is still produced and the volume of fluid required to return to full hydration must be at least 1.5 times that lost in sweat (assuming the individual was fully hydrated before the onset of activity). Women have a lower proportion of water in their bodies and may be at greater risk of dehydration than men.
Thirst is only stimulated when more than 2-3% dehydration has occurred. If an individual drinks only enough to satisfy their thirst they will be chronically dehydrated, particularly if they drink substantial amounts of caffeine-containing drinks, which act as diuretics. It is essential that personnel working hard in any environment are made aware of the need to drink water despite not feeling thirsty.
Thirsty = Dehydrated. Dehydrated does not = thirsty.
Hydration can be monitored by the colour and quantity of urine along with how often one needs to pass urine. Dark yellow urine is a sure indicator that the individual is dehydrated, as is the need to urinate less than twice a day.
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Hope they're of use...
