If you are dehydrated, your chance of suffering heat illness increases greatly. Loss of water affects blood flow from the core to the skin, and the availability of water for sweating. The effects can start at as little as 3% loss of body weight.
There are 3 simple methods of keeping track of dehydration in the field:
The table below gives very simple guidance based on loss of body weight in any period of time, for example before and after a period of exercise, or from one day to the next.
Body weight measurements should ideally be made in a minimum of dry clothing.
|Guidelines on fluid loss [ dehydration ]|
|Weight loss up to||3%||2.1||kgs||5||lbs||take drinks to rehydrate|
|before undertaking exercise|
|Weight loss up to||6%||4.2||kgs||9||lbs||take drinks to rehydrate|
|reduce levels of exercise|
|Weight loss more than||7%||4.9||kgs||11||lbs||no exercise|
|consult a medical specialist|
|Average||female||50||kgs||same guidelines on % weight loss as male|
|Weight loss up to||3%||1.5||kgs||3||lbs||rehydrate before exercise|
|Weight loss up to||6%||3||kgs||7||lbs||rehydrate and reduce exercise|
|Weight loss more than||7%||3.5||kgs||8||lbs||no exercise - consult a specialist|
Work by Dr Lawrence Armstrong has helped show that the colour of urine can be a reasonable guideline to dehydration in the field. An eight point scale of colour has been established from very pale yellow  to brownish green  which indicates different levels of hydration/dehydration. The table below indicates the end points of the scale.
1-3 = well hydrated
7-8 = dehydrated, you should consume fluids
The colours shown here are not accurate, and have not been reproduced in full as the colour table is available in Dr Armstrong's book "Performing in Extreme Environments" which contains many useful extra details, as well as reproducing 2 key Position Statements by the American College of Sports Medicine on "Heat and Cold Illness" and "Fluid Replacement".
Though rough and ready, a quick way of checking your dehydration levels if no other tests are available, is to pinch the back of your hand and watch how quickly the skin restores itself to
its normal smoothness.
Make sure you have practiced this when fully hydrated so you know what normal return from a pinch looks like.
If you are dehydrated the skin on the back of your hand takes significantly longer to return to normal.