Dehydration occurs when there is more water going out or being used than is going in. Additionally, if you're drinking too fast (more than a litre an hour for an average adult male), you're not absorbing the water and so it doesn't count as going in.
Confusingly, dehydration can also be classified loss of water, loss of electrolytes or loss of both. To add further confusion, electrolyte imbalance can be too high or too low. In one sense the electrolytes can be lost through something like sweating, and in another, the concentration can be too high due to lack of water.
It's generally the electrolyte imbalance that causes you the problem and to the symptoms of dehydration are really the symptoms of electrolyte imbalance.
For completeness, the electrolytes we're talking about are:
Let's assume you've not got to the kidney failure end of matters, which causes electrolyte imbalance and that we're not accounting for medical conditions or medications. Instead stick to those which are associated with dehydration and the causes of dehydration.
Dehydration reduces the ability of the kidneys to remove excess calcium from the body. This will cause mild hypercalcemia and is likely to by asymptomatic. Equally, an increase in chloride can have no symptoms, but in some cases can present with thirst, difficulty breathing and weakness. And high levels of potassium can cause heart rhythm problems.
Low levels of magnesium can be caused by sweating and the symptoms are normally associated with severe dehydration. These include muscle weakness, shakes & twitches, nausea, vomiting, lower limb cramps and changes in blood pressure which if low could cause fainting.
You are unlikely to get a phosphate imbalance due to dehydration as far as my research goes.
Sodium is the annoying one as you can have both low sodium because you sweat it out and high sodium due to lack of free water. Symptoms of low sodium include headache, nausea, vomiting, confusion and fatigue. High sodium can cause increased thirst, confusion, lethargy and muscle twitches.
So, my original list of symptoms of dehydration are actually symptoms of electrolyte imbalance, but for the first one. This is because they are so tightly linked.
- Your urine will be darker.
- You'll get a headache.
- You'll get a bunch of symptoms like lethargy and confusion which others are more likely to notice.
- You'll feel thirsty.
Something to be aware of is that many of the above symptoms are also associated with thing that can happen outdoors such as heat stroke, hypothermia, sleep deprivation, malnutrition and low blood sugar.
So to answer the question about "needing salts", you'll be looking for:
In terms of solutions. An imbalance where the concentration is too high can be solved by drinking more water. Where too low, you should go for oral rehydration and eating something. Six teaspoons of sugar, half a teaspoon of salt and a litre of water should do it. Don't forget to drink that little and often over an hour.
PS for generation interest, the sugar is there to help the salt be absorbed through the small intestine and was considered one of the most important medical advances of the 1960s
Sources: experience, training, training materials, NHS, Healthline, Medicinet and May Clinic