While simply having a headache is not an indication of dehydration it is one of many inital symptons. In the UK, you should drink about 1.2 litres (six to eight glasses) of fluid every day to prevent dehydration.(Climate reference) NHS.uk

Headache

A headache is a pain in the head with the pain being above the eyes or the ears, behind the head (occipital), or in the back of the upper neck. Headache, like chest pain or back ache, has many causes.

All headaches are considered primary headaches or secondary headaches. Primary headaches are not associated with other diseases. Examples of primary headaches are migraine headaches, tension headaches, and cluster headaches. Secondary headaches are caused by other diseases. The associated disease may be minor or major.

Tension headaches are the most common type of primary headache. As many as 90% of adults have tension headaches. Tension headaches are more common among women than men.

Migraine headaches are the second most common type of primary headache. An estimated 28 million people in the US have migraine headaches. Migraine headaches affect children as well as adults. Before puberty, boys and girls are affected equally by migraine headaches, but after puberty more women than men have them. Migraine often goes undiagnosed or is misdiagnosed as tension or sinus headaches.

Cluster headaches are a rare but important type of primary headache, affecting mainly men. The average age of cluster headache sufferers is 28-30 years, although headaches may begin in childhood.

Secondary headaches may result from innumerable conditions, ranging from life threatening ones such as brain tumors, strokes, meningitis, and subarachnoid hemorrhages to less serious but common conditions such as withdrawal from caffeine and discontinuation of analgesics (pain killing medication). Many people suffer from "mixed" headache disorders in which tension headaches or secondary headaches may trigger migraine.

Reference Fauci, Anthony S., et al. Harrison's Principles of Internal Medicine. 17th ed. United States: McGraw-Hill Professional, 2008.

Dehydration

"Dehydration occurs when the normal water content of your body is reduced, upsetting the delicate balance of minerals (salts and sugar) in your body fluid. If you are thirsty, you're already likely to be suffering from the effects of dehydration. The signs of dehydration include dry mouth, lips and eyes, clammy hands and feet, headaches, light-headedness, and concentrated, dark urine with a strong odour." NHS

Dehydration occurs when there is a 1% or greater reduction in body weight due to fluid loss. Depending on the percentage of body weight lost, dehydration can be described as mild, moderate or severe.

Mild to moderate dehydration

The first sign of dehydration is thirst. Other symptoms are:

• dizziness or light-headedness,
• headache,
• tiredness,
• dry mouth, lips and eyes,
• dark, concentrated urine, and
• passing only small amounts of urine, infrequently (fewer than three or four times a day).

Moderate dehydration (a 3-5% decrease in body weight due to fluid loss) causes you to lose strength and stamina and is the primary cause of heat exhaustion.

If dehydration is chronic (ongoing) it can affect kidney function and may lead to the development of kidney stones. It can also cause:

• harm to your liver, joints and muscles,
• cholesterol problems, and
• constipation.

Severe dehydration

Severe dehydration is a decrease of more than 5% of body weight due to fluid loss.

As well as severe thirst, you may also experience:

• dry, wrinkled-looking skin,
• an inability to urinate,
• irritability,
• sunken eyes,
• low blood pressure (hypotension),
• a weak pulse,
• a rapid heartbeat,
• cool hands and feet,
• seizures, and
• a low level of consciousness.

If body weight is reduced by more than 10%, this is extremely serious. If not treated immediately this level of dehydration can lead to death, as the blood stops circulating. You may need to go to hospital and be put on a drip to restore the substantial loss of fluids.

The International Life Sciences Institute published an excellent monograph, Hydration: Fluids for Life, which states: Children, and especially infants and toddlers, are at greater risk of dehydration than adults. Dehydration in children is not only serious, but can be life-threatening.

"The importance of consuming adequate fluids to avoid has been aggressively communicated to physically active people. ... Central nervous system (CNS) symptoms include headache, fatigue, anorexia, lethargy, confusion, disorientation, hyper-irritability, nausea, vomiting, seizures, and coma." -The International Life Sciences Institute - Hydration: Fluids for Life