Your body may be thirsty without telling you - Thirst and (De)hydration

a large, blue water bottle with regular time intervals marked with encouraging text about drinking water. They read, “7AM: rise and shine! / 10 AM: drink water! / 1PM: stay hydrated! / 4PM: keep it going! 7PM: don’t stop now! / 10 PM: great job!”

The brain alerts us that the body needs fluids via thirst.

Once again the hypothalamus plays an important role as the body’s primary “thirst center.” It does this using sensors that monitor the concentration of sodium and other substances in the blood and input from sensors in the blood vessels that monitor blood volume and pressure. If blood volume or pressure becomes too low, (such as from excessive loss of fluid due to sweat) the hypothalamus tells us to drink.

Age, a stroke, Parkinson’s disease, Alzheimer’s disease, diabetes, and several other conditions can dull the brain’s response to dehydration. As a result older people, disabled or not, may be less aware they are dehydrated and need to drink more fluids. This can also happen with certain conditions. As well, some people who’ve had aneurysms have lost their sense of thirst entirely. It is also worth remembering that heat has cognitive impacts and you might experience less than your usual level of executive functioning whatever that is.

When the body is low on fluid the hypothalamus also increases the creation of a hormone called vasopressin, which is secreted by the pituitary gland and travels to the kidneys. Vasopressin is an antidiuretic, meaning it signals for the body to hold onto and use the fluid it has. It causes water to be reabsorbed from the urine, which reduces fluid loss and conserves water until more fluids are consumed.

Tip: Mark a large jug with times of the day to make certain you drink throughout the day. You can also set alarms or purchase water bottles that are pre-marked or have alarms on them reminding you to drink. Water is by far the best hydrator, in part because sugary beverages are more work for your kidneys. Some research suggests that there can be kidney damage from rehydrating with soda depending on the level of dehydration.

The US National Academies of Sciences, Engineering, and Medicine estimates that men need 15.5 cups of fluid a day and women need 11.5 cups but this amount will vary depending on the amount of exertion and the amount you sweat. If your thirst is impaired you may need to look for other early warning signs of dehydration.

Additional Reading

The Neuroscience of Thirst.
Real Time Signals from Body to Brain Help Regulate Thirst
The Neural Regulation of Thirst

Dehydration

“Between about 55% to about 78% of your body is made of water. Newborn babies are about 78% water; a year-old baby is 65%; adult men are about 60%; and adult women are about 55%. Your brain is made up of 73% water, and so is your heart. Your bones are 31% water, muscles and kidneys are 79% and your skin is 64%. A whopping 83% of water makes up your lungs. Between about 55% to about 78% of your body is made of water. Newborn babies are about 78% water, a year-old baby is 65%, adult men are about 60% and adult women are about 55%. Your brain is made up of 73% water, and so is your heart. Your bones are 31% water, muscles and kidneys are 79% and your skin is 64%. A whopping 83% of water makes up your lungs.” Source

Dehydration is serious and can be life-threatening. It can occur more rapidly in the heat so being conscious of consuming fluids is of vital importance.

Dehydration can lead to electrolyte imbalance. Electrolytes are the bike couriers for cells, delivering messages from one to another. An imbalance can lead to involuntary muscle contractions, seizures and loss of consciousness.

Even non-life-threatening dehydration can lead to infections and kidney stones and put added stress on your body which can lead to worse acute and chronic health outcomes.

Signs of dehydration can include:

headache
change in mood (more anxious, grumpy)
memory or feeling confused
change in level of tiredness/fatigue
faster heart rate; loss of appetite (though possibly craving sugar)
dark-coloured or decreased urine

Diabetes can cause the body to lose water more quickly and complications often associated with the condition can alter blood vessels and the ability to sweat.

People with swallowing difficulties (dysphagia) can also face the challenge of maintaining sufficient fluids.

And a reminder that any condition that causes vomiting, diarrhea or greater than typical urination, can lead to dehydration with or without the added demands of heat.