Hunger, Satiety, and Thirst: Part 3
This is the last part of my three part series on hunger, satiety, and thirst. I'm going to focus on drinking water, because I think a conversation involving the consumption of coffee, alcohol, juice, etc. could become complex really quickly.
There are two types of drinking: homeostatic (the kind that addresses fluid loss or seeks to maintain the concentration of the blood) and non-homeostatic (the kind that doesn't). Homeostasis is simply an organism's need to keep all physiological processes in balance. Our bodies like to have a certain amount of fluids at all time for normal function. Sometimes that can change, like when we're exercising or during pregnancy. (1 p34)
Something to address right away: dry mouth isn't necessarily caused by thirst. It can signal thirst, but not always. Sure, decreased salivation is due to decreased hydration status. Most people have experienced this. However, simply wetting your mouth isn't going to shut down signals of thirst. Water still needs to reach the gastrointestinal tract in sufficient quantities (1 p37-39).
A great example of homeostatic drinking is the water intake that occurs with a meal. I had to track my fluid intake with meals earlier this term, and I was surprised to see that most of my water intake occurred with meals. That is totally normal for all people and animals. Similarly, the less food consumed results in less water consumed. We think this is due to an inherent need to balance the amount of food to water in the gastrointestinal tract. But water consumed before a meal is non-homeostatic, anticipating the need for water after a meal (1 p34-36).
Another example of homeostatic drinking is the need to restore blood volume or alter the concentration (number of particles per volume of fluid) of solutes (such as salt or glucose) in the blood. If there is a lot of blood loss, say from a traumatic accident, medical staff would restore blood volume with IV fluids (1 p35). Now, if there is a lot of glucose in the blood, as is the case in type 2 diabetes, the body may try to correct the blood concentration by stimulating thirst. Polydipsia, or greatly increased thirst, can be a symptom of type 2 diabetes (2).
I know I feel really thirsty after I eat a lot of a salty food, like french fries or potato chips. Water follows salt, so if there is a lot of salt in the blood stream, water will be pulled out of the body’s cells and into the blood stream. It’s hard to be hydrated while consuming foods with a lot of salt (3). This is a mechanism for increasing blood pressure, which is why people who have high blood pressure are advised to eat a diet low in salt.
Angiotensin is a chemical produced by a reaction cascade that begins in the kidneys and ends in the blood stream. It is heavily involved in homeostatic drinking. Angiotensin does three things: increases blood pressure by constricting blood vessels, helps the body hold onto sodium, and communicates with the brain to release more antidiuretic hormone (ADH). ADH results in the kidneys retaining more water. Not only does angiotensin ask the brain to release more ADH, but it asks the brain to signal thirst so the person will go drink some water (1 p40-42). Certain blood pressure medications (ex: ACE inhibitors) interfere with the reaction that creates angiotensin in an effort to lower blood pressure.
A decrease in blood glucose, like the kind that happens after insulin is released in anticipation of eating, can stimulate thirst too (1 p45). This is considered a form of non-homeostatic drinking. Anticipatory drinking in general is non-homeostatic. Hydrating before a workout is another good example.
Other forms of anticipatory drinking have been observed in specific people groups. People groups who live in deserts and travel long distances without carrying water too will drink as much as possible at a water source before continuing to travel, anticipating that water will be scarce (1 p36). Whether or not someone has access to a bathroom may influence fluid intake too. I have heard of this happening in busy, demanding workplaces that expect employees to work without breaks (3). Work breaks are important (and often mandatory anyway).
Thirst can be experienced differently too in specific people groups. For whatever reason, elderly people are not as consistent about replacing water lost. This can lead to higher concentrations of their prescription medications in their bodies and altered mental status (difficulty making decisions, confusion, memory problems, etc.) (1 p43). Please do not go home and start telling your grandparent or other family member to start drinking more water. There are certain medical conditions in which drinking more fluids can be dangerous. To be safe, please start by talking to a physician.
As to how best to quench thirst, cold water does a better job...but only for a few hours. To really quench thirst beyond that, it is the amount of water consumed that matters most (1 p39).
I hope you enjoyed my series and gain insight on what makes you feel hungry, full, and thirsty. If you're interested in learning more about why people choose to eat what they eat and when they eat it, The Psychology of Eating and Drinking by Dr. Alexandra Logue is written to be accessible to anyone and is a fascinating read.
As usual, if you have concerns about your diet and eating habits, please consult with a licensed healthcare provider first.
1. Logue A. Chapter 3: "You Never Miss the Water Till the Well Runs Dry." In: The Psychology of Eating and Drinking. 4th Edition. New York, NY. Routledge; 2015: 33-44.
2. McCulloch D. Clinical presentation and diagnosis of diabetes mellitus in adults. In: UpToDate. Mulder J (Ed), UpToDate, Waltham, WA. Accessed on February 14, 2018.
3. White W. Hunger, Satiety, Thirst. Psychology of Eating. January 2018.