Why is it that it takes few seconds for our body to decide that it’s had enough water not to be thirsty, but several minutes to recognize that no more food is needed cuz it’s not hungry?



Why is it that it takes few seconds for our body to decide that it’s had enough water not to be thirsty, but several minutes to recognize that no more food is needed cuz it’s not hungry?

In: Biology

Sensors in your mouth, back of the throat and oesophagus can detect how much water has been consumed and report this information to the brain. Fairly shortly after drinking water, the bode will start to behave as if that water is already in the bloodstream, even if it is still somewhere in the digestive tract.

Food is a completely different story. The brain relies on stretch receptors in the stomach, hormones produced by the process of digestion, and sometimes chemicals generated by the bacteria living in the intestines. This is a much more complicated process that takes longer, which leads to a lag between actually being full, and the feeling of fullness.

Feeling full is complicated and one of the reasons there’s no magic pill for weight loss.

Scientists are studying a variety of ways the body signals when it feels full. For example, one is called the “Ileal Brake.” Basically, there are sensors/receptors in your ~~ileal~~ ileum (the final part of your small intestine). When food and nutrients, particularly fats, are detected here, your body reports that it “feels full.” If you think about it, it’s a nice evolutionary measure. We eat until we know fats and nutrients have not just filled our stomach, but our small intestine.

Because it takes time to get from your stomach to the end of your small intestine, you can often overeat before you feel full.

There are other mechanisms at play, but this is a good example of how our body detects it has eaten enough and how it can lead to delays and overeating.

Thirst and Hunger appear similar, but work very differently!
Regulating how much water and salts you need in your body is really determined by your kidneys. Sensors in your heart and kidney measure if you have enough water in your vessels, enough pressure in your circulation and what salts need adjusting. These processes take a long time and are NOT directly coupled to your brains desire to drink.
You get thirsty when your mouth is dry! In some cases it’s because you are dehydrated and aren’t producing enough saliva. Most of the time it’s breathing through your mouth or dry heated air in winter.
Example 1: People who loose a lot of blood need more volume (water with salts) in their system. They do not get thirsty though.
Example 2: Patients with kidney failure have too much water in their bodies, but it is immensely difficult for them to drink less. Doctors need to rigorously explain to them, what amount of drinking is acceptable if you’re on dialysis.
Example 3: Palliative Care Patients who cannot swallow anymore (dysphagia) cannot drink or eat. Giving them i.v. fluids only prolongs death, it does not alleviate thirst however. That’s why these patients prefer coating their mouth with water/soda/beer because it’s delicious and stops thirst.

TL;DR Saturation is largely determined by the stretching of your stomach and complicated hormones. Thirst is usually a dry mouth

Edit: Spelling

Hormones have alot to do with hunger. Sated & starving dogs when given a blood transfusion to the opposing dog will display opposing characteristics. Starving dog will not eat, Sated dog will gorge and vomit.

For both hunger and thirst we have multiple signals that tell us whether we have ‘had enough’. For example, stomach dilation, salt or glucose levels in the bloodstream, etc. Unfortunately all of these have at least several minutes’ lag (for hydration) or even a hour’s lag (for glucose levels) and we often feel that we’ve had enough water or food before this happens. Psychologists believe therefore that the real reason we feel we’ve had enough is simply habit. We know roughly how much water to drink from previous experience, and how much food to eat. The reason you feel you’ve had enough water earlier that for food is simply that you have different drinking habits to eating habits. If you retrain yourself to stop eating after less food, you will feel full from food earlier.

How hungry you are is indicative of how soon you have to eat and not how much you have to eat

Early humans often had access to unlimited water, but rarely had access to unlimited food. You can also consume enough water within less than a minute, whereas eating takes a lot longer (or at least took longer, until we invented high-density foods like chocolate, energy bars etc.)

Thus there was an evolutionary need to quickly realize how much water to drink, but no pressure to realize the same for food. Overeating hasn’t been a problem until pretty recently.

Ive never experienced this. I feel full and don’t overeat unless I really want to. Maybe I’m just lucky.

Other people have answered it well enough but I want to throw out some notions of what ‘hunger’ is that may help people with weight gain problems.

Youve probably heard about hunger “being in the head” and well, thats pretty much true. Hunger is not exactly a “I need food” signal, its a “im currently not digesting food” signal. Just because you FEEL hungry doesnt mean you HAVE to eat. You may well be perfectly fine and fed, and overeating can definitely cause inconsistencies to worsen.

You can often ignore hunger and it will go away after some minutes, assuming you didnt actually need to eat. This is why its also healthier to eat less more often than the massive one off meals we normally eat, AND it helps lose weight because eating little bits keeps the hunger feeling away while potentially consuming less food overall.

The receptors for hunger are all over, stomach activity, intestine activity, even just the motion of eating all provide signals for the brain to say “im eating, or im fed” and the hunger dissipates. Its completely separate from “I need food im low on stuff”

But yes, as other said, with water its almost instant and is controlled mostly by the throat. And not particularly because you need water and have replenished it.

Ugh I’m exhibit A, I just ate too much corned beef because I thought I was hungry again after dinner but I should have just drank some water and went to bed.

People who are aware of how their own bodies actually feel can normally tell when they’re not hungry anymore really quickly or even when they feel they’re getting full. I’ve never thought, “damn I overate, I should’ve just stopped 5 minutes ago”… I can normally very easily tell when I am going to be full soon. I eat because I’m hungry and because I have to, not because I want to finish a whole dish or because it tastes good.

Biochemist here with speciality in hormones. When you are hungry your gut releases a hormone known as ghrelin which signals that it’s time to eat. When you start eating, leptin, a hormone that is produced by fat, checks your fat supply and if it is sufficient (so that you don’t die of starvation), is released from your fat cells to act on your brain to tell you to stop eating and to induce a feeling of being full. This usually takes around 20 minutes for it to enter the bloodstream and act on your brain. So I always advise people if they want to have seconds, wait 20 minutes, and if you are still hungry after then you good to go, otherwise you may just overeat. Leptin not functioning properly is linked to diseases such as type 2 diabetes etc. so it’s a pretty important hormone. Hope that helps!

Wait, what? If I feel “not hungry” after eating food just like “not thirsty” after drinking water.

Mine work the same way. I know immediately when I’ve eaten enough. Never understood how anyone can be unaware of this.

It’s not true.

Have you never taken a bite of food and then though, “OK that’s it I’m full”? That’s it, it happens instantly.

I think the confusing factor is that water isn’t “delicious” while food can be, also water doesn’t come in portions while food does, and so we can easily decide to eat more food because it’s yummy or because it’s on the plate, so we wind up eating more than we needed to feel full.

And I think a lot of people have learned to just completely ignore their internal feeling of fullness, because their parents insisted “finish your plate!” or that last chicken wing just looks really good or whatever.

But if you pay attention, your body tells you *exactly* when you’re full, to the bite, at the moment. Just like water.