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.

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.

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.

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