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