Most of us are good at identifying when to eat, when we’re satisfied and when we’re thirsty. But it is more than us making that decision that determines this process. Our metabolic systems are really smart; because the brain, gastrointestinal tract and hormones are the ones responsible for reminding us when to eat, stop eating and when we’re thirsty. This is an overview on how this all takes place.
We have all experienced a moment when we haven’t eaten anything for hours. Then we come across the smell of food and immediately our mouth starts to salivate and then our stomach starts to feel hungry, right? Well, there is more than just the smelling of food, salivating, and stomach growling that makes us want food. It is our hormones that are at work, sending signals back and forth to the brain, stomach and small intestine. These hormones are called ghrelin, leptin, cholecystonkinin (CCK) and insulin.
When we’re hungry our stomach starts to growl. This causes the release of a hormone called ghrelin (think of ghrelin is growling). Ghrelin send a signal to our brain to let us know it’s time to eat. Ghrelin is one of the hormones that forms at night when we are sleeping. However, when people get little sleep their ghrelin hormone will be higher in the day time and as a result they will overeat.1 When we start to eat our stomach stretches and ghrelin slows down. However, the type of food depends how soon we will reach that level. A well-balanced meal with fiber, protein and fat will help us reach fullness sooner than a bowl of ice cream.2 In addition, to ghrelin there are other hormones that also play a role that influence when we stop eating.
On average, it usually takes about 20 minutes for our brain to receive the signal from other hormones that we are full. The sense of feeling full (making us stop eating) is referred to as “satiated” and the time between being full and up until the repeat of the cycle of eating again is called “satiety” (withhold from eating in between meals).3 These two stages are initiated by the hormones called leptin, CCK and insulin.
When we start to digest our food, it enters the small intestine and leptin and CCK start to send signals to the brain that the stomach is getting food. Leptin is secreted when fats are ingested, CCK with proteins and insulin with glucose. Leptin’s job is to send a signal to the brain (hypothalamus) to stop eating. The more fat you ingest the higher the leptin levels; so, appetite decreases because leptin decreases hunger. However, the more fat we have on our bodies the higher amounts of leptin. However, if there’s too much leptin in our cells our bodies can eventually develop a leptin resistance.4 Leading to a hormonal imbalance. The CCK hormone is activated to help break down the amino acids (proteins). This happens when amino acids reach the top of the small intestine during digestion. Thus, CCK sends a signal to the brain to stop eating as well.2 At the same time insulin is also activated.
The digestion of glucose (carbohydrates) activates the pancreas to starts secreting insulin. Insulin regulates the blood glucose. When insulin levels go up ghrelin hormones go down. All these hormones work together and help our us determine when we should stop eating. However, as I mention earlier the foods we eat will play a role in how long our satiety levels last. For example, protein and fat are more filling than carbohydrates because it takes longer for our stomach to break it down. Eating a meal in solid form will last longer than if we were to have it in liquid form because it stretches the stomach and activates these hormones.2 Foods with lower glycemic index will provide us with higher satiety and we’ll eat less of it.
Thirst is more than our mouth’s feeling parched. It is similar to hunger, in that there are hormones that communicate with the brain that influence our thirst. There are several factors that can initiate thirst. This is a brief overview of what makes us thirsty. One way thirst is activated is when we are dehydrated. The hypothalamus, in the brain, sends a message to the pituitary gland to release an anti-diuretic hormone (ADH), also referred as vasopressin. When this happens, the kidneys preserved water, due to the dehydration, so instead of getting rid of it through urination it recycles it to prevent a deficit. When we eat salty foods the salt concentration in our blood increases. In order, to normalize this we get thirsty and this dilutes the salt concentration. Studies have also shown that cold water decreases thirst more than a hot beverage.2 Next time your thirsty think about what might be the reason for your thirst.
This post only touches on the surface of how hunger, satiety and thirst are regulated by our hormones. However, there are other factors that also influence this process; the environment, whether we’re eating in a stressed state (sympathetic) or rest and digest mode (parasympathetic state), and/or genetics all contribute on how we eat, reach satiety, or what makes us thirsty. Therefore, next you feel and hear your ghrelin hormone, feed it foods that will help you reach that satiated state sooner and when your thirsty reach out for a glass of cold water.
- Broussard JL, Kilkus JM, Delebecque F, et al. Elevated ghrelin predicts food intake during experimental sleep restriction. Obesity. 2016;24(1):132-138. doi:10.1002/oby.21321
- Logue AW. The Psychology of Eating and Drinking. New York: Routledge
- Whitney E, Rolfes SR. Understanding Nutrition. Eleventh. Thomson Wadsworth; 2008.
- Bergmann KE, Bergmann RL. [Control of food intake]. Monatsschr Kinderheilkd. 1986;134(6):387-392.