Hunger: Simple Biology, or is it?

Klara Zietlow
11 min readFeb 15, 2024

The Wobbly Balance of Hunger

Everyone experiences hunger to some extent throughout the day. How that manifests itself, however, can vary tremendously.

While I need three solid meals a day, my mom needs a snack on the hour, and my brother can go 24 hours on a carton of ice cream.

Hunger is defined as “the painful sensation or state of weakness caused by the need of food”. Yet, whether you feel a single strong pang around lunchtime or lots of little cravings throughout the day is due to a plethora of factors.

Since our hunter-gatherer days, the way we eat has radically shifted. Modern diets and restrictive eating schedules have permanently altered our nervous system and hormonal clock in ways that are responsible for many diet related health issues. Where we once relied solely on true physical demand to inform us to eat, we now frequently employ conditioned environmental cues to fulfill this role. In order for hunger to manifest itself, it must rely on a complex network of signals situated throughout the human body.

Caveman Hunger

Our hungry ancestors :) I Shutterstock

When we’re hungry due to a true physiological energy requirement, it’s due to our nervous and endocrine systems. These inform our brains about a caloric imbalance, triggering physical manifestations which urge us to seek food. These systems were important for our early ancestors to know when to burn energy on gathering food and when to lay low.

The hypothalamus sits at the base of the brain and is imperative to the generation of hunger signals. In its ventromedial (bottom center) portion lies the arcuate nucleus, which acts as a hub for triggering chemical release.

Brain Areas Responsible for Hunger and Satiety I Osmosis

Appetite Suppression

These chemicals either accelerate or break feeding urges. In the arcuate nucleus, proopiomelanocortin (POMC) neurons, stimulated by the protein leptin, call on the nearby pituitary gland to secrete α-melanocyte stimulating hormone (α-MSH). The role of this hormone is to activate and bind to appetite suppressing neurons. Although people sometimes take it in supplement form as a diet catalyst, α-MSH is naturally activated by UV-light exposure, which is why people tend to eat less in spring and summer.

Appetite Stimulation

On the other hand, the arcuate nucleus also contains AgRP neurons that stimulate a desire to eat. AgRP neurons are activated by the hormone ghrelin, which is secreted from the gut. Like a hormonal clock, ghrelin triggers hunger cues at certain times of the day.

Your body predicts when you’re about to eat and makes you feel hungry! I Original

When these systems first evolved in humans, food was scarcer and was consumed whenever it was available. Back then, the main driver for when ghrelin was expressed was whether glucose levels were low.

Today, most people have developed some sort of eating schedule, and as a result, ghrelin release has begun to match this, instead of relying solely on glucose levels. The AgRP neurons that ghrelin activates are so powerful that when they were artificially stimulated in mice, the mice gorged themselves to death.

Ghrelin is not the only thing capable of activating these neurons. Rather, simply approaching food causes our AgRP neurons to begin firing rapidly. This makes sense, since the hungrier we are, the more AgRP activity there is.

GI Tract

Other symptoms of hunger can be explained by activity in the gastrointestinal tract. After we eat, food is pushed through our stomach and intestines, emptying the gastrointestinal tract. Then, just over two hours later, the migrating motor complex (MMC) receives a signal to sweep up any undigested food. This is carried out using specialized contractions, which are what cause stomach rumbling and correspond with hunger pangs.

…Complicated Hunger

On your third pizza, haven’t moved all day, but munchy I iStock

Although the human body has developed a sophisticated process for expressing and suppressing hunger, it is not always complied by. Despite the negative health consequences that come with it, 74% of adults in the US are medically overweight, according to the CDC. This is due to a no longer useful tendency our brains evolved to possess.

In an effort to never run out of energy, our bodies have been trained to seek out excess calories, regardless of current physical need. This is called hedonic hunger: eating without the physical sensation of hunger, usually to obtain pleasure. Hedonic hunger can override feelings of fullness, leading us to consume food beyond this point leading to discomfort and future health complications if it becomes a pattern.

Exercise and Appetite

A good illustration of the power of hedonic hunger is a study by the University of Colorado on the effect of exercise on food consumption. As reported by the New York Times, there are “some studies that indicate that exercise, especially if it is strenuous and prolonged, tends to blunt people’s appetites”. Favorably, the University of Colorado also concluded that appetite hormones dropped after workouts.

Nevertheless, when they examined their data, they realized that the environmental factors surrounding meals were powerful enough to override the decrease in appetite. When we are presented with enticing foods and surrounded by others who are also hungry, we tend to eat the same amount as we normally would have had we not exercised prior to the meal.

Exercising is Great for Health, but Likely Won’t Reduce Your Appetite I Shutterstock

Different Foods, Different Responses

Curiously, the reason we are tempted by certain foods often has less to do with the amount of energy our bodies need in the moment, than with our bodies’ adaptations to seek out energy-rich foods.

This could explain why the global pizza market is worth $160 billion; pizza is full of fat, making it calorically dense and therefore subconsciously attractive (not to mention scrumptious).

In fact, we are so hardwired to seek out calories that a study reported on by Medical News Today revealed that “participants who were under the impression that they had eaten a smaller breakfast ate a larger lunch and more daily calories than those who thought that they had eaten a bigger breakfast,” despite both groups receiving the same meal.

As a result, often what we think are conscious decisions have been calculated by our evolution long ago.

An Important Gut Hormone

Regardless of varying caloric levels, certain foods inherently make us fuller than others based on their molecular composition. As a result, what we eat can create long-lasting effects on how and when our hunger cues reveal themselves. The gut hormone cholecystokinin (CCK) is released naturally around 15 minutes after a meal begins. Its release is triggered by neurons that detect levels of fatty acids, amino acids, and sugar in the gut and its mucosal lining. CCK, which acts as a sign to stop eating, will only be triggered once enough of these three molecules have passed through the gut.

This means that certain foods with a higher content of these compounds will be more filling. Appetite regulating foods include those high in Omega-3 fatty acids, conjugated linoleic acid (CLA), and protein, which is composed of amino acids.

You and Your Hormones states that very obese people usually have lower levels of CCK in their bloodstream. Although more research is needed, it is likely that “this low level of cholecystokinin may contribute to reduced feelings of fullness and difficulty in losing weight in very obese people”.

Blood Sugar Levels

Glucose levels also play a role in the frequency and occurrence of hunger. Every time we eat, our food is broken down into sugars and used as energy. Foods with high carbohydrate content are quickly absorbed into the bloodstream, resulting in a sudden blood sugar spike followed by a drop when the hormone insulin shuttles it into cells to be used as energy.

The Molecular Makeup of a Food Determines Its Effect on Blood Sugar I Gestational Diabetes UK

Volatile glucose levels increase hunger, which is one reason restaurants often serve bread before meals. The opposite can be said about steady glucose levels. If you had a meal of rice, salmon, and cabbage, and ate the cabbage first, the glucose spike you’d experience from eating would be blunted even after eating the rice because cabbage is low in carbohydrates. This steady, slower rise of glucose prevents the hunger pang that would have otherwise resulted from a sudden change.

Even the Order of How you Eat your Food Changes your Blood Sugar I Original

Glucose levels can also be modulated without changing diet. Exercise before or after a meal dampens glucose levels because of the activation of the protein GLUT4. GLUT4 transports glucose into cells, thereby removing it from the bloodstream and avoiding a spike.

But again, it’s important to remember that environmental factors may still override physiological shifts.

Lifestyle and Long-Term Effects

Aside from these temporary alterations, some lifestyle choices can permanently alter our hunger signals. Highly processed foods have been linked to weight gain many times, such as in a 2019 study conducted by the National Institutes of Health where “on [an] ultra-processed diet, people ate about 500 calories more per day than they did on [a] minimally processed diet”.

It’s not just that ultra-processed foods tend to have more calories; the emulsifiers found in many of these foods, which bind fat with water, strip away the gut mucosal lining just like a cleaning detergent, forcing hunger-regulating neurons to retract deeper into the gut. As a result, CCK and other hormones are unable to reach those neurons, preventing the onset of satiety. The structural damage to the mucosal lining takes a long time to repair, so the effects caused by damage are enduring.

Emulsifiers are Likely Found in Most of These Foods I Unsplash

Pavlovian Conditioning and Learned Overeating

Another long-lasting issue related to hunger regulation is learned overeating. “Learned overeating,” an example of Pavlovian conditioning, is when one trains themselves to associate a particular stimulus with pleasurable food, leading to cravings every time one is exposed to it. This can be dangerous as associations are usually formed unconsciously.

In the 1890s, Russian physiologist Ivan Pavlov discovered that by playing the sound of a metronome right before feeding his dogs, the dogs would eventually become conditioned to begin salivating just by hearing the sound of the metronome.

This response is present in humans as well. Every time we eat good food, we associate it with the internal and external environment it was consumed in. This could be anything from the emotional state we were in to our location. The next time we are in that situation, we anticipate a similar reward.

What once may have been a neutral stimulus results in heightened cephalic phase responses compared to other neutral stimuli, because you’ve conditioned yourself to expect a positive reward of food in its presence. These responses prepare the body for optimal processing of food by altering digestion, absorption, and metabolism.

This association is quick to form, needing only 3–6 pairings before it is experienced. After only five days, a study by Van den Akker et al. conditioned subjects to have heightened eating desires and expectations before completing a questionnaire.

Although it is possible to reduce sensitivity to conditioned cues via cue exposure therapy (where one is shown cues for extensive periods of time without the reward), this is difficult because of where cue responses are stored. The eating desires that come from a conditioned stimulus are stored in one’s memory making them difficult to undo and prone to returning if placed in the situation again. Learned overeating is a great example of when hedonic hunger can be so convincing, it displays all of the signs of true physiological hunger.

Not-So Anatomically Correct, but Helpful Illustration I The Awkward Yeti

Feeling Hungry Better

With so many factors that need to be taken into account, it’s no wonder most nutritional information on the internet is bullshit.

Two highly reputable sources reporting on the same topic could promote entirely different messages based on where they were published and their audience.

It’s pointless to talk about physiological and hedonic hunger separately because it’s how they work in conjunction with one another that makes the magic happen.

Because we can use food as a means to so many different ends, we often forget to enjoy the experience of it. Whether we’re eating as a coping mechanism or just to subdue a growling stomach, people today are often mentally absent during meals.

We live in a world where overriding natural hunger cues is commonplace; as a result, many humans have lost touch with their bodies’ signaling. Whether from mood eating or rigid meal times and amounts, the modern human is teetering precariously on the edge of losing their connection with natural eating.

Think About How your Food got to You I Unsplash

One solution that has been developed to reconnect humans with their biological hunger cues is called intuitive eating. Sometimes referred to as mindful eating, the idea behind the practice is to only eat when hungry and to stop as soon as one becomes full.

Another aspect of mindful eating involves contemplating all the steps one’s food has taken. Thinking about who prepared the meal, stocked the shelves, transported the produce, and planted the produce is an exercise in “gratitude to all of the people who gave their time and effort”.

This exercise provides one with more than connection to their community. Since CCK hormone, the sign to stop eating, isn’t released until 15 minutes into a meal, the extra time it takes to think about where one’s food comes from enables one to experience what true satiety feels like, as opposed to artificial signals to stop eating, like a lunch break ending.

Don’t Stress It

I realize that by now I’ve probably just overwhelmed you with all the ways you’re wrecking your metabolism. I promise that wasn’t my goal!

One last, very important reason we eat is as a social activity! Eating is the oldest and most powerful community builder.

Eating to Socialize I Unsplash

Food is a conversation starter. Food is a way of connecting with others who you might share little in common with. Food is a way of sharing cultures.

At the end of the day, you should be eating because it makes you feel satisfied, happy, and grounded. If it takes tapping into your hedonic hunger a bit more than not, so be it.



Klara Zietlow

Passionate about the future of food and the environment. Likes animals too :)