What happens in your body when you’re hungry?

We’re all familiar with hunger — that empty, gnawing sensation in your gut and the drive to empty your fridge. 

But have you ever wondered how and why your body makes you feel like that?

It won’t come as a surprise to any of you, but you need to eat to survive. And the same goes for all other animals. 

So, long ago, evolution designed a way to make hunger difficult to ignore: Your brain makes sure that if you're low on energy, you know about it.

Here, we’ll explain how that feeling comes about. Among other things, we’ll also discuss tummy rumbles, feeling “hangry,” and what’s going on in your brain.

How does your brain know you need food?

Your brain is the queen of hunger. From the safety of its skull-shaped palace, it hollers, “eat something!” But how does it know you need food? Well, there are a few signals it pays attention to.

For instance, when your stomach is empty, it contracts, sending signals to the brain to tell you to eat. On the other hand, when you’re full, stretch receptors in the stomach tell your brain that you’ve eaten enough.

Also, a few hours after eating, blood sugar levels dip. This may provide the brain with another signal that it's time to eat.

Some hormones are involved, too. For instance, your stomach produces a hormone called ghrelin just before it expects your next meal. 

Ghrelin activates receptors in a part of your brain involved in hunger called the hypothalamus — more on this brain region later. As ghrelin levels rise, so do your hunger pangs.

Although best known as the hunger hormone, ghrelin also plays a role in sleep, glucose metabolism, anxiety, and much more.

Scientists have identified many hormones that reduce feelings of hunger. These include leptin, insulin, and the dramatically titled cholecystokinin.

But ghrelin is one of very few that trigger hunger. The second to be identified as a hunger hormone is insulin-like peptide 5 (ILP 5), which is produced in the gut. 

More recently, some scientists have suggested that asprosin, a hormone that’s released from fat cells, might also make us feel hungry.

Tummy rumbles

What causes that growling deep in your bowels that accompanies feelings of hunger?

When you eat food, it’s gently pushed along your intestines by rings of muscles. This process is called peristalsis. Peristalsis also occurs a while after eating, when your gut is emptier.

These so-called hunger contractions clear out your stomach of any remaining debris and cause the infamous stomach rumbling noises you're familiar with. 

Although peristalsis mostly happens when your gut is full of food, the sounds are muffled by your meal. When you're empty, it’s easier to hear the resonant growls.

As an aside, the proper word for stomach rumbling is borborygmi. No, we’re not sure how you pronounce it, either.

Feeling hangry?

Before we dive into the brain, we should talk about feeling angry when you’re hungry — also called being “hangry.” 

This isn’t just a made-up term. People really can enter a negative mindset when they’re ravenous.

If this happens to you, don’t worry, you’re not alone. Even fruit flies and female zebra fish get aggressive when they’re peckish.

A few theories help explain why some people feel this way when they’ve gone too long between meals. 

The first takes us back to our evolutionary past. When food is in short supply, it might benefit our survival if we become more aggressive. That way, we can outcompete our rivals and win dinner.

At least one study backs up this theory — well, in a species of goat-antelope, anyway.

Another explanation for pre-dinner rage might be that our blood sugar levels are getting lower. This triggers the release of cortisol and adrenaline, which help increase blood sugar levels. 

Cortisol is a stress hormone, and adrenaline is involved in your fight-or-flight response. So, in combination, they might make some people hangry.

However, not all experts agree with the low blood sugar theory, and studies have been inconclusive.

Also, a compound called neuropeptide Y is released when you’re hungry. And some research shows that this can increase aggression, too.

But scientists don’t know why some people get hangry while others are just hungry. Unsurprisingly, there’s not a whole lot of research into it.

What’s going on in the brain?

Your brain, as you’ll remember, sits comfortably in its skull palace. And the physical and chemical messages outlined above mostly end up in a part of your brain called the hypothalamus.

This structure sits deep inside your brain. It receives messages from the body — via hormones and nerves — and ensures things are all running smoothly. 

To use the scientific terminology, it helps maintain homeostasis.

This includes keeping an eye on body temperature, blood pressure, sleep, mood, and, of course, hunger.

Scientists have known that the hypothalamus is involved in hunger for more than 70 years. For instance, in 1951, scientists destroyed part of the hypothalamus in rats, and they stopped eating.

Soon after, other researchers stimulated the same region in cats and found that they increased their food intake by 1,000%.

Within the hypothalamus, the part that deals with hunger is called the arcuate nucleus. It houses two types of cells:

1. orexigenic — these stimulate your desire to eat

2. anorexigenic — these suppress your desire to eat

As a geeky aside, “orexis” is the Latin (and Greek) word for “appetite.”

These cells project to other areas of the hypothalamus. And, depending on what signals they’ve received from the body, they decide whether you should start looking for food or not.

If the signals suggest to the hypothalamus that you should be peckish, neurons traveling from the hypothalamus carry melanin-concentrating hormone (MCH) and orexin (OX) to other parts of the brain. 

MCH and OX promote feeding behavior, such as walking about and sniffing if you’re a rat or looking in the fridge if you’re you.

We should note that MCH and OX aren’t the only signaling compounds involved in feeling hungry and feeding. It’s an incredibly complicated system. 

The above is a fairly stripped-down account of the hunger response. When you start getting into the brain, it gets bewildering pretty fast.

More than survival

As far as survival goes, the hypothalamus helps keep us fed. But other bits of the brain get involved, too. 

After all, we don’t always eat because we’re hungry. Sometimes, we eat because we want to eat.

In these circumstances, brain areas and neurotransmitters involved with reward and pleasure — such as the amygdala and dopamine — can also get involved.

To make things more complex, there’s an overlap between these two systems. For example, sometimes we eat because we're hungry, but we also want to eat food that we think is delicious.

And sometimes we keep eating after we’ve stopped feeling hungry because it makes us feel good.

Hunger, as a concept, is pretty straightforward, but the biology behind it is fascinatingly involved. Nothing is simple when it comes to the brain, especially where food and pleasure are involved.