How do our brains know when the time for our next meal has come?
Hunger serves an obvious purpose: it tells us that we need to eat to keep our bodies fueled. Yet most of us live in a world where food is ever-present and meals are scheduled around social conventions. Do we really need a reminder to eat breakfast, lunch, and dinner?
Apparently so. While our society may have evolved to provide us with endless opportunities to eat, our bodies are primarily concerned with keeping the well-oiled machine going.
This means that we start to feel hungry once our stomach is empty. However, the sighting of a tasty Halloween treat being shared in the office may tempt us, even though we're not technically hungry.
That's because our brains are on the lookout for energy-rich foods, just in case we need to go without later on.
And the key word in this sentence is "brain," because our gray matter is in charge of hunger.
Hunger and an empty stomach
After a meal, our gastrointestinal tracts slowly empty by pushing food through the stomach and the small and large intestine.
Specialized contractions called the migrating motor complex (MMC) sweep up undigested food, which is a process that takes around 130 minutes. The final phase of the MMC is regulated by a hormone called motilin. Motilin-controlled contractions cause the rumbling in our stomachs and coincide with hunger pangs in humans.
Another hormone implicated in hunger control is ghrelin. In mice, ghrelin activates neurons called agouti-related peptide (AgRP)-expression neurons in the hypothalamus region of the brain, which tell us that we are hungry.
These neurons are the control center for hunger. When AgRP neurons are artificially switched on in mice, they gorge themselves on food.
So, our brains pick up messages from our stomachs and tell us that it's time for our next meal, occuring around 2 hours after we've eaten. But that doesn't explain the irresistible draw of a delicious snack between meals.
Appealing snacks and the brain
Here, we need to differentiate between homeostatic hunger, which is related purely to balancing our energy reserves short-term, and hedonic hunger, which makes use of opportunities to gather extra energy. Hedonic hunger is less well understood than homeostatic hunger.
When our eyes detect something that we have previously enjoyed eating, our brain is notified.
If we are full, we might take a rain check. However, our brains are hardwired to avoid running out of energy. The offer of extra food can therefore override our feeling of fullness and lead us to grab that tasty snack after all.
How we feel about our previous meal may also have something to do with it.
Medical News Today recently reported on a study that showed 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.
Hunger and overeating
So, our brains control our hunger based on what we eat, whether or not we feel that what we've eaten is enough to make us full, and the availability of extra calories.
This system may have worked while humans were hunter-gatherers, but these days, it contributes to overeating and the steady rise of obesity.
Maintaining a healthful diet and weight may therefore be a battle between what we tell our brains and what our brains tell us. In that spirit, I've decided to opt for the more healthful Halloween treats.