Eating When Full

A scientific explanation

January 25, 2020


When thinking about a topic to write about, we remembered about Nutrition week that happened in school last semester. We were informed on topics such as eating healthy, what food is good for your body and in what way a healthy diet can help you in your daily life. Many of us are vowing to make a change in our diet and cut back on the school’s delicious chocolate-chip cookies, but the question is how? Most people tend to find themselves in a vicious cycle of bad eating habits that they cannot seem to get out of. When talking about eating habits, we usually encounter topics such as the different food groups and how much of each we should be having per day. However, we very rarely think about consuming habits and how to fix these in order to have a healthier consuming schedule.

Have you ever found yourself reaching for a pack of chips while watching Netflix even though you were not hungry? Or perhaps you have noticed that you tend to eat more, less or differently during exam season? In this article we will be looking into the psychological reasons for consuming habits and answering these questions. The following sections will explore the psychological and scientific explanations for occurrences such as eating when full and stress eating.




The Struggle Between Brain Signals

Why does our desire to keep eating overpower the signal from our brain saying we are full? A new study has shown that the reason for this involves a battle between two neighbouring groups of brain cells, in which the brain’s opioid system also has a role. The study was carried out on mice; however, scientists believe that this will help us to understand similar mechanisms in humans. Professor Huda Akil, a neuroscientist in the Department of Psychiatry at the University of Michigan Molecular and Behavioural Neuroscience Institute in Ann Arbour, explains the study; “Our work shows that the signals of satiety — of having had enough food — are not powerful enough to work against the strong drive to eat, which has strong evolutionary value.”

Throughout the study, Professor Akil and her colleagues focused on two small groups of adjacent nerve cells, or neurons, in the hypothalamus, a small brain region that is involved in several functions, such as the control of “motivated behaviours”. The hypothalamus is also known as the arcuate nucleus (Arc). The two cells groups are called pro-opiomelanocortin (POMC) and agouti-related peptide (AgRP) cells. Scientists already knew that the Arc was involved in the ‘control of feeding’. Previous work had revealed that when receiving certain signals, POMC neurons act ‘like a break’ on eating and AgRP neurons act as the gas pedal – especially if it has been a while since the last meal.

The hypothalamus (red circle)


However, it was unclear how these two groups interacted with one an another. The investigators used a tool, called optogenetics, to map the mechanism’s signals by using laser light to activate and deactivate selective cells in the mice that were overeating. This revealed that when they activated the POMC, the AgRP cells were activated. Professor Akil explained that “When both are stimulated at once, AgRP steals the show.” Using another optogenetic method, scientists then found that they were able to activate the POMC cells without activating the AgRP cells, which lead to a significant decrease in the mice’s eating.

In a final set of experiments, the scientists revealed that activating AgRP also switches on the brain’s opioid system. Upon giving the mice the opioid receptor blocker naloxone, the feeding behaviour stopped. Professor Akil stated that “This suggests that the brain’s own endogenous opioid system may play a role in wanting to eat beyond what is needed”. Previous studies on this matter have shown that metabolic drivers of eating and overeating tend to focus on hormones, such as ghrelin ad leptin. However, this study has shown that the brain circuits, or neural systems, also play an important role. These could be reacting to emotional, social and perceptual signals.

Professor Akil urges scientists to further study this aspect of overeating as she says, “There’s a whole industry built on enticing you to eat, whether you need it or not, through visual cues, packaging, smalls, emotional associations. People get hungry just looking at them, and we need to study the neural signals involved in those attentional, perceptional mechanisms that drive us to eat”.



The Hormone responsible for overeating – Ghrelin

Have you ever been in a restaurant, shoved the last bite of your delicious meal into your mouth, despite knowing you are full, but then finding the waiter walking towards you with the dessert menu?  You know you shouldn’t eat anymore. You know you shouldn’t get the triple chocolate cake. But you can’t help yourself. You get it and you end up eating the whole thing. As you loosen your belt, you wonder why you just did that.

The University of Texas Southwestern Medical Center conducted a study that suggested that ghrelin, the hormone your body secretes when you are hungry, might act on the brain, thus influencing the hedonistic aspects of eating behaviour – the result being that we continue to eat ‘pleasurable’ foods even when we are full. Researchers Jeffrey Zigman, Mario Perello, and Michael Lutter suggest that ghrelin increases specific rewarding aspects of eating. Previous studies have shown that ghrelin levels are linked with pleasurable feelings one gets from substances, such as alcohol. Zigman explained that the rewards give us sensory pleasure and motivates us to work to obtain them. They also facilitate the reorganisation of our memory, causing us to remember how to obtain the rewards – in other words eating.

In order to study the effects of ghrelin on overeating, the researchers conducted two studies using mice as test subjects. In the first study, the scientists observed whether the mice that were satiated preferred a room where they previously found high-fat pellets versus a room that had regular, bland food. The mice without the ghrelin had no preference, however when the mice were injected with ghrelin, they preferred the room with the fatty food. In the second study, the scientists measured how long the mice would continue to poke their heads into a hole to receive a pellet of high-fat food. This study, similar to the first, was conducted with both ghrelin-injected mice and unaffected mice. The mice with ghrelin spent more time poking their heads into the hole, while the mice without ghrelin gave up sooner.

So, the next time you reach for the dessert menu, ask yourself if you want the triple chocolate cake or if the ghrelin is making you think that you do.


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