Eating: what memories are made of

The memory of what we believe we have eaten in a recent meal is now considered an important part of regulating our appetite and hunger.

What drives us to desire food is a complex mix of hormones, psychology and physiology. One new research frontier being explored is how our recent memory of what we have eaten (termed episodic memory) can modify future food intake.

If asked, most people could easily remember what they ate for their last few meals, but this is not something that is foremost on our minds for much of the time. An interesting observation is that reminding people of what they have recently eaten can reduce how much food they eat at a subsequent meal.

Likewise, introducing a distraction when eating such as television watching or playing a video game can result in over-eating at subsequent meals. The explanation behind distracted eating is that people over eat because they took little note of what they were eating in the first place.

One further line of evidence into how memory of what has been eaten in recent meals can affect subsequent eating comes from studies in people with retrograde amnesia. Certain forms of brain impairment causing amnesia can mean a person will eat a meal to fullness despite having recently eaten a very large meal. It appears their memory of having eaten and even the hunger suppression that would normally occur from a large meal has been lost.

Soup is served

Building on the theory of episodic memory being an influence on appetite regulation, researchers recruited 100 volunteers to take part in a laboratory eating study. Before being provided with a lunch meal, half of the volunteers were shown a 300 mL bowl of soup and the other half a 500 mL bowl.

From each group, half again were provided with 300 mL of soup for lunch and the other half 500 mL. This meant there were four different groups all together: those who saw a small bowl of soup and then went on to eat the same amount or more, and those that saw a large bowl of soup and went on to eat the same amount or smaller.

Just to add a devious twist to the experiment, when it came time to eat the soup, none of the participants knew how much they there given. This was achieved thanks to a cunning piece of apparatus that could keep the soup bowl topped up until the person had eaten down to an indicated mark on the bowl.

Now to the results. How hungry each person felt after lunch was directly related to how much soup they ate so those that had the 500 mL bowl felt more full. Where things get interesting is when the participants were quizzed about their hunger two to three hours after lunch, people who thought they consumed the 500 mL bowl of soup (because that is what they were shown before lunch) felt less hungry, even when they went on to only eat 300 mL. This effect of greater perceived satiety was experienced for 24 hours.

The research was published in the open access journal PLOS One

What it all means

Our memory of recent eating can be a strong influence on how much we go on to eat. As researchers build on this research, it may well be that a greater focus on mindfulness of what we are eating and have recently eaten will become a cornerstone of dietary advice for behaviour change.