Why the obese keep right on eating: It’s a memory thing

How can you say no to a second plate of pasta, if you don't remember eating the first one?

How can you say no to a second plate of pasta, if you don't remember eating the first one? Photo: Getty

What makes us start and stop eating? It’s an important, if obvious question for researchers trying to better understand the complexities of obesity.

The answers seem obvious too.

We eat because we’re hungry, we stop when we feel full, more or less.

People with obesity don’t respond to the hormonal signals governing hunger and satiety in the same way as healthy people.

Short version:  they don’t feel as full, and keep eating well beyond their nutritional needs.

Case solved, right?

Not really

As I mentioned, it’s complex.

Indeed, increasing evidence has found that the cognitive processes that either govern or play into decision-making add yet another layer of complexity.

Short version: obesity damages the brain, in children as well as adults.

Part of this damage is caused by inflammation (which is caused by eating too much, but also eating moreish highly processed foods).

In the day to day, it’s now known that obesity compromises executive function skills – the complex ability to initiate, plan and carry out tasks. In the longer term, it significantly increases the risk of dementia.

All of this is relatively new knowledge.

In a January paper, neurologists from McGill University, Montreal, for the first time found that obesity-related neurodegeneration mimics damage found in Alzheimer’s disease. In other words, the same parts of the brain were losing neurons.

The good news: losing weight might reverse this damage.

This is where it gets a little bit weird

Over the last 20 years or so, scientists have turned their microscopes onto the close links between memory and appetite.

And this brings us back to the question: what makes us start and stop eating?

The most intriguing answer comes in the form of another question: how can you say no to a meal, if you don’t remember eating one half an hour ago?

The 1998 study

In 1998, University of Pennsylvania psychologists conducted a somewhat outlandish experiment on two brain-damaged men. These chaps had such profound amnesia, they couldn’t remember events more than a minute in the past.

The experiment was run on three separate occasions.

It went like this: The men were given a meal, which they ate. Ten to 30 minutes later, they were offered a second meal, which they both ate without complaint or query.

Then a third lunch was offered!

A third lunch was offered 10 to 30 minutes after the second: both men tucked in.

As The New York Times reported at the time:

“One of the men, called R.H. by the researchers, ate much of the third lunch and, 20 minutes later, announced that he planned to ‘go for a walk and get a good meal’.

“In contrast, when two control subjects were offered lunch a second time… they replied, in effect, ‘What are you, crazy? I just ate’.”

In a US study, men with amnesia were fed three meals in a row, with their hunger intact..

In their paper, the researchers concluded:

“These findings suggest that memory for what has recently been eaten is a substantial contributor to the onset or cessation of eating of a meal.”

Overweight people and episodic memory

Overweight or obese people generally don’t suffer from this sort of profound amnesia. But they do suffer a subtler kind of amnesia which is just another term for memory loss.

For one thing, they have poorer episodic memory than healthy people.

I mentioned earlier that people, including children, with obesity have compromised executive function skills. One of these skills is episodic memory – the ability to recall past events.

One of the first studies to demonstrate people with higher BMI (body mass index) suffering a deficit in their episodic memory came out of Cambridge University in 2016.

The brain damage suffered by people with obesity includes a kind of amnesia.

At that time, scientists were thinking that excess bodyweight was probably  “associated with changes to the structure and function of the brain and its ability to perform certain cognitive tasks optimally”.

In particular, obesity had been linked in previous studies “with dysfunction of the hippocampus, an area of the brain involved in memory and learning”.

The frontal lobe was also thought to be similarly affected. This is the part of the brain involved in decision making, problem solving and emotions.

But actual evidence for memory impairment in obesity was limited. That began to change.

The 2016 study

Seven years ago, Dr Lucy Cheke – a researcher from Cambridge’s Department of Psychology – conducted a fiendishly simple experiment that found an association between high body mass index (BMI) and poorer performance on a test of episodic memory.

The study involved 50 participants aged 18-35, with body mass indexes (BMIs) ranging from 18 through to 51.

A BMI of 18-25 is considered healthy, 25-30 overweight, and over 30 obese.

The participants took part in a memory test known as the ‘Treasure-Hunt Task’, where they were asked to hide items around complex scenes.

They were then asked to remember which items they had hidden, where they had hidden them, and when they were hidden.

Overall, the participants with higher BMI performed more poorly on the these memory tasks.

It was a small study, but a pretty compelling one.

What the researcher said

In a statement from Cambridge, published alongside the paper, Dr Cheke said:

“Understanding what drives our consumption and how we instinctively regulate our eating behaviour is becoming more and more important given the rise of obesity in society,”

“We know that to some extent hunger and satiety are driven by the balance of hormones in our bodies and brains, but psychological factors also play an important role – we tend to eat more when distracted by television or working, and perhaps to ‘comfort eat’ when we are sad, for example.”

Increasingly, she said, researchers were beginning to see that memory – especially episodic memory, the kind where you mentally relive a past event – “is also important”.

She said it was possible “that becoming overweight may make it harder to keep track of what and how much you have eaten, potentially making you more likely to overeat”.

The 2018 review

Two years later, Birmingham researchers conducted a review of existing evidence and concluded that episodic memory – and working memory – were “core cognitive processes that are critical for food-related decision-making”.

The authors wrote that “disruption to these processes contributes to problems with appetite control and weight gain, which suggests that weight loss programs might be improved by the addition of cognitive training”.

Working memory is the retention of a small amount of information in a readily accessible form. It facilitates planning, comprehension, reasoning, and problem-solving.

Brain scans and a chocolate milkshake

Last month, researchers from the University of Pennsylvania School of Medicine identified the “link between memory and appetite in the human brain to explain obesity”.

This was the finding of a study that involved scanning the brains of 34  female patients “prone to binge eating”. Their body types ranged from lean to overweight to obese.

As the scans were in play, the women were offered a sweet treat (a chocolate milkshake) which was then given to them. This allowed the researchers to see in real time the neural flush of anticipation and reward.

They found that participants who were obese had impaired connections between the dorsolateral hippocampus (dlHPC) and the lateral hypothalamus (LH).

It’s these damaged circuits that may impact the ability to control or regulate emotional responses when anticipating treats.

The dlHPC is located in the region of the brain that processes memory. The LH is in the region of the brain that is responsible for keeping the body in a stable state.

Previous research has found an association with loss of function in the human hippocampus and disordered eating.

Higher BMI and brain damage go hand in hand

The most compelling finding? The level of disruption to the neural connections between memory and appetite were “directly proportional to body mass index (BMI)”.

What this means: the greater a patient’s body mass, the greater this neural network was out of whack.

This was notably the case in patients who suffered from disordered or overeating.

The researchers suggest interventions could be developed to improve “the function of this critical circuit”.

In other words, tinkering with the brain might reset the relationship between memory and appetite. This might turn things around for people whose eating is out of control.

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