Weight gain makes food-related memories more enduring

The World Obesity Federation predict that, within the next decade, half the world’s population will be overweight or obese. And all the evidence points to this being a one way street: once people put on weight, it becomes very hard to shift. One reason might be the way the brain makes food-related memories. Speaking with Chris Smith, Henrike Scholz, from the University of Cologne, has been looking at this using flies; their brains are wired for food in the same way ours are, making them excellent study subjects. The crux of her discovery is that lean people - and flies - make less persistent food-related memories than fatter individuals, which also tend to attach a reduced reward to food consumption. Hey presto, the toxic combination of an enduring impression that foods pack a lower calorie punch than they really do. And that adds up to over-eating and long term weight gain…

Henrike - When animals or humans feed, the energy contents is measured. The insulin levels in the blood tell the brain how much they have eaten, and if they have eaten enough, they stop eating. This system is conserved between humans and the genetic model system we are working with - the flies - and we use flies as a model to understand behaviour and movement.

Chris - So basically our bodies are counting the calories that we take in and then they're storing a memory snapshot of, when I eat this, I get this much calorie reward?

Henrike - Yes.

Chris - So how does dieting or weight gain and weight loss fit into that then?

Henrike - So it depends on your physical condition. So if you're a lean person and have periods where you don't eat, you think about the food, you then might eat the food. And the food is so rewarding that you lose the memory. However, an obese person goes through this phase of starvation, the memory gets much more stable. And if these obese person then eats food, the food is not perceived as rewarding and the obese person still feels hungry. And that despite the fact that the energy level is full and that results in overeating.

Chris - So if we unpick that a bit: a person who is overweight, they have a memory that when they eat certain calories, they don't get as much of a calorie reward as say a leaner person has for that food. The person who's overweight then attempts to lose weight, they go on a diet. So how does that affect that memory in the first instance? What does that do to the memory?

Henrike - Because the memory is much more stable. So there are different phases of memory. You can have a very short lasting memory and a long lasting memory, and these memories have different stabilities. In obese people, this longer lasting form of memory is much more stable. And if they eat, they don't lose the memory.

Chris - And is that memory one telling them, eat this, it's good, or is it a memory that says eat this, but it doesn't give you many calories, and that's what encourages overeating?.Which way round is it?

Henrike - So it's not about that the body is not measuring the energy content, it's about the memory, which is basically giving them the feeling that the food was not rewarding or not enough for them.

Chris - They've basically stored a memory that the food has fewer calories in it than it really does for them. And that memory is entrenched. It doesn't shake off very easily?

Henrike - Yes.

Chris - And they weren't obviously overweight to start with. So were they always making memories that way and then the memories become a problem because they make them gain weight? Or does the pattern of memory formation change with weight gain?

Henrike - So the pattern of memory formation changes due to increasing body weight. So when you are lean at the start, you form a different form of memory than if you have accumulated body weight basically, then you form a different form of food memory.

Chris - So why is it not working in reverse? If you then shed the calories, why do you not make the lean form of the memory again and, and then lose the weight?

Henrike - Because the organism adapted to this kind of memory because it's so stable, it is not erased.

Chris - And how did you put this to the test in the flies?

Henrike - So you can basically generate genetically modified organism, which have different increases in body weight, and then ask how good they remember food related cues and how long the memory lasts. And then in turn how these memory influence their future food intake.

Chris - So you overfeed a fly, so you get a sort of a fly with a beer belly if you like, and it lays down memories that are very long lived that when it eats this sort of food, it doesn't return much value. So it therefore tends to overeat?

Henrike - Yes.

Chris - You mentioned that it's tied up to insulin and insulin encourages us to store calories. So does that mean it's purely linked to carbohydrates, this then. And is the fact that our, our modern day diets are so dominated by sugars, people have often made low fat versions of foods by substituting sugars instead of some of the fats, for example, is that then at the heart of why so many people across the world now more than half the world population is dangerously overweight?

Henrike - So we linked it to carbohydrates because carbohydrates are enriched in the western diets, and this mechanism is triggered by carbohydrates and it is extended to other foods. So, for example, these obese flies were deprived of carbohydrates, but they also overeat protein-rich food. So basically this mechanism triggers overeating and it's it's independent of what kind of food it is.

Chris - What are the implications then? Now you've got this association and you've unpicked how these food related memories work that must point towards possible interventions?

Henrike - So the idea would be that so one thing is that these neurotransmitter, which normally suppress a memory could be basically activated in overweight people to block this mechanism or this formation of long-term memory, when they basically go through a diet.