There's an intriguing psychological reason it's so hard to stick to a diet

When’s the last time you were caught choosing a decadent dessert over a healthy snack despite previously promising yourself to eat healthier?

Chances are you’re not alone. And some new research reveals what’s going on in the brain when you opt for the cake instead of the fruit platter.

When people see a thing that reminds them of something that was rewarding in the past — like a slice of chocolate cake — the part of their brains linked with pleasure may be flooded with the brain chemical dopamine, even if they’re not paying attention to it, finds a study published Thursday in the journal Current Biology.

The findings could explain why it’s so hard to stick to a diet.

Even when you’ve decided not to eat junk food anymore, your brain is still rewarding you for past bad behaviour, Brian Anderson, a neuroscientist at Johns Hopkins University and one of the study’s authors, told Business Insider.

“When you have a difficult time getting your mind off something that was rewarding in the past but you don’t want to do anymore, you can’t help but pay attention,” he said.

The brain remembers past rewards

Previous research has shown that when we have a rewarding experience such as eating a piece of cake or having sex, our brains are flooded with the brain chemical dopamine. But the reason we continue to experience these rewards even when we’re not consciously seeking out the thing that causes them has been a mystery.

To find out, Anderson and his colleagues recruited 20 people for a brain-scanning study. They had to perform two tasks while having their brains scanned in a PET machine, which measures brain activity via a radioactive chemical injected into their blood.

In the first task, the volunteers had to find coloured shapes on a screen. They were paid $1.50 for finding red objects and 25 cents for finding green ones.

In the second task, the participants had to do the same thing, except they were no longer paid for finding shapes of a particular colour.

Here’s the surprising part: Even when they were no longer being paid for finding the shapes, the participants automatically focused on red objects when they appeared, and the brain scans revealed a release in dopamine in a region at the base of the brain called the ventral striatum, which is known to be involved in experiencing rewarding things.

They were also slower to find shapes of other colours, perhaps because they were distracted by the red ones, the researchers theorised.

What the findings showed was that a previously rewarding experience (finding red shapes) was still linked with the brain’s release of feel-good chemicals, even when the person no longer expected a reward from the behaviour.

While the experiments involved a fairly abstract paradigm with shapes on a computer screen, we may be able to extrapolate the findings to real-world scenarios, such as trying to stick to a diet, Anderson said. When you see a doughnut, for example, your brain may already start releasing dopamine because of your previous experience of a delicious doughnut, even though you may not even be thinking about eating one.

Of course, self-control “is a complicated process, and many things contribute to your inability to overcome a desire to do [something] that’s not good for you,” he said.

How to resist temptation

So given this knowledge of how our brains work, what can we do to resist temptation?

Anderson suggests two strategies:

  1. Avoid situations in which you know you’re going to encounter the temptation, such as going to a doughnut shop.
  2. Reward yourself for doing things you want to keep doing, like letting yourself watch your favourite TV show after going to the gym.

But when it comes to self-control, not all of us are created equal. And the strength of the reward signal in your brain can predict how much you struggle to avoid paying attention to it, Anderson explained.

In a small previous study of people with drug addiction, he has found that addicts may be more strongly affected by temptations of any kind, not just drugs. When he and his colleagues gave 17 opioid addicts and 17 healthy people the same colour-finding task as the one in the new study, the addicts were much slower than the healthy volunteers to recognise the colours that weren’t rewarded with money. The addicts also scored higher on a test designed to measure their impulsivity.

These findings suggest that addiction may be part of a much broader problem in how the brain makes decisions, which is something the researchers plan to look into in future.

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