Eating a diet of junk food for just one week was enough to damage part of the brain that stops us eating more when we are already full, research suggests.
Study participants who ate an abundance of fast food and high-fat milkshakes had increased cravings for more after seven days.
They performed worse on cognitive tests, with results suggesting an area of the brain called the hippocampus was impaired.
The hippocampus normally stops us from gorging on more food when we are full by suppressing memories of how tasty it is.
When it's not working properly, the memories are more powerful and we are left unable to resist more cake, chocolates and crisps in front of us, the researchers believe.
The discovery sheds light on why people reach for the biscuit tin out of habit - rather than due to being hungry - and why it's so hard to get out a cycle of bad eating.
Experiments on animals have shown poor diets rapidly mar the hippocampus - an area of grey matter that regulates memory and appetite.
Now the phenomenon has been demonstrated for the first time in humans.
Lead author Professor Richard Stevenson, of Macquarie University in Australia, said: 'When we see cake, chocolate or crisps, for example, we remember how nice they are to eat.
'When we are full the hippocampus normally suppresses these memories, reducing our desire to eat.
'We found lean healthy young people exposed to one week of a junk food diet developed impaired hippocampal function and relatively greater desire to eat junk food when full.'
He explained: 'Junk food may then act to undermine self-control by increasing desire.'
In the study 110 male and female undergraduates from the campus were split into two groups - half of whom were randomly selected to gorge on a western-style diet for seven days.
They began by receiving a laboratory breakfast of a toasted sandwich and a milkshake, high in saturated fat and added sugar.
Their daily diet then included waffles and the main meal, dessert and drink from a popular fast food restaurant amounting to an intake of more than 4,000 calories. They were handed cash to pay for it.
The others acted as a control, beginning with a breakfast of a toasted sandwich and a milkshake, low in saturated fat and added sugar. They continued on their normal diet.
Before and afterwards, the participants rated their liking and wanting of tasty sugary cereals such as Coco pops and Frosties in milk and snack foods like Vegemite and Nutella on toast.
Rankings went up among those who had been eating the junk food - even when presented with the images having been sated.
They also performed worse in memory and learning tests targeted to the hippocampus.
Interestingly, when these were repeated three weeks later after they returned to their regular eating pattern these abilities returned to normal - suggesting the damage can be reversed.
Because the damaged part of the brain is also involved in memory, researchers say it adds to evidence that an unhealthy diet raises the risk of Alzheimer's disease.
Professor Stevenson, a food psychologist, said a number of studies have found the hippocampus is especially vulnerable to environmental risks - such as unhealthy diets.
He said: 'It may be for this reason that a Western-style diet - along with other factors - are known correlates of Alzheimer's disease.
'In conclusion, a large animal literature demonstrates a Western-style diet adversely affects the hippocampus.
'The current study suggests something similar occurs in humans, in that one week's exposure causes a reduction in memory and learning performance, in addition to alterations in appetitive control, as measured by the wanting and liking test.'
The study, published in the Royal Society's journal Open Science, follows research that found rats fed a diet containing 25 per cent sugar were thrown into a state of anxiety when it was removed.
Their symptoms included chattering teeth and the shakes - similar to those seen in people withdrawing from nicotine or morphine, said the US team.
Other studies in rodents have shown sweet foods stimulate opioids or 'pleasure chemicals' in the brain - suggesting people can become overly dependent on them.
The Daily Mail
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