Data from a study with laboratory rats indicated that the development of obesity was accompanied by a break-down in brain chemistry linked to pleasure responses. According to findings published in Nature Neuroscience, the very same changes occur when rats over-consume heroin or cocaine.
"These findings confirm what we and many others have suspected that overconsumption of highly pleasurable food triggers addiction-like neuroadaptive responses in brain reward circuitries, driving the development of compulsive eating," said lead researcher Dr Paul Kenny, from The Scripps Research Institutein Florida.
"Common mechanisms may therefore underlie obesity and drug addiction,” he added.
The data appears to refocus attention on the formulation of foods, and the Western diet in particular – the researchers fed the rats easy-to-obtain high-calorie, high-fat foods like sausage, bacon, and cheesecake.
Of mice and men
However, despite a slew of newspaper coverage, experience in the area of ‘sugar addiction’ should fan the flames of caution. Results from animal studies recently likened the brain activity and behaviour of rats bingeing on sugar to those seen in drug addicts. Researchers from Princeton University reported their findings at the American College of Neuropsychopharmacology in Scottsdale, Arizona in December 2008.
However, a professor from Swansea University in the UK who specialises in dietary influences on mood and cognitive function challenged the findings. Professor David Benton told our sister publication ConfectioneryNews.com that the neurobiological changes observed in the animals would be unlikely to be observed in humans, noting that “only when sugar is administered in a highly prescribed and unusual manner is it reported that signs of addiction occur”.
Rats like junk food
Dr Kenny and graduate student Paul Johnson divided rats into three groups: One group was fed normal rat chow, while the second and third groups were given restricted or extended access to a so-called ‘cafeteria-style’ diet, defined by the researchers as “consisting of palatable energy-dense food readily available for human consumption”.
Results showed that animals in the extended feeding group consumed twice the amount of calories as rats in the control (chow) group. Rats in the restricted group (with some access to chow) were found to consume over 66 per cent of their calories during the one hour restricted access to the cafeteria-style food.
“When we removed the junk food and tried to put them on a nutritious diet – what we called the 'salad bar option' – they simply refused to eat,” noted Kenny. “The change in their diet preference was so great that they basically starved themselves for two weeks after they were cut off from junk food. It was the animals that showed the ‘crash’ in brain reward circuitries that had the most profound shift in food preference to the palatable, unhealthy diet. These same rats were also those that kept on eating even when they anticipated being[given an electric shock]."
Brain receptors
The researchers focussed their attention on dopamine receptors in the brains of the animals, with specific attention on the dopamine D2 receptor. Dopamine is a neurotransmitter that is released in the brain by pleasurable experiences like food or sex or drugs like cocaine.
Similar to the effects of cocaine, the junk food-fed animals displayed significant reduction in the activity of these D2 dopamine receptors. Additional experiments looked at knocking out these receptors using a specialized virus, and the results showed a dramatic acceleration of addiction-like eating, said Kenny and Johnson.
"This addiction-like behavior happened almost from the moment we knocked down the dopamine receptors," said Kenny. "The very next day after we provided access to the palatable food, their brains changed into a state that was consistent with an animal that had been overeating for several weeks. The animals also became compulsive in their eating behaviors almost immediately.
“These data are, as far as we know, the strongest support for the idea that overeating of palatable food can become habitual in the same manner and through the same mechanisms as consumption of drugs of abuse,” he added.
The work was funded by a Bank of America Fellowship, the Landenberger Foundation, and the US National Institutes of Health.
Source: Nature Neuroscience
Published online ahead of print, doi: 10.1038/nn.2519
“Dopamine D2 receptors in addiction-like reward dysfunction and compulsive eating in obese rats”
Authors: P.M. Johnson, P.J. Kenny
A full copy of the paper is freely available here. http://www.nature.com/neuro/journal/vaop/ncurrent/pdf/nn.2519.pdf