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Obesity makes it harder to lose weight

9th October 2012

Researchers in the United States say they have discovered brain changes linked to a diet high in saturated fat and refined sugar that may explain why weight loss can sometimes be very difficult.

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A team led by psychology professor Terry Davidson, director of American University's Center for Behavioral Neuroscience, found over-consumption of such foods may lead people along a path towards obesity, making it very hard for them to turn back again.

Davidson said his team had uncovered a "vicious cycle" which could explain today's obesity epidemic.

The effect is epitomised in a well-known advertisement for Lay's potato crisps, which challenges people with the slogan: "Betcha can't eat just one!"

Writing in the journal of Physiology & Behavior, Davidson said overweight and obese children are more likely to over-eat even when they are not hungry, because of the effects of saturated fat and refined sugar on the hippocampus, part of the brain which controls memory and learning.

The team tested trained rats fed a low-fat diet on two problems linked to hippocampal-dependent learning and memory abilities, as well as a problem that did not involve this area of the brain.

After the training was finished, the rats were divided into a group that was given unlimited access to low-fat food, and another group that was given unlimited access to high-energy food high in calories and saturated fat.

High levels of saturated fat in the diet have been linked to cardiovascular disease and some cancers.

The rats were given the same problems a second time. Davidson and his team found  the rats that had become obese from the high-energy food performed at a much lower level on the tests than did the group that had been given low-fat food.

Both groups of rats had similar results on the third test that didn't involve the hippocampus.

Using a dye that does not usually cross the blood-brain barrier, the researchers were able to show that the blood-brain barrier in the rats fed unlimited high-fat food had been impaired, and allowed a significantly larger amount of dye to pass into the brain than was found in the non-obese rats.

Some rats given the high-fat diet actually ate less of it, suggesting individuals have different levels of appetite for high-fat foods.

These rats also had better blood-brain barrier performance and less memory impairment than others in the group.

The factor that enabled them to eat less also helped to keep their brains cognitively healthy, the team concluded.

Assuming Davidson's findings apply to people, the study could show humans who have limited ability to suppress thoughts (via the hippocampus) are also more vulnerable to the affects of saturated fat and refined sugar in their diet.

Such an effect would be endlessly repeated in obese people, meaning they were more likely to eat fatty foods, and less likely to stop eating them, producing a vicious cycle of obesity and cognitive decline.

A high fat and sugar diet, in other words, sabotages the brain's natural ability to stop eating. The same inhibitory mechanism prevents cognitive decline by supporting memory.

Previous studies have found a link between obesity in humans in middle age and a higher chance of Alzheimer's in later life.

While massive weight loss is possible in obese people, they may need to permanently change their lifestyle in order not to put lost weight back on again.

The struggle may last a lifetime owing to the changes in the brain prompted by the obesity in the first place.

However, Davidson said it was still unclear at what point in the process such damage to the brain becomes permanent.

 

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