December 30, 2008

Sugar-Addicted Rats!

When rats are denied access to sugar after having had easy access to it, they show classic signs of addiction - bingeing and withdrawal...

In this holiday season with sweets staring you down everywhere you go, you are likely to be well aware of the power of sugar. A recent study shows that, at least in rats, sugar is an addictive substance.

Rats that had their sugar supply cut off would drink large amounts of sugar water when it was once again made available to them.

The word addiction has several different meanings, both in common usage and in the language of scientists. Researchers with a background in biochemistry tend to focus on physical dependence, while behavioral scientists think more in terms of psychological dependence. The American Psychiatric Association defines addiction as including three stages: craving, bingeing, and withdrawal. While sugar bingeing and withdrawal have been previously demonstrated in rats, this is the first study to demonstrate all three stages of sugar addiction.

In the study, rats that had their sugar supply cut off would drink large amounts of sugar water when it was once again made available to them. This is bingeing. Taking away their sugar caused several signs of withdrawal in the rats: they exhibited anxiety, their teeth chattered and their behavior changed. It's normal rat behavior (in a laboratory) to explore a maze when placed in one; sugar−deprived rats would cower in the sheltered area of the maze, which is taken to be an expression of rat anxiety. After sugar denial, the rats ate more sugar than ever before when it was re−introduced, and also worked harder to get it, indicating craving. Together, that's the whole addiction package.

The study also showed that bingeing on sugar caused long−term changes in the structure of the rats brains. The bingeing caused release of large amounts of the neurotransmitter dopamine in an area of the brain associated with motivation and reward. After a month, the rat's brains reacted to the excess dopamine; they had less of a particular type of dopamine receptor and more total opioid receptors. This is similar to the changes seen in rats given heroin or cocaine.

While the findings have possible implications for humans, particularly in the field of eating disorders, it's important to realize that there are significant differences between rats and humans. For one thing, humans live in nicer cages. A good deal of the study relies on interpretation of the behavior and motivation of caged rats. Most humans display profound differences in both motivation and behavior from that seen in rats. What the study does show is that sugar can produce changes in both the rat brain and rat behavior that are similar to the changes addictive drugs produce in the human brain and in human behavior.

The basic method employed in the study was to allow the rats access to a sugar solution for half the day, but deny access while they slept and for the first four hours after awakening. It wasn't the sugar alone that caused the observed effects, it was the sugar combined with the cycle of deprivation and unlimited access.

The study was conducted by the team of Bart Hoebel, a professor of psychology at Princeton University and long−time researcher into the mental rewards of eating. He was scheduled to present the findings at the annual meeting of the American College of Neuropsychopharmacology in Scottsdale, Arizona on December 10, 2008. A paper on the study has also been submitted to the Journal of Nutrition.

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