Although the term "sugar addiction" often appears in magazines and on television, scientists had not demonstrated that such a thing as sugar dependency really exists, said neuroscientist Bart Hoebel, who led the study. Hoebel and colleagues studied rats that were induced to binge on sugar and found that they exhibited telltale signs of withdrawal, including "the shakes" and changes in brain chemistry, when the effects of the sweets were blocked. These signs are similar to those produced by drug withdrawal.
Sugar, said Hoebel, triggers production of the brain's natural opioids. "We think that is a key to the addiction process," he said. "The brain is getting addicted to its own opioids as it would to morphine or heroin. Drugs give a bigger effect, but it is essentially the same process."
Hoebel emphasized that there are more elements to addiction than bingeing and withdrawal, and that further studies will be needed to complete the picture. Also, it is not clear how closely the findings might apply to humans, he said.
The greatest value of the research, Hoebel said, is that it provides an animal model of sugar dependency, allowing scientists to probe more deeply the connections between food cravings and brain physiology.
As far as the plight of the many people who feel powerless in the face of sugar, it also is not clear how the findings will help, he noted. "Unfortunately, it's very difficult to treat addictions," he said. "But it does change the way the person might look at it. It puts it in the realm of an addictive disorder rather than a failure of willpower."
Hoebel's research was published in the June issue of Obesity Research. His co-authors are former undergraduates Carlo Colantuoni, Joseph McCarthy, Caroline Patten and Andrew Chadeayne, visiting researcher Pedro Rada and graduate student Nicole Avena.
"I think the results are interesting and exciting and provide a new way of looking at overeating," said Harry Kissileff, a psychologist and specialist in human food intake at Columbia University. Kissileff agreed that Hoebel's rats offer an important model system, but said he would be cautious about using them to put sugar in the same category as drugs.
"There is some overlap between the systems that control food intake and addiction," Kissileff said. "I am not sure they necessarily make food addictive."
In their experiments, Hoebel and colleagues in his lab started rats on a pattern of bingeing by withholding food for 12 hours when the rats were sleeping and through breakfast time, then giving them nutritionally balanced food plus sugar water. The animals gradually increased their daily sugar intake until it doubled, consuming most of it in the first hour it was available.
When the researchers suddenly removed the sugar portion of the rats' diet, the animals exhibited teeth chattering, a common sign of withdrawal. For some animals, the researchers removed the sugar and also administered a dose of a drug that blocks the opioid receptors in the brain. In addition to teeth chattering, those animals showed anxiety and a reversal in the usual balance of neurochemicals in the brain's motivation system.
Animals that binged on normal food with no sugar and received the opioid blocker did not show these withdrawal signs. Animals that were given a steady diet of food and sugar water without binging also did not show signs of withdrawal.
"The implication," said Hoebel, "is that some animals, and some people, can become overly dependent on sweet food, particularly if they periodically stop eating and then binge. This may relate to eating disorders such as bulimia."
Hoebel noted that his rats should be termed "sugar-dependent" rather than addicted, because he has not yet proven that they exhibit all the elements that make up the definition of addiction, which is three-fold: a behavioral pattern of increased intake and changes in brain chemistry; then signs of withdrawal and further changes in brain chemistry upon deprivation; and third, signs of craving and relapse after withdrawal is over. Experiments in Hoebel's lab so far have shown the first two points, while ongoing experiments are investigating craving and relapse in the rats.