Rats show the perils of sugar addiction, researchers say
Sugar can be addictive, wielding power over the brains of lab animals much like a craving for drugs, according to Princeton University scientists who say their findings may eventually have implications for the treatment of humans with eating disorders.
Psychologist Bart Hoebel and colleagues at the university's neuroscience institute have studied what they call sugar addiction in rats for years.
They say their rats have met two of the three elements of addiction — they show a pattern of increased intake and then signs of withdrawal. But Hoebel's most recent experiments also demonstrate a third element — craving and relapse.
"If binging on sugar is really a form of addiction, there should be long-lasting effects in the brains of sugar addicts," Hoebel said in a news release. "Craving and relapse are critical components of addiction, and we have been able to demonstrate these behaviours in sugar-binging rats in a number of ways."
Hoebel presents his findings Wednesday to the American College of Neuropsychopharmacology in Scottsdale, Ariz., just in time to give humans pause during their holiday feasting.
The researchers encouraged sugar binging by denying the rats food for four hours after waking. "It's a little bit like missing breakfast," Hoebel said. "As a result, they quickly eat some chow and drink a lot of sugar water."
Previous research showed hungry rats that binge on sugar experience a surge of dopamine in their brains. Dopamine is a neurotransmitter that plays a major role in addiction and affects movement, emotional response and the ability to experience pleasure and pain.
After a month of binging, the rats' brains adapt to increased dopamine levels and have fewer of a certain type of dopamine receptors than before. Similar changes have been observed in the brains of rats fed cocaine and heroin.
Sugar withdrawal
Once the rats were hooked, researchers induced symptoms of withdrawal by taking away their sugar. The level of dopamine in the rats' brains dropped and they exhibited anxious behaviour such as chattering teeth and an unwillingness to venture into the open arm of their maze, preferring to stay in a tunnel area. Normally rats like to explore their environment, but the researchers say the rats experiencing sugar withdrawal were too anxious to explore.
Finally, Hoebel and his colleagues found that rats denied sugar for a prolonged period after learning to binge worked harder to get it when it was reintroduced to them. In fact, they ate more sugar than ever, suggesting craving and relapse behaviour.
Hoebel said such behaviour shows their motivation for sugar had grown. "Abstinence makes the heart grow fonder," he said.
Perhaps most disturbingly, the rats drank more alcohol than normal after their sugar supply was cut off. Hoebel suggests changes in their brains served as "gateways" to other destructive behaviour, such as increased alcohol intake.
Previous research at Laurentian University in Sudbury, Ont., showed similar results.
When cognitive neuroscientist Dr. Michael Persinger fed his rats sucrose, he found they liked sugar, a lot. They drank up to 30 per cent of their body weight every day when the sugar water was available, sucking on the bottle every five minutes. They also ate more and had disturbed sleeping patterns. All are symptoms of addiction.
In an interview with CBC's Marketplace in 2002, Persinger mused about how his findings might illuminate addiction in people.
"The important aspect of addictive behaviours is that the time between the craving and the reinforcement is immediate. That’s why fast foods are so effective," he said.