Common Mechanisms of Drug Abuse and Obesity
Research Suggests Food Availability Could Prompt Addiction
Some of the same brain mechanisms that fuel drug addiction in humans accompany the emergence of compulsive eating behaviors and the development of obesity in animals, according to research funded by the National Institute on Drug Abuse (NIDA), a component of the National Institutes of Health.
The study, conducted by researchers at the Scripps Research Institute, was released today in the online version of Nature Neuroscience and will also appear in the journal's May 2010 print issue. When investigators gave rats access to varying levels of high-fat foods, they found unrestricted availability alone can trigger addiction-like responses in the brain, leading to compulsive eating behaviors and the onset of obesity.
"Drug addiction and obesity are two of the most challenging health problems in the United States," said Dr. Nora D. Volkow, director of NIDA. "This research opens the door for us to apply some of the knowledge we have gathered about drug addiction to the study of overeating and obesity."
Both obesity and drug addiction have been linked to a dysfunction in the brain's reward system. In both cases overconsumption can trigger a gradual increase in the reward threshold - requiring more and more palatable high fat food or reinforcing drug to satisfy the craving over time.
Researchers conducted this study in three groups of male rats over a 40-day period. Each day, the three groups had unlimited access to standard lab food. In addition, two of the groups also had access to high-fat, cafeteria style foods for short (one-hour) or long (18-23 hours) periods.
After 40 days, all groups were denied access to the high-fat foods. Throughout the study, researchers observed the feeding behaviors of each group, noting caloric intake, weight gain, and brain response.
The results support the notion that type 2 dopamine receptors (D2DR) - brain receptors that have been shown to play a key role in addiction - also play a key role in the rats' heightened response to food. In fact, as the rats became obese, the levels of D2DR in the brain's reward circuit decreased. This drop in D2DR is similar to that previously seen in humans addicted to drugs like cocaine or heroin.
"The results of this study could provide insight into a mechanism for obesity," said Paul J. Kenny, one of the study's co-authors and an associate professor at the Scripps Jupiter, Fla., research facility. "It's possible that drugs developed to treat addiction may also benefit people who are habitual overeaters."
Study results also suggest that environmental factors, such as increased or unlimited access to high-fat food options, can contribute to the problem of obesity.
"Hopefully, this study will change the way people think about eating," said Paul Johnson, a co-author and graduate student in the department of molecular therapeutics. "It demonstrates how just the availability of food can trigger overconsumption and obesity."
The study titled: "Addiction-like reward dysfunction and compulsive eating in obese rates: Role for dopamine D2 receptors," by Paul M. Johnson and Paul J. Kenny in Nature Neuroscience can be found online at: http://www.nature.com/neuro/journal/vaop/ncurrent/index.html
Fatty foods may cause cocaine-like addiction
Scientists have finally confirmed what the rest of us have suspected for years: Bacon, cheesecake, and other delicious yet fattening foods may be addictive.
A new study in rats suggests that high-fat, high-calorie foods affect the brain in much the same way as cocaine and heroin. When rats consume these foods in great enough quantities, it leads to compulsive eating habits that resemble drug addiction, the study found.
Doing drugs such as cocaine and eating too much junk food both gradually overload the so-called pleasure centers in the brain, according to Paul J. Kenny, Ph.D., an associate professor of molecular therapeutics at the Scripps Research Institute, in Jupiter, Florida. Eventually the pleasure centers "crash," and achieving the same pleasure--or even just feeling normal--requires increasing amounts of the drug or food, says Kenny, the lead author of the study.
"People know intuitively that there's more to [overeating] than just willpower," he says. "There's a system in the brain that's been turned on or over-activated, and that's driving [overeating] at some subconscious level."
In the study, published in the journal Nature Neuroscience, Kenny and his co-author studied three groups of lab rats for 40 days. One of the groups was fed regular rat food. A second was fed bacon, sausage, cheesecake, frosting, and other fattening, high-calorie foods--but only for one hour each day. The third group was allowed to pig out on the unhealthy foods for up to 23 hours a day.
Not surprisingly, the rats that gorged themselves on the human food quickly became obese. But their brains also changed. By monitoring implanted brain electrodes, the researchers found that the rats in the third group gradually developed a tolerance to the pleasure the food gave them and had to eat more to experience a high.
They began to eat compulsively, to the point where they continued to do so in the face of pain. When the researchers applied an electric shock to the rats' feet in the presence of the food, the rats in the first two groups were frightened away from eating. But the obese rats were not. "Their attention was solely focused on consuming food," says Kenny.
In previous studies, rats have exhibited similar brain changes when given unlimited access to cocaine or heroin. And rats have similarly ignored punishment to continue consuming cocaine, the researchers note.
The fact that junk food could provoke this response isn't entirely surprising, says Dr.Gene-Jack Wang, M.D., the chair of the medical department at the U.S. Department of Energy's Brookhaven National Laboratory, in Upton, New York.
"We make our food very similar to cocaine now," he says.
Coca leaves have been used since ancient times, he points out, but people learned to purify or alter cocaine to deliver it more efficiently to their brains (by injecting or smoking it, for instance). This made the drug more addictive.
According to Wang, food has evolved in a similar way. "We purify our food," he says. "Our ancestors ate whole grains, but we're eating white bread. American Indians ate corn; we eat corn syrup."
The ingredients in purified modern food cause people to "eat unconsciously and unnecessarily," and will also prompt an animal to "eat like a drug abuser [uses drugs]," says Wang.
The neurotransmitter dopamine appears to be responsible for the behavior of the overeating rats, according to the study. Dopamine is involved in the brain's pleasure (or reward) centers, and it also plays a role in reinforcing behavior. "It tells the brain something has happened and you should learn from what just happened," says Kenny.
Overeating caused the levels of a certain dopamine receptor in the brains of the obese rats to drop, the study found. In humans, low levels of the same receptors have been associated with drug addiction and obesity, and may be genetic, Kenny says.
However, that doesn't mean that everyone born with lower dopamine receptor levels is destined to become an addict or to overeat. As Wang points out, environmental factors, and not just genes, are involved in both behaviors.
Wang also cautions that applying the results of animal studies to humans can be tricky. For instance, he says, in studies of weight-loss drugs, rats have lost as much as 30 percent of their weight, but humans on the same drug have lost less than 5 percent of their weight. "You can't mimic completely human behavior, but [animal studies] can give you a clue about what can happen in humans," Wang says.
Although he acknowledges that his research may not directly translate to humans, Kenny says the findings shed light on the brain mechanisms that drive overeating and could even lead to new treatments for obesity.
"If we could develop therapeutics for drug addiction, those same drugs may be good for obesity as well," he says.
Dear Drugs-Forum readers: We are a small non-profit that runs one of the most read drug information & addiction help websites in the world. We serve over 4 million readers per month, and have costs like all popular websites: servers, hosting, licenses and software. To protect our independence we do not run ads. We take no government funds. We run on donations which average $25. If everyone reading this would donate $5 then this fund raiser would be done in an hour. If Drugs-Forum is useful to you, take one minute to keep it online another year by donating whatever you can today. Donations are currently not sufficient to pay our bills and keep the site up. Your help is most welcome. Thank you.