February 22, 2010 — Vol. 26 No. 8
The obesity epidemic cannot be blamed on people’s genetic makeup, but can be pinned to environmental factors, suggests a variety of studies by Idaho State University associate professor of psychology Erin Rasmussen.
Rasmussen, with the help of ISU undergraduate and graduate students, is studying the epidemic from several different angles and has also completed some studies of drugs that may someday be used in the United States to fight the malady.
“There has been an increase in prevalence of obesity the last 30 years, although those rates have begun to level off a bit,” Rasmussen said. “It’s a problem globally, too, not just here in the United States. The increase in obesity rates appears to be caused by environmental factors.”
The ISU psychology professor has developed several experimental models to help test her hypotheses. In a couple of her more prominent experiments she has used or is using Zucker rats, a strain of rats that is genetically predisposed to obesity. Zuckers as adults weigh two to three times as much as normal rats. Because of this trait, this strain has been used in a variety of studies on obesity for about 50 years.
“Zuckers rats may literally eat themselves to an early death, just as humans may do in the wrong circumstances,” Rasmussen said.
However, Zuckers will only eat themselves to premature deaths when food is freely available. Zuckers will eat twice as much and increase their daily intake of food by more than twice as much as normal lean rats when food is easily available to them in an unlimited supply.
But Rasmussen found, through clever experiment, that when the price of food goes up and it isn’t free, at a certain point even genetically-predisposed-to-obesity Zucker rats behave like their leaner brethren.
Rasmussen designed some experiments in which both lean and obese rats had to press a lever a given amount of times to receive a 45-milligram pellet of food, which is about the size of a crumb. Both sets of rats had to press the lever from one to 300 times to get the reward.
“We, in effect, manipulate the price for food, by making them press the lever at a range of values to get the food,” Rasmussen said. “The more difficult it is to get the food, the less they are willing to get it. When both sets of rats had to press the lever 50 or more times their behavior is virtually identical.” At lower prices, though, between 1 and 50 responses, the obese rats will earn more food pellets than lean rats.
This study points out that accessibility to food is a major factor in the obesity epidemic. It is also probably a clinically stronger study than some on obesity that were based on self-reporting by human participants or by laboratory animals with free access to food. In this study the schedule of how much food was delivered to a rat played a far greater role in predicting how much food rats would eat than the genetics of the animal.
Food availability obviously plays a major role in obesity and another study Rasmussen has published, this time a translational human study conducted with her colleague, Dr. Steven Lawyer, showed that obese people are less likely to wait as long for food compared to healthy-weight individuals, which may also contribute to their weight status.
Another major factor in the obesity epidemic is the role a sedentary lifestyle plays. TV, the Internet, office jobs, fast and easy food can combine to conspire against us and lead to an inactive lifestyle. Rasmussen and her students have developed a study in which motivation for exercise in her obese and lean Zucker rats has been examined. The animals have to press a lever at different response requirements to gain access to 2 minutes in an exercise wheel. Results from this study indicate that the obese Zucker rats find exercise rewarding, but only about half as rewarding as the lean rats. The higher the cost to exercise, the less motivated they were to do it.
Rasmussen’s published studies indicate that Zucker rats find food more reinforcing and find exercise less reinforcing than lean rats. Not coincidentally, Rasmussen is studying a drug, rimonabant, which addresses the double-whammy that contributes to obesity noted above. Giving Zucker rats rimonabant reduces the animal’s food intake and food reward, resulting in increased weight loss and improved health.
“Rimonabant also reduced the reinforcing value of cues associated with food, which to a human would be things like the kitchen or images of food that you see on a commercial…stimuli that make you more likely to search for and eat food. We also have some preliminary evidence that shows that at low doses, rimonabant may also enhance the rewarding properties of exercise,” Rasmussen said.
The drug has not been approved for use in the United States and has some potential side effects. However, the class of drugs rimonabant is in, the cannabinoids offers an alternative to amphetamine-based drugs that are prescribed for treating obesity and holds some promise as a treatment. The more studies done on this class of drugs, the greater the knowledge base on this drug will be, increasing the potential for using it to effectively treat obesity in humans.
As a side note to Rasmussen’s research, it is perhaps ironic that years ago Rasmussen developed an allergy to working with rats called “rats lung,” caused by spending too much time with the animals. She has to strictly limit the time she spends with the animals.
“If I am near them too much I can have a severe reaction, so my students and trained staff spend most of the time with the rats,” Rasmussen said. “I’m told by my allergist I shouldn’t be doing rat research, but it is fun and important work. I love doing it.”
Rasmussen’s research has been funded by the National Science Foundation’s WeLEAD program, the Idaho INBRE program, which is funded through the National Institutes of Health, and various internal grants at Idaho State University. She currently also has grants submitted to the National Institutes of Health.