Although obesity was seen as a sign of social status during the Renaissance, it’s been known since long before then that being overweight is actually unhealthy.
We all know that carrying around extra pounds increases the risk for high blood pressure, diabetes and cardiovascular disease, yet one out of three Americans is obese.
New research, published online today in Science, suggests that for some obese people, genetic variations may desensitize the brain’s reward centers, making food less satisfying and leading to the consumption of more calories followed by the predictable result – weight gain.
Dopamine
When we eat, feelings of pleasure are produced by the release of a chemical called dopamine into the brain. Recent research has shown that the brains of obese people have fewer dopamine-responsive receptors; this deficit may decrease the pleasure obese people get from food and prompt them to overeat in order to compensate.
“Individual differences in how the brain processes food reward have been postulated to play a role in explaining why some, but not all, people are gaining weight in an environment where calories are plentiful. Our finding is exciting because it supports this possibility by demonstrating an association between an abnormal response to food and future weight gain — and it shows that this relationship depends on your genetic make-up,” said study author Dana Small in a statement.
The Study
Small and her colleagues examined 43 college women and 33 adolescent girls using functional magnetic resonance imaging scans, which analyze brain activation by measuring blood flow in the brain, while the study subjects tasted either a chocolate milkshake or a flavorless solution.
In both study groups, women with higher body mass indices (BMI) had lower levels of activation in dopamine-responsive areas of their brains as they tasted the milkshake. This relationship was significantly stronger in women with one or two copies of a variation in the dopamine receptor gene DRD2 that scientists call the “Taq1A A1 allele”.
The A1 allele has been associated with fewer dopamine receptors in the brain. Previous research has shown that people with one or two copies of the A1 allele are more likely to be obese. Other traits – such as alcoholism, obsessive-compulsive disorder, and error avoidance — have also been linked to the A1 allele and thus the brain’s reward pathways.
After one year, the researchers found, women with one or two copies of the A1 allele who had also shown lower brain activation in response to food gained more weight, possibly because of their diminished pleasure response.
“Identifying changes in behavior or pharmacological options could correct this reward deficit to prevent and treat obesity,” said the study’s lead author Eric Stice.