An unhealthy diet is considered a key contributor to obesity. When it comes to cravings for sweet treats, however, impairments in the brain's reward system might be to blame.
Researchers say the reward system in the brains of obese individuals appears to be impaired in response to sweet foods.
In a new study published in the journal Diabetes, researchers found age and receptor levels of the reward-associated chemical dopamine influence preference for sweet foods among people of a healthy weight, but not for people who are obese.
First author M. Yanina Pepino, Ph.D., of the Washington University School of Medicine, and colleagues reached their findings by enrolling 44 adults aged 20-40 years.
A total of 24 of the participants were obese - defined in the study as having a body mass index (BMI) of 30 or higher - while 20 were a healthy weight.
The researchers asked the participants to consume a number of drinks, each containing different sugar contents, and rate which ones they preferred.
Subjects then underwent positron emission tomography (PET) scans, which allowed the researchers to assess dopamine receptor levels in each subject's brain.
Dopamine is a neurotransmitter - a chemical that enables communication between nerve cells - that regulates the reward and pleasure centers of the brain.
Insulin resistance may impair brain's reward response to sugar
On analyzing the brains of the healthy-weight participants, the researchers found that younger age and fewer dopamine receptors were associated with a greater preference for sugar.
"We found disparities in preference for sweets between individuals, and we also found individual variations in dopamine receptors - some people have high levels and some low," explains study co-author Tamara Hershey, Ph.D., a professor of psychiatry, neurology, and radiology at Washington.
"But when we looked at how those things go together, the general trend in people of normal weight was that having fewer dopamine receptors was associated with a higher preference for sweets."
However, this was not the case in the brains of the obese participants, suggesting that the brains of obese individuals are altered in some way to influence preference for sweet foods.
"We believe we may have identified a new abnormality in the relationship between reward response to food and dopamine in the brains of individuals with obesity."
M. Yanina Pepino, Ph.D.
Hershey notes that some of the obese participants had high blood glucose and insulin concentrations, which may have altered the brain's response to sugar.
"There is a relationship between insulin resistance and the brain's reward system, so that might have something to do with what we saw in obese subjects," she explains.
"What's clear is that extra body fat can exert effects not only in how we metabolize food but how our brains perceive rewards when we eat that food, particularly when it's something sweet."
It is unclear exactly what these findings may mean for individuals who are obese, but the researchers say they shed light on the neurological mechanisms that influence sweet preferences.