Women who are obese showed faulty learning when the reward was food, but not when the reward was money in a small lab experiment.

Obese women struggled to make accurate predictions about when a reward would appear compared with normal-weight individuals or obese men, Ifat Levy an assistant professor of comparative medicine and neurobiology at Yale University in New Haven, Conn., and her team found.

M&M candy

Obese women showed impaired learning when associating colours with reward foods like peanut M&M's in an experiment. (Brian Snyder/Reuters)

In the experiment, 135 normal-weight and obese men and women were tested on how well they were able to predict when they'd see a photo of a reward in the form of food or money on coloured squares.

The subjects knew they'd receive the snacks or cash pictured on the reward cards.

Part way through, the colour/reward association was switched.

But obese women continued to consider both colours to be predicative of a food reward.

So what could be going on?

"It is possible that once food is in the environment, obese individuals have a hard time ignoring its distracting properties," Levy said in an email.

Previous research points to differences in reward processing between obese women and obese men. In one study for example, obese women weren’t able to block out their desire to eat when stimulated by food compared with men.

In another, obese women showed a greater preference for immediate reward in the face of longer-term negative consequences compared with their normal-weight counterparts.

What’s more, in animal research, some types of high-fat diets seemed to lead to neural changes in parts of the brain that are involved in the type of learning that Levy tested.

The new study, published Thursday in the journal Current Biology, doesn’t offer information about any potential cause and effect relationships, such as whether impaired learning is a consequence of obesity in some women or a cause. Or both.

"When we are hungry, food is rewarding. However, once we reach satiety, the same food should not be rewarding anymore. An individual who fails to react to the change in his/her internal environment may continue to see the food as rewarding and overeat. This may lead to a vicious cycle, in which overeating leads to impaired learning and impaired learning leads to overeating, such that the learning impairment is both a cause for obesity and its effect," Levy said.
 
Sex differences in body image between men and women, hormones, and differences in the function of prefrontal brain areas involved in learning might also be important.

With files from CBC's Amina Zafar