When it comes to controlling hunger urges, a groundbreaking brain-imaging study from the US suggests that men’s brains are better able to do so, possibly explaining gender differences in rates of binge eating and obesity and why women find it harder to lose weight.

The study, which was led by researchers at the Brookhaven National Laboratory in the US Department of Energy, is to be published later this week as an early online issue in the Proceedings of the National Academy of Sciences.

Lead author of the study, Gene-Jack Wang, a Nuclear Medicine PET Medical Imaging scientist at Brookhaven Lab, said:

“Our findings may help us understand the neurobiological mechanisms underlying the ability to control food intake, and suggest new pharmacological methods or other interventions to help people regulate eating behavior and maintain a healthy weight.”

“The surprising finding of a difference between genders in the ability to inhibit the brain’s response to food and hunger will certainly merit further study,” he added.

Paul A M Smeets, who works at the Image Sciences Institute at the University Medical Center Utrecht, in the Netherlands, said:

“Something happens in the brains of men which does not happen in the brains of women. It’s quite amazing.”

According to CNN, Smeets, who uses brain imaging techniques to study the effects of hunger and fulness but was not involved in this study, told the press that these findings:

“Suggest that in real life there might be a difference in how good men and women are at suppressing this kind of desire.”

For the study, Wang and colleagues observed brain activity in 13 female and 10 male volunteers using PET (positron emission tomography) scans where glucose tagged with a radioactive tracer was injected into their bloodstream while they lay in the scanner. The brain feeds on glucose so the reserachers were able to observe the uptake by different regions of the brain in each of the subjects.

Each volunteer was asked to pick their favorite food from a list (eg bacon-egg-and-cheese sandwiches, pizza, cinnamon buns, barbecued ribs, chocolate), and then they underwent three scans at separate times. Before each scan they fasted for up to 20 hours.

During two of the scans, they were given food stimulation, where they could see and smell their stated favourite food and were given a taste of it on a swab. For one scan they were given no instructions on how to react. For the other, they were instructed beforehand to suppress their response to the food when it appeared. They also underwent a third scan where they were not presented with any food at all. The scans were done in random order.

The participants were also asked to rate the foods and describe their hunger sensations and desire to eat during food stimulation. Their responses to these questions were then compared to the brain activity observed on the PET scanner.

The results showed that for both men and women, various regions of the brain associated with controlling emotions, conditioning, and motivation “lit up” more in those scans where they had food stimulation compared to scans where they were not presented with any food. (When a brain region “lights up” during a PET scan it means that part of the brain is active because the radioactively tagged glucose is being metabolized). This confirmed earlier findings in another study at the Brookhaven Lab.

When they were asked to suppress their desire for the food, both men and women reported feeling less hungry and less interested in the food, compared to when they were not told to inhibit their response. But only the activity in the men’s brains reduced when they were asked to keep their desire for food in check, matching what they said about feeling less hungry. This was not so for the women.

As Wang explained:

“Even though the women said they were less hungry when trying to inhibit their response to the food, their brains were still firing away in the regions that control the drive to eat.”

Wang and colleagues think this is the firt time that such a gender difference has been shown in subjective reports versus observed brain activity of emotional and motivational states.

Perhaps this reflects a difference between men and women in the way they perceive and respond to internal body signals, said Wang.

“The finding of a lack of response to inhibition in women is consistent with behavioral studies showing that women have a higher tendency than men to overeat when presented with palatable food or under emotional distress,.” he added, suggesting that:

“This decreased inhibitory control in women could be a major factor contributing to the observed differences in the prevalence rates of obesity and eating disorders such as binge eating between the genders, and may also underlie women’s lower success in losing weight while dieting when compared with men.”

Speculating on what mechanisms might underpin such a difference, the researchers suggested estrogen might hold a key and warrants further exploration. Other studies have shown sex hormones help to regulate food intake, body weight and fat distribution, and they also exchange signals with molecules that regulate eating behaviour, they explained.

The study did not take into account possible influences from the women’s menstrual cycle, a factor that ought to be studied further, as co-investigator Nora Volkow, who is Director of the National Institute on Drug Abuse (NIDA), explained:

“A woman’s menstrual cycle can be an important factor in responsiveness to reward and in successful quit attempts for smoking.”

“Its role in inhibiting food-related brain activation will be important to address in future studies,” she added.

The study was sponsored by the National Institute on Drug Abuse, the National Institute on Alcohol Abuse and Alcoholism, and the General Clinical Research Center at Stony Brook University.

Click here for Proceedings of the National Academy of Sciences.

Sources: Brookhaven National Laboratory News, CNN.

Written by: Catharine Paddock, PhD