Overweight More Likely to Make Unhealthier Choices When Faced with Real Food

Brain activity was a good predictor of which foods participants would choose when later faced with a selection of real food choices. But the presence of real food influenced choices differently across the groups.

In a related study published recently in the International Journal of Obesity, the researchers show that the brain structure in obese people differs from that in lean individuals in key regions of the brain involved in processing value judgements.
Previous studies have suggested that obesity is associated with a greater consumption of unhealthy foods – those with high sugar and/or fat content – even though lean and overweight people do not appear to differ in their judgements of the relative healthiness of foods.

 

To help explore further this apparent contradiction, researchers from the University of Cambridge and the Medical Research Council Human Nutrition Unit examined the relationship between how people judge the healthiness and tastiness of food and whether this predicts their food choices at a buffet lunch.

 

The researchers asked 23 lean and 40 overweight individuals to rate 50 common snack foods, presented on a computer screen, on a five-point scale for their healthiness and tastiness independently. They then examined the degree to which each of these individually-rated attributes appeared to influence a person's willingness to swap a particular food for one that had previously been rated as "neutral".

 

Neither choice behavior nor accompanying brain activity differed measurably according to participants' body weight. For both groups, taste was a much better guide to whether a person might choose to swap a food than healthiness.

 

The participants were then presented with an all-you-can-eat buffet. For each type of food, there were healthier and less healthy options, such as chicken sandwich and a BLT (bacon, lettuce and tomato) sandwich, or cola and diet cola.
Brain activity predicted the proportion of healthy food consumed in both lean and overweight individuals, and both groups selected a greater proportion of foods that they had rated as tasty. However, the overweight participants consumed more of the unhealthier foods than the lean participants.

 

"There's a clear difference between hypothetical food choices that overweight people make and the food they actually eat," says Dr Nenad Medic from the Department of Psychiatry. "Even though they know that some foods are less healthy than others and say they wouldn't necessarily choose them, when they are faced with the foods, it's a different matter.”

 

In a second study, the researchers looked at the brain structure of over 200 healthy individuals using an MRI scanner and found an association between BMI and brain structure. Strikingly, one of the regions showing this relationship overlapped with the region responding to food value in the first study – the ventromedial prefrontal cortex. The grey matter layer in this region was thinner in people with greater BMI.

"While the region is clearly responding in a way that is not distinct from leaner people, perhaps the structural differences suggest a reduced ability to translate what one knows into what one chooses,” explained Professor Paul Fletcher from the Department of Psychiatry.

 

"Although we can only speculate at this stage, and we really don't know, for example, whether this brain change is a cause or a consequence of increased weight, this could help explain why this same group of people found it harder to stick to their original, healthier food choices when presented with a buffet selection."

 

Professor Theresa Marteau, Director of the Behavior and Health Research Unit at the University of Cambridge, a co-author of the study, added: "These findings attest to the power of environments in overwhelming many people's desires and intentions to eat more healthily. The findings also reinforce the growing evidence that effective obesity policies are those that target food environments rather than education alone."