Genetic effects are of little importance in switching on your appetite; rather it's all a matter of blood sugar levels, finds a study.
However, when it comes to feeling full, genes do matter, says Dr Ellen Schur of the University of Washington, who presents her team's work at the annual meeting of the Society for the Study of Ingestive Behavior in Seattle this week.
The team looked at 21 pairs of identical twins, investigating whether there was an inherited influence in the way their brains responded to photos of food, and in how that response changed when they were full.
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The twins ate identical breakfasts and had functional MRI brain scans 3.5 hours later, while they looked at photos of high and low calorie foods. The scanner recorded the activity in the appetite centers of their brains.
By using identical twins, the researchers ensured that each pair effectively had the same genes and the same upbringing.
"We're asking whether twins are more similar to each other than to other unrelated individuals [in terms of their response to the food photos]," says Schur.
"If you put a bunch of people into an fMRI scanner, even though we controlled how they ate... there is a wide range to how people respond to the pictures," she explains. "In fact the twins were not more similar to each other in the way they responded."
She concludes genes and upbringing have little influence on brain appetite centers after several hours without food. "Instead what we found was that activation was strongly related to the individual glucose levels measured in their blood at the time."
People with lower glucose levels showed more brain activation, especially when looking at photos of high-calorie fattening foods.
Feeling full runs in the family
The twins then ate macaroni cheese for lunch and had another fMRI scan immediately afterwards, again measuring their response to food photos.
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Importantly, the meal affected the activation levels of each member of a pair of twins more similarly than the others doing the experiment.
However, people whose glucose levels remained low still had more brain activation in areas that motivate eating.
As if they hadn't had enough, the twins were then let loose on a buffet, where the amount they consumed was secretly measured. Twins ate similar amounts and also reported similar levels of fullness, suggesting that genetics and upbringing plays a role in satiety.
Results make sense
Australian expert Dr Zane Andrews of Monash University says it is "very interesting" that Schur and colleagues found the greatest activation of appetite centers in the brain were in people with low blood sugar.
"That's absolutely as we'd predict," he comments.
He explains that maintaining blood sugar levels is key to survival, and that low blood glucose can lead to coma.
"If the brain senses that blood glucose is getting low it will do all it can to get you to eat," says Andrews.
Andrews has not seen the results, but suspects that the part of the brain that is turning on is the motivation and reward pathway. "This is activated by drugs like cocaine but it is also activated by food."
Schur says her team's work supports other studies that find skipping meals is not a good way to diet. They now want to look at how being obese affects the appetite centers.