Scientists led by Dr. Rachel Batterham at University College London's Metabolism and Experimental Therapeutics and colleagues recruited 359 healthy male volunteers to examine the "real life" effects of the FTO variation in humans.
They studied two groups of participants -- those with two copies of the high obesity-risk FTO variant, or the AA group, and those with the low-obesity-risk version, or the TT group.
The researchers matched the volunteers perfectly for body weight, fat distribution and social factors such as educational level to ensure that any differences they saw were linked to FTO, and not to other physical or psychological characteristics, the study said.
A group of 20 participants -- 10 AA and 10 TT -- were asked to rate their hunger before and after a standard meal, while blood samples were taken to test levels of ghrelin, a hormone released by cells in the stomach that stimulates appetite.
The study, published in the Journal of Clinical Investigation, found while normally ghrelin levels rose before meals and fell after eating, this study found men with the AA variation had much higher circulating ghrelin levels and felt hungrier after the meal than the TT group. This suggested the obesity-risk variant, or the AA group did not suppress ghrelin in a normal way after a meal.
"What this study shows us is that individuals with two copies of the obesity-risk FTO variant are biologically programmed to eat more," Batterham said in a statement. "Not only do these people have higher ghrelin levels and therefore feel hungrier, their brains respond differently to ghrelin and to pictures of food."