Nightly News | May 09, 2013
>>> with two-thirds of americans either officially obese or over weight , weight loss is a struggle for millions. it turns out there may be a genetic reason for that. what some lightly call the couch potato gene. our chief medical editor dr. nancy snyderman has our report.
>> reporter: ariella jackson has a love-hate relationship with exercise.
>> i don't want to get up. i don't want to exercise.
>> reporter: when she stopped training she gained weight. the 21-year-old blames herself. but a new study indicates it could be another culprit. the gene fto, already linked to obesity risk is now linked to something else -- the will to exercise.
>> so the robot is pulling dna from this plate.
>> reporter: for a decade general 'tis dr. molly bray has isolated millions of dna samples from thousands of subjects.
>> when people tell you, exercise is hard, it hurts for me, they are not lazy. they're reporting to you exactly what they are feeling.
>> reporter: in their dna she found the person with this fto sequence is more likely to keep exercising. with this sequence, more likely to drop out.
>> good job.
>> reporter: during final trials at the university of alabama at birmingham subjects are asked to train for 15 weeks and are measured, tested, and interviewed.
>> the hardest part was just actually deciding to do it.
>> reporter: dr. bray says that is fto talking.
>> do you think physicians are going to embrace the idea of genetics and obesity?
>> well, certainly for early onset severe obesity genetics will be very helpful. it will help physicians be more understanding and less biased toward obese patients.
>> reporter: the study also found.
>> it is empowering. they no longer feel i'm weak. they say, i do have a challenge. so i'm going to do something about it.
>> reporter: it worked for shane day.
>> i couldn't run a solid mile. couldn't run a solid quarter mile . now i can run six miles.
>> reporter: proof you can fight and conquer your genes. dr. nancy snydermannbc news, birmingh birmingham, alabama.