There's a lot more to feeling hungry than you might think. And the science behind our cravings could help explain why Americans are getting bigger.

Some people can't seem to lose weight. Experts at Johns Hopkins predict three out of four adults in the U.S. will either be overweight or obese by year 2015 - that's just seven years away is being fat in our genes? Jeannette Standard, who is morbidly obese, believes she's proof.

"I always figured I got it from my parents 'cause daddy was overweight, mama was heavier than what she should have been," said Standard.

Standard weighed 150 pounds in fifth grade, 180 pounds when she started high school, 250 by the time she got married. At 30, that number doubled. Then it doubled again. Her heaviest weight is 1,200.

Could whether we're fat or thin be determined by something more than willpower?

"it actually may start at conception," said Dr. Melinda Sothern, an exercise physiologist at Louisiana State University.

Sothern says our genes may pre-determine the future weight of our children, and the first few years of life are critical.

"They are born with a certain number of genes. They inherit and then during their development, they have genes that will either turn on or turn off depending upon their environment and depending upon what behaviors they are allowed to participate in," said Sothern.

Researchers have also found a gene that may influence obesity.

"If you do have the FTO gene, it does put you at risk for becoming obese and having Type 2 diabetes and extra body fat," said Emily Rubin, RD, weight loss dietician.

Standard worries she's passed her weight problem to her children. She just hopes they'll learn how to reverse the trend before it's too late.

Losing weight is a life long battle. Only one-third of morbidly obese people are able to keep the weight off for more than year.

Exercise is key at any age. Studies show how much physical activity children are exposed to in the first three years of life has a lot to do with whether or not they become overweight.