Feeling unhappy with your appearance is nothing new. 

But as the country's population grows more overweight and obese, it's becoming more normal to have a negative body image, says Tiffany Stewart, a professor at the Pennington Biomedical Research Center.

"Most people are dissatisfied with the way their bodies look because the gap between what we look like and what we want to look like is bigger," she said.

And contrary to popular opinion, it's not just women. Men also have negative images of their bodies. Only 28 percent of men — compared with 26 percent of women — say they are extremely satisfied with their appearance, according to a study published in the Body Image journal last year. 

Stewart has dedicated much of her career to studying the way Americans view their bodies. And she has learned that changing the way we see our appearance can also improve our lives.

Growing up a gymnast, Stewart often saw powerful, strong girls succeed in competition but still feel pressure from coaches to fit an ideal body type. They were told to lose weight and get lean to fit the ideal gymnast's physique.

"There was this idea that you are supposed to perform a certain way, but you were supposed to look a certain way," she said. 

That experience and Stewart's interest in psychology led her to specialize in body image, working with collegiate athletes, soldiers in the Army and patients in weight loss surgery clinics. 

Our culture dictates what the optimal body is, Stewart says. We should be thin, people seem to believe, but not too thin, and strong, but not overly muscular.

"There is a cultural ideal, and everyone knows what it looks like," she said.

But everyone is shaped differently, and many people aren't capable of reaching that cultural ideal.

"There are so many things that play into how our bodies look — genetics, training," she said. "We could all do exactly the same exercise training plan and look very different."

A multibillion-dollar fitness industry has emerged around Americans' desires to change the way they look. Gyms, magazines, supplement companies and even mattress stores sell their products based on how they help you look young and healthy.

"People are being marketed to based on their insecurities about their appearance," Stewart said.

But appearance is a poor motivator, she says. Stewart has found most people cannot stick with an exercise plan or another healthful lifestyle change if their only goal is changing the way they look.

"Feeling bad about your body does not motivate you to do things that are healthy," she said. "It just makes you feel worse."

Instead, Stewart encourages athletes and patients to focus on what they need their bodies to do — not how they look.

When she was invited to speak about body image at the TEDx event at LSU last month, she compared this idea to a principle architects follow when designing buildings: Form must follow function. A building must first fulfill a purpose, which the exterior will complement.

"People really need to ask themselves, 'What is my body capable of?' versus 'What should my body look like?'" she said.

This idea developed after Stewart's gymnastics career led to numerous injuries and caused ankle, shoulder and neck pains in adulthood. A few years ago, she had her foot reconstructed, a painful surgery with a difficult recovery.

While recuperating, she worked out to keep the rest of her body strong so she could continue doing the things she loves, like traveling and bicycling.

"When you're in that state, you're not looking at what your body looks like," she said. "You're looking at how it works."

That state should be the norm, she says. Psychologists call it body appreciation, where you live a healthful lifestyle in order to maintain the body's function.

"You are willing to appreciate its function and kind of be grateful for that and recognize the benefits of health behaviors aside from what it does to your appearance," Stewart said.

Those who live in that state are more likely to engage in a healthy lifestyle. They work out, eat fruits and vegetables, avoid smoking and wear sunscreen, she says.

Other than just studying the effects of body image on our society, Stewart is speaking out. She has helped devise technologies that help gauge a person's body image, and she lobbies to make women's athletic uniforms more practical and less skimpy when the sport doesn't call for form-fitting or revealing clothing. 

"We must convince our younger generation that a healthy body is an ideal body," she said, "and just because something is standard does not make it ideal."

Follow Kyle Peveto on Twitter, @kylepeveto.