Editor's note: This is the fourth story in CNN's series exploring the issues surrounding childhood obesity.
(CNN) -- Lyn McDonald is doing everything right.
After losing more than 80 pounds, she taught her kids how to control their portion sizes, shop at the farmers market, eat vegetables with every meal and avoid a lot of sugar.
Her efforts are working. At a time when approximately one-third of American children are overweight or obese, McDonald's kids are at healthy weights.
So why is every day still a struggle for the blogger and mother of five?
"I have had to deal with teachers who hand out Skittles, candy bars, lollipops and giant frosted sugar cookies to the children in class ... before 10 a.m.," McDonald says. "I think this is setting kids up for failure and un-teaching the healthy habits I have instilled."
The fact that doughnuts and cupcakes are given out as a reward after soccer practice or dance class is a paradoxical hurdle in the fight against childhood obesity. As doctors and parents struggle to encourage healthy behaviors, our sugar-filled, sedentary surroundings resist every step.
Think about it, says Dr. Stephen Daniels, chief pediatrician at Children's Hospital Colorado. Every day kids are exposed to advertising about fast food instead of home-cooked meals. They're surrounded by vending and soda machines at school. They have hundreds of channels on TV, own three video game systems and live in neighborhoods that were built without sidewalks.
"Our environment is constantly pushing kids in the wrong direction."
Childhood obesity isn't just a cosmetic issue, although studies have shown overweight children are often isolated and bullied by their peers.
Obese youth are more likely to suffer from cardiovascular disease, diabetes, sleep apnea, liver disease and bone and joint problems, according to the Centers for Disease Control and Prevention. Excess fat has also been linked to many types of cancer. About two-thirds of obese children grow up to be obese adults.
Gary Evans is an environmental and developmental psychologist at Cornell University. His latest study, published this year in the journal Pediatrics, analyzed the effects of stress on weight gain in children and adolescents.
Researchers know that both adults and children seek higher fat foods in response to stress. Evans and his team found that stress also damages a child's ability for self-control, which leads to a higher body mass index as a teen.
Evans examined children who were dealing with stressful situations, such as poverty, single parenthood, housing problems and domestic violence. In the study, stress hormones hurt the brain's pre-frontal cortex -- the one responsible for our ability to plan and avoid temptations -- at the cellular level.
See also: How to stop your kids from stressing
It's kind of a quadruple whammy, Evans notes. Lower income children have less healthy food stores nearby, more junk food available because it's cheap, fewer places to play outdoors and, as his team found out, a harder time curbing bad impulses.
"If you are born poor, your life expectancy is less," Evans wrote in an e-mail. "Perhaps even more striking ... upward mobility does not remove the ill effects of early childhood poverty on subsequent health and well-being."
For parents trying to raise healthy kids, this is all kind of depressing.
"What we need to do as a society is work to make the healthier choice the easier choice," says Daniels.
There has been movement in that direction. Policymakers are issuing new rules for healthier food in schools and local programs are encouraging more activity. But realistically, an environmental overhaul could take years.
There's a danger in being too pessimistic about the influence we have on the ways our kids live, Daniels says. Research shows that children who lose weight are less likely to gain it back than teenagers or adults.
"As hard as it is to make a change at age 10, it's that much easier than at 30 or 40."
Twins Molly and Chris McGann, 15, are perfect examples of this. In third grade, Molly was bullied for being overweight. The McGanns started attending the Shape Down program at Children's Hospital Colorado.
Shape Down's instructors taught the whole family how to measure their food, cook with different colors -- broccoli, red peppers, carrots -- and include exercise in their daily lives. Molly dropped the extra pounds and is still at a healthy weight.
Her twin Chris hit a tough spot in middle school when undiagnosed sleep apnea caused his weight to creep up. As a teenager he is finding it more difficult to stay on track because of peer pressure. His school cafeteria, for instance, has a pizza buffet and a long line of desserts available every day.
"My friends eat the pizza and the Little Debbie cakes and they're all as thin as rails," he says. "It's really hard to walk by that stuff because it looks so good. I just think I want to be healthy, I want to lose weight and I know if I eat those things it's not going to happen."
Daniels doesn't talk about dieting or weight loss with his patients. He talks about getting the entire family on board to eat healthier and be more active.
"You have to understand what kinds of behaviors are leading to the problem and the changes to take," he says. "It's helpful to go slow. It's about simple goals. You don't have to get to a perfect weight in order to have the health benefits."
For more help conquering your environment, the Mayo Clinic has suggestions on making weight loss a family affair.