Given today's environment, it is surprising that there are still thin people around.When I was at Raging Waters water park this weekend, I was reminded yet again of the obesity epidemic in the U.S. Almost everyone there (myself included) was at the very least overweight.

The origins of this epidemic are pretty easy to spot– lots of food and less opportunity for exercise. And yet, not everyone in the U.S. is overweight. So why is one person fat and the next thin?

One big reason is genetics. A number of twin, family and adoption studies have found that somewhere between 45-60% of body mass index (BMI) comes from the genes we inherit. In other words, some people are more likely to be sucked into a Super-Sized meal because of their genes.

So how might genes affect someone’s chances of succumbing to the mountain of food now available? Lots of ways.

Some people burn energy more slowly than other people. These folks need to eat less to maintain their weight. Not an easy thing to do!

Some people take longer to realize they are full. Others get hungrier more quickly after eating. Still others need more sweets and fat to get enjoyment from their food.

The last example was addressed by a study last year. One of the reasons people eat is that they get a hit of dopamine when they do. The dopamine makes us feel good and once we get it, we feel less inclined to keep eating.

The study found that people with a certain version of the DRD2 gene needed more food to get enough dopamine to stop eating. So they ate more on average.

There are more and more studies finding gene variations just like this one. Finding these gene variations might be useful in creating new medicines to help people eat less by decreasing hunger, burning calories faster, etc.

Knowing about these gene variations might also help doctors identify who is at a greater risk for obesity. These folks can get early help in maintaining their optimal body weight.

Now none of this is an excuse for getting fat (although I wish it was). For the most part, genes that affect our BMI make maintaining a healthy weight harder, not impossible.

But what it also means is that the thin should be a bit nicer to the overweight. Recognize that it might be easier for the thin person to not overeat.

This is not to take away from the thin person's accomplishment. In a world awash in high calorie foods and with work and play involving a lot of sitting, it takes will power not to become overweight. Just remember that it is easier for some people to be thin.

http://www.cdc.gov/nccdphp/dnpa/obesity/trend/maps/index.htm

I'm the third from left to right.I'm in my late teens in this undated photo. I'm the third from left to right. It's very likely one of the last times I went camping as a member of the Girl Guide and Boy Scout Association of Costa Rica, which I joined when I was 11. I was very lucky growing up in Costa Rica because the association's national campground, called Campo Escuela Iztarú, where this photo was taken, was in the hills near my house. From the backyard of my childhood home in Tres Ríos, you can see the national campground. It's the hill in the background, dotted with a few trees. I thought a lot about this campground while I was working on our QUEST Nature Deficit Disorder TV segment about how kids nowadays in the United States aren't spending enough time out in nature.

Every year, during the decade or so that I was a Girl Guide in Costa Rica, my fellow guides and I would trek up the steep hill, usually carrying our own sleeping bags and supplies. Once we got to the top, we chopped up wood with machetes and cooked over open fires. We woke up at the crack of dawn and showered in icy-cold water. During the rainy season, we got very wet. During the dry season, we got sunburned.

Camping took precedence over almost everything else. The day I graduated from high school I was in the middle of an international camping trip with girl guides from around the world. My parents drove up the hill, put some ointment on my sunburned ears and whizzed me over to the theater to pick up my diploma. Then they drove me back up the hill to finish the event.

My childhood home in Costa Rica, with the
campground ridge in the backgroundOur trips were always full of that sense of adventure you can only experience as a kid when you're out in nature and away from your parents. We ran up and down the mountains, crawled in muddy pits, climbed trees and cut ourselves with our machetes. By the end of our trips we were always completely worn out. Once, I sat on my bed and fell asleep with my backpack still on my back.

But all this happiness came to a crashing halt. On Aug. 20, 1988, when I was 16, a young couple my age was murdered in a coffee field on the road to the campground. My sister and I were supposed to go camping a few days later. But my parents wouldn't hear of it, no matter how much we begged. And who could blame them? The way he had killed his 14 victims was so cruel that it was impossible to incorporate his methods into his moniker, à la The Boston Strangler. So the press simply called him el psicópata, The Psychopath. Fear of el psicópata marked my adolescence and early adult life.

What was near-miraculous was that in time I was allowed to go camping again at Campo Escuela Iztarú. This photo of me is proof, I guess, although it doesn't make any sense, because my parents were so fearful of everything when it came to their daughters. Perhaps they let me go because they both had good memories of the time they spent outdoors, my mother as a little girl camping all summer long in New England and my father as a teenager pedaling up and down Costa Rica's mountains to make it to the Pacific coast by sundown. I'm so grateful to them for overcoming their fears (or not – I'm sure they had many sleepless nights). Those fleeting moments of freedom and that sense that anything is possible that I felt when I was camping are so much a part of me that I can't even really consider them memories. So thank you, Mummy and Daddy, for letting go and letting us go up the mountain.

Watch the "Nature Deficit Disorder" TV Story online, as well as find additional links and resources. Also, please share your own photos of childhood nature experiences in our Flickr Photo Pool.

Gabriela Quirós is a Segment Producer for KQED-TV, and is the producer for this story.