When thin people see someone who is overweight, many of them assume it's because all overweight people are slothful gluttons.
Anyone intelligent person knows this is not true. Some people are born lucky, with a metabolism that lets them eat almost anything without putting on weight. Other people are born with a metabolism that would be great in a world where calories were scarce: they are very efficient at converting calories to fat, which can store the calories until food is scarce. Then these people can live off their fat while the skinny ones expire.
We all know intuitively that body build tends to run in families. And studies have shown that adopted children tend to have a body build that is more similar to that of their birth parents than that of their adopted parents.
Despite this, many health care people continue to believe that if you're overweight, you've been stuffing yourself, and all you need to do to lose is to eat reasonable amounts of healthy foods. Most of us know that in order to lose weight, some people need to cut back so far that their calories are close to a starvation level.
Now a couple of studies have been published that provide some evidence for the genetic basis of obesity and fat distribution. These are not the first studies to find genes linked to obesity, but the more often these studies are published, the more health care people may come to change their attitudes.
This particular study identified 18 new genes associated with overall obesity. A second study described in the same article found 13 genes associated with fat distribution. The latter is important, as fat around the middle (the "apple shape," more common in men) has been found to be more dangerous than fat around the hips and legs ("pear shape," more common in women).
What is interesting is that Joel Hirschhorn, lead author of the first study, said, "Different people have different susceptibilities to obesity. Some don't rigorously watch what they eat or how much they exercise and still resist gaining weight, while others constantly struggle to keep their weight from skyrocketing. Some of this variability is genetic, and our goal was to increase understanding of why different people have different inherited susceptibility to obesity."
One of the gene variants encodes a protein that influences insulin levels and metabolism. Another encodes a protein that affects appetite.
One problem researchers face is that there are so many of these gene variants, and you might need a lot of them to affect your tendency to gain weight. For example, they found that people who had more than 38 variants associated with easy weight gain were on average 15 to 20 pounds heavier than those who had fewer than 22 variants. That's a lot of genes and a relatively small difference in weight.
We still know less than we don't know about what really causes obesity. Clearly environment also plays a role. But I think we do know enough to say that if you've always been careful about your diet and you get a reasonable amount of exercise and you're still very overweight, it's not your fault.