Quote:
Obesity is a complex disease resulting from the interactions of a wide variety of hereditary and environmental factors. The combined progress in quantitative genetics, genomics and bioinformatics has contributed to a better understanding of the genetic and molecular basis of obesity.
Clustering of cases within a family, the congruence of body weight for monozygotic twins, and the discovery of genes associated with obesity are all arguments reinforcing the genetic dimension of obesity
It is now well established that overweight and the different forms of obesity are conditions tending to concentrate within a family. Obesity risk is two to eight times higher for a person with a family history as opposed to a person with no family history of obesity, and an even higher risk is observed in cases of severe obesity. Heritability of obesity may vary depending on the phenotype studied, however it tends to be higher for phenotypes linked to adipose tissue distribution (40-55%) and for weight or body fat excess (5-40%). Weight gain and adiposity increase with age, an effect also influenced by heredity.
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Several genes have been identified as influencing common obesity.
Notice how the majority of Downs Syndrome children/adults are overweight?
Also many prescription drugs including anti-depressants cause weight gain, some by increasing appetite and others it's not understood why.
There was some controversial research/studies about diet sodas years ago, not a shocker that people who drink diet soda are often obese, since they also eat a lot of junk food but the interesting part of the research is that food/calorie intake was taken into consideration, those participants that had the same calorie intake but drank diet soda remained the same weight as those who drank sugared sodas. So that suggested that the brain was tricked into believing the artificial sweetener was sugar and the body went through the same routine as if the person ingested all that sugar. Again, it was controversial since they couldn't provide the exact mechanism by which it happens.
The brain and body is too complex and incompletely understood to make simple cause and effect statements. Saying that it's also true that for the vast majority of people taking in less energy than they put out all people including the obese will lose weight.
Then there's the neuroscience axiom 'All psychology reduces to biology'.