There are tons of genetic reasons why people are obese - after all, it's important that we eat, and we didn't evolve to live in a world of plentiful food. But why would anyone be genetically predisposed to extreme thinness?
After all, while it isn't that big deal to be very thin in today's world - hell, it's positively prized - it seems like a strange thing to have in our evolutionary makeup if you look at our past as hunter-gatherers. After all, a genetically small appetite could be downright dangerous in a world where life was largely defined by the constant search for more food.
Now researchers may have found one possible genetic cause for extreme thinness, and it's actually an "overdose" of genes. Normally, we inherit a copy of each of our 23 chromosomes from both our parents, giving us a complete set. But sometimes specific parts of chromosomes are duplicated or deleted, creating an imbalance. In this case, part of chromosome 16 is duplicated in about 1 in 2,000 people. Those affected are 23 times more likely to be very thin if male and 5 times more likely if female.
Head researcher Phillippe Frouguel of Imperial College London explains:
"The dogma is that we have two copies of each gene, but this isn't really true. The genome is full of holes where genes are lost, and in other places we have extra copies of genes. In many cases, duplications and deletions have no effect, but occasionally they can lead to disease. So far, we have discovered a large number of genetic changes that lead to obesity. It seems that we have plenty of systems that increase appetite since eating is so important – you can suppress one and nothing happens. This is the first genetic cause of extreme thinness that has been identified.
"One reason this is important is that it shows that failure to thrive in childhood can be genetically driven. If a child is not eating, it's not necessarily the parents' fault. It's also the first example of a deletion and a duplication of one part of the genome having opposite effects. At the moment we don't know anything about the genes in this region. If we can work out why gene duplication in this region causes thinness, it might throw up new potential treatments for obesity and appetite disorders. We now plan to sequence these genes and find out what they do, so we can get an idea of which ones are involved in regulating appetite."