A middle school student from California
January 10, 2008
An obsessive fear of weight gain
is a symptom of anorexia.
A lot of work has been done on the genetics of eating disorders and scientists are starting to figure out the genes involved. But there are too many different kinds of eating disorders to deal with in such a short answer. So I'll focus on the one we hear about most -- anorexia nervosa
People with anorexia usually have an obsessive fear about gaining weight. They also have a distorted idea about how their body looks -- they think they're fat when they are really painfully thin. Because of this they don't eat enough and so put their health at risk.
Genetics does seem to play a role in anorexia. But it has been really hard to figure out the gene(s) involved. One reason for this is that there is probably more than one gene that can cause anorexia. Another reason is that there are a lot of external things in a person's life that help determine whether that person gets the disease or not.
For a long time scientists thought that society alone could be blamed for anorexia. They pointed out that anorexia is more common in cultures that value thinness. And they are right in that environment and culture definitely play a role. But we now know that there is definitely biology involved too.
How do we know this? Well for one thing, eating disorders run in families. A person who has a family history of an eating disorder is 14 times more likely to develop one themselves!
OK, OK, you could claim that this is just another example of societal pressure. A parent with unrealistic ideas about thinness could easily pass on these feelings to their children.
When researchers want to figure out if a disorder runs in a family because of genes or environment they frequently look at twins. Why twins? Because both identical and non-identical twins are likely to have similar environments. BUT identical twins have the same genetics and fraternal twins have different genetics.
So if genes are involved, then both twins in an identical twin pair will be more likely to share the disease than both twins will in a fraternal twin pair. And this is the case with anorexia.
If one identical twin has anorexia, then about 6 out of 10 times the other twin in that identical twin pair will get it too. For non-identical twins that number is only about 1 in 10.
Because both identical twins have anorexia more often than any two fraternal twins, genetics is probably involved. Now comes the hard part...scientists need to piece together which genes are to blame. Anorexia is so complicated that researchers have focused on neurotransmitter genes because these are the most likely suspects.
A gene for a serotonin receptor
has been linked to anorexia.
Neurotransmitters are chemicals found in the brain. Nerves use neurotransmitters to talk with each other. In a disease like anorexia, it is likely that the symptoms come from miscommunication between nerve cells. There are lots of different types of neurotransmitters but eating disorder researchers have focused on two: dopamine and serotonin.
Serotonin has been implicated in a lot of different behaviors including anger, aggression, body temperature, mood, sleep, and appetite. That's right, I said appetite. Makes sense it is involved in anorexia, right?
The role of dopamine in eating disorders is a little less obvious. Dopamine has been implicated in motivation, sleep, mood, attention, and learning.
What is interesting to researchers is that people who make too much dopamine seem more likely to have obsessive behaviors. Remember when I said that people with anorexia often have obsessive feelings about food and weight loss?
So genes related to dopamine and serotonin seem like a good place to start for researchers studying eating disorders. But findings so far have been mixed. The most promising results have been in genes involved with serotonin (the appetite one). In particular a gene called 5-HT2a
has been getting a lot of attention.
is one of many genes that make proteins for what is called a serotonin receptor. Serotonin receptors sit on the outside of neurons and tell the nerve that a message has been sent.
You can think of the receptors as the people sending a letter to each other and the neurotransmitter as the letter. If there is a mistake in the letter, then the message may not make it. But if the person at the other end misreads the message, then the message will not be understood as well.
It is this last case that scientists have focused on. Receptors come in many different versions. Some know what to do when they get a neurotransmitter. Others get confused and can overreact.
Scientists have tried hard to find a version of 5-HT2a
that is confused by serotonin and so is involved in anorexia. At my last count there have been at least 15 different studies that have investigated the role of 5-HT2a
in eating disorders!
About half of these studies see the link between 5-HT2a
and anorexia. But half of them do not. This does not mean that the link does not exist- but it does mean that if it is there, it is hard to see.
A number of other genes that deal with serotonin and dopamine have been looked at but none of them look as promising as 5-HT2a
. Most likely there are a number of genes involved. And don't forget about the environment too.
This complicated mix of many genes and environment make it difficult to identify the exact genes involved. But we do have some clues. And the closer we come to knowing the underlying genetics, the closer we are to knowing how to treat anorexia.