Why am I always hungry and therefore overeat? Is there a genetic component to our appetites?
-A curious adult from New JerseyDecember 8, 2006
Genes definitely play a role in appetite. And in how much we can get away with eating, how good food tastes, how much food it takes to be full, and how long we stay full. Genes are involved in every step of wanting, needing, and eating food.
Now of course genes aren't the whole story. Life style choices like how much someone exercises or what they choose to eat play an important role too. (Although some of this is most likely influenced by genes as well!) But let's focus on genes involved in appetite.
Appetite is too complicated to deal with fully in this short answer. So we'll focus on the genes involved in the gut hormones that our brain uses to know the body is hungry.
Remember in this discussion that we are only talking about a few of the players in appetite. We are leaving out a whole lot of other genes that might control when we get hungry as well. But this discussion will give you an idea about how genes can play a role in appetite.
As our blood sugar level drops, our gut releases certain hormones into the blood. These hormones travel to the brain and interact with proteins called receptors. These receptors then trigger brain cells so that our brain tells our body it is time to eat.
There are lots of these hormones and lots of these receptors. They are set up as a system of checks and balances that determine when we're hungry and when we're full.
Genes are involved because each one of these hormones and receptors has its own gene (or set of genes). Each gene has a set of instructions for making a specific receptor or hormone.
And there are different versions of these genes. You might have a different version of a hormone gene than I do. Which means your hormone will work a bit differently.
Let's use an example to show how specific differences in genes might make someone become hungry sooner. Imagine Bob gets hungry an hour after a meal but Sam doesn't get hungry until two hours after. There are lots of ways to explain this with just the receptors and hormones.
Sam might not start making a hormone until his blood sugar gets lower. Or Sam might make the hormone more slowly. Or Sam might make a weaker hormone so it takes more to trigger the hunger. Or Sam's receptor might not be as sensitive as Bob's so it takes less to set it off.
And those are just a few off the top of my head. We haven't even involved all of the proteins in the brain cells that the receptors talk to. Or hormones and receptors involved in feeling full. Or their brain cell proteins. Or...
As you can see, lots of genes get involved very quickly. But let's narrow things down even more and look at just one of these hormones, ghrelin (great name, huh?).
Ghrelin is almost certainly involved in appetite. Ghrelin levels go up before we eat and go down after we eat. If you give extra to a person, they eat more and report that they are hungry.
So if Bob's ghrelin gene has instructions for making a ghrelin that likes its receptor better, he might get hungry before Sam. This is because it would take less ghrelin to activate the receptor and cause hunger.
And there are lots of different forms of the ghrelin gene out there. But none have really been nailed down as causing an increase in appetite yet. A few versions have been associated with obesity but later studies seemed to throw that into doubt.
Now this doesn't mean there aren't gene differences out there that affect appetite. It just means we haven't found them yet with ghrelin (or any other gene).
This isn't really surprising. We didn't discover the first of these hormones until 2000. This just isn't enough time to find the DNA differences that explain why Bob gets hungry before Sam. And things keep getting more complicated as we discover new receptors and hormones to add to an already long list.
We do know though that obesity in general is very strongly influenced by genetics. Some twin and adoption studies have shown that up to 70% of BMI comes from genes. Appetite is almost certainly at least a part of this.
So most likely, genetics will explain some of the differences between people's appetites. We're not there yet but we can see how it might happen.
As a final note, last year scientists identified a close relative of ghrelin called obestatin. Obestatin is apparently the opposite of ghrelin -- extra obestatin causes people not to be hungry.
This exciting hormone may be a boon to all of the people who suffer from an overactive appetite. Perhaps medicines can be made from obestatin in the future that will control our hunger. Finally, a magic bullet for obesity!
This project was supported by the Department of Genetics, Stanford School of Medicine. Its content is solely the responsibility of the authors and does not necessarily represent the official views of Stanford University or the Department of Genetics.