Your jeans size? It’s in your genes

Some of the first clues of a link between genes to obesity came in 1952 from a group of researchers from the University of Michigan. They carried out a study involving 81 pairs of twins from mostly local high schools. The twins were measured in many ways including foot length, forearm length and even nose height.

The measurements enabled the researchers to figure out how likely it was that these different traits had been inherited from their parents – this is called heritability. Out of all the different traits that the researchers measured, they found that the traits with the highest heritability rate were body weight and waist circumference.

Many similar studies have been carried out since, which also compared twins that have not grown up together. Identical twins were equally similar in weight, regardless of whether they have been raised together or raised apart from birth. This shows the power of genes over environment in determining body weight. Together, these findings provide indisputable evidence that genetics play a central role in obesity. Some evidence suggests that the genetic contribution is between 40 and 70 percent. This means that the genes you inherit from your parents may increase your risk of developing obesity.

The research in this field is ongoing. But what we do know, is that genes influence:

• How much food we tend to eat at a sitting
• How we respond to the sensation of fullness
• How much enjoyment we get from certain types of food
• How much energy we need to run our body’s basic functions
• How and where excess calories are stored as fat in our bodies

We now know that these things might have less to do with our personalities and lifestyle choices and more to do with our genes.

But if that’s the case, why there were very few people with obesity a hundred years ago? As the geneticist Francis Collins puts it: “Genetics loads the gun, and environment pulls the trigger”.

Our genes haven’t changed over the last hundred years. In fact, they have remained largely unchanged over the last 50,000 years. What has changed is our environment. And just as some of us develop allergies in certain environments, some of our genes can be activated and changed by the environment too.

We now live in a different environment, with different types of stress, food and technology. They interact with our genes in a new way. Obesity is part of the result.

Professor Joseph Proietto, a researcher and clinician specialising in obesity, explains the genetic basis of obesity by asking us to think about two pots. They have different sizes: one pot holds five litres while the other pot holds fifty. The pots stand in the rain overnight, and in the morning both are full of water.

It’s no surprise that the larger pot is holding more water than the smaller pot. Professor Joseph Proietto explains that this is because the bigger pot was made to hold more water. “In other words, you need both your genetic make-up (how the pot was made) and the environment (the rain) to develop obesity,” he says.

So, how can we use this information? After all, we can’t change our genes. But our individual genetic differences can make us more or less at risk of developing obesity. So, the more we know about our genes, the more information we have to make informed decisions about weight management. For example, we can try to minimise our exposure to environmental factors that increase the risk of developing obesity.

And because of our individual genetic make-up, we may respond differently to different types of treatment. What works for one person, might not work for another. That’s why we each need an individual approach to weight management.