Obesity is one of the biggest threats to public health. The number of people with obesity, defined as having a body mass index (BMI) of more than or equal to 30, has tripled between 1975 and 2016.

Concerningly, children are also becoming more obese – in the UK today, nearly a third of children are overweight or have obesity. Obesity is connected to numerous health problems such as heart disease, diabetes, and cancer.

More recently, obesity has also been shown to increase the risk of severe illness after COVID-19 infection.

We know that there are many factors that can contribute to obesity, and one of them is genetics. Have scientists managed to identify an 'obesity gene', or is the genetics of this condition more complicated?

We asked 5 experts in genetics, nutrition, obesity, and neuroscience, "Is there an 'obesity' gene?" Here is what they said.

What do we know about the genetics of obesity?

In order to investigate the genetics behind obesity, researchers scan lots of peoples' DNA to see if they can identify certain genes or variations of genes that correlate with obesity.

Professor Cornelie Nienaber-Rousseau, an expert in nutrition from North-West University in South Africa, says, "Large-scale genome-wide association studies (GWAS) have identified several loci associated with body mass index and not just a single loci/gene". However, in rare cases, obesity is caused by a particular syndrome due to a mutation in one gene.

Genes that correlate with body mass index or obesity vary widely in their function. Dr. Ivor Ebenezer, an expert in neuroscience from the University of Portsmouth in the UK, says these "genes control the hormones, neurotransmitters, neuromodulators, and enzymes involved in the regulation of food intake and energy homeostasis."

Why are some genes linked to obesity?

From an evolutionary perspective, it doesn't make sense that our genes, which we pass down from generation to generation, contribute to a dangerous condition such as obesity.

However, Dr. Ebenezer explains that "if we look back in history, to a time when our hunter-gatherer ancestors roamed the earth, we find that early man lived mainly on root vegetables (which are high in carbohydrate) and hunted animals for proteins and fats. There were periods of many days and sometimes weeks when they went hungry. Those who survived were able to store body fat during times of plenty to provide energy during times of hardship. Thus, the majority of us have inherited genes from our ancestors that predispose us to accumulate body fat."

These genes, which were essential for survival when food was scarce, are now detrimental to those of us who have easy access to food and don't need to physically exert ourselves.

What other factors contribute to obesity?

Whilst genetics is a factor that influences obesity, it is not the only one. Dr. Richard Edwards, an expert in bioinformatics and genetics from UNSW Sydney in Australia, says, "Obesity is an extremely complex trait controlled by interactions between many genetic and environmental factors. One only needs to look at the changes in obesity levels we are seeing in modern populations versus our recent ancestors – our genes have not changed notably during this period."

We know that our environment strongly influences obesity, as Dr. Ebenezer says, "Obesity is due to an excess of energy (food) intake coupled with a decrease in energy expenditure." Interestingly, however, our environment also interacts with our genetics.

Dr. Edwards points out that "the biggest genetic factor is most likely an 'epigenetic' one – not to do with what genes you have, but rather how they are expressed and controlled."

Dr. Ebenezer explains that "environmental factors, such as maternal starvation or consumption of foods with high sugar content, can produce epigenetic changes in the fetus that can results in obesity in the offspring."

There may be other factors that influence obesity which are less widely appreciated. For example, Dr. Ebenezer says, "More recently, it has been found that the gut microbiome plays an important role in the control of hunger and satiety by modulating the hormones and neurotransmitters involved."

The takeaway: Whilst genetics does contribute to obesity, it is not the only factor involved, and there is not just one 'obesity gene'.

Article based on 5 expert answers to this question: Is there an obesity gene?