Researchers from Imperial College London found that obesity may be genetically linked to how our bodies digest carbohydrates.
The study investigated the relationship between body weight and a gene called AMY1, which is responsible for an enzyme present in our saliva known as salivary amylase.
This enzyme is the first to be encountered by food when it enters the mouth, and it begins the process of starch digestion that then continues in the gut.
People usually have two copies of each gene, but in some regions of our DNA there can be variability in the number of copies a person carries, which is known as copy number variation.
The number of copies of AMY1 can be highly variable between people, and it is believed that higher numbers of copies of the salivary amylase gene have evolved in response to a shift towards diets containing more starch since prehistoric times.
Researchers looked at the number of copies of the gene AMY1 present in the DNA of thousands of people from the UK, France, Sweden and Singapore.
They found that people who carried a low number of copies of the salivary amylase gene were at greater risk of obesity.
The chance of being obese for people with less than four copies of the AMY1 gene was approximately eight times higher than in those with more than nine copies of this gene.
The researchers estimated that with every additional copy of the salivary amylase gene there was approximately a 20 per cent decrease in the odds of becoming obese.
"I think this is an important discovery because it suggests that how we digest starch and how the end products from the digestion of complex carbohydrates behave in the gut could be important factors in the risk of obesity," said Professor Philippe Froguel, Chair in Genomic Medicine in the School of Public Health at Imperial College London, and one of the lead authors on the study.
The study was published in the journal Nature Genetics.