Interesting new research indicates that obesity may be genetically linked to how our bodies digest carbohydrates.
Scientists investigated the relationship between body weight and a gene called AMY1, which is responsible for an enzyme found in saliva known as salivary amylase. This is the first enzyme that food encounters when it enters your mouth, beginning the process of starch digestion that continues in your gut.
Typically everyone has two copies of each gene, but sometimes the number of copies in DNA can change. For instance, the number of copies of the AMY1 gene varies significantly between people - higher numbers of copies of this gene may have evolved in response to our shift towards diets containing more starch, relative to prehistoric times.
Researchers from Imperial College London looked at the number of copies of the AMY1 gene present in the DNA of thousands of people from the UK, France, Sweden and Singapore. Overall, they found that people who had a low number of copies were at greater risk of obesity.
First, genetic data from a Swedish family sample of 481 participants - recruited on the basis of sibling-pairs where one was obese and the other non-obese - was analyzed. The researchers used these data to identify all genes whose copy number differences influence body mass index (BMI).
They found that the gene coding for the enzyme salivary amylase (AMY1) had the greatest influence on body weight. They then investigated the relationship between the number of times the AMY1 gene was repeated in the DNA of each individual and their risk of obesity in approximately 5,000 subjects from France and the UK.
The researchers also expanded their study to include approximately 700 obese and normal-weight people from Singapore, and demonstrated that the same relationship between the number of copies of the AMY1 gene and the risk of obesity also existed in non-Europeans.
Scarily, the chance of being obese for people with less than four copies of the AMY1 gene was a staggering eight times higher than in people with more than nine copies. Every additional copy lowered the odds of being obese by nearly 20 per cent.
This is an important discovery because it suggests that how we digest starch and how the end products of complex carbohydrate digestion behave in the gut are important factors that determine the risk of obesity.
Further research is needed to understand whether or not altering starch digestion can improve the ability to lose weight or even prevent the onset of obesity. Also, there might be a link between such genetic variations and risk of metabolic disorders such as diabetes.
Health experts are now starting to develop a clearer picture of genetic factors affecting the psychological and metabolic processes that contribute to obesity. Previous studies have found rare genetic variations that cause extreme forms of obesity, but they only occur in only a small number of people. This study is novel in that it identifies a genetic variation that is both common and has a relatively large effect on the risk of obesity in the general population.