There's a gene in your saliva that determines how your body breaks down carbs: scientists

British researchers have discovered a link between a gene that breaks down carbohydrates and obesity, which may pave the way for more effective, individually tailored diets for people wanting to lose weight.

Researchers at King's College London and Imperial College London found that people with fewer copies of a gene responsible for carbohydrate breakdown may be at higher risk of obesity. The findings, published in Nature Genetics, suggest that dietary advice may need to focus more on a person's digestive system, based on whether they have the genetic predisposition and necessary enzymes to digest different foods.

The salivary amylase gene plays a significant role in breaking down carbohydrates in the mouth at the start of the digestion process. The new study suggests that people with fewer copies of the AMY1 gene have lower levels of this enzyme and so have more difficulty breaking down carbohydrates than those with more copies.

Previous research has found a genetic link between obesity and food behaviours and appetite, but the discovery highlights a genetic link between metabolism and obesity.

The study suggests that people's bodies may react differently to the same type and amount of food, leading to weight gain in some and not in others. The effect of the genetic difference found in the latest study appears to be a much stronger link than in any of those found before, say the researchers.

Professor Tim Spector, head of the department of twin research and genetic epidemiology at King's and joint lead investigator, raised the prospect of more customised diets for overweight and underweight people. "In the future, a simple blood or saliva test might be used to measure levels of key enzymes such as amylase in the body and therefore shape dietary advice for both overweight and underweight people," he said. "Treatments are a long way away but this is an important step in realising that all of us digest and metabolise food differently – and we can move away from 'one-size fits all diets' to more personalised approaches.'

"While studies to date have mainly found small effect genes that alter eating behaviour, we discovered how the digestive 'tools' in your metabolism vary between people – and the genes coding for these – can have a large influence on your weight. The next step is to find out more about the activity of this digestive enzyme, and whether this might prove a useful biomarker or target for the treatment of obesity." Researchers arrived at their conclusions after first studying 149 Swedish families with differences in the levels of obesity and found unusual patterns around two amylase genes (AMY1 and AMY2), then 972 twins from TwinsUK, the biggest UK adult twin registry.

The researchers went on to estimate the precise copy numbers of the amylase gene in the DNA of a further 481 Swedish subjects, 1,479 subjects from TwinsUK and 2,137 subjects from the DESIR project. They found that the number of copies of the AMY1 gene was consistently linked to obesity. Further analysis in French and Chinese patients with and without obesity showed the same patterns.

"A lower estimated AMY1 copy-number showed a significantly increased risk of obesity in all samples and this translated to an approximate eightfold difference in the risk of obesity between those subjects with the highest number of copies of the gene and those with the lowest," said the study.

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