Updated 05 November 2014

Why some people don't get fat eating carbs

Take two people. They eat exactly the same amount of carbohydrates, and do the same amount of exercise. But why does the one pick up weight and the other not?

Ever wondered why certain people who consume lots of carbohydrates remain thin and skinny, while the others plump up into obesity?

It has got to do with your genes and believe and it or not, your saliva.

According to the journal Nature Genetics, a study published in March 2014 looked at the relationship between body weight and a gene called AMY1, which is responsible for an enzyme present in our saliva known as salivary amylase.

As soon as we eat food, it comes into contact with this enzyme which starts to digest the food, and this continues into the gut. 

But here's the rub, and what could explain why some of us digest carbs more efficiently than others.

According to the researchers at the Imperial College of London, we don't all have the same number of copies of the AMY1 gene. The more you have, the better you'll digest carbohydrates and the knock-on effect is that you are skinnier.

Read: The Tim Noakes experiment comparing high fat and high carb diets

Professor Philippe Froguel, Chair in Genomic Health at Imperial College London, and one of the lead authors on the study, said:

“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. 

The research showed that the fewer copies of the AMY1 gene you carry, the higher you are at risk of being overweight.

They believe that higher numbers of copies of the salivary amylase gene seen in some people have evolved in response to a shift towards diets containing more starch since prehistoric times. 

The precise role of salivary amylase is unclear but it is believed that by breaking down sugars in the mouth the digestive system becomes prepared to receive starch, speeding up the rate of absorption.

ReadShould you try the Paleo diet?

Another interesting possibility is that different levels of salivary amylase can change how food tastes. People with low levels of salivary amylase will find that starches taste creamier and thus more appealing. This could increase the desire for carb-heavy foods and thus drive obesity.

They go on to say that the chance for people with less than four copies of the AMY1 gene to be obese was about eight times higher than those with more than nine copies of the gene. 

ReadThe physiology of food cravings
ReadWhat your saliva says about your health

Graph from The Imperial College of London: the more copies of the gene you have, the less likely it is that carbs will make you fat

AMY1 copy number and BMI
The research could be a valuable tool for dieticians. By focusing on a person's digestive system, their genetic make-up and number of AMY1 gene copies they have, they could better recommend dietary changes to lose weight, including reducing carbohydrates as part of a healthy eating plan. 

Read more

FAQ: Eating meat made our ancestors smarter
Researchers identify the gene that turns carbs into fat
Check the Glycaemic index of different foods to see how they react in the body
Tim Noakes on carbohydrates

Sources: The Imperial College of London, Nature Genetics
Image: fat and thin from Shutterstock

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