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One way to know how much soda people drink is to ask them.

The problem? We tend to underestimate, lie or forget what we’ve consumed.

And this is a challenge for researchers who study the links between sugar­sweetened beverages and obesity.

A new study published in the Journal of Nutrition explains a technique that could help researchers get a good measurement of sugary beverage consumption — by analyzing a piece of hair or a blood sample.

Researcher Diane O’Brien of the University of Alaska and her colleagues have used carbon isotope (同位素)analysis to develop their measuring tool. “We’re isolating the [carbon] isotope ratio in a specific molecule,” explains O’Brien. The molecule is an amino acid called alanine, which captures carbon from sugars.

It turns out that when you consume sweetened soda, slightly more of a particular kind of carbon called C­13 gets trapped in alanine and incorporated into proteins. And proteins hang around in the body much longer than sugar does. So the scientists say they can sample proteins to look for extra amounts of C­13 in alanine. People with a lot of C­13 are likely to be people who have consumed a lot of corn syrup and cane sugar.

Using this technique, O’Brien says, you can capture a longer­term picture of sugar consumption compared with urine samples — which only reveal how much sugar a person has consumed in the past day or so.

Carbon isotope analysis has helped scientists piece together ancient dietary patterns, explains Dale Schoeller of the University of Wisconsin, Madison, in a commentary about the study: “The use of stable isotope signatures has even provided information about the diet of Otzi aka The Iceman, the 5,000­year­old natural mummy found in the Alps in 1991.”

And he writes that he thinks the technique will be helpful for researchers studying the obesity epidemic.

“This should be a major step toward resolving the controversy over the role of caloric sweetener intake in the development of obesity,” writes Schoeller.