Quick: How many ounces of soda have you consumed in the past week? Not sure? Or not willing to admit it?
It's a problem obesity researchers face when trying to tease out connections between soda and sugary drinks and obesity.
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New research published in the Journal of Nutrition aims to provide more exact data: in strands of hair.
Using a technique that's helped researchers determine the diets of ancient peoples called carbon isotope analysis, scientists can track how much sugar has been consumed over a longer period than a urine test would show.
"We're isolating the isotope ratio in a specific molecule," researcher Diane O'Brien of the University of Alaska told NPR.
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When people drink soda, a type of carbon called C-13 gets trapped in alanine, an amino acid. So the more C-13 present, the more likely it is you've been gulping corn syrup and cane sugar.
"This should be a major step toward resolving the controversy over the role of caloric sweetener intake in the development of obesity," Dale Schoeller of the University of Wisconsin, Madison, wrote in a commentary.
Others not associated with the study deemed it hopeful, but preliminary and expensive, NPR reported.