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The acetone measuring chip used in the study. (Credit: ETH Zurich/Andreas Güntner)

Experts advise anyone looking to shed extra pounds is to eat less and exercise more. One way is with endurance training, during which the body burns not only carbohydrates such as sugar, but also fat. When exactly the body begins burning fat can now be determined by analyzing, for example, biomarkers in the blood or urine. Researchers have developed a method for the highly convenient, real-time monitoring of lipolysis by testing a person’s exhalations during exercise.

When burning fat, the body produces by-products that find their way into the blood. In the pulmonary alveoli, these molecules — especially the volatile ones — enter the air exhaled by the person. The most volatile of these lipid metabolites is acetone. A small gas sensor measures the presence of this substance. The sensor can detect a single acetone molecule in hundred million molecules. It also measures acetone exclusively, so the more than 800 other known volatile components in exhalations do not affect the measurement.

The researchers were able to show how the acetone concentration in the exhalations varies greatly from person to person. The measurements taken by the researchers showed that lipolysis in some test subjects did, in fact, only start toward the end of the one-and-a-half-hour training session. In the other volunteers, the measurements showed that their bodies began burning fat much sooner.

The sensor uses a chip coated with a porous film of special semiconducting nanoparticles. The particles are tungsten trioxide that the researchers have implanted with single atoms of silicon. Development began when researchers discovered that tungsten trioxide nanoparticles interact with acetone if the atoms of the nanoparticles are arranged in a certain crystalline structure. The interaction reduces the electrical resistance of the chip coated with the nanoparticles, and this phenomenon can then be measured.

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