Biomarkers are needed in order to better estimate habitual patterns of food and nutrient intake.  Recent work has demonstrated the utility of carbon isotope ratios in blood sugar as a biomarker of recent sugar intake.  The carbon isotope ratios of cane and corn sugar are high because these plants use the C4 photosynthetic pathway.  Because glucose is metabolized to carbon dioxide, O’Brien and colleagues postulated that it may be possible to use the carbon isotope ratios in breath to measure short-term added sugar intake.  They report the results of their study in the March 2021 issue of The Journal of Nutrition.

Subjects in the randomized, crossover feeding study included 6 men and women who were assigned to 5 dietary treatments that were given with a 1-2 week washout period between treatments.  They included: a low (0 g/d), medium (75 g/d), or high (150 g/d) level of added sugar during breakfast and lunch one day, or 2 treatments where the high and low amounts were switched between breakfast and lunch.  Breath was collected every 2 hours for 8 hours, starting at 08:00, which was a fasting measurement time.

There was an increase in carbon isotope ratio at all sampling times in response to increasing added sugar intake.  Post-feeding carbon isotope ratios were affected by those determined at fasting, and the effect was greatest in the morning samples.  Added sugar intake from lunch led to a 4-fold greater increase in carbon isotope ratios than the effect observed from breakfast. 

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These observations led the authors to conclude that breath carbon isotope ratio may serve as a biomarker of short-term added sugar intake, but that more work is needed in order to determine if multiple collections could be used to estimate longer-term intakes.

In a commentary on this article, Kuhnle notes the potential benefit of this approach to determining better estimates of short-term added sugar intake and agrees that more work is needed to determine if it would work for long-term dietary assessments.  Kuhnle also points out that although this approach may work in the United States, it would not in Europe where most added sugar is derived from sugar beets, which are C3 plants.


O’Brien DM, Niles KR, Black J, Schoeller DA.  The breath carbon isotope ratio reflects short-term added-sugar intake in a dose-response, crossover feeding study of 12 healthy adults. The Journal of Nutrition, Volume 151, Issue 3, March 2021, Pages 628–635,

Kuhnle GGC. Take my breath away:  Measuring sugar intake in exhaled air.  Journal of Nutrition, Volume 151, Issue 3, March 2021, Pages 457–458,