(P13-052-24) Carbon Isotope Ratio (CIR) Measured in 24-H Urine Is Associated With Multiple Dietary Exposures in a 15-d Controlled Feeding Study

Objectives: We explored the potential of the carbon isotope ratio (CIR) measured in 24-h urine to serve as a biomarker of either animal protein or added sugars, measured as the ratio of animal to total protein intake (APR) and added to total sugars intake (ASR), respectively, using data from a 15-d feeding study.

Methods: Ninety-nine participants (55% women) from the Phoenix Metropolitan Area were recruited from March 2016 – June 2019 for a 15-d controlled feeding study simulating participants’ usual intakes. All foods consumed by the participants were prepared in a metabolic kitchen and intake was carefully measured. Over the 15-d study period, participants collected 6 non-consecutive 24-h urines measured for CIR, nitrogen isotope ratio (NIR) and C:N (g/g, a measure reflecting urine composition). A linear mixed model regressing daily urine CIR on daily and 15-day mean intake of APR, ASR and corn was fitted while controlling for age, sex, BMI, physical activity and daily urine NIR and C:N.

Results: Animal protein intake represented on average 66% of participants’ total protein intake, while approximately 60% of total sugars intake came from added sugars (9.4% of total energy). Urinary CIR was strongly correlated with APR (r = 0.79, p < 0.001) and log animal protein intake (r = 0.67, p < 0.001), while the correlation with ASR and added sugars was moderate (0.37, < 0.001; 0.29, 0.004; respectively). In the final linear mixed model, daily and 15-d mean intakes of APR, ASR and corn intake, along with urine NIR and C:N were all found to be significant predictors of urinary CIR, explaining a large portion of its variance (Model marginal R2 = 0.73). While all dietary predictors were highly significant, the coefficient estimates for APR were the largest, identifying APR intake as the most influential predictor of the biomarker.

Conclusions: Despite its strong association with APR, urine CIR may not be useful as a sole biomarker of animal protein due to its strong association with other factors. Nonetheless, its potential to contribute helpful intake information when combined with other carbon isotope measures needs to be further explored.

Funding Sources: This work was funded by NIH - National Cancer Institute.