The carbon isotope ratio of alanine is a biomarker of added sugar and sugar-sweetened beverage intakes: a pooled analysis of 4 studies

Background: The alanine carbon isotope ratio (CIR) biomarker was positively associated with added sugar (AS) and sugar-sweetened beverage (SSB) intakes in multiple studies from the United States. Association strengths varied, and Ala CIR also correlated with protein source in certain studies. Objectives: This study examined Ala CIR associations with AS and SSB intakes and animal protein ratio (APR; animal protein/total protein), and adjustment for APR, by pooling data from 4 previous studies. Methods: We pooled diet and biomarker data from 4 studies (n = 346). These included a cross-sectional study of Yup'ik Alaska Native adults (n = 62), a 12-wk randomized controlled feeding study in males (n = 32), a 2-wk habitual intake controlled feeding study in postmenopausal females (n = 153), and a 15-d habitual intake controlled feeding study of adults (n = 99). We estimated correlations between amino acid CIRs and diet and performed multivariable regression of Ala CIR on standardized intake variables to determine simultaneous associations with AS (g/d) or SSBs (servings/d) and APR. We included study by intake interactions to allow for heterogeneity among studies. We then performed models where leucine (Leu) CIR was included to adjust for APR. Results: There were positive correlations between Ala CIR and AS intake (r = 0.54; 95% CI: 0.46, 0.61), log-SSB intake (r = 0.63; 95% CI: 0.56, 0.69), and APR (r = 0.32; 95% CI: 0.22, 0.41). Study-specific slopes for the relationship between Ala CIR and AS or SSB intake were similar in models with and without adjustment for APR. Across studies, slopes ranged from 0.34 (95% CI: 0.08, 0.61) to 1.75 (95% CI: 1.29, 2.20) for AS intake in models with APR and from 0.35 (95% CI: 0.01, 0.68) to 1.11 (95% CI: 0.81, 1.40) for SSB intake in models with APR. Replacing APR with Leu CIR resulted in similar slopes between Ala CIR and AS/SSB intake. Conclusions: The Ala CIR is a robust biomarker of AS/SSB intake. Potential associations with APR can be adjusted for using a simultaneously measured biomarker. The DBD study was registered at clinicaltrials.gov as NCT01237093 and the NPAAS, an ancillary study of the Women's Health Initiative, at clinicaltrials.gov as NCT00000611.