| Summary: | Previous studies suggest that amino acid carbon stable isotope ratios (CIR<sub>AA</sub>s) may serve as biomarkers of added sugar (AS) intake, but this has not been tested in a demographically diverse population. We conducted a 15-day feeding study of U.S. adults, recruited across sex, age, and BMI groups. Participants consumed personalized diets that resembled habitual intake, assessed using two consecutive 7-day food records. We measured serum (<i>n</i> = 99) CIR<sub>AA</sub>s collected at the end of the feeding period and determined correlations with diet. We used forward selection to model AS intake using participant characteristics and 15 CIR<sub>AA</sub>s. This model was internally validated using bootstrap optimism correction. Median (25th, 75th percentile) AS intake was 65.2 g/day (44.7, 81.4) and 9.5% (7.2%, 12.4%) of energy. The CIR of alanine had the highest, although modest, correlation with AS intake (<i>r</i> = 0.32, <i>p</i> = 0.001). Serum CIR<sub>AA</sub>s were more highly correlated with animal food intakes, especially the ratio of animal to total protein. The AS model included sex, body weight and 6 CIR<sub>AA</sub>s. This model had modest explanatory power (multiple <i>R<sup>2</sup></i> = 0.38), and the optimism-corrected <i>R<sup>2</sup></i> was lower (<i>R<sup>2</sup></i> = 0.15). Further investigations in populations with wider ranges of AS intake are warranted.
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