The carbon isotope ratios of serum amino acids in combination with participant characteristics can be used to estimate added sugar intake in a controlled feeding study of US postmenopausal women

Background: The carbon isotope ratio (CIR) is a proposed biomarker for added sugar (AS) intake in the United States; however, because the CIR is also associated with meat intake in most populations the need for specificity remains. The CIR of amino acids (AAs) has the potential to differentiate sugars from meat intakes, because essential AAs must derive from dietary protein whereas certain nonessential AAs can be synthesized from sugars. Objectives: We tested whether serum CIR-AAs in combination with participant characteristics could meet a prespecified biomarker criterion for AS intake in the Nutrition and Physical Activity Assessment Study Feeding Study (NPAAS-FS) of the Women’s Health Initiative, a population in which the whole-serum CIR was not associated with AS intake. Methods: Postmenopausal women (n = 145) from Seattle, WA, were provided with individualized diets that approximated their habitual food intakes for 2 wk. Dietary intakes from consumed foods were characterized over the feeding period using the Nutrition Data System for Research. The CIR of 7 AAs—Ala, Gly, Val, Leu, Ile, Pro, and Phe—were measured in fasting serum collected at the end of the 2-wk feeding period, using gas chromatography–combustion isotope ratio mass spectrometry. Biomarker models were evaluated using regression R² ≥ 0.36 as a major biomarker criterion, based on the benchmark R² values of well-established recovery biomarkers in the NPAAS-FS. Results: AS intake was associated with CIR-Ala (ρ = 0.32; P < 0.0001). A model of AS intake based on CIR-Ala, CIR-Gly, CIR-Ile, smoking, leisure physical activity, and body weight met the biomarker criterion (R² = 0.37). Biomarker-estimated AS intake was not associated with meat or animal protein intake. Conclusions: Results support serum CIR-AAs in combination with participant characteristics as potential biomarkers of AS intake in US populations, including those with low AS intake. The Women’s Health Initiative is registered at clinicaltrials.gov (NCT00000611). J Nutr 2020;150:2764–2771.