Evaluating amino acid isotopic biomarkers of added sugar and animal protein intakes

Abstract

Diet is an important risk factor for chronic disease, but identifying precise relationships between diet and disease remains challenging because of the high error in self-reported dietary measurements. Objective biomarkers of diet can help account for this error. Natural abundance amino acid carbon isotopes ratios (AA CIRs) and nitrogen isotope ratios (AA NIRs) are candidate biomarkers of multiple dietary intakes in the US, due to their natural variation in the diet. In this dissertation, I evaluate whether AA CIRs and AA NIRs are sensitive and specific biomarkers of intakes, such as added sugar and animal protein, in the context of two controlled feeding studies. In the first controlled feeding study, participants resided at the Phoenix branch of the National Institute of Diabetes and Digestive and Kidney Diseases and consumed study diets for a 12-week duration. These diets had the same macronutrient profile but varied in the presence or absence of 3 intakes with isotopic variation: sugar-sweetened beverages (SSBs), meat, and fish. I demonstrate that most nonessential AA CIRs, in plasma and red blood cells, have specific responses to SSB intake and that essential AA CIRs have specific responses to meat intake. I also estimate the turnover rates of AA CIRs. Next, I show that both fish and meat intake influence AA NIRs in plasma and red blood cells. In the second controlled feeding study, participants were recruited in Phoenix, Arizona across sex, age, and BMI groups. Participants consumed study diets reflecting their usual intake for a 15-day period, and I present the correlations between various intakes and AA CIRs from serum collected at the end of the feeding period. I find a moderate correlation between the alanine CIR and added sugar intake and also multiple moderate to high correlations between protein-related intakes and all AA CIRs. I present a model of added sugar intake, which has modest explanatory power. The work in this dissertation further attests to the biomarker potential of AA CIRs and NIRs in the US, and it suggests that AA CIRs are especially promising biomarkers of SSB and animal protein intakes.