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association between protein intake and nitrogen loss. In a study involving 8 days of


collection and 18 days of intake observation, the coefficient of variation for urine


nitrogen excretion was approximately 13% and nitrogen output was about 5%


(Bingham 2003 ). Due to the high participant burden of 8 days of 24-h urine col-


lection, partial 24-h urine collections have been suggested. However, even repeated


overnight urine collections cannot replace the accuracy of a full 24-h urine col-


lection and a single 24-h collection amidst several repeated overnight collections


can strengthen the validity of reported protein intake (Ogawa 1986 ). The use of


24-h urine nitrogen as a biomarker of intake assumes that subjects are in nitrogen


balance, i.e., there is no accumulation of nitrogen due to growth or tissue repair or


conversely no loss of nitrogen secondary to injury, dieting, or starvation (Bingham


2003 ).


Other Isotope-Specific Recovery Biomarkers


Dual-isotope models based on red blood cell (RBC), carbon (δ(13)C), and nitrogen


(δ(15)N) isotope ratios have been used to explain a large percentage of the variation


in self-reported sugar intake (Nash et al. 2014 ). The dual-isotope model is proposed
to be an accurate biomarker for assessment of this dietary exposure. Importantly,


these isotope ratios can be assessed using a variety of human biosamples including


RBC, plasma, and hair samples. In addition, the carbon isotope ratio^13 C/^12 C(δ(13)


C) has been determined to be an accurate predictive biomarker of sweetened


beverage intake (Choy et al. 2013 ). In a study of a Yup’ik population, RBC alanine


δ(13)C was demonstrated to have a correlation of 0.70 with sweetened beverage


intake (O’Brien et al. 2014 ). Other stable isotopes, including the nitrogen isotope


ratio,^15 N/^14 N(δ(15)N), of RBCs which has been proposed as a biomarker of


marine food intake, have also been employed as recovery biomarkers. The nitrogen


isotope ratio (δ(15)N) in particular has shown a strong association with concen-


tration biomarkers offish intake such as RBC eicosapentaenoic acid (EPA) and


docosahexaenoic acid (DHA) (O’Brien et al. 2009 ). Important to its relevance for


nutrition research, this biomarker has been inversely associated with blood pressure


(O’Brien et al. 2014 ).


Predictive Dietary Biomarkers


Predictive biomarkers are commonly used to validate self-reported intake in that


these measures are time-sensitive to intake and generally demonstrate a reasonable


dose–response relationship with intake of specific food constituents/nutrients. The


overall amount of the dietary biomarker recovered with predictive biomarkers is


lower than that of recovery biomarkers, likely related to differences in nutrient
metabolism and bioavailability (Jenab et al. 2009 ). Predictive biomarkers are


182 T.E. Crane and C.A. Thomson

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