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