​​AS423 – Validating Multiple Stable Isotope Measures of Diet Within the NPAAS Feeding Study (AS272)

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Investigator Names and Contact Information

Diane M. O'Brien [dmobrien@alaska.edu]

Introduction/Intent

This study evaluates dietary biomarkers of sweetener, meat and fish intake based on the stable isotope ratios of carbon, nitrogen and sulfur, using serum specimens from the NPAAS Feeding Study​.  The NPAAS Feeding Study is a 2-week controlled feeding study of 169 women ages 60-80 in Seattle, WA, in which each participant's controlled diet is matched to her usual intake in order to maintain intake distributions similar to those of the study population and achieve the level of control needed for biomarker validation.  

The carbon isotope ratio (¹³C/¹²C, expressed as d¹³C as described below) is a candidate biomarker of sweetener intake because it is high in corn and sugar cane, the major sources of dietary sweeteners in the US. The nitrogen isotope ratio (¹⁵N/¹⁴N, expressed as d¹⁵N as described below) is a candidate biomarker of meat intake, because it is elevated in animals relative to plants.  The sulfur isotope ratio (³⁴S/³²S, expressed as d³⁴S as described below) is a candidate biomarker of fish intake, because marine foods are elevated relative to terrestrial foods.  However, preliminary data suggest that the specificity of these biomarkers needs to be improved for them to be validated for use in public health research. 

We test two approaches to validating isotopic measures for foods of interest.  Specific Aim 1 will test whether single or multi-isotopic models based on serum carbon, nitrogen, and sulfur stable isotope ratios can provide valid and specific measures of sweetener, fish and meat intake in the NPAAS Feeding Study.  Specific Aim 2 will test whether the carbon isotope ratio of specific serum amino acids can provide alternatives to multi-isotopic models of similar or improved validity and specificity.  The data being provided herein are for Specific Aim 1; data for Specific Aim 2 will be provided at a later date as those analyses are considerably more time-consuming. 

A synopsis of participant selection criteria

All 169 participants in the NPAAS Feeding Study were included in this study.  Pre and post intervention serum samples were analyzed, for a total of 338. 

A synopsis of laboratory methods used

Whole serum d¹⁵N and d¹³C:  Serum d¹⁵N and d¹³C were analyzed at the Alaska Stable Isotope Facility (ASIF) at the University of Alaska Fairbanks (UAF).  Both isotope ratios are measured from a single, ~ 5 ml sample, using an elemental analyzer interfaced to an isotope ratio mass spectrometer (Delta series, Thermo Scientific, Inc).  Isotope ratios are presented in permil (‰) abundance of heavy isotope relative to reference values as follows: dX = (R_sample – R_standard)/(R_standard) Χ1000‰,  where X is the heavy isotope (¹⁵N or ¹³C), R is the ratio of heavy to light isotope (¹⁵N/¹⁴N or ¹³C/¹²C) and the standards are IAEA-certified reference materials calibrated to V-PDB (¹³C/¹²C_V-PDB = 0.01124) and atmospheric nitrogen (¹⁵N/¹⁴N_atm-N = 0.003677).

Whole serum d³⁴S: Serum d³⁴S was analyzed at the Stable Isotope Facility (SIF) at the University of California, Davis.  Instrumentation and methods are similar to those given above for d¹⁵N and d¹³C, except that larger samples are used (~45 ml) and vanadium pentoxide is added as a catalyst.  Isotope ratios are presented in permil (‰) abundance of heavy isotope relative to reference values as given above.  The standard for sulfur analysis is an IAEA-certified reference material calibrated to Canyon Diablo Triolite (³⁴S/³²S = 0.0450).​