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Stability During Processing and Storage


The stability of the nutrient or bioactive during processing and storage is another


aspect to consider when selecting and/or evaluating nutritional biomarkers. For


example, carotenoids are light sensitive, and as such, blood sampling and pro-


cessing should be conducted in a darkened room and phlebotomy tubes should be


covered with foil or amber covers to minimize light exposure. Alternately, stability
of ascorbic acid concentrations in blood is highly dependent on collection and


processing conditions including room temperature and humidity. Current recom-


mendations include collection in heparin tubes, immediate centrifugation, and


plasma acidification prior to storage at−80 °C (Karlsen et al. 2007 ). Importantly,


some nutrients and diet-derived compounds will denature over time even if stored at


the proper temperature. Examples of bioactives that show low tolerance to


long-term frozen storage are the anthocyanins (de Ancos et al. 2000 ). On the other


hand, phospholipid fatty acids have been shown to be stable for up to 10 years at


−80 °C (Matthan et al. 2010 ). Storage temperature also influences the stability of


nutrient measures over time. In a study evaluating storage temperature and folate


measurement, samples stored at−20 °C were deemed unreliable for measurement,


while those stored at−80 °C were stable over 12 months; B12 concentrations were


stable at both storage temperatures (Jansen et al. 2012 ). Standardized laboratory


procedures and methods for a wide range of nutrients are available through the


Centers for Disease Control and Prevention.


Advancements in Dietary Biomarkers


In recent years, expanded emphasis on the need for more reliable, valid, and


sophisticated biomarkers in nutrition research has led to innovative approaches and
methodological advances. Certainly, the re-emphasis on isotopes for quantification


of exposure—afield that was productive in relation to earlier work defining nutrient


requirements—has gained popularity. But even beyond stable isotope labeling, the


area of metabolomics has advanced our understanding of nutrient/dietary exposures


(Swann and Claus 2014 ). Metabolomics, which involves the measurement of all


metabolite concentrations, has been applied to assess disease risk in relation to a


comprehensive array of metabolites, whereas metabonomics is more specifictoa


systems biology approach that aims to assess metabolic response to biological,


environmental (diet), and genetic exposures (Nicholson and Lindon 2008 ).


Metabolomics requires the use of more sophisticated instrumentation such as


nuclear magnetic resonance (NMR) as well as higher level statistical modeling to


achieve greater precision in relation to food intake and true biological exposure and


bioactivity relative to disease risk. The end product of such biomarker approaches is


a metabolic phenotype that can be assigned at the individual or population sample


level to evaluate health risks and outcomes. Further, this metabolic phenotype can


9 Biomarkers of Diet and Nutritional Health 187

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