Biomarkers: background, classification and guidelines for applications in nutritional epidemiology.

One of the main problems in nutritional epidemiology is to assess food intake as well as nutrient/food component intake to a high level of validity and reliability. To help in this process, the need to have good biomarkers that more objectively allow us to evaluate the diet consumed in a more standardized, valid and precise way has often been commented upon. There are various definitions of biomarkers and also different classifications of the same. In general a biomarker can be defined as a characteristic that can objectively measure different biological samples and that can be evaluated as an exposure marker of normal or pathogenic biological processes or of responses to a certain intervention. The biological samples most commonly used in nutritional epidemiology are blood, red blood cells, plasma, serum, urine, nails, saliva, faeces and samples of different tissues. Exposure biomarkers (dietary intake), biomarkers of effects and biomarkers of disease status can be determined from these samples. In turn, exposure biomarkers can be temporarily categorized into markers of acute, medium term or chronic effects. Many difficulties arise in identifying good biomarkers. Currently, advances in omics are opening up new possibilities for obtaining new biomarkers of various kinds, using genomics, epigenomics, transcriptomics, lipidomics, proteomics and metabolomics. We shall review the present situation of biomarkers in nutritional epidemiology as well as the future trends of the new omic biomarkers.