Assessing vitamin status in large population surveys by measuring biomarkers and dietary intake – two case studies: folate and vitamin D

The National Health and Nutrition Examination Survey (NHANES) provides the most comprehensive assessment of the health and nutrition status of the US population. Up-to-date reference intervals on biomarkers and dietary intake inform the scientific and public health policy communities on current status and trends over time.The main purpose of dietary assessment methods such as the food-frequency questionnaire, food record (or diary), and 24-hr dietary recall is to estimate intake of nutrients and, together with supplement usage information, describe total intake of various foods or nutrients. As with all self-reporting methods, these tools are challenging to use and interpret. Yet, they are needed to establish dietary reference intake recommendations and to evaluate what proportion of the population meets these recommendations. While biomarkers are generally expensive and, to some degree, invasive, there is no question as to their ability to assess nutrition status. In some cases biomarkers can also be used to assess intake or function, although rarely can one biomarker fulfill all these purposes. For example, serum folate is a good indicator of folate intake, red blood cell (RBC) folate is a good status indicator, and plasma total homocysteine is a good functional indicator of one-carbon metabolism.Using folate and vitamin D - two vitamins that are currently hotly debated in the public health arena - as two case studies, we discuss the complexities of using biomarkers and total intake information to assess nutrition status. These two examples also show how biomarkers and intake provide different information and how both are needed to evaluate and set public health policy. We also provide guidance on general requirements for using nutrition biomarkers and food and supplement intake information in longitudinal, population-based surveys.