*Geek Box: Dietary Biomarkers
The term “biomarker” means use of a specific biochemical measure that provides an indication of nutrient intakes. This isn’t always as straightforward as “nutrient in = nutrient measured”, because nutritional status is influenced by variations in the digestion, absorption, metabolism, distribution, and excretion of a nutrient, which differs from nutrients to nutrient.
For example, when measuring fatty acids, whether it is the phospholipid content of cell membranes, lipoproteins, or adipose tissue measured, each will provide different indications of dietary intake. Biomarkers may be classified according to what measurement they allow for.
A biomarker for which there is a quantitative relationship between dietary intake and the value of the biomarker, such that absolute intake over a 24hr period can be measured accurately, is known as a “recovery biomarker”. “Recovery” reflects the fact that all intake over a 24hr period is excreted, usually through urine, with minimal losses through other excretory pathways. These are very rare in nutrition science: only 24hr urinary sodium, 24hr urinary potassium, 24hr urinary nitrogen, and total energy measured by doubly-labeled water, are considered recovery biomarkers.
The most commonly used biomarkers, when measuring the concentration of a nutrient in a plasma, red blood cells, or adipose tissue, are known as “concentration biomarkers”, as they are measuring the concentration of that particular nutrient in the circulation or tissue. The use of biomarkers is very attractive for nutritional epidemiology, as it allows for an objective assessment of the validity of dietary questionnaires, and quantification of dietary intake that is independent of measurement error.
However, there remain limitations to their application. First, there is not a reliable biomarker for every nutrient of interest to nutrition science. Secondly, many non-dietary factors may influence the status of a biochemical indicator, thus introducing a potential measurement error that is unrelated to actual dietary intake. Nonetheless, for exposures of interest like sodium, potassium, fatty acids, or total energy expenditure, biomarkers are reliable, and provide a means of quantifying accurate dietary intake.