*Geek Box: Biomarkers

*Geek Box: Biomarkers of Dietary Intake

“Biomarkers” are an attractive tool for nutrition science, particular epidemiology, as biomarkers may provide useful complementary data to dietary questionnaires used in cohort studies. However, there are a number of important criteria which must be considered to determine whether a particular biomarker is useful or not.

The levels of any given nutrient in the blood or in a tissue, for example adipose tissue, may be influenced not only be the actual levels of dietary intake of that nutrient, but also by factors like absorption, distribution, competition with other nutrients for transport, metabolism and excretion.

For example, some nutrients may be synthesised in the body – an example of this is saturated fatty acids, which may be synthesised from carbohydrate intake. As a result, biomarkers of saturated fatty acids are generally a poor reflection of dietary intake of saturated fat. However, these saturated fatty acids are even-chain [i.e., 8, 12, 16, 18 carbon fatty acids]. The two biomarkers for dairy intake mentioned above have an odd number of carbons – 15 and 17. And these fatty acids cannot be synthesised in the body, and are only found in dairy foods. Therefore, these biomarkers are a reliable reflection of dietary intake of dairy.

Another example is the omega-6 essential fatty acid linoleic acid – as an essential fat, humans require dietary intake, and therefore measuring tissue levels of linoleic acid is reliable reflection of dietary intake. Biomarkers can be classified in two broad categories: recovery biomarkers and concentration biomarkers. Recovery biomarkers have a quantitative relationship between the biomarker and dietary intake in a specific period. For example, sodium is excreted from the body to maintain sodium balance, and therefore urinary sodium [if enough collections are made over the course of a 24hr day] can be used to ‘recover’ data on sodium intake.

A concentration biomarker has a correlation between dietary intake and the levels [‘concentrations’] – of the nutrient in a sample, for example plasma, red blood cells, or adipose tissue. Most nutrient biomarkers are concentration biomarkers, such as the examples of the C15:0 and C17:0 dairy fatty acids, and linoleic acid, above.