Biomarker.

A measurable indicator used to track a biological state or response.

A biomarker is any measurable characteristic used as an indicator of a normal process, a disease process, or a response to an intervention. Blood glucose, blood pressure, IGF-1, and inflammatory markers are all biomarkers. They are indispensable in research because they are objective, can be measured quickly, and often move long before a clinical outcome would become visible. The critical distinction is between a biomarker and an outcome that matters to a person. A surrogate endpoint is a biomarker used as a stand-in for a real outcome, and the validity of that substitution has to be demonstrated rather than assumed. The history of medicine contains several cases where a drug reliably improved a surrogate marker while making the clinical outcome worse, because the marker was correlated with the disease without being on the causal path to it. This is directly applicable to peptide claims, where a great deal of promotional material reports marker changes, in inflammatory markers, hormone levels, or body composition proxies, as though they were established benefits. The honest reading is that a biomarker moving in the expected direction is a reason to keep investigating, and only occasionally a reason to conclude anything about how someone will actually fare.