Disease or non-disease - about the identification of metabolic conditions that require no treatment

Many inborn errors of metabolism (IEM) fulfil clinically relevant criteria and therefore are considered metabolic diseases. Typically, these conditions are due to impaired biosynthesis or breakdown of substances within specific pathways. However, there is also a growing number of metabolic conditions that are understood to represent merely biochemical phenotypes but are not linked to typical clinical findings and do not require treatment. The distinction between disease and non-disease is often not straightforward, but may have several nuances as well as overlaps, and may even be impossible to definitely classify as in many instances there is a continuum from non-disease to disease rather than categories. This article aims for deeper insights into the characteristics of conditions under discussion in this regard.