Closing the Border on a New Frontier The Problem With Salivary Nerve Growth Factor

In 2013, a study appeared in Psychosomatic Medicine showing that salivary levels of nerve growth factor (sNGF) rapidly increase (to +45% of baseline levels) in response to an acute psychosocial stress task that involved discussing an unresolved conflict with a romantic partner (1). NGF, also referred to as mature NGF-β, is a neurotrophic factor that, among other functions, mediates neuronal growth and differentiation, although it may also drive cell death (2). The study presented a genuinely novel finding, which was emphasized by the authors' branding of sNGF as a “new frontier for stress research” that could “represent a
resilience factor protecting individuals from adverse effects of stress” (1,3). Secondary analyses of these seminal data were in line with these speculations, showing, for example, that greater sNGF reactivity to conflict was related to stronger appraisals of coping ability and agency, lower anticipatory stress appraisals, and higher well-being (1,3). However, there is one problem, which is substantial: saliva appears to contain little, if any, NGF. Therefore, on grounds that the findings are biologically implausible, we think that the validity of these data (and related data published in other journals) need to be reconsidered.