Research paper
Adam EK, Quinn ME, Tavernier R, McQuillan MT, Dahlke KA, Gilbert KE. Diurnal cortisol slopes and mental and physical health outcomes: A systematic review and meta-analysis. Psychoneuroendocrinology. 2017 Sep;83:25–41. doi:10.1016/j.psyneuen.2017.05.018. Epub 2017 May 24.
Why we cite it
The first systematic review and meta-analysis of the diurnal cortisol slope literature — 179 associations, 80 studies, 36,823 participants (26,167 unique individuals) — and the source that lets the corpus state the “flattened curve” claim quantitatively rather than as a generality. Three things it contributes that nothing else in the library does.
First, it says what flattening physically is: flatter diurnal cortisol “rhythms contain elements of both hypo- and hyper-cortisolism (including low morning and/or high evening cortisol levels)” — which permits describing an elevated evening as one way the curve flattens, while forbidding the common overstatement that chronic stress simply keeps cortisol high all night.
Second, it sizes the health link (r = .147 overall; r = .288 for immune/inflammation, in 10 of 12 outcome subtypes) and is candid that 91.1% of the underlying findings are cross-sectional, so the direction of causation is unsettled.
Third — and most directly useful to a publication about eating — it puts a number on the association between a flattened rhythm and obesity/BMI/adiposity (r = .093), which is significant but small, and which concerns a different construct from the elevated-cortisol-to-visceral-fat line the corpus carries in `ksc02`. Flattening is not the same as high cortisol.
It complements `ccy01` (mechanism and the awakening response) and `ccy02` (experimental evidence that the rhythm is endogenous) with population-scale evidence on the rhythm’s shape and its correlates.
DOI: 10.1016/j.psyneuen.2017.05.018 · PubMed 28578301 · Publisher