Phase difference between serum prolactin and cortisol rhythms is related to body mass index in humans

Prolactin (PRL) has many effects in animals and man. For example, it regulates fat mass in fish, birds, and mammals. In particular, the timing of the serum PRL acrophase in relation to the light-dark cycle or to serum cortisol in constant light conditions determines whether the fat mass increases or decreases, as part of the adaptation to seasons. The role of PRL in this respect has been less well studied in man.

We hypothesized that the timing of the PRL acrophase (time point of peak amplitude of the rhythm) with respect to that of cortisol may be correlated with fat mass [or body mass index (BMI) as proxy] in the human, as observed in animals.

Seventy-four subjects were available [mean age, 43 (22-77) yr; mean BMI, 26.8 (18.7-38.4) kg/m(2)].

Immunofluorometric PRL assay and cortisol RIA of 10-min blood samples collected for 24 h were followed by cosinor analysis for the estimation of the acrophase.

The time difference between the cortisol and PRL acrophases was positively correlated with BMI (P = 0.002), but not with sex, age, or season.

In various species, a wide gap between the serum cortisol acrophase and that of PRL leads to fat storage. Our finding is consistent with this observation, although we used BMI as proxy. If an advance shift of the PRL acrophase in relation to that of cortisol is indeed responsible for increased fat mass in man, manipulating the PRL phase may offer an alternative means to treat obesity.