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LETTER TO THE EDITOR |
Department of Animal Physiology University of Bayreuth Germany
Address correspondence to Dr. Martin Fenske, Department of Animal Physiology, University of Bayreuth, 95440 Bayreuth, Germany. E-mail: martin.fenske{at}uni-bayreuth.de
To the Editor:
I read with great interest the paper of Rideout and colleagues (1) who claimed that "our finding of higher cortisol excretion in postmenopausal women with high cognitive dietary restraint suggests that high dietary restraint may be a source of stress for generally healthy, normal-weight postmenopausal women" (p. 631). A shortcoming may confound their work, making the interpretation of their endocrine results difficult, if not impossible. The shortcoming is: The authors measured urinary "cortisol" (probably: urinary free "cortisol") by a competitive chemiluminescent immunoassay (Bayer ADVIA Centaur) that lacks specificity. Gray and colleagues (2) had evaluated this immunoassay and compared their values of urinary free cortisol (UFF) with those obtained by a specific gas chromatography–mass spectrometry method. They observed that cortisone, a major metabolite of UFF in the urine, demonstrated a cross-reactivity of 44%, and they concluded that this method overestimates urinary cortisol, giving results up to twice those obtained by a specific gas chromatography–mass spectrometry method. Thus, we may assume that Rideout and colleagues (1) measured the sum of free cortisol and urinary free cortisone (UFE) excretion [for discussion, see (3)], and that the observed changes were mainly due to changes of UFE excretion; this may especially be true if urine volume is increased [see also (4,5)]. Thus, their finding "we were surprised to find a relatively strong relationship between mean total water intake and cortisol excretion" [(1), p. 632] should be changed as follows: "we were surprised to find a relatively strong relationship between mean total water intake and corticosteroid (UFF + UFE) excretion." Unfortunately, the results of Rideout and colleagues (1) will not be improved if results would be reworked and the volume of each 24-hour urine would be taken into calculation since urinary steroid excretion depends also on daytime: An increase of urine volume in the morning (zenith of adrenocortical secretion) will have a more pronounced effect on steroid excretion than an increase of urine volume in the evening [nadir of adrenocortical secretion (3,6)].
Conclusion: Specific methods are necessary to measure true UFF excretion in 24-hour urines [for discussion, see (7)]. UFF excretion should be determined only if urine volume is kept as constant as possible—this demands that patients or volunteers strictly observe their fluid intake (volume, daytime). Future work will then show whether dietary restraint may be a source of stress in postmenopausal women.
References
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