| HOME | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|---|
| ||||||||||||||
LETTER TO THE EDITOR |
1 Human Nutrition
2 Psychology The University of British Columbia Vancouver Canada
Address correspondence to Susan I. Barr, PhD, RD, Human Nutrition, The University of British Columbia, 2205 East Mall, Vancouver, BC, V6T 1Z4, Canada. E-mail: susan.barr{at}ubc.ca
To the Editor:
We appreciate Dr. Fenske's interest in our recent report of higher 24-hour urinary cortisol excretion in postmenopausal women with high versus low dietary restraint (1). Dr. Fenske raised two main points in his letter: First, that our results are limited by the lack of specificity of the Bayer ADVIA Centaur method for the measurement of urinary cortisol, and second, that potential differences in the time and volume of urine excretion could account for the difference in cortisol observed between our two groups. While his comments are noteworthy, it is unlikely that they explain the difference in cortisol excretion observed between postmenopausal women with high and low dietary restraint.
As indicated by Dr. Fenske, Gray and colleagues showed that the Bayer ADVIA Centaur method of urinary cortisol measurement, like most immunoassay methods, lacks specificity due to its cross-reactivity with cortisol metabolites such as cortisone (2). Given that our study was designed as a between-groups comparison and cortisol excretion was measured by the same method in both groups, this tendency to overestimate cortisol excretion is only pertinent to our results if there might be reason to believe that the compounds with which cross-reactivity occurs could be present in systematically different proportions between the two groups of women we compared.
In his second point, Dr. Fenske cites his recently published data (3) to suggest that this could be the case. Specifically, because short-term water diuresis appears to increase urinary cortisone, but not cortisol, excretion in young men (3,4), he suggests that the higher urine volume we observed in women with high versus low dietary restraint (2.4 ± 0.8 L versus 2.1 ± 0.8 L, p =.04) could mean that urinary cortisone, rather than cortisol, was higher in women with high dietary restraint. This would be especially problematic if the higher urine volume was produced in the morning, because the amount of cortisol excreted following the morning peak of hypothalamic–pituitary–adrenal (HPA) axis activity typically makes a substantial contribution to the total amount of cortisol excreted in a day. However, whether Dr. Fenske's results from a small group of young men are applicable to the postmenopausal women in our study is unclear.
First, short-term water diuresis was obtained in that study by the ingestion of 1 L of water between 7:00 AM and 8:00 AM and an additional 1 L of water at 10:00 AM (3), a pattern which does not represent habitual daily fluid intake. This water-loading significantly increased urine excretion; in fact, the difference in urine output over 2 hours between control participants (who ingested 1 L of water at the first time-point only) and participants undergoing diuresis (who also ingested a second liter of water at the second time-point) appears to have been almost 900 mL. This is approximately three times the difference in urine volume observed over an entire 24-hour period for the two groups in our study (1). The extent to which differences may exist in urinary cortisol and cortisone excretion at more typical levels of fluid ingestion and urine output remains to be determined. In addition, differences in cortisol secretion have been observed in association with both age and gender (5). Thus, whether a similar pattern of results associated with water diuresis would be obtained across age and gender groups is open to question.
We disagree with Dr. Fenske that urinary cortisol excretion should be measured only under conditions that keep urine volume as constant as possible. This would impose artificial limits on participants' dietary patterns and preclude investigations of HPA activity associated with habitual diet-related behavior, which was the very aim of our study. Our study participants were not advised to alter their dietary patterns; however, they did strictly observe their fluid intake (including volume and time of day) as part of the food records completed on the days of the 24-hour urine collections.
Thus, in response to Dr. Fenske's comments, we undertook an additional analysis to determine whether our high-restraint and low-restraint groups may have differed in the proportion of urine they excreted in the morning (when HPA activity and cortisol excretion is at its peak). Because the time and volume of each urine void throughout the 24-hour collection was not recorded, we used the volume of fluid ingested in the morning as a proxy for morning urine excretion. We calculated all fluids ingested between 5:01 AM and 10:59 AM on the day of each participant's 24-hour urine collections (including all beverages and soup) and expressed this as a proportion of the participant's total volume of urine voided throughout the day. We found that high and low dietary restraint groups did not differ in the estimated proportion of urine voided in the morning (36.7% versus 36.2%, p =.90). Thus, it is unlikely that the overall ratio of cortisol to cortisone differed between groups as a result of the slightly higher 24-hour urine volume in women with high dietary restraint.
In light of the considerations mentioned above and given that 1) the proportion of urine voided in the morning did not differ between groups, and 2) our original analysis included total urine volume in the regression for predictors of urinary cortisol excretion, it is unlikely that the points raised by Dr. Fenske alter the main finding of our study.
References
| ||||||||||||||
| HOME | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|---|
