| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Endocrine Care |
Department of Medicine, University of Hull (V.J., S.L.A.), York District General Hospital (P.E.J.), and Department of Clinical Biochemistry and Immunology, Hull Royal Infirmary (E.S.K., S.H.), Hull, United Kingdom HU3 2RW
Address all correspondence and requests for reprints to: Dr. V. Jayagopal, University of Hull, Michael White Center for Diabetes and Endocrinology, Hull Royal Infirmary, Anlaby Road, Hull, United Kingdom HU3 2RW. E-mail: . v.jayagopal{at}hull.ac.uk
Abstract
Increased insulin resistance (IR) is a cardinal feature of overweight patients with polycystic ovarian syndrome (PCOS). However, there are no data on the variability of IR for subjects with PCOS. The biological variation of IR (homeostasis model assessment model) was assessed by measuring IR at 4-d intervals on 10 consecutive occasions in 12 overweight PCOS patients (median age, 28 yr; range, 18–31 yr) and 11 weight-matched control women having regular menses and without PCOS (median age, 30 yr; range, 19–33 yr). The distribution of IR was log Gaussian in PCOS and Gaussian distribution in the control group. The IR in PCOS subjects was significantly greater than in the controls [mean (range), 5.85 U (1–42.1) vs. 1.67 U (0.48–3.49); P = 0.001]. After accounting for analytical variation, the mean intraindividual variance was also substantially greater in PCOS patients than in controls (mean, 1.19 vs. 0.23). As a consequence, at any level of IR, a subsequent sample must rise by more than 322% or fall by more than 31% to be considered significantly different from the first. IR, measured using the homeostasis model assessment model, is significantly greater and more variable for overweight patients with PCOS. Therefore, this inherent variability needs to be accounted for in studies of IR in PCOS.
POLYCYSTIC OVARY SYNDROME (PCOS) is a common disorder of women of reproductive age and is characterized by chronic anovulation and androgen excess with the clinical manifestation of oligomenorrhea, hirsutism, and acne (1). Increased insulin resistance (IR) has been shown to be an important feature in PCOS and may play a pathogenetic role (2). Hyperinsulinemia may contribute to the androgen excess seen in women with PCOS by stimulating androgen synthesis in thecal cells (3), decreasing SHBG synthesis in the liver leading to increasing free androgen concentrations (4), and potentiation of ACTH-stimulated adrenal androgen secretion (5). It may also contribute to the increased risk of developing type 2 diabetes (6, 7) and an adverse cardiovascular risk profile (2, 8) potentially leading to accelerated coronary and vascular disease. Hyperinsulinemia may be an important factor in the infertility associated with PCOS, as improvements in fertility rates have been reported after treatment with insulin-sensitizing agents (9, 10). These observations have led to the use of insulin-sensitizing agents such as metformin (9) and troglitazone (10) to attenuate the metabolic and hormonal consequences of PCOS. At present there are no data on the variability of IR for an individual with PCOS; therefore, this study was undertaken to establish whether an individual’s IR remains within narrow biological limits or varies more widely over a given time period.
Subjects and Methods
Subjects
Twelve overweight [body mass index (BMI), >25] Caucasian women diagnosed with PCOS (median age, 28 yr; range, 18–31 yr) and 11 weight-matched Caucasian women (controls) having regular menses (every 28–30 d) and without PCOS (median age, 30 yr; range, 19–33) participated in the study. The diagnosis of PCOS was based on evidence of hyperandrogenemia (free androgen index, >8; mean ± SD: PCOS, 21.85 ± 7.95; controls, 4.68 ± 2.05), with a history of oligomenorrhea and hirsutism or acne. Nonclassical 21-hydroxylase deficiency, hyperprolactinemia, and androgen-secreting tumors were excluded by appropriate tests before the diagnosis of PCOS was made (2). No subject was taking any medication currently or for the preceding 6 months, and there was no concurrent illness. All subjects were on an unrestricted diet and were instructed not to modify their usual eating patterns during the period of sampling. Fasting plasma glucose, age, BMI, and current smoking status were obtained. The BMI in the PCOS group (median, 29.74; range, 27.21–43.28) was nonsignificantly greater (P = 0.151) than that in the control group (median, 28.71; range, 23.24–33.56). Fasting venous blood was collected into serum gel tubes (Becton Dickinson and Co., Franklin Lakes, NJ) and one fluoride oxalate tube at the same time each day (0800–0900 h) on 10 consecutive occasions at 4-d intervals. Samples were separated by centrifugation at 2000 x g for 15 min at 4 C, and two aliquots of the serum were stored at -20 C within 1 h of collection. Plasma glucose was analyzed in singleton within 4 h of collection. The serum samples were split before assay. All subjects gave their informed written consent before entering the study that had been approved by the Hull and East Riding local research ethics committee.
Reagents
Serum insulin was assayed using a competitive chemiluminescent immunoassay, supplied by Euro/DPC (Llanberis, UK). The assay was performed on a DPC Immulite 2000 analyzer (Euro/DPC), using the manufacturer’s recommended protocol. The coefficient of variation of this method was 8%, calculated as described below using study samples. The analytical sensitivity was 2 µU/ml, and there was no stated cross-reactivity with proinsulin. Plasma glucose was measured using a Synchron LX 20 analyzer (Beckman Coulter, Inc., High Wycombe, UK), using the manufacturer’s recommended protocol. The coefficient of variation for this assay was 1.2% at a mean glucose value of 5.3 mmol/liter during the study period. Before analysis, all of the serum samples were thawed and thoroughly mixed. The duplicate samples (i.e. two per visit) were randomized and then analyzed in a single continuous batch using a single batch of reagents.
Statistical analysis
IR was calculated using the homeostasis model assessment (HOMA) method [resistance = insulin/(22.5e-ln glucose)] (11). Biovariability data were analyzed by calculating analytical, within-subject, and between-subject variances (SDA2, SDI2, and SDG2, respectively) according to the methods of Fraser et al. (12). By this technique, analytical variance (SDA2) was calculated from the difference between duplicate results for each specimen (SDA2 = 
d2/2N, where d is the difference between duplicates, and N is the number of paired results). The variance of the first set of duplicate results for each subject on the 10 d of assessment was used to calculate the average biological intraindividual variance (SDI2) by subtraction of SDa2 from the observed dispersion (equal to SDI2 + SDA2). Subtracting SDI2 + SDA2 from the overall variance of the set of first results determined the interindividual variance (SDG2).
Results
The clinical and biochemical details of the individual subjects are shown in Table 1
. The distribution of IR was found to be log Gaussian (by Kolmogorov-Smirnov test) in the PCOS group and Gaussian for the control population. Figure 1 shows the mean and range of IR for the individuals in the two groups.
|
|
Discussion
This is the first study to show that the absolute as well as the intraindividual variation in IR are much greater in overweight women with PCOS. In accord with other studies, the absolute IR in individuals with PCOS was significantly greater than that in weight-matched individuals without PCOS (2, 13). However, for the PCOS subjects there was a wide degree of variation in the value of IR both as a group and between each individual within the group, that is the interindividual variation and intraindividual variation were high. Conversely, for the group without PCOS, IR remained within a narrow range over a period of time both for the group as a whole and for each individual within the group; that is the interindividual variation and intraindividual variation were low. These data show that variability data on IR collected from normally cycling women (14, 15) are not comparable to those with PCOS due to the significant difference for both the intra- and interindividual comparisons. Furthermore, the high degree of intraindividual variability seen in women with PCOS means that a change in two single measurements of IR may be more a consequence of biological variation than of any treatment effect. Because of the log Gaussian distribution of IR in the PCOS group, the values have to rise by more than 322% or fall by more than 31% before a significant critical difference can be assured. As an illustration, we evaluated the difference in IR between two time points in the series (visits 1 and 5) for the PCOS group. Using the Wilcoxon sign-rank test, we found a significant difference in the measured IR between visit 1 (mean ± SD, 4.38 ± 2.24) and visit 5 (mean ± SD, 9.15 ± 10.98; P = 0.015). The difference illustrated is within the biological variability for IR in these patients and therefore is not significant. However, had these two measurements been made before and after treatment, then the difference found may have been incorrectly attributed to a treatment effect. Until now there have been no longitudinal data for the variability of IR in subjects with PCOS over intervals of time longer than the present study. In a previous study (13) the fasting glucose/insulin ratio was a good measure of insulin sensitivity in obese women with PCOS and has both high sensitivity and specificity for detecting insulin-resistant women; however, there were insufficient time points to determine biological variability.
It is known that age, BMI, ethnicity, oral contraceptive use, and the presence of diabetes can influence an individual’s IR (1, 16, 17, 18, 19), and that only PCOS women with excess body fat generally exhibit IR compared with equally obese controls (20, 21). The two groups in this study were closely matched for age, BMI, and ethnicity, and no subject in either group was diabetic or receiving treatment.
Caution needs to be expressed in extrapolating the results of IR variability determined by the HOMA method reported here to repeated measures of IR by other methods, such as insulin clamps or iv glucose tolerance tests. However, the strong correlation shown between HOMA and other methods (11, 22) would suggest that the biological variation in PCOS would also be seen in other assessment methods. The very low biological variability in IR seen in women without PCOS using the HOMA method would indicate that the wider biological variation seen in PCOS is real rather than a methodological flaw of the HOMA method. This study used the HOMA model to assess IR for two reasons. Firstly, it is the technique most likely to be used in clinical practice, and secondly, 10 longitudinal measurements required over the 36-d sampling period using the other more labor-intensive and invasive techniques such as hyperinsulinemic-euglycemic clamps or iv glucose tolerance tests were considered unsuitable in view of the potential associated complications, such as thrombophlebitis, from these invasive methods. The advantage of the type of study performed here is that it eliminated the effect of any analytical imprecision and concentrated on the biological variation.
In conclusion, we have found that IR measured using the HOMA model is both greater in overweight individuals with PCOS and has more intrinsic variability than in women without PCOS. Therefore, this inherent variability needs to be accounted for in studies of IR in PCOS.
Acknowledgments
Footnotes
Abbreviations: HOMA, Homeostasis model assessment; IR, insulin resistance; PCOS, polycystic ovarian syndrome.
Received July 30, 2001.
Accepted January 6, 2002.
References
- hydroxycorticosteroid intermediates response to adrenocorticotropin in hyperandrogenic women: apparent relative impairment of 17,20-lyase activity. J Clin Endocrinol Metab 81:881–886[Abstract]
This article has been cited by other articles:
 |
|
 |
|
  L. W. Cho, V. Jayagopal, E. S. Kilpatrick, and S. L. Atkin The Biological Variation of C-Reactive Protein in Polycystic Ovarian Syndrome Clin. Chem., October 1, 2005; 51(10): 1905 - 1907. [Full Text] [PDF]
|
|
|
|
 |
|
 C. Ortega-Gonzalez, S. Luna, L. Hernandez, G. Crespo, P. Aguayo, G. Arteaga-Troncoso, and A. Parra Responses of Serum Androgen and Insulin Resistance to Metformin and Pioglitazone in Obese, Insulin-Resistant Women with Polycystic Ovary Syndrome J. Clin. Endocrinol. Metab., March 1, 2005; 90(3): 1360 - 1365. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
V. Jayagopal, E. S. Kilpatrick, S. Holding, P. E. Jennings, and S. L. Atkin Orlistat Is as Beneficial as Metformin in the Treatment of Polycystic Ovarian Syndrome J. Clin. Endocrinol. Metab., February 1, 2005; 90(2): 729 - 733. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
V. Jayagopal, E. S. Kilpatrick, P. E. Jennings, D. A. Hepburn, and S. L. Atkin The Biological Variation of Testosterone and Sex Hormone-Binding Globulin (SHBG) in Polycystic Ovarian Syndrome: Implications for SHBG as a Surrogate Marker of Insulin Resistance J. Clin. Endocrinol. Metab., April 1, 2003; 88(4): 1528 - 1533. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
