1 Division of Endocrinology, Department of Internal Medicine, San Juan de Dios Hospital, School of Medicine, University of Chile, 2 Laboratory of Energy Metabolism and Stable Isotopes, INTA, University of Chile, Santiago, Chile, 3 Laboratory of Animal Physiology and Endocrinology, School of Veterinary Medicine, University of Concepción, Chillán, Chile, 4 Division of Gynecological Endocrinology and Reproductive Medicine, Department of Obstetrics and Gynecology and Reproductive Medicine, University of Erlangen, Erlangen, Germany
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Abstract |
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Key words: insulin resistance/lactation/PCOS
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Introduction |
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During lactation, mammary glucose utilization is increased, which may lead to a lowering of serum glucose and insulin concentrations, as has been observed in many species including humans (
Illingworth et al.1987). In lactating rats, one possible mechanism proposed to face the high glucose requirements of the lactating mammary gland would be a decrease in glucose utilization in other tissues by developing an insulin-resistant state (
Burnol et al.1983
,
1987
) However, in normal lactating women, in contrast to rodents, the increase in the rate of glucose utilization by the mammary gland appears not to alter the whole body insulin sensitivity, as changes in insulin sensitivity of peripheral tissues are not involved in promoting the glucose flux to the mammary gland (
Neville et al.1994
;
Stanley et al.1998
).
In normal women, lactation reduces glucose and insulin concentrations without affecting insulin sensitivity. In lactating PCOS women, the impact of lactation on these parameters is not known. The aim of the present study was to evaluate the effect of lactation on insulin resistance, glucose and insulin metabolism, and SHBG and IGFBP-1 concentrations in normal and PCOS lactating women during different states of the postpartum and after weaning.
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Materials and methods |
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PCOS patients included in this study were selected from a group of 20 PCOS patients who were seeking treatment for infertility at the Unit of Reproductive Medicine, and were placed on a 6 month diet and exercise programme which consisted of a 1000 kcal low-fat diet and a daily walk of 30 min. During this programme, six patients ovulated and became pregnant. Another six patients became pregnant after pharmacological induction of ovulation with clomiphene citrate 100 mg/day from cycle day 59.
By design, six normal lactating (NL) women of similar age and BMI (Table I
)
acted as a control group. Each one had a history of regular 2832 day menstrual cycles, absence of hirsutism and other manifestations of hyperandrogenism, absence of galactorrhoea, thyroid dysfunction and family history of diabetes. They were healthy, not receiving any drug therapy and had a normal term pregnancy with vaginal delivery of a healthy infant. These women were recruited from the maternity unit of the same hospital.
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In order to compare the values observed in lactating women with the preconceptional state, two reference groups were constituted by: (i) 10 non-pregnant non-lactating normal cycling women comparable in age and BMI with the lactating groups and (ii) the same PCOS patients before they become pregnant (pre-pregnant PCOS women).
Prior to the study, informed consent was obtained from all subjects. This study was approved by the local ethics committee.
Study protocol
The women were admitted to the Clinical Research Centre in the morning (8:309:00 am) on the 4th and 8th week postpartum and 8 weeks after weaning. The study was initiated at the 4th week postpartum to avoid the effect of placental steroids and peptide hormones.
The first testing consisted of a 2 h, 75 g oral glucose tolerance test (oGGT) performed in accordance with the National Diabetes Data Group standards. Glucose and insulin were measured in each sample. SHBG and IGFBP-1 were determined in the fasting sample. The women were not allowed to nurse during the 2 h oGGT.
The second evaluation consisted of an insulin tolerance test, performed according to established criteria (
Grulet et al.1993), 2 days later and under the same conditions described for the oGTT. For the insulin tolerance test, 0.1 IU/kg regular human insulin was injected i.v. and blood samples were drawn to measure glucose at 0, 3, 5, 7, 10 and 15 min after injection, and also 5 min before injection.
Data analysis
The measurements derived from the oGTT included the following: (i) Serum fasting glucose, serum fasting insulin and the fasting glucose to insulin ratio (G:I); (ii) Serum 2 h glucose and 2 h insulin. The insulin sensivity index (SI) was calculated using the delta glycaemia/initial glycaemia (DG/G0), in which G0 is the initial serum glucose level calculated from the mean glycaemic value at 5 and 0 min before injection, and DG is the variation between G0 and the glycaemia value obtained after 15 min by calculation of the regression plot.
Assays
Serum glucose was determined by the glucose oxidase method (Photometric Instrument 4010; Roche, Basel, Switzerland). The intra-assay coefficient of variation of this method was <2.0%. Serum insulin was assayed by radioimmunoassay (RIA) (Diagnostic Products Corp. LA, USA), SHBG and IGFBP-1 concentrations were measured using kits obtained from Diagnostic Laboratories System, Inc. (Webster, TX, USA). The intra- and inter-assay coefficients of variation were 5 and 8% for insulin; 3.8 and 7.9% for SHBG; 3.5 and 4.2% for IGFBP-1.
Statistical evaluation
Comparisons within groups were performed by analysis of variance (ANOVA) followed by Newman-Keul's multiple range test. Differences between groups were sought by MannWhitney
U-test. The significance level was set at
P < 0.05. Results are expressed as means together with ranges.
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Results |
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Table
II
shows the metabolic and endocrine characteristics during the three study periods in NL women. In NL women, fasting glucose and fasting insulin levels, fasting G:I ratio and SI did not change during the study and were comparable with the values observed in non-pregnant non-lactating normal women.
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Table
III
shows the metabolic and endocrine characteristics during the three study periods in LPCOS women. In LPCOS women, fasting glucose levels and SI did not change during the study and were comparable with values observed in pre-pregnant PCOS women. Fasting insulin concentrations increased significantly between the 4th week postpartum and after weaning, but remained lower than those observed in prepregnant PCOS women.
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Comparing both groups of lactating women at the 4th week postpartum, 2 h insulin was significantly higher in LPCOS women, compared with NL women, while SHBG and IGFBP-1 concentrations were significantly lower. At the 8th week postpartum, 2 h insulin remained significantly higher and SHBG and IGFBP-1 significantly lower in LPCOS women, compared to NL women. After weaning, IGFBP-1 and SHBG concentrations remained lower and 2 h insulin concentrations remained higher than those observed in normal lactating women.
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Discussion |
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During lactation, the demand for glucose by the mammary gland increases to synthesize milk lactose, favouring a decrease in serum glucose concentrations and, in turn, decreasing insulin release by the pancreas. This, at the same time, increases the SHBG and IGFBP-1 concentrations in blood, because the levels of both proteins are negatively regulated by insulin. On the other hand, the increase in IGFBP-1 has a protective role on glucose levels because it binds IGF-1, preventing IGF-induced hypoglycaemia and therefore avoiding stages of hypoglycaemia in lactating women (
Baxter, 1995). It is interesting to point out that in a pathological condition like PCOS, this regulatory mechanism is similar to that observed in normal lactating women.
Early in the lactation period, a decrease in basal insulin concentrations was observed together with an increase in binding proteins. This effect was transitory since at the end of the lactating period, these parameters tended to reach the values observed in the non-lactating reference groups, with a delay in the PCOS group. Even when these parameters changed during lactation, tissue insulin sensitivity, evaluated by the insulin tolerance test, remained constant in both groups and with values similar to those of the corresponding non-lactating reference group. These findings are interesting and similar to those previously published (
Neville et al.1994), in which it was observed that in normal lactating women there was no modification in insulin sensitivity, contrary to what had been described in animals (
Burnol et al.1987
;
Vernon et al.1989
). Moreover, in our lactating PCOS group, peripheral insulin sensitivity was not modified. This observation is unique and suggests that independent of the metabolic changes that could be occurring during pregnancy and lactation, the insulin resistance state of PCOS is invariable, indicating that the insulin resistance of the PCOS syndrome is intrinsic and unrelated to other factors as previously described (Dunaif 1987). Accordingly, although during lactation the binding proteins increased in both groups, in lactating PCOS women the values remained low in comparison to those observed in normal lactating women, suggesting again that lactation is able to modify the biological markers of insulin resistance but is not able to correct them. Although the lactating and non lactating PCOS groups were matched, fasting insulin concentrations were lower after weaning in the lactating PCOS group than in the same patients before pregnancy. We have no explanation for this phenomenon, nevertheless lactation can be considered as a critical period for the improvement of the metabolic control of these patients.
The WHR is a simple method for estimating intra-abdominal fat mass (
James, 1996), which predisposes to obesity-related diseases such as insulin resistance. During lactation the fat mass decreases (
Butte et al.1999
), probably promoting a redistribution of the fat content; this fact was observed in lactating PCOS women who presented a decrease in WHR during lactation and an increment after weaning.
In summary, lactation has no benefits on peripheral insulin resistance in PCOS lactating women, however a transitory effect on insulin concentrations and biological markers of insulin resistance is observed.
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Acknowledgements |
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Notes |
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References |
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Submitted on November 30, 2000; accepted on March 23, 2001.