Clinical and Investigative Medicine

 

Glucose turnover and gluconeogenesis during pregnancy in women with and without insulin-dependent diabetes mellitus

Jean-Louis Chiasson, MD
Ghassan G. El Achkar, MD
Francine Ducros, BSc
Josée Bourque, BSc
Pierre Maheux, MD

Clin Invest Med 1997;20(3):140-151

[résumé]

From the Research Group on Diabetes and Metabolic Regulation, Institut de recherches cliniques de Montréal and Hôtel-Dieu de Montréal Hospital, Department of Medicine, University of Montreal, Montreal, Que. Dr. Maheux is now with the Centre universitaire de santé de l'Estrie, Sherbrooke, Que.

(Original manuscript submitted Dec. 6, 1995; received in revised form Feb. 6, 1997; accepted Feb. 10, 1997)

Reprint requests to: Dr. Jean-Louis Chiasson, Centre de Recherche, Hôtel-Dieu de Montréal, 3850 Saint-Urbain St., Montreal QC H2W 1T8; fax 514 843-2709; chiassoj@ere.umontreal.ca

Contents
See also p.152

Abstract

Objective: To characterize the effect of pregnancy on glucose turnover and gluconeogenesis in healthy women and in women with well-controlled insulin-dependent diabetes mellitus (IDDM).

Design: Prospective clinical study.

Setting: Clinical research unit of the Hôtel-Dieu de Montréal hospital.

Participants: Five healthy pregnant women and 6 pregnant women with IDDM.

Interventions: Glucose turnover and gluconeogenesis in the postabsorptive state at 16 and 32 weeks' gestation and at 24 weeks postpartum were studied
with the use of a double stable isotope technique (D[2,3,4,6,62H]-glucose and L[1,2,313C]-alanine). In the women with IDDM, plasma glucose levels were controlled by continuous subcutaneous insulin infusion throughout pregnancy and with a Biostator on the morning of the study.

Results: In the women without IDDM, hepatic glucose production was 11.6 (standard error of the mean [SEM] 2.2) µmol/kg per minute at 16 weeks' gestation, 12.5 (SEM 1.8) µmol/kg per minute at 32 weeks' gestation, and 13.2 (SEM 1.9) µmol/kg per minute at 24 weeks postpartum. In the women with IDDM, it was 10.7 (SEM 2.4) µmol/kg per minute, 10.5 (SEM 1.2) µmol/kg per minute and 12.3 (SEM 0.5) µmol/kg per minute at the same respective periods. The difference in levels between the 2 groups was not significant. Levels of the gluconeogenic precursors alanine and lactate were increased during pregnancy in both the women without IDDM (from 0.18 [SEM 0.02] mmol/L and 0.64 [SEM 0.09] mmol/L, respectively, to 0.25 [SEM 0.02] mmol/L and 1.15 [SEM 0.17] mmol/L, respectively, p < 0.01) and in those with IDDM (from 0.15 [SEM 0.01] mmol/L and 0.47 [SEM 0.04] mmol/L, respectively, to 0.19 [SEM 0.02] mmol/L and 0.70 [SEM 0.01] mmol/L, respectively, p < 0.05). After an overnight fast, gluconeogenesis from alanine was not affected by pregnancy in both groups of women. In the women without IDDM, the plasma insulin level was low in early pregnancy (33.6 [SEM 3.6] pmol/L) and increased in late gestation (87.6 [SEM 9.6] pmol/L) compared with postpartum levels (60.0 [SEM 7.8] pmol/L). Plasma glucagon levels tended to rise in late gestation (from 95.1 [SEM 6.7] ng/L to 116.0 [SEM 36.0] ng/L). In the women with IDDM, the free plasma insulin and plasma glucagon levels were higher in early pregnancy (55.2 [SEM 6.6] pmol/L and 196.1 [SEM 29.8] ng/L, respectively) and did not change significantly during pregnancy.

Conclusion: Basal glucose turnover and gluconeogenesis are not increased during pregnancy in women without IDDM or in women with well-controlled IDDM. The decrease in the plasma glucose level during pregnancy suggests that the use of glucose by the growing fetus is augmented and that this is not totally compensated for by a rise in postabsorptive hepatic glucose production. The glucose requirement by the growing fetus is probably supplied by the increased postprandial plasma glucose level.

Résumé

Objectif : Décrire l'effet de la grossesse sur le cycle de reconstitution du glucose et la gluconéogenèse chez des femmes en bonne santé et chez des femmes atteintes de diabète sucré insulinodépendant (DSID) bien contrôlé.

Conception : Étude clinique prospective.

Contexte : Unité de recherche clinique de l'hôpital Hôtel-Dieu de Montréal.

Participantes : Cinq femmes enceintes en bonne santé et 6 femmes enceintes atteintes de DSID.

Interventions : On a étudié le cycle de reconstitution du glucose et la gluconéogenèse après l'absorption, après 16 et 32 semaines de gestation, et 24 semaines après l'accouchement, au moyen de la technique à 2 isotopes stables (D[2,3,4,6,62H]-glucose et L[1,2,313C]-alanine). Chez les femmes atteintes de DSID, on a contrôlé la concentration du glucose dans le plasma au moyen d'une infusion sous-cutanée continue d'insuline pendant toute la grossesse et d'un Biostator le matin de l'étude.

Résultats : Chez les femmes sans DSID, la production hépatique de glucose s'est établie à 11,6 (écart-type de la moyenne [ETM] 2,2) µmol/kg par minute après 16 semaines de gestation, à 12,5 (ETM, 1,8) µmol/kg par minute après 32 semaines de gestation, et à 13,2 (ETM, 1,9) µmol/kg par minute 24 semaines après l'accouchement. Chez les femmes atteintes de DSID, elle s'est établie à 10,7 (ETM, 2,4) µmol/kg par minute, à 10,5 (ETM, 1,2) µmol/kg et à 12,3 (ETP, 0,5) µmol/kg par minute aux mêmes périodes respectives. La différence entre les groupes n'était pas important sur le plan statistique. Les concentrations des précurseurs de la gluconéogenèse, l'alanine et le lactate, ont augmenté au cours de la grossesse à la fois chez les femmes sans DSID (pour passer de 0,18 [ETM, 0,02] mmol/L et 0,64 [ETM, 0,09] mmol/L, respectivement, à 0,25 [ETM, 0,02] mmol/L et 1,15 [ETM, 0,17] mmol/L respectivement, p < 0,01) et chez celles qui étaient atteintes de DSID (pour passer de 0,15 [ETM, 0,01] mmol/L et 0,47 [ETM, 0,04] mmol/L respectivement, à 0,19 [ETM, 0,02] mmol/L et 0,70 [ETM, 0,01] mmol/L, respectivement, p < 0,05). Après 12 heures de jeûne, la grossesse n'a pas affecté la gluconéogenèse à partir de l'alanine chez les 2 groupes de femmes. Chez les femmes sans DSID, la concentration d'insuline dans le plasma était basse au début de la grossesse (33,6 [ETM, 3,6] pmol/L) et a augmenté vers la fin de la gestation (87,6 [ETM, 9,6] pmol/L) comparativement aux concentrations après l'accouchement (60,0 [ETM, 7,8] pmol/L). Les concentrations de glucagon dans le plasma ont eu tendance à augmenter vers la fin de la gestation (pour passer de 95,1 [ETM, 6,7] ng/L à 116,0 [ETM, 36,0] ng/L). Chez les femmes atteintes de DSID, les concentrations d'insuline libre et de glucagon dans le plasma ont été plus élevées au début de la grossesse (55,2 [ETM, 6,6] pmol/L et 196,1 [ETM, 29,8] ng/L, respectivement) et n'ont pas changé de façon significative au cours de la grossesse.

Conclusion : La reconstitution du glucose de base et la gluconéogenèse n'augmentent pas pendant la grossesse chez les femmes sans DSID ou chez les femmes atteintes de DSID bien contrôlé. La diminution de la concentration de glucose dans le plasma au cours de la grossesse indique que la consommation de glucose par le fœtus en croissance augmente et que l'augmentation n'est pas compensée entièrement par une augmentation de la production hépatique de glucose après l'absorption. Le glucose dont a besoin le fœtus en croissance est probablement fourni par l'élévation de la concentration postprandiale du glucose dans le plasma.

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Introduction

Normal pregnancy induces a state of hyperinsulinemia secondary to a decrease in insulin sensitivity during the latter half of gestation.1 This impairment in glucose disposal remains poorly understood, but it is believed that increases in levels of human placental lactogen (hPL),2 free cortisol,3 progesterone4 and possibly prolactin5 may contribute to insulin resistance. A postreceptor defect has been suggested as the site of impairment of insulin-mediated glucose disposal in normal pregnancy.6

However, during a normal pregnancy, fasting plasma glucose concentrations are lower than they are outside of pregnancy.7,8 It has been hypothesized that these low levels are due to increased placental glucose uptake, decreased renal absorption or subnormal hepatic glucose production.9 The latter has never been demonstrated but is hypothesized mainly on the basis of indirect evidence.10­12 In fact, recent studies have indicated that glucose turnover is either increased or unchanged in late normal pregnancy, compared with turnover outside of pregnancy.13­16

Although normal pregnancy has been described as a state of "accelerated stavation," 17 there are very few reports on gluconeogenesis during pregnancy. On the basis of urinary urea excretion18and plasma levels of gluconeogenic substrates in pregnant rats,19 it has been suggested that gluconeogenesis is increased in late gestation. This view is supported by liver perfusion studies in fasting rats in late gestation, in which alanine conversion to glucose was found to be increased.20 However, Kalhan and associates21 reported that, in human pregnancy, the rate of urea synthesis is lowered, indicating decreased gluconeogenesis from amino acids. More recently, these investigators have also shown that gluconeogenesis from alanine is reduced in late normal human pregnancy.22

Whereas several studies have looked at glucose metabolism during normal pregnancy, little is known about the impact of diabetes on this variable during pregnancy. Two studies of glucose turnover in pregnant women with diabetes using 13 C-glucose as tracer have suggested that, if blood glucose is well controlled, overall glucose turnover is comparable to that in pregnant women without diabetes.13,14 In addition, Kalhan and associates23 have shown that glucose turnover and gluconeogenesis from alanine in women with well-controlled insulin-dependent diabetes mellitus (IDDM) are not affected by pregnancy.

The goal of this study was to evaluate the effect of pregnancy on glucose turnover and gluconeogenesis in healthy pregnant women and in pregnant women with well-controlled IDDM, using a double stable isotope technique.

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Materials and methods

Materials

The stable isotopes D[2,3,4,6,6-2H]-glucose and L[1,2,3-13C]-alanine were purchased from Merck Sharp & Dohme (Pointe Claire, Que.). Eugly insulin pumps were provided by Travenol Inc. (Toronto, Ont.).

Subjects and procedures

Five healthy pregnant women and 6 pregnant women with IDDM participated in this study (Table 1). All 11 subjects were investigated on 3 different occasions: at 16 (standard deviation [SD] 1) and 32 (SD 2) weeks' gestation and at 24 (SD 10) weeks postpartum, at least 3 months after cessation of lactation. All 6 women with IDDM had previously been admitted to the Clinical Research Unit at Hôtel-Dieu de Montréal hospital at the beginning of pregnancy to receive continuous subcutaneous insulin infusion. They were given instruction on conducting home blood-glucose monitoring and on adjusting their basal rates and premeal insulin boluses according to specific algorithms to maintain normoglycemia, as described elsewhere.24 This protocol was approved by the ethics committee of the Institut de recherches cliniques de Montréal, and each volunteer provided informed consent.

All subjects were studied after an overnight fast (10 hours) at the Institut de recherches cliniques de Montréal. Before the study, the participants' body weight was measured, and a blood sample was drawn for measurement of HbA1c and plasma concentrations of estrogen, progesterone, cortisol, prolactin and hPL. On the morning of the study, the women with IDDM were switched to a Biostator GCISS (model 3000 S/N, Life Science Instruments, Miles, Elkhardt, Indiana) after discontinuation of subcutaneous insulin infusion. Glucose turnover and gluconeogenesis were measured by a double stable isotope (D[2,3,4,6,6-2H]-glucose and L[1,2,3-13C]-alanine) technique, described elsewhere.25 Briefly, D[2,3,4,6,6-2H]-glucose was administered intravenously as a prime-constant (200 mg ­ 2.0 mg per minute) infusion over 3 hours. D[13C3]-alanine was given as a constant infusion (3 mg per minute) throughout the study. The first 2 hours were allowed for equilibration of the tracers. Blood samples were drawn every 15 minutes during the last 60 minutes of the experiment from a peripheral hand vein warmed at 68°C.26 The following variables were measured throughout the experiment: labelled and unlabelled plasma glucose levels, labelled and unlabelled plasma alanine and lactate levels, free plasma insulin level and plasma glucagon level.

Analysis

The isotopes D[2H5]-glucose, D[13C3]-glucose, D[13C3]-alanine and D[13C3]-lactate were determined by combined gas chromatography and mass spectrometry under electron impact in the selected ion monitoring mode.25 The plasma samples were purified by ion exchange chromatography as described by Kreisberg, Siegal and Owen.27 The glucose isotopes were measured as their 6-acetyl-[1,2,3,5]-bis-butylburonyl-alphaD-glucofuranose derivative, and alanine and lactate as their bis-t-butyldimethylsilyl derivatives. In each case, the m-butyl+ ions were monitored: m/z 297, 300 and 302 for glucose, m/z 260 and 263 for alanine and m/z 261 and 264 for lactate. Ions related to molecules containing less than 3 13C or less than 5 2H atoms or both were also monitored, but their levels, after natural abundance subtraction, were negligible or below the confidence limit.

The plasma glucose level was measured by the hexokinase method after deproteinization with 6% perchloric acid.28 The free plasma insulin level was measured with a commercial radioimmunoassay kit after precipitation with polyethylene glycol.29 The plasma lactate and alanine levels were assayed enzymatically by spectrophotometry.30,31 The levels of plasma glucagon, estrogen, progesterone, cortisol, prolactin and hPL were also measured by radioimmunoassay.32­37

Calculations

The rates of total glucose appearance (Ra) and disappearance (Rd) were calculated according to Cowan and Hetenyi38 and Cherrington and Vranic39 using the equation proposed by Steele and associates,40 as modified by De Bodo and associates.41 Specifically, plasma enrichment in D[2H5]-glucose was used to measure endogenous hepatic glucose production. In addition, distribution volumes were determined from the decrement over time of plasma-specific activities after the initial bolus of the tracer. These volumes of distribution were used to compute all the turnover data and were, on average, around 20% of total body weight. D[13C3]-alanine enrichment was used to calculate the postabsorptive alanine turnover rate. The rate of appearance of D[13C3]-glucose divided by the level of D[13C3]-alanine enrichment during steady state was used as an index of gluconeogenesis. Our calculations of turnover took into account the amount of tracer infused, which cannot be consider "massless," and the background levels of naturally occurring isotopes. Plasma enrichment of all isotopes was very stable during the last 60 minutes of the experiment (Fig. 1). All turnover data are expressed per total body weight. Statistical analyses were performed with the paired Student's t-test and analysis of variance. Differences with a p value of less than 0.05 were considered significant.

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Results

Carbohydrate metabolism

The fasting plasma glucose concentrations in the women without IDDM were lower at 16 weeks (4.2 [SEM 0.1] mmol/L) and 32 weeks (4.2 [SEM 0.2] mmol/L) of pregnancy than postpartum (4.6 [SEM 0.2] mmol/L, p < 0.01) (Fig. 2). In the women with well-controlled IDDM, the fasting plasma glucose levels were 4.6 (SEM 0.6) mmol/L at 16 weeks and 3.9 (SEM 0.3) mmol/L at 32 weeks of pregnancy, but were not statistically different from the postpartum fasting plasma glucose value of 4.4 (SEM 0.5) mmol/L (Fig. 2). The glycosylated hemoglobin confirmed the good metabolic control of IDDM in the pregnant women; values were 5.5% (SEM 0.4%) at 16 weeks' gestation, 4.9% (SEM 0.4%) at 32 weeks' gestation and 6.0% (SEM 0.7%) at 24 weeks postpartum, compared with 4.2% (SEM 0.2%) in the women without IDDM (Table 1).

Hepatic glucose production in the women without IDDM was 11.6 (SEM 2.2) µmol/kg per minute at 16 weeks' gestation, 12.5 (SEM 1.8) µmol/kg per minute at 32 weeks' gestation and 13.2 (SEM 1.9) µmol/kg per minute at 24 weeks postpartum (Fig. 2). The women with well-controlled IDDM had similar hepatic glucose production values: 10.7 (SEM 2.4) µmol/kg per minute at 16 weeks' gestation, 10.5 (SEM 1.2) µmol/kg per minute at 32 weeks' gestation, and 12.3 (SEM 0.5) µmol/kg per minute at 24 weeks postpartum (Fig. 2). There was no statistical difference over the course of pregnancy or between the groups of women with and without IDDM. Since these women were studied under steady state conditions, at a time when Ra equals Rd, the Ra of glucose expressed per kilogram of total body weight was unchanged, indicating that there was a parallel increase in glucose appearance with the increase in total body weight.

Alanine metabolism

In the women without IDDM, levels of the gluconeogenic precursors alanine and lactate rose from 0.18 (SEM 0.02) mmol/L and 0.64 (SEM 0.09) mmol/L to 0.25 (SEM 0.02) mmol/L (p < 0.01) and 1.15 (SEM 0.17) mmol/L (p < 0.01) from the 16th to the 32nd week of pregnancy and returned to 0.18 (SEM 0.02) mmol/L and 0.70 (SEM 0.06) mmol/L at 24 weeks postpartum(Fig. 3). Postabsorptive alanine and lactate levels were lower in the pregnant women with IDDM than in the women without IDDM at 16 weeks' gestation (0.15 [SEM 0.01] mmol/L and 0.47 [SEM 0.04] mmol/L) and 32 weeks' gestation (0.19 [SEM 0.02] mmol/L and 0.70 [SEM 0.01] mmol/L, p < 0.01). Postpartum, the concentrations of both gluconeogenic precursors were not significantly different between the 2 groups (0.18 [SEM 0.02] mmol/L and 0.70 [SEM 0.06] mmol/L in the women without IDDM v. 0.19 [SEM 0.02] mmol/L and 0.70 [SEM 0.01] mmol/L in the women with IDDM). In the women with IDDM, both gluconeogenic precursors remained well below the levels recorded in the women without IDDM throughout pregnancy (p < 0.01) (Fig. 3). Similarly, gluconeogenesis from alanine tended to be lower in the women with IDDM (0.78 [SEM 0.14] and 0.68 [SEM 0.07] µmol/kg per minute at 16 and 32 weeks' gestation, respectively) than in the women without IDDM (1.41 [SEM 0.33] and 1.15 [SEM 0.17] µmol/kg per minute at the respective times). These differences were small but statistically significant at 32 weeks' gestation (p < 0.03). In both groups, however, gluconeogenesis was not affected by pregnancy and was not statistically different from that during the postpartum period (Fig. 3). The mean overall alanine turnover rate in all subjects in the postabsorptive state was 5.2 (SEM 0.7) µmol/kg per minute for both the women with IDDM and the women without IDDM, and this rate was not influenced by pregnancy(Table 2).

Pancreatic hormones (Table 3)

In the women without IDDM, the free plasma insulin level was significantly lower at 16 weeks' gestation (33.6 [SEM 3.6] pmol/L) than at 32 weeks' gestation (87.6 [SEM 9.6] pmol/L, p < 0.01) and at 24 weeks postpartum (60.0 [SEM 7.8] pmol/L, p < 0.01) (Fig. 4). In the women with IDDM, although the free plasma insulin level tended to follow the same profile, it did not show any significant difference: the values were 55.2 (SEM 6.6) pmol/L at 16 weeks and 71.4 (SEM 13.8) pmol/L at 32 weeks of pregnancy, compared with 72.0 (SEM 8.4) pmol/L 24 weeks postpartum (Fig. 4).

The glucagon levels in the women without IDDM were 95.1 (SEM 6.7) ng/L in early pregnancy, 116.0 (SEM 36.0) ng/L in late pregnancy and 89.9 (SEM 8.6) ng/L postpartum (Fig. 4). There was no significant difference among these concentrations. In the women with IDDM, the glucagon level was high at 16 weeks' gestation (196.1 [SEM 29.8] ng/L) and unchanged in late pregnancy (180.3 [SEM 71.5] ng/L) and postpartum (192 [SEM 62] ng/L) (Fig. 4). The glucagon concentrations were significantly higher in the women with IDDM only at 16 weeks of pregnancy (p < 0.01).

Gestational hormones (Table 3)

The estradiol levels increased gradually as pregnancy progressed in both groups (from 18.5 [SEM 4.8] nmol/L and 9.7 [SEM 1.0] nmol/L to 51.9 [SEM 13.9] nmol/L and 48.5 [SEM 8.0] nmol/L in women without IDDM and women with IDDM, respectively). The levels returned to baseline after delivery (0.2 [SEM 0.1] and 0.3 [SEM 0.2] nmol/L, respectively). The progesterone levels followed the same profile, rising during pregnancy (from 120.8 [SEM 22.2] nmol/L and 91.6 [SEM 12.1] nmol/L to 269.2 [SEM 19.9] nmol/L and 296.6 [SEM 61.0] nmol/L in the women without IDDM and the women with IDDM, respectively) and declining to baseline postpartum (7.6 [SEM 5.6] nmol/L and 8.6 [SEM 5.7] nmol/L, respectively). Plasma hPL levels were 1.1 (SEM 0.3) µg/L and 1.1 (SEM 0.06) µg/L in the women without IDDM and the women with IDDM at 16 weeks' gestation, rising to 6.4 (SEM 0.7) µg/L and 6.1 (SEM 0.6) µg/L, respectively, at 32 weeks. The prolactin level at 16 weeks was 16.0 (SEM 7.0) µg/L and 18.4 (SEM 9.2) µg/L in the women without IDDM and the women with IDDM, respectively; it increased to 176.5 (SEM 30.2) µg/L and 163.4 (SEM 45.8) µg/L at 32 weeks of pregnancy and returned to baseline (13.0 [SEM 2.5] µg/L and 9.0 [SEM 2.1] µg/L) postpartum. The plasma cortisol level showed the same profile, with an increase in both groups as pregnancy progressed (from 755.7 [SEM 120.9] nmol/L and 652.3 [SEM 66.9] nmol/L to 1140.2 [SEM 203.8] nmol/L and 1035.0 [SEM 126.5] nmol/L in the women without IDDM and in the women with IDDM, respectively) and a return to baseline (644.6 [SEM 355.7] nmol/L and 801.0 [SEM 228.1] nmol/L) 24 weeks postpartum. The gestational hormone levels were not statistically different between the 2 groups.

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Discussion

This study was designed to evaluate the effect of pregnancy on glucose turnover and gluconeogenesis in healthy women and in women with well-controlled IDDM. The data show that, in both groups of women, the increased energy demand of the growing fetus is not paralleled by a rise in postabsorptive hepatic glucose production or by an increase in basal gluconeogenesis from alanine.

In the pregnant women without IDDM, the fasting plasma glucose level was about 10% lower in early and late gestation than postpartum. This statistically significant reduction is a well-known phenomenon,7,8,15,16 and it has been suggested that it is due to reduced hepatic glucose production, to renal loss through glycosuria or to increased glucose consumption by the growing fetus.9 Our inability to demonstrate a similar reduction in plasma glucose concentrations in pregnant women with well-controlled IDDM is mainly due to the fact that the day-to-day variation of their fasting plasma glucose concentrations exceeded 10%. In either group, we and others1,13,14 have not been able to confirm a reduction in hepatic glucose production, despite higher plasma insulin concentrations. Although the renal glucose threshold is low during pregnancy, it is still above 6 mmol/L and, thus, cannot explain the lower fasting plasma glucose levels observed. Finally, the rate of fetal glucose consumption under normal conditions depends mainly on the supply of glucose from the mother. Experiments in pregnant sheep have shown that there is a direct relationship between maternal blood glucose concentrations and the rate of fetal glucose use.42 Therefore, an increased consumption of glucose by the fetus appears to be the most likely explanation for the lower plasma glucose concentrations in pregnant women.

The possibility that glucose disposal is increased in pregnant women with and without IDDM is in itself an interesting phenomenon if one sees no change in glucose turnover. At first glance, this finding could be intriguing, since glucose kinetics were measured in steady-state conditions, when the rate of appearance of glucose (Ra) was equal to its rate of disappearance (Rd). We therefore postulate that the lower postabsorptive plasma glucose concentrations could result from an increase in postprandial Rd, without an appropriate response in Ra. Such a postprandial increase could reset the plasma glucose at a lower level without affecting the overall glucose turnover. Indeed, it is not unusual to observe a slight decrease in plasma glucose to below baseline by the third and fourth hour after a meal; it is believed that this is corrected by a small and temporary increase in hepatic glucose production. In pregnant woman, this slight decrease in late postprandial plasma glucose is more likely to occur because of fetal glucose disposal. Although we did not specifically examine this possibility in the study, we suggest that the increased Rd is not entirely corrected by an increase in hepatic glucose production during pregnancy. Consequently, plasma glucose concentration remains lower in the postabsorptive period because hepatic glycogenolysis and gluconeogenesis are impeded in the late postprandial period. Several factors could explain this lack of response in hepatic glucose production. First, elevated estradiol and progesterone concentrations could prevent an efficient compensation of the overall hepatic glucose production. Second, the liver could be resistant to the effect of glucagon. Indeed, in this study we observed normal postabsorptive hepatic glucose production despite a tendency for glucagon concentrations to increase during pregnancy. These possibilities need to be tested.

Our observation that postabsorptive hepatic glucose production is not increased during pregnancy confirms those of other investigators who could not show any increased glucose turnover (expressed per kilogram body weight) in women without IDDM and with well-controlled IDDM in late gestation.1,2,13,14 The same finding has been made in anesthetized pregnant rats in the postabsorptive state during late gestation, when hepatic glucose production is expressed per kilogram of body weight.43 Only 1 study showed a significant increase in absolute as well as relative hepatic glucose production.15 In our study, there was an increase of approximately 10% to 15% if the data are not corrected for body weight; a small rise in absolute glucose turnover was observed in the group of women without IDDM (between the 16th and the 32nd weeks of gestation, from 644 [SEM 113] µmol per minute to 755 [SEM 83] µmol per minute), but this difference did not reach statistical significance. A similar tendency was seen in the women with well-controlled IDDM. It is unlikely, however, that this increase in absolute glucose turnover could fulfil the energy demand of the growing fetus. How, then, can we account for the necessary fuel needed for fetal development? Although minor changes in glucose homeostasis in the postabsorptive state are apparent, even in late gestation, major alterations are observed during meals and in the postprandial period.44 Owing to the insulin resistance of pregnancy, ingestion of a mixed meal induces postprandial hyperglycemia and an increase in free fatty acids. Since both energy substrates cross the placenta in a concentration-dependent fashion, and since insulin resistance prevails in late gestation, elevation of these fuels in response to meals could provide a readily available source of energy for the normal development of the fetus.

Contrary to the concept of pregnancy as a state of "accelerated starvation,"9,17 we were unable to find any increase in gluconeogenesis from alanine in either women without IDDM or in women with well-controlled IDDM. In fact, there was a slight tendency for gluconeogenesis to decrease slightly throughout pregnancy in the women with IDDM, but this difference was not statistically significant. This finding contrasts with the increased conversion of alanine to glucose observed in the perfused liver of 21-day pregnant rats.18 These experiments, however, were conducted after a 24-hour fast using high concentrations of gluconeogenic precursors. Our results are therefore consistent with the only clinical available data on gluconeogenesis during pregnancy, by Kalhan and associates, in women with IDDM23 and in women without IDDM22 in late gestation. Not only did they find no increase in gluconeogenesis, but they also suggested that this process could be decreased, at least in pregnant women without IDDM.22 The lack of an increase in gluconeogenesis during pregnancy could in fact be due to elevations of estradiol and progesterone, as suggested by Matute and Kalkhoff.44 These investigators showed that parenteral administration of estradiol and progesterone in female rats could suppress the incorporation of 14C-alanine and 14C-pyruvate into glucose. Thus, the well-known rise in these 2 hormones during pregnancy could explain the lack of increase in gluconeogenesis in late gestation. The limitation of our tracer method has already been discussed.25 Hetenyi45 noted that dilution of labelled carbon in the oxaloacetate pool results in underestimation of the true rate of gluconeogenesis. It still remains, however, a valid index of gluconeogenesis in a comparative study such as ours.

Our data indicate that alanine turnover is not affected by pregnancy in the postabsorptive state in women without IDDM and in women with well-controlled IDDM (Table 2). These observations are similar to those of Rushakoff and Kalkhoff,46 who did not show any difference in alanine efflux from the skeletal muscles of pregnant and nonpregnant rats. They are also similar to the recent findings reported by Kalhan and associates,22,23 and discussed earlier. Our results are also consistent with our previous data concerning men without diabetes mellitus, studied in a fasting state.47 Therefore, despite major changes in pancreatic hormones during pregnancy, alanine turnover does not seem to be affected.

Significant increases in the 2 major gluconeogenic precursors lactate and alanine in late gestation were observed (Fig. 3); these changes were, however, less pronounced in pregnant women with IDDM who were receiving intensive insulin therapy. These changes are different from those observed by Phelps, Metzger and Freinkel,48 who reported a decrease in all amino acids, with the exception of threonine, during late pregnancy. Our results are, however, consistent with data from Metzger, Hare and Freinkel19 concerning pregnant rats; in this study, alanine levels were lower in the fasting state but higher in the fed state. Our results are consistent with those of Kalhan and associates,22 who recorded an increase in both alanine and lactate after an overnight fast in late normal pregnancy. This rise in lactate and alanine could be due to either an increased production in skeletal muscles or a decreased uptake by the liver. The fact that there is no change in alanine turnover suggests that peripheral production is not increased, at least in the postabsorptive state. It is still possible that fractional alanine uptake by the liver is decreased and that an increase in the amino acid would maintain a normal absolute uptake. Under these conditions, alanine ingested with mixed meals would also tend to further elevate plasma alanine levels postprandially. The insulin resistance of pregnancy may also favour an enhanced conversion of glucose to lactate.49 Interestingly, in early pregnancy, both alanine and lactate levels were lower in the women with IDDM than in those without IDDM. They rose significantly in late gestation but remained well below the concentrations found in pregnant women without IDDM. Hyperinsulinemia induced by intensive insulin therapy may be responsible for the lower gluconeogenic precursor levels in the women with IDDM. Free plasma insulin concentrations were indeed higher at 16 weeks in the women with IDDM than in the women without IDDM.

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Conclusion

Our observations can easily be reconciled with the state of "accelerated starvation" that is characteristic of pregnancy.9,17 First, there is an efficient diversion of postprandial glucose to the developing fetus, facilitated by maternal insulin resistance, mainly in skeletal muscles. Second, fasting plasma glucose concentrations are reduced because the overall hepatic glucose production is restrained in the late postprandial period. Finally, gluconeogenic precursors may be increased owing to impaired fractional uptake of alanine and lactate by the liver. Because of the delicate balance between fetal and maternal glucose supply, minor dietary deprivation in pregnant women can lead to a greater reduction in plasma glucose concentrations. Therefore, the absence of significant changes in glucose and alanine turnovers in the postabsorptive state suggests that energy substrates necessary for the growing fetus are probably provided by meals.

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Acknowledgements

This study was supported by a grant from the Medical Research Council of Canada. We thank Lucie Germain for her technical help and Susanne Bordeleau-Chénier for preparing the manuscript.

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