The set point for maternal glucose homeostasis is lowered during late pregnancy in the rat: the role of the islet beta-cell and liver

The Role of Fatty Acid Signaling in Islet Beta-Cell Adaptation to Normal Pregnancy

BACKGROUND

Maintenance of a normal fetal nutrient supply requires major adaptations in maternal metabolic physiology, including of the islet beta-cell. The role of lipid signaling processes in the mechanisms of islet beta-cell adaptation to pregnancy has been minimally investigated.

OBJECTIVE

To determine the effects of pregnancy on islet fatty acid (FA) metabolic partitioning and FA augmentation of glucose-stimulated insulin secretion (GSIS).

METHODS

Age matched virgin, early pregnant (gestational day-11, G11) and late pregnant (G19) Sprague-Dawley rats were studied. Fasted and fed state biochemistry, oral glucose tolerance tests (OGTT), and fasted and post-OGTT liver glycogen, were determined to assess in vivo metabolic characteristics. In isolated islets, FA (BSA-bound palmitate 0.25 mmol/l) augmentation of GSIS, FA partitioning into esterification and oxidation processes using metabolic tracer techniques, lipolysis by glycerol release, triacylglycerols (TG) content, and the expression of key beta-cell genes were determined.

RESULTS

Plasma glucose in pregnancy was lower, including during the OGTT (glucose area under the curve 0-120 min (AUC_0-120); 655±24 versus 849±13 mmol.l^-1.min; G19 vs virgin; P<0.0001), with plasma insulin concentrations equivalent to those of virgin rats (insulin AUC_0-120; 97±7 versus 83±7 ng.ml^-1.min; G19 vs virgin; not significant). Liver glycogen was depleted in fasted G19 rats with full recovery after oral glucose. Serum TG increased during pregnancy (4.4±0.4, 6.7±0.5; 17.1±1.5 mmol/l; virgin, G11, G19, P<0.0001), and islet TG content decreased (147±42, 172±27, 73±13 ng/µg protein; virgin, G11, G19; P<0.01). GSIS in isolated islets was increased in G19 compared to virgin rats, and this effect was augmented in the presence of FA. FA esterification into phospholipids, monoacylglycerols and TG were increased, whereas FA oxidation was reduced, in islets of pregnant compared to virgin rats, with variable effects on lipolysis dependent on gestational age. Expression of Ppargc1a, a key regulator of mitochondrial metabolism, was reduced by 51% in G11 and 64% in G19 pregnant rat islets compared to virgin rat islets (P<0.001).

CONCLUSION

A lowered set-point for islet and hepatic glucose homeostasis in the pregnant rat has been confirmed. Islet adaptation to pregnancy includes increased FA esterification, reduced FA oxidation, and enhanced FA augmentation of glucose-stimulated insulin secretion.

Apolipoprotein A-I Protects Against Pregnancy-Induced Insulin Resistance in Rats

Objective- Insulin resistance and inflammation in pregnancy are risk factors for gestational diabetes mellitus. Increased plasma HDL (high-density lipoprotein) and apo (apolipoprotein) A-I levels have been reported to improve glucose metabolism and inhibit inflammation in animals and humans. This study asks whether increasing plasma apoA-I levels improves insulin sensitivity and reduces inflammation in insulin-resistant pregnant rats. Approach and Results- Insulin-resistant pregnant rats received intravenous infusions of lipid-free apoA-I (8 mg/kg) or saline on days 6, 9, 12, 15, and 18 of pregnancy. The rats were then subjected to a euglycemic-hyperinsulinemic clamp. Glucose uptake was increased in white and brown adipose tissue by 57±13% and 32±10%, respectively ( P<0.05 for both), and in quadriceps and gastrocnemius muscle by 35±9.7% and 47±14%, respectively ( P<0.05 for both), in the apoA-I-treated pregnant rats relative to saline-infused pregnant rats. The pregnant rats that were treated with apoA-I also had reduced plasma TNF-α (tumor necrosis factor-α) levels by 57±8.4%, plasma IL (interleukin)-6 levels by 67±9.5%, and adipose tissue macrophage content by 54±8.2% ( P<0.05 for all) relative to the saline-treated pregnant rats. Conclusions- These studies establish that apoA-I protects against pregnancy-induced insulin resistance in rats by increasing insulin sensitivity in adipose tissue and skeletal muscle and inhibiting inflammation. This identifies apoA-I as a potential target for preventing pregnancy-induced insulin resistance and reducing the incidence of gestational diabetes mellitus.

Effects of dietary fat subtypes on glucose homeostasis during pregnancy in rats

BACKGROUND

Dietary n-3 and n-6 polyunsaturated fatty acids (PUFAs) have an impact on insulin secretion and sensitivity but whether and how these may be related to maternal glucose homeostasis during pregnancy is unclear.

METHODS

Female Wistar rats (240-250 g) were assigned to laboratory CHOW or high fat diets rich in either n-6 (safflower oil; n-6 group) or n-6 + n-3 (safflower oil + fish oil; n-3 group) PUFAs. After 10 days half of the animals in each diet group were inseminated and confirmed pregnant. An overnight fasted intravenous glucose tolerance test (500 mg glucose/kg body weight) was performed on chronically cannulated non-pregnant and 20-day pregnant rats. Indices of insulin secretion (β) and insulin sensitivity (S) were calculated from the plasma glucose and insulin responses. The fatty acid composition of phospholipids was determined in samples of liver and two skeletal muscles (soleus and red quadriceps).

RESULTS

Pregnancy in the CHOW group significantly increased β (P < 0.001) and decreased S (P < 0.01). In contrast, both n-6 and n-3 diets abolished both the pregnancy-induced decrease in S and pregnancy-induced increase in β with the n-3 diet having a more potent effect on both S and β. S was positively correlated with the sum of n-3 fatty acids, with docosahexaenoic acid (22:6 n-3) the major contributor, in liver (r = 0.485; P < 0.001), red quadriceps (r = 0.421; P = 0.004) and soleus (r = 0.476; P < 0.001). In contrast S was inversely related to arachidonic acid (20:4n-6) levels in liver and red quadriceps across all groups and these relationships were particularly powerful in pregnancy (liver: r = -0.785; red quadriceps: r = -0.754, both P < 0.0001).

CONCLUSIONS

The results demonstrate potent effects of dietary fat amount and profile on glucoregulation during pregnancy and emphasize the importance of the balance between dietary n-3 and n-6 PUFAs.

Corticosterone alters materno-fetal glucose partitioning and insulin signalling in pregnant mice

Glucocorticoids affect glucose metabolism in adults and fetuses, although their effects on materno-fetal glucose partitioning remain unknown. The present study measured maternal hepatic glucose handling and placental glucose transport together with insulin signalling in these tissues in mice drinking corticosterone either from day (D) 11 to D16 or D14 to D19 of pregnancy (term = D21). On the final day of administration, corticosterone-treated mice were hyperinsulinaemic (P < 0.05) but normoglycaemic compared to untreated controls. In maternal liver, there was no change in glycogen content or glucose 6-phosphatase activity but increased Slc2a2 glucose transporter expression in corticosterone-treated mice, on D16 only (P < 0.05). On D19, but not D16, transplacental (3) H-methyl-d-glucose clearance was reduced by 33% in corticosterone-treated dams (P < 0.05). However, when corticosterone-treated animals were pair-fed to control intake, aiming to prevent the corticosterone-induced increase in food consumption, (3) H-methyl-d-glucose clearance was similar to the controls. Depending upon gestational age, corticosterone treatment increased phosphorylation of the insulin-signalling proteins, protein kinase B (Akt) and glycogen synthase-kinase 3β, in maternal liver (P < 0.05) but not placenta (P > 0.05). Insulin receptor and insulin-like growth factor type I receptor abundance did not differ with treatment in either tissue. Corticosterone upregulated the stress-inducible mechanistic target of rapamycin (mTOR) suppressor, Redd1, in liver (D16 and D19) and placenta (D19), in ad libitum fed animals (P < 0.05). Concomitantly, hepatic protein content and placental weight were reduced on D19 (P < 0.05), in association with altered abundance and/or phosphorylation of signalling proteins downstream of mTOR. Taken together, the data indicate that maternal glucocorticoid excess reduces fetal growth partially by altering placental glucose transport and mTOR signalling.

Accretion of visceral fat and hepatic insulin resistance in pregnant rats

Insulin resistance (IR) is a hallmark of pregnancy. Because increased visceral fat (VF) is associated with IR in nonpregnant states, we reasoned that fat accretion might be important in the development of IR during pregnancy. To determine whether VF depots increase in pregnancy and whether VF contributes to IR, we studied three groups of 6-mo-old female Sprague-Dawley rats: 1) nonpregnant sham-operated rats (Nonpreg; n = 6), 2) pregnant sham-operated rats (Preg; n = 6), and 3) pregnant rats in which VF was surgically removed 1 mo before mating (PVF-; n = 6). VF doubled by day 19 of pregnancy (Nonpreg 5.1 +/- 0.3, Preg 10.0 +/- 1.0 g, P < 0.01), and PVF- had similar amounts of VF compared with Nonpreg (PVF- 4.6 +/- 0.8 g). Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp in late gestation in chronically catheterized unstressed rats. Glucose IR (mg.kg(-1).min(-1)) was highest in Nonpreg (19.4 +/- 2.0), lowest in Preg (11.1 +/- 1.4), and intermediate in PVF- (14.7 +/- 0.6; P < 0.001 between all groups). During the clamp, Nonpreg had greater hepatic insulin sensitivity than Preg [hepatic glucose production (HGP): Nonpreg 4.5 +/- 1.3, Preg 9.3 +/- 0.5 mg.kg(-1).min(-1); P < 0.001]. With decreased VF, hepatic insulin sensitivity was similar to nonpregnant levels in PVF- (HGP 4.9 +/- 0.8 mg.kg(-1).min(-1)). Both pregnant groups had lower peripheral glucose uptake compared with Nonpreg. In parallel with hepatic insulin sensitivity, hepatic triglyceride content was increased in pregnancy (Nonpreg 1.9 +/- 0.4 vs. Preg 3.2 +/- 0.3 mg/g) and decreased with removal of VF (PVF- 1.3 +/- 0.4 mg/g; P < 0.05). Accretion of visceral fat is an important component in the development of hepatic IR in pregnancy, and accumulation of hepatic triglycerides is a mechanism by which visceral fat may modulate insulin action in pregnancy.

Protein restriction during early development enhances insulin responsiveness but selectively impairs sensitivity to insulin at low concentrations in white adipose tissue during a later pregnancy

Poor early nutrition may elicit long-term detrimental effects on adult health, including susceptibility to non-insulin-dependent diabetes mellitus. We investigated the impact of moderate maternal protein restriction during pregnancy and lactation on the action of insulin on adipocyte glucose uptake in female offspring during their own pregnancies. Offspring of dams provided with diets containing either 200 g protein/kg or 80 g protein/kg during pregnancy and lactation (termed C and EPR groups respectively) were weaned on to 200 g protein/kg diet at 24 d of age. At 9–12 weeks of age both groups were time-mated and studied at day 19 of gestation. Rates of glucose utilization (assessed using the 2-deoxy-d-- [1-3H- ]glucose technique) measured in five distinct adipose tissue depots (parametrial (PM), mesenteric (MES), perirenal (PR), subcutaneous (SC), interscapular (IS)) in vivo in the post-absorptive state were consistently lower in early-protein-restricted (EPR) pregnant rats compared with control (C) pregnant rats. In C pregnant rats, insulin significantly increased glucose utilization only in the IS depot. In contrast, significantly increased glucose utilization rates in response to hyperinsulinaemia were evident in all five adipose-tissue depots of the EPR pregnant group. Consequently, glucose utilization rates in PM and SC depots during hyperinsulinaemia were significantly higher in EPR pregnant rats compared with C pregnant rats. Adipocytes were isolated from PM and MES depots to determine whether altered responses to insulin in vivo were retained in vitro. Rates of insulin-stimulated glucose uptake at sub-maximal (15 μU/ml) and maximal (15 mU/ml) insulin concentrations were significantly higher in both MES and PM adipocytes from EPR pregnant rats, but the sensitivity of glucose uptake to insulin at low concentrations was blunted compared with adipocytes from C pregnant rats. The results demonstrate that early protein restriction enhances the capacity for adipocyte glucose uptake at high insulin concentrations, but dampens the response to insulin at low physiological concentrations.

Dietary protein concentration correlates in a complex way with glucose metabolism and growth performance in pregnant rats

We studied the influence of dietary protein concentration (PC) on the adaptation mechanism of maternal glucose metabolism to gestation, and on maternal body weight (BW) gain using pregnant rats fed 4% (low protein, LP), 10% (medium protein, MP) or 20% (high protein, HP) dietary protein concentration. Feed (FI), energy (EI) and protein (PI) intakes were recorded. Plasma glucose, insulin and GH were determined, and insulin:glucose (insulin resistance) and insulin:GH ratios computed. Correlation and regression analyses were used to determine the physiological relationships between metabolic and ponderal variables. MP dams with greater FI and EI (12%), and lower PI (44%) than HP dams showed similar maternal and fetal growth performance because of the maintained energy efficiency (EE). LP dams, with similar FI and EI, and lower PI (81%) than HP dams, failed to catch up due to the low EE. PC exerted a non-linear influence on BW gain, insulin and GH, through EI and EE modifications. The EI linearly influenced hormone concentrations with the maximum and minimum hormone response in MP and LP dams, respectively. Insulin resistance increased as PC decreased, whereas the GH lipolytic effect appeared to predominate over insulin action. Circulating hormones and metabolites affected nutrient partitioning between dam and fetus, with the result that the competing demands for materno-fetal growth of HP and MP dams, but not the LP dams, were satisfied. A deeper knowledge of the nature of the materno-fetal energy relationships will enhance the manipulation of the growth performance of the fetus in several animal species.

Potential role of peroxisome proliferator-activated receptor-alpha in the modulation of glucose-stimulated insulin secretion

In this review, we discuss the influence of peroxisome proliferator-activated receptor (PPAR)-alpha on islet insulin secretion and develop the hypothesis that modulation of PPAR-alpha function may be important for the regulation of compensatory insulin secretion. We have attempted to analyze the role of PPAR-alpha-linked fatty acid metabolism in islet function in health and in insulin-resistant states linked to lifestyle factors, in particular pregnancy and a diet inappropriately high in saturated fat. We have emphasized the potential for both actions of PPAR-alpha on insulin sensitivity that may be relayed systemically to the islet, leading to modulation of the insulin response in accordance with changes in insulin sensitivity, and direct effects of PPAR-alpha action on the islet itself. Finally, we have developed the concept that compensatory insulin secretion may have a function not only in glucoregulation but also in liporegulation. Thus, augmented insulin secretion may reflect a requirement for lipid lowering as well as for increased glucose disposal and is perceived to aim to compensate for impaired suppression of islet lipid delivery by insulin. This introduces the possibility of a continuum between liporegulation with islet compensation and lipodysregulation leading to islet decompensation in the development of type 2 diabetes.

Peroxisome proliferator-activated receptor-alpha activation during pregnancy attenuates glucose-stimulated insulin hypersecretion in vivo by increasing insulin sensitivity, without impairing pregnancy-induced increases in beta-cell glucose sensing and responsiveness

We investigated the effects of acute (24-h) peroxisome proliferator-activated receptor (PPAR)alpha activation by WY14,643 (pirinixic acid) treatment on glucose-stimulated insulin secretion (GSIS) during pregnancy, in the rat, in relation to insulin sensitivity. GSIS after iv glucose challenge (500 mg/kg) was increased at d 15 of pregnancy but was attenuated by WY14,643 treatment in vivo, with decreases in acute insulin response (51%; P < 0.001) and total suprabasal 30-min area under the insulin curve (deltaI) (55%; P < 0.001). GSIS was unaffected by WY14,643 treatment in unmated rats. Islet perifusions were employed to identify persistent effects of PPARalpha activation. GSIS was enhanced, and the glucose threshold was reduced in perifused islets from pregnant rats, but WY14,643 treatment failed to reverse these effects. WY14,643 treatment of 15-d-pregnant rats significantly lowered (by 63%; P < 0.01) the insulin resistance index [total suprabasal 30-min area under insulin curve x suprabasal 30-min area under glucose curve (deltaI x deltaG)]. A strong positive linear relationship (r = 0.92) between acute insulin response and deltaI x deltaG was evident between groups. Our studies show that acute PPARalpha activation reverses the augmented GSIS evoked by pregnancy in vivo, whereas the isolated islets retain pregnancy-induced enhancement of beta-cell glucose sensing and responsiveness. Normalization of maternal GSIS to that found in the nonpregnant state is observed in association with alleviation of maternal insulin resistance.

Sir David Cuthbertson Medal Lecture. The impact of dietary protein restriction on insulin secretion and action

The goal of this review is to develop the hypothesis, and review the evidence, that protein restriction, through synergistic effects on multiple organ systems predisposes to loss of normal regulation of fuel homeostasis that plays the central role in the development of type 2 (non-insulin-dependent) diabetes mellitus. The ability of insulin to regulate glucose production and disposal varies between individuals. These differences, together with the various compensatory mechanisms that are invoked to attempt to normalize fuel homeostasis, are of fundamental importance in the development and clinical course of type 2 diabetes mellitus. Protein deprivation impacts on both insulin secretion and insulin action. These effects may persist even when a diet containing adequate protein is presented subsequently. Data are presented that suggest that protein restriction results in an impaired ability of pancreatic beta-cells to compensate adequately for the defect in insulin action in insulin-resistant individuals. This persistent impairment of insulin secretion resulting from protein restriction predisposes to loss of glucoregulatory control and impaired insulin action after the subsequent imposition of a diabetogenic challenge. This inability to maintain the degree of compensatory hyperinsulinaemia necessary to prevent loss of glucose tolerance may have relevance to the increased incidence of diabetes on changing from a nutritionally-poor diet to a Western diet, and to the hypothesis that some cases of type 2 diabetes in adulthood may be related to poor early nutrition.

Antecedent protein restriction exacerbates development of impaired insulin action after high-fat feeding

The study investigated whether a persistent impairment of insulin secretion resulting from mild protein restriction predisposes to loss of glucoregulatory control and impaired insulin action after the subsequent imposition of the diabetogenic challenge of high-fat feeding. Offspring of dams provided with either control (20% protein) diet (C) or an isocaloric restricted (8%) protein diet (PR) were weaned onto the maintenance diet with which their mothers had been provided. At 20 wk of age, protein restriction enhanced glucose tolerance despite impaired insulin secretion and an augmented and sensitized lipolytic response to norepinephrine in adipocytes. C and PR rats were then transferred to a high-fat diet (HF, 19% protein, 22% lipid, 34% carbohydrate) and sampled after 8 wk. These groups are termed C-HF and PR-HF. Glucose tolerance was impaired in PR-HF, but not C-HF, rats. Insulin-stimulated glucose disposal rates were significantly lower (by 30%; P < 0.01) in the PR-HF group than in the C-HF group, and a specific impairment of antilipolytic response of insulin was unmasked in adipocytes from PR-HF, but not C-HF, rats. The study demonstrates that antecedent protein restriction accelerates and augments the development of impaired glucoregulation and insulin resistance after high-fat feeding.

Moderate protein restriction during pregnancy modifies the regulation of triacylglycerol turnover and leads to dysregulation of insulin's anti-lipolytic action

Moderate protein restriction throughout pregnancy in the rat leads to relative hyperlipidaemia and blunted insulin responsiveness of lipid fuel supply, and impairs foetal growth. The present study examined the basis for these changes. Isocaloric 8% (vs 20%) protein diets were provided throughout pregnancy. Rats were sampled at 19-20 days of gestation. Protein restriction enhanced triacylglycerol (TAG) secretion rates (estimated using Triton WR 1339) 1.6-fold (P < 0.05) in the post-absorptive state. Insulin infusion (4.2 mU/kg per min) decreased plasma TAG concentrations by 33% (P < 0.05) and 48% (P < 0.05) in control (C) and protein-restricted (PR) pregnant groups, an effect associated with suppression of TAG secretion by 42% (P < 0.05) and 51% (P < 0.01) respectively, in the C and PR groups. Since TAG concentrations decline more rapidly, while TAG secretion is enhanced, TAG utilisation during hyperinsulinaemia is enhanced in the PR group. We evaluated whether these changes were associated with dysregulation of lipolysis using adipocytes from two abdominal depots (mesenteric and parametrial). Noradrenaline-stimulated glycerol release was enhanced in parametrial adipocytes (by 40%; P < 0.05) from PR pregnant rats. The anti-lipolytic action of insulin at low concentrations (< or = 15 microU/ml) was impaired by protein restriction (adipocytes from both depots). There was no evidence for altered intra-hepatic regulation of fatty acid (FA) disposal at the level of carnitine palmitoyltransferase. Our results demonstrate increased post-absorptive production of non-carbohydrate energy substrates (TAG and FA) as a consequence of mild protein restriction during pregnancy. These adaptations contribute to a homeostatic strategy to reduce the maternal requirement for gluconeogenesis from available amino acids, optimising the foetal protein supply. Protein restriction also enhances TAG turnover during hyperinsulinaemia. This effect is not a consequence of abnormal regulation of hepatic lipid metabolism by insulin.

Impaired pancreatic beta cell function in the fetal GK rat. Impact of diabetic inheritance

The Goto-Kakisaki (GK) rat is a genetic model of non-insulin-dependent diabetes. At 21.5 d of age we found that GK fetuses had an increased plasma glucose concentration, a decreased plasma insulin level, and a reduced pancreatic beta cell mass. To investigate the beta cell function during fetal life we used a hyperglycemic clamp protocol applied to the mothers, which allowed us to obtain a steady-state hyperglycemia in the corresponding fetuses. At variance, with Wistar (W) fetuses, plasma insulin concentration in GK fetuses did not rise in response to hyperglycemia. In contrast, GK fetal pancreas released insulin in response to glucose in vitro to the same extent as W fetal pancreas. Such a discrepancy between the in vivo and in vitro results suggests that the lack of pancreatic reactivity to glucose as seen in vivo is extrinsic to the fetal GK beta cell. Finally, the importance of gestational hyperglycemia was investigated by performing crosses between GK and W rats. Fetuses issued from crosses between W mother and GK father or GK mother and W father had a beta cell mass close to normal values and were still able to increase their plasma insulin levels in response to hyperglycemia in vivo. Our data suggest that hyperglycemia in utero does not influence the severity of the decrease of the beta cell mass or the lack of the insulin secretory response to glucose in the fetal GK rat. Moreover they indicate that conjunction of GK genes originating from both parents is necessary in order for these defects to be fully expressed.

Glucose turnover and gluconeogenesis in human pregnancy

The rate of appearance (Ra) of glucose in plasma and the contribution of gluconeogenesis were quantified in normal pregnant women early ( approximately 10 wk) and late ( approximately 34 wk) in gestation. Their data were compared with those of normal nonpregnant women. Glucose Ra was measured using the [U-13C]glucose tracer dilution method. Gluconeogenesis was quantified by the appearance of 2H on carbon 5 and 6 of glucose after deuterium labeling of body water pool. Weight-specific glucose Ra was unchanged during pregnancy (nonpregnant, 1.89+/-0.24; first trimester, 2.05+/-0.21; and third trimester 2.17+/-0.28 mg/kg.min, mean+/-SD), while total glucose Ra was significantly increased (early, 133.5+/-7.2; late, 162.6+/-16.4 mg/min; P = 0.005). The fractional contribution of gluconeogenesis via pyruvate measured by 2H enrichment on C-6 of glucose (45-61%), and of total gluconeogenesis quantified from 2H enrichment on C-5 of glucose (i.e. , including glycerol [68-85%]) was not significantly different between pregnant and nonpregnant women. Inasmuch as total glucose Ra was significantly increased, total gluconeogenesis was also increased in pregnancy (early pregnancy, 94.7+/-15.9 mg/min; late pregnancy, 122.7+/-9.3 mg/min; P = 0.003). These data demonstrate the ability of the mother to adapt to the increasing fetal demands for glucose with advancing gestation. The mechanism for this unique quantitative adjustment to the fetal demands remains undefined.

Glucoregulation during progressive starvation in late pregnancy in the rat

The response of glucose utilization (transport and phosphorylation) by individual skeletal muscles to progressive starvation in late pregnancy in the rat was investigated in relation to changes in whole body glucose turnover. Compared with insulin-stimulated values, the decline in muscle glucose utilization evoked by short-term (6-h) starvation was about twofold greater in pregnancy. Suppression of glucose utilization by slow-twitch muscles was observed as the starvation period was extended from 6 to 24 h only in unmated rats. Extending starvation to 24 h did not further reduce glucose utilization by fast-twitch skeletal muscles in either group. Suppression of whole body glucose disposal was observed between 6 and 24 h of starvation in unmated, but not pregnant, rats. The results demonstrate that metabolic adaptation of almost complete suppression of glucose utilization by slow-twitch muscle, normally elicited only by prolonged (24-h) starvation, is already established after acute (6-h) starvation in late pregnancy. The present study supports the concept of "accelerated starvation" in late pregnancy with respect to muscle glucose utilization after short-term food withdrawal but demonstrates that further glucose conservation cannot be achieved after more prolonged starvation.