Physiological significance of altered insulin metabolism in the conscious rat during lactation

Maternal adaptations of the pancreas and glucose homeostasis in lactation and after lactation

During lactation, the maternal physiology adapts to bear the nutritional requirements of the offspring. The exocrine and endocrine pancreas are central to nutrient handling, promoting digestion and metabolism. In concert with prolactin, insulin is a determinant factor for milk synthesis. The investigation of the pancreas during lactation has been scattered over several periods. The investigations that laid the foundation of lactating pancreatic physiology and glucose homeostasis were conducted in the decades of 1970-1980. With the development of molecular biology, newer studies have revealed the molecular mechanisms involved in the endocrine pancreas during breastfeeding. There has been a surge of information recently about unexpected changes in the pancreas at the end of the lactation period and after weaning. In this review, we aim to gather information on the changes in the pancreas and glucose homeostasis during and after lactation and discuss the outcomes derived from the current discoveries.

Breastfeeding duration and subsequent risk of mortality among US women: A prospective cohort study

BACKGROUND

Breastfeeding has been associated with a reduced maternal long-term risk of chronic diseases, but its association with mortality is poorly known.

METHODS

We included 166,708 female United States (US) nurses from the Nurses' Health Study (1986-2016) and the Nurses' Health Study II (1989-2019) who experienced at least one pregnancy lasting at least six months across their reproductive lifespan. Hazard ratios and 95% confidence intervals (CI) for mortality according to lifetime breastfeeding duration were estimated with time-dependent Cox proportional hazards regression models.

FINDINGS

During 4,705,160 person-years of follow-up, 36,634 deaths were documented in both cohorts, including 9880 from cancer and 7709 from cardiovascular disease (CVD). Lifetime total breastfeeding duration was associated with a lower subsequent risk of all-cause mortality in a non-linear manner (p-value for non-linearity=0.0007). The pooled multivariable-adjusted hazard ratios of all-cause mortality were 0.95 (95% CI: 0.92 to 0.98), 0.94 (95% CI: 0.91 to 0.98), 0.93 (95% CI: 0.90 to 0.97), and 0.93 (95% CI: 0.89 to 0.97), respectively, for women reporting lifetime total breastfeeding duration of 4-6, 7-11, 12-23, and ≥24 months, compared to women who breastfed for ≤3 months over their reproductive lifespan. Cause-specific analysis showed a similar pattern of non-linear inverse associations between lifetime total breastfeeding duration and CVD and cancer mortality (both p-values for non-linearity <0.01). There was no evidence of interactions between breastfeeding duration and pre-pregnancy lifestyle factors on mortality risk.

INTERPRETATION

Parous women with longer lifetime breastfeeding duration had a modestly lower risk of mortality.

FUNDING

The National Institutes of Health grants.

Lactation alters the relationship between liver lipid synthesis and hepatic fat stores in the postpartum period

In mothers who are nursing their infants, increased clearance of plasma metabolites into the mammary gland may reduce ectopic lipid in the liver. No study to date has investigated the role of lactation on liver lipid synthesis in humans, and we hypothesized that lactation would modify fatty acid and glucose handling to support liver metabolism in a manner synchronized with the demands of milk production. Lactating (n = 18) and formula-feeding women (n = 10) underwent metabolic testing at 6-week postpartum to determine whether lactation modified intrahepatic triacylglycerols (IHTGs), measured by proton magnetic resonance spectroscopy. Subjects ingested oral deuterated water to measure fractional de novo lipogenesis (DNL) in VLDL-TG during fasting and during an isotope-labeled clamp at an insulin infusion rate of 10 mU/m²/min. Compared with formula-feeding women, we found that lactating women exhibited lower plasma VLDL-TG concentrations, similar IHTG content and similar contribution of DNL to total VLDL-TG production. These findings suggest that lactation lowers plasma VLDL-TG concentrations for reasons that are unrelated to IHTG and DNL. Surprisingly, we determined that the rate of appearance of nonesterified fatty acids was not related to IHTG in either group, and the expected positive association between DNL and IHTG was only significant in formula-feeding women. Further, in lactating women only, the higher the prolactin concentration, the lower the IHTG, while greater DNL strongly associated with elevations in VLDL-TG. In conclusion, we suggest that future studies should investigate the role of lactation and prolactin in liver lipid secretion and metabolism.

Reduction of litter size during lactation in rats greatly influences fatty acid profiles in dams

This study aimed to determine in lactating rats how fatty acid profiles are affected by litter size. On day 2 after parturition, litters of lactating rats were adjusted to a normal litter size of 9 pups/dam (NL) or to a small litter of 4 pups/dam (SL), and dams were studied at day 21 of lactation. Plasma glucose, insulin, and docosahexaenoic acid (DHA) concentrations were higher in SL than in NL dams, whereas the concentrations of most other fatty acids, triacylglycerols (TAG), and non-esterified fatty acids were lower in the SL dams. In the liver, the concentration of TAG was lower in SL than in NL dams as was the concentration of most fatty acids, with the exception of stearic acid (STA), arachidonic acid (ARA), and DHA concentrations that were higher in SL. Both plasma and liver Δ⁹ desaturase indices were lower in SL than in NL dams, whereas both Δ⁵ and Δ⁶ desaturase indices were higher in SL dams. In the liver, the expression of acetyl CoA carboxylase was lower in SL than in NL dams, and among the different adipose tissue depots, only mesenteric adipose tissue showed a higher concentration of most fatty acids in SL than in NL dams. It is proposed that reduction of litter size during lactation decreases liver lipogenesis de novo, although the synthesis of long-chain polyunsaturated fatty acids from their corresponding precursors increases, and lipolytic activity in mesenteric adipose tissue decreases probably as result of increased insulin responsiveness.

Central actions of insulin during pregnancy and lactation

Pregnancy and lactation are highly metabolically demanding states. Maternal glucose is a key fuel source for the growth and development of the fetus, as well as for the production of milk during lactation. Hence, the maternal body undergoes major adaptations in the systems regulating glucose homeostasis to cope with the increased demand for glucose. As part of these changes, insulin levels are elevated during pregnancy and lower in lactation. The increased insulin secretion during pregnancy plays a vital role in the periphery; however, the potential effects of increased insulin action in the brain have not been widely investigated. In this review, we consider the impact of pregnancy on brain access and brain levels of insulin. Moreover, we explore the hypothesis that pregnancy is associated with site-specific central insulin resistance that is adaptive, allowing for the increases in peripheral insulin secretion without the consequences of increased central and peripheral insulin functions, such as to stimulate glucose uptake into maternal tissues or to inhibit food intake. Conversely, the loss of central insulin actions may impair other functions, such as insulin control of the autonomic nervous system. The potential role of low insulin in facilitating adaptive responses to lactation, such as hyperphagia and suppression of reproductive function, are also discussed. We end the review with a list of key research questions requiring resolution.

Fenugreek Stimulates the Expression of Genes Involved in Milk Synthesis and Milk Flow through Modulation of Insulin/GH/IGF-1 Axis and Oxytocin Secretion

We previously demonstrated galactagogue effect of fenugreek in a rat model of lactation challenge, foreshadowing its use in women's breastfeeding management. To assess longitudinal molecular mechanisms involved in milk synthesis/secretion in dams submitted to fenugreek supplementation, inguinal mammary, pituitary glands and plasma were isolated in forty-three rats nursing large 12 pups-litters and assigned to either a control (CTL) or a fenugreek-supplemented (FEN) diet during lactation. RT-PCR were performed at days 12 and 18 of lactation (L12 and L18) and the first day of involution (Inv1) to measure the relative expression of genes related to both milk synthesis and its regulation in the mammary gland and lactogenic hormones in the pituitary gland. Plasma hormone concentrations were measured by ELISA. FEN diet induced 2- to 3-times higher fold change in relative expression of several genes related to macronutrient synthesis (Fasn, Acaca, Fabp3, B4galt1, Lalba and Csn2) and energy metabolism (Cpt1a, Acads) and in IGF-1 receptor in mammary gland, mainly at L12. Pituitary oxytocin expression and plasma insulin concentration (+77.1%) were also significantly increased. Altogether, these findings suggest fenugreek might extend duration of peak milk synthesis through modulation of the insulin/GH/IGF-1 axis and increase milk ejection by activation of oxytocin secretion.

Impact of Fenugreek on Milk Production in Rodent Models of Lactation Challenge

Fenugreek, a herbal remedy, has long been used as galactologue to help mothers likely to stop breastfeeding because of perceived insufficient milk production. However, few studies highlight the efficacy of fenugreek in enhancing milk production. The aims of our study were to determine whether fenugreek increased milk yield in rodent models of lactation challenge and if so, to verify the lack of adverse effects on dam and offspring metabolism. Two lactation challenges were tested: increased litter size to 12 pups in dams fed a 20% protein diet and perinatal restriction to an 8% protein diet with eight pups’ litter, with or without 1 g.kg⁻¹.day⁻¹ dietary supplementation of fenugreek, compared to control dams fed 20% protein diet with eight pups’ litters. Milk flow was measured by the deuterium oxide enrichment method, and milk composition was assessed. Lipid and glucose metabolism parameters were assessed in dam and offspring plasmas. Fenugreek increased milk production by 16% in the litter size increase challenge, resulting in an 11% increase in pup growth without deleterious effect on dam-litter metabolism. Fenugreek had no effect in the maternal protein restriction challenge. These results suggest a galactologue effect of fenugreek when mothers have no physiological difficulties in producing milk.

High-intensity breastfeeding improves insulin sensitivity during early post-partum period in obese women with gestational diabetes

AIM

To investigate whether high-intensity breastfeeding (HIB) reduces insulin resistance during early post-partum period in women with gestational diabetes (GDM), independent of post-partum weight change (PWC).

MATERIALS AND METHODS

In this multicentre prospective study, we included Japanese women with GDM who underwent a 75-g oral glucose tolerance test (OGTT) during early post-partum. We measured plasma insulin during OGTT to obtain a homeostasis model of assessment of insulin resistance (HOMA-IR). We defined the condition in which infants were fed by breastfeeding alone or greater than or equal to 80% of the volume as HIB, and other statuses, including partial and nonbreastfeeding, as non-HIB. We investigated the association between post-partum HOMA-IR and the breastfeeding status after adjusting for confounders including PWC.

RESULTS

Among 222 women with GDM who underwent the OGTT at 7.9 ± 2.3 weeks post-partum with a PWC of -7.8 ± 3.4 kg, although the rate of abnormal glucose tolerance (prediabetes and diabetes) did not differ between the groups (33% vs 32%), the HOMA-IR in the HIB women (n = 166) was significantly lower than that in the non-HIB women (n = 56) (1.12 ± 0.85 vs 1.72 ± 1.43, P = 0.0002). The effect of the HIB was independently associated with lower HOMA-IR after adjusting for confounders including PMC. However, the subgroup analysis according to their pre-pregnancy obesity states showed that the effect was seen only in the obese subjects (BMI ≥ 25).

CONCLUSIONS

In obese Japanese women with GDM, HIB has a significant effect in reducing insulin resistance during early post-partum, independent of the post-partum weight loss.

Long-Term Effect of Lactation on Maternal Cardiovascular Function and Adiposity in a Murine Model

OBJECTIVE

Epidemiological studies suggest that lactation is associated with long-term maternal health benefits. To avoid confounders in human studies, we used a previously characterized murine model to investigate the long-term effect of lactation on both cardiovascular function and adiposity.

STUDY DESIGN

After the delivery of the pups, CD-1 female mice were randomly divided into two groups: lactated and nonlactated (NL). Before pregnancy and at 9 months postdelivery, blood pressure was measured using a tail cuff, visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) were assessed by computed tomography (CT), echocardiography was performed using microultrasound, and cholesterol panels and fasting blood glucose were measured. The data were analyzed using Student's t-test (significance at p < 0.05).

RESULTS

There were no differences in baseline parameters between the two groups. At 9 months postdelivery, the NL group weighed significantly more (p = 0.03) and demonstrated a significantly lower cardiac output (p = 0.05) and ejection fraction (p = 0.03). The mice in the NL group also had higher VAT (p < 0.01) and SAT percentiles (p = 0.03). Fasting glucose (p = 0.01) and low-density lipoprotein (p = 0.01) were significantly higher in the NL group at 9 months.

CONCLUSION

Our results show the benefit of lactation is not just limited to the immediate postpartum period but it also extends into midlife in a murine model.

Metabolic effects of breastfeed in women with prior gestational diabetes mellitus: A systematic review and meta-analysis

This study was undertaken to provide comprehensive analyses of current research developments in the field of breastfeed (BF) and metabolic-related outcomes among women with prior gestational diabetes mellitus (GDM). Database PubMed, Embase, BIOSIS Previews, Web of Science, and Cochrane Library were searched through December 3, 2017. Odds ratio (OR) and weighted mean difference (WMD) with 95% confidence interval (CI) were pooled by random-effects model using Stata version 12.0. Twenty-three observational studies were included in quantitative synthesis. Reduced possibility of progression to type 2 diabetes mellitus (T2DM; OR = 0.79; 95% CI, 0.68-0.92) and pre-DM (OR = 0.66; 95% CI, 0.51-0.86) were found among women with longer BF of any intensity after GDM pregnancy. The positive effect of longer BF on progression to T2DM gradually became prominent with the extension of follow-up period. Compared with women with shorter BF, those with longer BF manifested more favourable metabolic parameters, including significant lower body mass index, fasting glucose, triglyceride, and higher insulin sensitivity index. The findings support that BF may play an important role in protection against the development of T2DM-related outcomes in midlife of women with prior GDM. However, further studies are needed to reveal the etiological mechanism.

Association between Breastfeeding and Prevalence of Diabetes in Korean Parous Women: The Korea National Health and Nutrition Examination Survey, 2010–2014

Background

It is well known that breastfeeding has a significant impact on the health of mothers and children. With the growing importance of breastfeeding, the present study aimed to investigate the relationship between breastfeeding and the prevalence of diabetes in Korean parous women.

Methods

The data of 5,448 premenopausal parous women aged 20–49 years who agreed to participate in the 5th–6th Korea National Health and Nutrition Examination Survey were analyzed in this study. Control group included women who had not breastfed. The subjects who had breastfed were classified into three groups based on the duration of breastfeeding: 0–6 months, 6–12 months, and >12 months. The variables included age, body mass index, education level, income, alcohol drinking, smoking, family history of diabetes, use of oral contraceptives, the number of pregnancies, and regular exercise.

Results

Among the subjects, the prevalence of diabetes was significantly lower in women who had breastfed compared to those who had not, with an odds ratio of 0.534 (95% confidence interval [CI], 0.289–0.976) in women who breastfed for 0–6 months and 0.575 (95% CI, 0.321–0.990) in women who breastfed for 6–12 months (both P<0.05).

Conclusion

The present study found a reduced prevalence of diabetes in women who had breastfed compared to those who had not. However, no association between the duration of breastfeeding and the prevalence of diabetes could be found.

Lactation and progression to type 2 diabetes in patients with gestational diabetes mellitus: A systematic review and meta-analysis of cohort studies

Aims/Introduction

To explore the association between lactation and type 2 diabetes incidence in women with prior gestational diabetes.

Materials and Methods

We searched PubMed, Embase and the Cochrane Library for cohort studies published through 12 June 2017 that evaluated the effect of lactation on the development of type 2 diabetes in women with prior gestational diabetes. A random effects model was used to estimate relative risks (RRs) with 95% confidence intervals (CIs).

Results

A total of 13 cohort studies were included in the meta‐analysis. The pooled result suggested that compared with no lactation, lactation was significantly associated with a lower risk of type 2 diabetes (RR 0.66, 95% CI 0.48–0.90, I² = 72.8%, P < 0.001). This relationship was prominent in a study carried out in the USA (RR 0.66, 95% CI 0.43–0.99), regardless of study design (prospective design RR 0.56, 95% CI 0.41–0.76; retrospective design RR 0.63, 95% CI 0.40–0.99), smaller sample size (RR 0.52, 95% CI 0.30–0.92, P = 0.024) and follow‐up duration >1 years (RR 0.75, 95% CI 0.56–1.00), and the study used adjusted data (RR 0.69, 95% CI 0.50–0.94). Finally, by pooling data from three studies, we failed to show that compared with no lactation, long‐term lactation (>1 to 3 months postpartum) was associated with the type 2 diabetes risk (RR 0.69, 95% CI 0.41–1.17).

Conclusions

The present meta‐analysis showed that lactation was associated with a lower risk of type 2 diabetes in women with prior gestational diabetes. Furthermore, no significant relationship between long‐term lactation and type 2 diabetes risk was detected. The impact of long‐term lactation and the risk of type 2 diabetes should be verified in further large‐scale studies.

Physiological, mitochondrial, and oxidative stress differences in the presence or absence of lactation in rats

BACKGROUND

Human epidemiological data show that breastfeeding reduces the mother's probability of developing several disease conditions, including obesity and type II diabetes compared to mothers that give birth but do not breastfeed. The goal of this investigation was to characterize how lactation changes a rat's body composition, metabolism, mitochondrial function, and oxidative stress.

METHODS

Ten-week old female Sprague-Dawley rats were divided into three groups (n = 8 per group): 1) non-reproductive (NR), 2) those that were allowed to mate and give birth, but were not allowed to suckle their pups (PP), and 3) those that were allowed to mate and give birth, and suckled their young until weaning at 21 days (PL). All animals were sacrificed at a time corresponding to 7 days following the weaning of pups (i.e., day 28 postpartum).

RESULTS

The body mass of PL rats was similar to NR rats, but the body mass of PP rats was higher than NR rats. Importantly, PL rats had lower retroperitoneal white adipose tissue mass compared to both NR and PP rats. The difference in fat mass was accompanied by higher protein levels of PPARδ, SOD2, and reduced oxidative damage. Furthermore, the liver of PL rats had higher mitochondrial function with NADH-linked substrates, and higher expression of PGC-1α, PPARδ, and SOD2.

CONCLUSIONS

These acute differences observed between female rats that did and did not suckle their young could be used as the foundation for future research investigating the prolonged and sustained benefits of lactation.

Lactation has persistent effects on a mother's metabolism and mitochondrial function

Human epidemiological data show that breastfeeding reduces the prevalence of numerous diseases compared to mothers that give birth but do not participate in lactation. The goal of this study was to determine if differences in metabolism, mitochondrial function, and oxidative stress underlie the protective phenotype found in lactating women. Ten-week old female Sprague-Dawley rats were divided into three groups (n = 8 per group): 1) rats that did not reproduce (NR), 2) rats that were allowed to mate and become pregnant but did not suckle their pups after giving birth (NL), and 3) rats that were allowed to mate and become pregnant and suckled their pups for 21 days before weaning (L). All animals were sacrificed at approximately 7 months of age, a time corresponding to 15 weeks after the NL and L females gave birth. Liver mitochondrial respiration was higher in L rats when using NADH-linked substrates and these rats had lower serum glucose concentration. Additionally, the L group exhibited changes in liver, skeletal muscle, and white adipose tissue PPARδ protein levels that may, in part, explain the observed lower serum glucose concentration. These novel animal findings provide evidence of differences in metabolic processes that persist months after weaning.

The effect of high-intensity breastfeeding on postpartum glucose tolerance in women with recent gestational diabetes

BACKGROUND

Although breastfeeding is expected to reduce the incidence of diabetes in women with gestational diabetes, the effect has not been clearly confirmed. We examined whether or not high-intensity breastfeeding reduces the incidence of abnormal glucose tolerance and investigated the effect of high-intensity breastfeeding on insulin resistance during the first year postpartum in Japanese women with current gestational diabetes.

METHODS

In this retrospective study, we included women with gestational diabetes who underwent postpartum 75 g oral glucose tolerance test during the first year (12-14 months) postpartum from 2009 to 2011 at a single tertiary perinatal care center in Japan. High-intensity breastfeeding was defined as the condition in which infants were fed by breastfeeding alone or 80% or more of the volume. We investigated the effect of high-intensity breastfeeding on the prevalence of postpartum abnormal glucose tolerance and the postpartum homeostasis model of assessment of insulin resistance (HOMA-IR), after controlling for confounders, including prepregnancy obesity and weight changes during pregnancy and postpartum.

RESULTS

Among 88 women with gestational diabetes, 46 (52%) had abnormal glucose tolerance during the postpartum period. High-intensity breastfeeding women (n = 70) were significantly less likely to have abnormal glucose tolerance than non-high-intensity breastfeeding women (n = 18) (46% vs. 78%, p = 0.015). High-intensity breastfeeding was also associated with a lower HOMA-IR at 12-14 months postpartum than non-high-intensity breastfeeding (1.41 ± 1.02 vs. 2.28 ± 1.05, p = 0.035). Those associations remained significant after controlling for confounders. At least six months of high-intensity breastfeeding had a significant effect on lowering both the abnormal glucose tolerance prevalence and HOMA-IR compared with non-high-intensity breastfeeding.

CONCLUSIONS

In Japanese women with gestational diabetes, high-intensity breastfeeding ≥6 months had a protective effect against the development of abnormal glucose tolerance during the first year postpartum through improving insulin resistance, independent of obesity and postpartum weight change.

Breastfeeding and maternal hypertension and diabetes: a population-based cross-sectional study

OBJECTIVE

This study aimed to assess the association of breastfeeding and maternal hypertension and diabetes in Beijing, China.

SUBJECTS AND METHODS

A cross-sectional study was conducted in four urban communities of Beijing, China, with 9,128 parous women 40-81 years of age who had had only one lifetime birth. Each participant completed a detailed survey and accepted blood pressure measurement and blood glucose testing. Moreover, self-reported hypertension and diabetes were confirmed by review of medical records.

RESULTS

After the analysis was adjusted for the potential confounders, including age, body mass index (BMI), waist to hip ratio (WHR), working status, educational level, drinking, smoking, family history of hypertension, age of menarche, menopause, oral contraceptive use, age of child-bearing, and postpartum BMI, the odd ratio (OR) of hypertension was 1.18 (95% confidence interval [CI], 1.05-1.32) for women who did not breastfeed, compared with women who did. In addition, the ORs for >0 to 6 months, >6 to 12 months, and >12 months of breastfeeding were 0.87 (95% CI, 0.76-0.99), 0.83 (95% CI, 0.68-1.00), and 0.79 (95% CI, 0.65-0.97), respectively, compared with women who did not breastfeed. With adjustment for age, WHR, working status, educational level, family history of diabetes, and postpartum BMI, women who did not breastfeed increased the risk of diabetes (OR=1.30; 95% CI, 1.11-1.53) compared with women who did. Moreover, women who breastfed for >0 to 6 months (OR=0.81; 95% CI, 0.67-0.98) and >6 to 12 months (OR=0.46; 95% CI, 0.26-0.84) had a lower risk of diabetes, compared with women who did not breastfeed.

CONCLUSIONS

Chinese mothers who did not breastfeed were more likely to develop hypertension and diabetes in later life.

Comparison of the physiological relevance of systemic vs. portal insulin delivery to evaluate whole body glucose flux during an insulin clamp

To understand the underlying pathology of metabolic diseases, such as diabetes, an accurate determination of whole body glucose flux needs to be made by a method that maintains key physiological features. One such feature is a positive differential in insulin concentration between the portal venous and systemic arterial circulation (P/S-IG). P/S-IG during the determination of the relative contribution of liver and extra-liver tissues/organs to whole body glucose flux during an insulin clamp with either systemic (SID) or portal (PID) insulin delivery was examined with insulin infusion rates of 1, 2, and 5 mU·kg(-1)·min(-1) under either euglycemic or hyperglycemic conditions in 6-h-fasted conscious normal rats. A P/S-IG was initially determined with endogenous insulin secretion to exist with a value of 2.07. During an insulin clamp, while inhibiting endogenous insulin secretion by somatostatin, P/S-IG remained at 2.2 with PID, whereas, P/S-IG disappeared completely with SID, which exhibited higher arterial and lower portal insulin levels compared with PID. Consequently, glucose disappearance rates and muscle glycogen synthetic rates were higher, but suppression of endogenous glucose production and liver glycogen synthetic rates were lower with SID compared with PID. When the insulin clamp was performed with SID at 2 and 5 mU·kg(-1)·min(-1) without managing endogenous insulin secretion under euglycemic but not hyperglycemic conditions, endogenous insulin secretion was completely suppressed with SID, and the P/S-IG disappeared. Thus, compared with PID, an insulin clamp with SID underestimates the contribution of liver in response to insulin to whole body glucose flux.

Effect of lactation on maternal postpartum cardiac function and adiposity: a murine model

OBJECTIVE

Lactation is associated with reduction in maternal metabolic disease and hypertension later in life; however, findings in humans may be confounded by socioeconomic factors. We sought to determine the independent contribution of lactation on cardiovascular parameters and adiposity in a murine model.

STUDY DESIGN

Following delivery, CD-1 female mice were randomly divided into 2 groups: lactated (L; nursed pups for 3 weeks, n = 10), and nonlactated (NL; pups were removed after birth, n = 12). Blood pressure (BP) was assessed prepregnancy and at 1 and 2 months' postpartum. Visceral and subcutaneous adipose tissue determined by computed tomography and left ventricular ejection fraction, cardiac output, and the E/A ratio determined by microultrasound were evaluated at 1 and 2 months' postpartum. The results were analyzed using a Student t test (significance at P < .05).

RESULTS

We observed a significantly different maternal BP at 2 months' postpartum with relatively greater BP in NL (systolic BP: NL, 122.2 ± 7.2 vs L, 96.8 ± 9.8 mm Hg; P = .04; diastolic BP: NL, 87.0 ± 6.8 vs L, 65.9 ± 6.2 mm Hg; P = .04). Visceral adipose tissue was significantly increased in NL mice at 1 (22.0 ± 4.1% vs 10.7 ± 1.8%, P = .04) and 2 months' postpartum (22.9 ± 3.5% vs 11.2 ± 2.2%, P = .02), whereas subcutaneous adipose tissue did not differ between the groups. At 2 months' postpartum, ejection fraction (51.8 ± 1.5% vs 60.5 ± 3.8%; P = .04), cardiac output (14.2 ± 1.0 vs 18.0 ± 1.3 mL/min; P = .02) and mitral valve E/A ratio (1.38 ± 0.06 vs 1.82 ± 0.13; P = .04) were significantly lower in NL mice than L mice.

CONCLUSION

Our data provide evidence that interruption of lactation adversely affects postpartum maternal cardiovascular function and adiposity.

Breast-feeding and maternal risk of type 2 diabetes: a prospective study and meta-analysis

AIMS/HYPOTHESIS

We aimed to examine the association between breast-feeding and maternal risk of type 2 diabetes and to investigate whether this association is mediated by anthropometric and biochemical factors.

METHODS

A case-cohort study nested within the European Prospective Investigation into Cancer and Nutrition (EPIC)-Potsdam Study between 1994 and 2005 including 1,262 childbearing women (1,059 in a random sub-cohort and 203 incident cases) mainly aged between 35 and 64 years at baseline was applied. Self-reported lifetime duration of breast-feeding was assessed by questionnaire. Blood samples were used for biomarker measurement (HDL-cholesterol, triacylglycerols, C-reactive protein, fetuin-A, γ-glutamyltransferase, adiponectin). A systematic literature search and meta-analysis was conducted of prospective cohort studies investigating breast-feeding and risk of type 2 diabetes.

RESULTS

The HR for each additional 6 months of breast-feeding was 0.73 (95% CI 0.56, 0.94) in EPIC-Potsdam. Meta-analysis of three previous prospective studies and the current study revealed an inverse association between breast-feeding duration and risk of diabetes (pooled HR for lifetime breast-feeding duration of 6-11 months compared with no breast-feeding 0.89; 95% CI 0.82, 0.97). Adjustment for BMI and waist circumference attenuated the association (HR per six additional months in EPIC-Potsdam 0.80; 95% CI 0.61, 1.04). Further controlling for potentially mediating biomarkers largely explained this association (HR 0.89; 95% CI 0.68, 1.16).

CONCLUSIONS/INTERPRETATION

Longer duration of breast-feeding may be related to a lower risk of diabetes. This potentially protective effect seems to be reflected by a more favourable metabolic profile; however, the role of body weight as a mediator or confounder remains uncertain.

Changes in mRNA expression of arcuate nucleus appetite-regulating peptides during lactation in rats

The contribution of hypothalamic appetite-regulating peptides to further hyperphagia accompanying the course of lactation in rats was investigated by using PCR array and real-time PCR. Furthermore, changes in the mRNA expression for appetite-regulating peptides in the hypothalamic arcuate nucleus (ARC) were analyzed at all stages of pregnancy and lactation, and also after weaning. Food intake was significantly higher during pregnancy, lactation, and after weaning than during non-lactation periods. During lactation, ARC expression of mRNAs for agouti-related protein (AgRP) and peptide YY was increased, whereas that of mRNAs for proopiomelanocortin (POMC) and cholecystokinin (CCK) was decreased, in comparison with non-lactation periods. The increase in AgRP mRNA expression during lactation was especially marked. The plasma level of leptin was significantly decreased during the course of lactation, whereas that of acyl-ghrelin was unchanged. In addition, food intake was negatively correlated with the plasma leptin level during lactation. This study has clarified synchronous changes in the expression of many appetite-regulating peptides in ARC of rats during lactation. Our results suggest that hyperphagia during lactation in rats is caused by decreases in POMC and CCK expression and increases in AgRP expression in ARC, the latter being most notable. Together with the decrease in the blood leptin level, such changes in mRNA expression may explain the further hyperphagia accompanying the course of lactation.

Beneficial effects of breastfeeding in women with gestational diabetes mellitus

Gestational diabetes mellitus (GDM) increases the future risk of developing type 2 diabetes mellitus (T2DM). There is now a growing evidence that breastfeeding has short- and long-term health benefits for mothers with GDM. Mothers with GDM who breastfeed have improved lipid and glucose metabolic profiles for the first 3 months after birth. However, women with GDM are less likely to breastfeed and, if they do, breastfeeding is usually continued for a shorter duration compared with women without GDM. One long-term prospective study followed women with GDM from delivery for up to 19 years postpartum, and found that breastfeeding for ≥3 months reduced the risk of T2DM and delayed the development of T2DM by a further 10 years compared with breastfeeding for <3 months. However, the physiological mechanisms underlying the protective effects of breastfeeding are still unknown, even though it is important to gain a full understanding of the pathways involved in these effects. Therefore, the purpose of this review is to provide a comprehensive analysis of the recent developments in the field of GDM and breastfeeding. We reviewed data from animal experiments and human studies. We also provide insight into the molecular pathways and describe promising topics for future research.

The insulin receptor plays an important role in secretory differentiation in the mammary gland

Insulin is known to be an important regulator of milk secretion in the lactating mammary gland. Here we examine the role of insulin signaling in mammary development in pregnancy using a mouse with a floxed insulin receptor (IR) crossed with a mouse expressing Cre specifically in the mammary gland. In the mammary glands of these IR(fl/fl) Cre(+) mice, expression of IR is significantly diminished throughout development. Glands from these mice had 50% fewer alveoli at midpregnancy; casein and lipid droplets were diminished by 60 and 75%, respectively, indicating a role for IR both in alveolar development and differentiation. In an acinar preparation from mammary epithelial cells (MEC) isolated from pregnant mice, insulin stimulated lumen formation, mammary cell size, acinar size, acinar casein content, and the formation of lipid droplets with a Km of ∼1.7 nM. IGF-I and IGF-II had no effect at concentrations below 50 nM, and a function blocking antibody to the IGF type 1 receptor did not alter the response to insulin. We conclude that insulin interacting with IR is essential for mammary differentiation during murine pregnancy. Using array analysis, we then examined the expression of genes up- or downregulated >1.5-fold in the IR(fl/fl) Cre(+) MECs, finding significant downregulation of differentiation specific genes and upregulation of cell cycle and extracellular matrix genes. We conclude that insulin fosters differentiation and may inhibit cell proliferation in the mammary gland of the midpregnant mouse.

Pharmacodynamics of cibenzoline-induced hypoglycemia in rats

Hypoglycemia is one of the serious adverse effects induced by cibenzoline (CBZ), an antiarrhythmic agent. In order to clarify the pharmacodynamics of CBZ-induced hypoglycemia, CBZ was administered intravenously to conscious rats at a dose of 5, 10 or 20 mg/kg and serum samples were collected periodically to determine the concentrations of CBZ, insulin and glucose. The pharmacokinetics of CBZ showed nonlinear characteristics and could be described by a two-compartment model with Michaelis-Menten elimination kinetics. CBZ induced a rapid increase in the serum concentration of insulin. As the CBZ dose was increased, a greater hypoglycemic effect occurred. The indirect response model was applied to account for the CBZ-induced increase in insulin secretion and the subsequent decrease in serum glucose. A linear relationship was assumed between the serum concentration of CBZ and its stimulating effect on insulin secretion. A nonlinear relationship was assumed between the serum concentration of insulin and its stimulating effect on the elimination of serum glucose. The time courses of serum concentrations of CBZ, insulin and glucose after intravenous injection of CBZ could be described by the pharmacokinetic and pharmacodynamic model developed. This approach will be useful for the identification of variable factors related to CBZ-induced hypoglycemia.

Lactation and maternal risk of type 2 diabetes: a population-based study

BACKGROUND

Lactation has been associated with improvements in maternal glucose metabolism.

METHODS

We explored the relationships between lactation and risk of type 2 diabetes in a well-characterized, population-representative cohort of women, aged 40-78 years, who were members of a large integrated health care delivery organization in California and enrolled in the Reproductive Risk factors for Incontinence Study at Kaiser (RRISK), between 2003 and 2008. Multivariable logistic regression was used to control for age, parity, race, education, hysterectomy, physical activity, tobacco and alcohol use, family history of diabetes, and body mass index while examining the impact of duration, exclusivity, and consistency of lactation on risk of having developed type 2 diabetes.

RESULTS

Of 2233 women studied, 1828 were mothers; 56% had breastfed an infant for > or =1 month. In fully adjusted models, the risk of type 2 diabetes among women who consistently breastfed all of their children for > or =1 month remained similar to that of women who had never given birth (odds ratio [OR] 1.01; 95% confidence interval [CI], 0.56-1.81). In contrast, mothers who had never breastfed an infant were more likely to have developed type 2 diabetes than nulliparous women (OR 1.93; 95% CI, 1.14-3.27) [corrected]. Mothers who never exclusively breastfed were more likely to have developed type 2 diabetes than mothers who exclusively breastfed for 1-3 months (OR 1.52; 95% CI, 1.11-2.10).

CONCLUSIONS

Risk of type 2 diabetes increases when term pregnancy is followed by <1 month of lactation, independent of physical activity and body mass index in later life. Mothers should be encouraged to exclusively breastfeed all of their infants for at least 1 month.

Duration of lactation and incidence of myocardial infarction in middle to late adulthood

OBJECTIVE

We assessed the relation between duration of lactation and maternal incident myocardial infarction.

STUDY DESIGN

This was a prospective cohort study of 89,326 parous women in the Nurses' Health Study.

RESULTS

During 1,350,965 person-years of follow-up, 2540 cases of coronary heart disease were diagnosed. Compared with parous women who had never breastfed, women who had breastfed for a lifetime total of 2 years or longer had 37% lower risk of coronary heart disease (95% confidence interval, 23-49%; P for trend < .001), adjusting for age, parity, and stillbirth history. With additional adjustment for early-adult adiposity, parental history, and lifestyle factors, women who had breastfed for a lifetime total of 2 years or longer had a 23% lower risk of coronary heart disease (95% confidence interval, 6-38%; P for trend = .02) than women who had never breastfed.

CONCLUSION

In a large, prospective cohort, long duration of lactation was associated with a reduced risk of coronary heart disease.

The reset hypothesis: lactation and maternal metabolism

For maternal metabolism, pregnancy ends not with delivery, but with weaning. In several recent epidemiological studies, authors have reported an association between duration of breast-feeding and reduced maternal risk of metabolic disease. These findings parallel data from animal models showing favorable changes in metabolism associated with lactation. During gestation, visceral fat accumulates, and insulin resistance and lipid and triglyceride levels increase. These changes appear to reverse more quickly, and more completely, with lactation. In this article, we review animal and human studies regarding the effects of lactation on adiposity, lipid, and glucose homeostasis. We hypothesize that lactation plays an important role in "resetting" maternal metabolism after pregnancy.

Duration of breast-feeding and the incidence of type 2 diabetes mellitus in the Shanghai Women's Health Study

AIMS/HYPOTHESIS

The aim of this study was to examine the association between lifetime breast-feeding and the incidence of type 2 diabetes mellitus in a large population-based cohort study of middle-aged women.

METHODS

This was a prospective study of 62,095 middle-aged parous women in Shanghai, China, who had no prior history of type 2 diabetes mellitus, cancer or cardiovascular disease at study recruitment. Breast-feeding history, dietary intake, physical activity and anthropometric measurements were assessed by in-person interviews. The Cox regression model was employed to evaluate the association between breast-feeding and the risk of type 2 diabetes mellitus.

RESULTS

After 4.6 years of follow-up, 1,561 women were diagnosed with type 2 diabetes mellitus. Women who had breastfed their children tended to have a lower risk of diabetes mellitus than those who had never breastfed [relative risk (RR)=0.88; 95% CI, 0.76-1.02; p=0.08]. Increasing duration of breast-feeding was associated with a reduced risk of type 2 diabetes mellitus. The fully adjusted RRs for lifetime breast-feeding duration were 1.00, 0.88, 0.89, 0.88, 0.75 and 0.68 (p trend=0.01) for 0, >0 to 0.99, >0.99 to 1.99, >1.99 to 2.99, >2.99 to 3.99 and >or=4 years in analyses adjusted for age, daily energy intake, BMI, WHR, smoking, alcohol intake, physical activity, occupation, income level, education level, number of live births and presence of hypertension at baseline.

CONCLUSIONS/INTERPRETATION

Breast-feeding may protect parous women from developing type 2 diabetes mellitus later in life.

Leptin levels and gene expression during the perinatal phase in the rat

The role of leptin in controlling food intake and adiposity has been the aim of many different investigations in the last 3 years. Pregnancy and lactation are two physiological situations associated with a clear hyperphagia (together with important changes in metabolism and adipose mass) to sustain the different and varying demands for foetal growth and milk production respectively. We therefore focused on the role of leptin in perinatal hyperphagia. The circulating leptin levels and leptin gene expression in adipose tissue of both pregnant and lactating rats were examined. Pregnant rats showed unchanged adipose tissue leptin mRNA levels but increased circulating leptin; this probably reflects the high fat carcass content characteristic of pregnancy. Conversely, lactating rats did not show any change either in circulating leptin or adipose tissue mRNA levels. Litter-removal caused a significant increase in both circulating leptin levels and gene expression. The results obtained permit us to suggest that leptin does not seem to have a role in controlling food intake during the perinatal phase.

The role of pyruvate dehydrogenase, phosphofructo-1-kinase and acetyl-CoA carboxylase in the regulation of fatty acid synthesis in the lactating rat mammary gland during the starved to re-fed transition

Re-feeding 24-h-starved lactating rats resulted in a rapid (within 0.5 h) restoration of glucose uptake by the mammary gland and a slower (within 3 h) restoration of fatty acid synthesis. The rapid reactivation of glucose uptake (82% of fed value within 0.5 h of re-feeding) correlated with a rapid reactivation of 6-phosphofructo-1-kinase (6-PF-1-K) and glycolysis (as determined by a 97% decrease in the [fructose-6-phosphate]/[fructose-1,6-bisphosphate] ratio). This could not be fully explained by a fall (29%) in the tissue concentration of its allosteric inhibitor, citrate. The delayed reactivation of pyruvate dehydrogenase (PDH) correlated very closely with the delayed reactivation of fatty acid synthesis and explained the continued output of pyruvate and lactate within the first 0.5 h of re-feeding. PDH reactivation preceded the reactivation of acetyl-CoA carboxylase (ACC), which did not occur significantly until 1.5 h of re-feeding. ACC reactivation correlated with a decrease in the tissue concentration of citrate and a second late phase of 6-PF-1-K activation. It is clear that the important regulatory steps 6-PF-1-K, PDH and ACC, are reactivated asynchronously in the lactating mammary gland in response to re-feeding starved rats and that PDH is more important than ACC in the regulation of fatty acid synthesis.

Nutritional control of amino acid supply to the mammary gland during lactation in the pig

The paucity of data relating to lactation physiology of the sow has frustrated researchers in estimating nutrient needs for production and mammary maintenance functions. The nutritional control of amino acid supply for milk synthesis is influenced by factors that have yet to be measured, such as blood flow and amino acid contribution from the body protein pool. The interaction or role of hormones such as insulin, glucagon or prolactin in amino acid dynamics and inter-organ exchange during lactation in the sow, are not well understood. The discrepancy existing between milk and mammary amino acid uptake profiles relative to lysine may be indicative of mammary metabolism and possibly maintenance requirements for specific amino acids. Hence, amino acid metabolism in the mammary gland, regardless of arterial blood substrate supply, may play an important role in a factorial approach to determining requirements. Mammary amino acid uptake ratios rather than milk amino acid ratios should provide a better tool to estimate amino acid requirements relative to lysine. Although lysine has typically been limiting in maize-soyabean-meal-based diets fed to lactating sows, current production trends are bringing a new dimension to the formulation of lactating-sow diets. Other amino acids may become limiting if dietary crystalline lysine is added without concern for the whole essential amino acid profile. Formulations based on an ideal amino acid profile for the lactating sow will, therefore, become critical.

Relationships between hypothalamic neuropeptide Y and food intake in the lactating rat

NPYergic neurons in the hypothalamic arcuate nucleus (ARC) that project to the paraventricular nucleus (PVN) are postulated to regulate food intake and energy balance. This projection is overactive in lactation and is thought to drive hyperphagia in this condition. We have explored further the relationship between hypothalamic NPY and food intake in lactation and tested the hypothesis that hypoinsulinemia is the stimulus to NPY neuronal activity. Compared with nonlactating controls (n = 10), freely fed lactating rats (n = 9) showed significantly increased (p < 0.05) NPY levels in the ARC and medial preoptic area (MPO), but there was no significant increase in whole hypothalamic NPY mRNA levels. Lactating rats (n = 8) that were restricted to control rats' food intake for 3 days showed generally higher hypothalamic NPY levels, with significantly higher concentrations than controls (p < 0.05) in the ARC, MPO, PVN, and lateral hypothalamic area (LHA); NPY mRNA levels were also significantly increased (p < 0.05). Across all three experimental groups, there was a significant inverse correlation between plasma insulin concentration and hypothalamic NPY mRNA levels (r = -0.39, p < 0.01). We conclude that the ARC-PVN projection is overactive in lactation and that this is not a consequence of hyperphagia. Hypoinsulinemia may stimulate these neurons, as it is thought to do in other conditions of energy deficit.

Increased neuropeptide Y concentrations in specific hypothalamic regions of lactating rats: possible relationship to hyperphagia and adaptive changes in energy balance

Lactation is accompanied by hyperphagia and a reduction in brown adipose tissue (BAT) thermogenesis, which are unexplained. Neuropeptide Y (NPY) powerfully stimulates feeding and inhibits BAT thermogenesis when injected into the paraventricular nucleus and other specific regions of the rat hypothalamus. We have tested the hypothesis that hypothalamic NPY activity is increased in lactating rats. Lactating rats consumed over four times as much food as nonlactating controls (n = 10; p < 0.001). Final plasma insulin concentrations in lactating rats were lower than in controls (6.8 +/- 0.8 vs. 11.7 +/- 2.1 pmol/l; p < 0.05) although plasma glucose and corticosterone concentrations were comparable (p > 0.05). Lactating rats showed significantly higher NPY levels than controls in specific hypothalamic regions, namely the arcuate nucleus-median eminence complex (a 41% rise; p < 0.001), paraventricular nucleus (35%; p < 0.001), ventromedial nucleus (66%; p = 0.003), and dorsomedial nucleus (78%; p < 0.001). Other hypothalamic regions showed no significant differences between groups. Increased NPY concentrations in specific hypothalamic regions, particularly the arcuate nucleus where NPY is synthesized, suggest increased activity of the hypothalamic NPYergic system in lactation. Neuropeptide Y may mediate hyperphagia and reduced BAT thermogenesis in lactation. Hypoinsulinemia may be a stimulus to hypothalamic NPY in lactation, as has been postulated in other conditions of negative energy balance.

Epidermal growth factor in mammary glands and milk from rats: the influence of insulin

Epidermal growth factor (EGF) is one of the major growth-promoting agents in milk. Using immunohistochemistry we localized EGF in the mammary glands of lactating rats to the luminal border of the secretory cells. Following proteolytic pretreatment of the histological sections, the EGF-immunoreactivity was revealed homogeneously in the cytoplasm of the secretory cells, which might suggest that EGF is present as a precursor molecule in the mammary glands. Altered glucose metabolism during lactation results in secondary hypoinsulinaemia in the lactating rat. As insulin is also known to affect lactation in several species, we treated normal lactating rats daily with insulin and studied the effect on the composition of milk. A significant increase in the content of total protein and milk fat was observed after a few days of insulin-treatment, as compared to a control group [total protein: 50 (36-97) g/l vs. 42 (35-72) g/l], [milk fat: 35 (22-40)% vs. 29 (23-36)%], [median (range)]. On day 16 the EGF concentration in milk was significantly increased in insulin-treated rats, as compared to controls [2.66 (1.40-5.08) nM vs. 1.98 (1.04-3.16) nM]. A similar significant increase was found for the secretion of the cobalamin-binding protein, haptocorrin (HC) [37.7 (15.8-110.4) nM vs. 23.5 (15.5-70.1) nM]. In conclusion, the highly insulin-sensitive lactating mammary glands were affected by exogenous insulin, since the milk concentrations of EGF, HC, total protein and the fat percentage were increased.

Effects of exogenous insulin or vanadate on disposal of dietary triacylglycerols between mammary gland and adipose tissue in the lactating rat: insulin resistance in white adipose tissue

The effects of exogenous insulin or vanadate (an insulin mimetic) on the disposal of dietary [14C]lipid between oxidation to 14CO2, deposition in adipose tissue or uptake by mammary gland and transfer to suckling pups were studied in virgin and lactating rats. After an oral load of [1-14C]triolein, virgin rats treated with a supraphysiological dose of insulin over 24 h showed a decrease (58%) in 14CO2 production and increased accumulation of [14C]lipid in carcass and white adipose tissue. There was a 2.5-fold increase in lipoprotein lipase activity in the latter. Chronic vanadate administration (12 days) had no effect on these parameters. In lactating rats, the stimulation of the deposition of [14C]lipid in adipose tissue by exogenous insulin was about 10% of that in virgin rats. In prolactin-deficient lactating rats there was no stimulation of [14C]lipid deposition in adipose tissue by insulin. However, both insulin and vanadate treatment increased the accumulation of [14C]lipid in mammary gland to the values seen in the mammary glands plus pups of normal lactating rats. Lipoprotein lipase activity in the gland was also restored to normal values. It is concluded that in lactation there is resistance to insulin stimulation of dietary lipid deposition in adipose tissue, and that this is not due to circulating prolactin. In addition, exogenous insulin plays a role in the regulation of lipoprotein lipase and hence of dietary lipid uptake into lactating mammary gland.

Metabolism of pregnant-lactating rats is adapted to pregnancy rather than to lactation

In pregnant-lactating rats implantation was induced on day 4 of lactation so that, as an exception, lactation coincided with the period of high fetal growth. The already present suckling litters of these animals lagged behind in growth, but the "second" litters were at birth normal in size and weight. Such pregnant-lactating rats were tested in vivo with intravenous glucose loads and compared with cyclic and lactating rats. Glucose tolerance was unaffected by the reproductive state. Pregnant-lactating rats showed, just as during their first pregnancy, low basal glucose levels. Their basal insulin levels and insulin responses, however, were decreased in comparison with the first pregnancy and resembled those of lactating rats. This may be due to an increased insulin turnover, because in vitro insulin responsiveness and insulin content of both "pregnant-lactating" and "pregnant" islets were increased in comparison with "cyclic" and "lactating" islets. It was concluded that the metabolism of pregnant-lactating rats is adapted to the pregnant rather than to the lactational state.

Maternal diet and insulin secretion in islets of lactating, nonlactating, and young rats

The effects of dietary phosphorus (P) on insulin secretion by pancreatic islets of rats at peak lactation (14 days after parturition) and by islets of nonlactating previously pregnant rats have been determined. In addition, the effects of changed dietary P in the maternal diet on insulin secretion by islets of the 1- and 14-day-old offspring were studied. Lactating rats had increased food intake relative to nonlactating rats and correspondingly increased P intake on the 0.4 and 0.7% P diets. Serum P levels were elevated in lactating rats consuming the 0.7% P feed relative to those on the 0.4% P feed and also relative to the nonlactating rats on the 0.7% P feed. Lactating rats had decreased bone calcium compared with the nonlactating rats. Islets from lactating rats on the 0.7% P feed secreted more insulin in response to glucose or to 3-isobutyl-1-methylxanthine (IBMX) than lactating rats on the 0.4% P feed. Additionally, lactating rats on the 0.7% P feed secreted more insulin in response to glucose than nonlactating rats on the same 0.7% P feed. The maternal diet had no influence on the endocrine pancreatic responses of the 1- and 14-day-old offspring in response to glucose or IBMX.

Insulin resistance of hind-limb tissues in vivo in lactating sheep

1. The effects of varying the plasma insulin concentration by infusion while maintaining euglycaemia by infusion of glucose on nutrient arterio-venous differences across the hind-limb and mammary gland in lactating and non-lactating sheep were investigated. 2. Insulin infusion increased the glucose arterio-venous difference across the hind-limb; this effect of insulin was decreased by lactation, suggesting that lactation induces insulin resistance in skeletal muscle. 3. Lactation increased but insulin infusion decreased the plasma concentrations of acetate, beta-hydroxybutyrate and non-esterified fatty acids. 4. Insulin infusion decreased the arterio-venous differences of acetate and hydroxybutyrate across the hind-limb; this effect of insulin is probably indirect, resulting from the decrease in plasma concentrations of these metabolites. 5. Infusion of insulin had no effect on the glucose arterio-venous difference across the mammary gland, but did decrease the oxygen arterio-venous difference. 6. The results suggest that lactation results in insulin resistance in skeletal muscle, at least with respect to glucose utilization; this should facilitate the preferential utilization of glucose by the mammary gland.

Glucose and insulin responses during mixed meals or infusion of glucose in pregnant and lactating rats

We studied the glucose tolerance in freely moving rats throughout pregnancy and lactation and during the first week after weaning. Dioestrous virgin rats served as controls. Basal glucose and insulin levels were determined after a 2-hr fasting period. Subsequently, the changes of the insulin and the glucose levels were determined during ingestion of a mixed ad lib meal or a 2 g mixed test meal, or during infusion of glucose (7.4 mg/min for 20 min) into the vena cava. Basal glucose levels were high during early pregnancy, low during late pregnancy, and in the normal range throughout lactation and after weaning. Basal insulin levels were decreased at the end of lactation. The results of the ad lib meal and test meal experiments were essentially the same. Glucose tolerance during meals was somewhat decreased early in pregnancy. The corresponding insulin responses greatly increased during the last week of pregnancy. Glucose tolerance during IV infusion of glucose was normal during pregnancy, but increased during lactation. Insulin responses to the infusion were increased during pregnancy and decreased during lactation. We concluded that glucose tolerance is hardly affected by pregnancy and even increases in the course of lactation. This is effected by an increased responsiveness of the B-cells to glucose during late pregnancy and by an increased turnover of glucose during lactation. We discuss to what extent the actions of progesterone, placental lactogen and prolactin may explain these adaptions of maternal metabolism.

Re-examination of the putative roles of insulin and prolactin in the regulation of lipid deposition and lipogenesis in vivo in mammary gland and white and brown adipose tissue of lactating rats and litter-removed rats

1. The effects of various treatments to alter either plasma prolactin (bromocryptine administration or removal of litter) or the metabolic activity of the mammary gland (unilateral or complete teat sealing) on the disposal of oral [14C]lipid between 14CO2 production and [14C]lipid accumulation in tissues of lactating rats were studied. In addition, the rates of lipogenesis in vivo were measured in mammary gland, brown and white adipose tissue and liver. 2. Bromocryptine administration lowered plasma prolactin, but did not alter [14C]lipid accumulation in mammary gland or in white and brown adipose tissue. 3. In contrast, complete sealing of teats results in no change in plasma prolactin, but a 90% decrease in [14C]lipid accumulation in mammary gland and a 4-fold increase in white and brown adipose tissue. The rate of lipogenesis in mammary gland was decreased by 95%, but there was no change in the rate in white and brown adipose tissue. Unilateral sealing of teats resulted in a decrease in [14C]lipid accumulation in white adipose tissue. 4. Removal of the litter for 24 h (low prolactin) produced a similar pattern to complete teat sealing, except that there was a 6-fold increase in lipogenesis in white adipose tissue. Re-suckling for 5 h increased plasma prolactin, but did not alter the response seen in litter-removed lactating rats. 5. Changes in lipoprotein lipase activity and in plasma insulin paralleled the reciprocal changes in [14C]lipid accumulation in white and brown adipose tissue and in mammary gland. 6. It is concluded that the plasma insulin is more important than prolactin in regulating lipid deposition in adipose tissue during lactation, and that any effects of prolactin must be indirect.

Effect of dietary fat and lactation status on insulin binding to bovine milk fat globule membranes

Lactating cows were used to examine the relationship between lactation status and insulin binding to milk fat globule membranes. Variables evaluated were daily milk yield, stage of lactation, breed, age, lactation number, daily milk fat and protein yields, milk fat and protein percentages, breeding status, body weight, body weight.75, and mammary health. Milk yield was correlated with insulin binding and accounted for 20% of the binding variability. No other variables were related to insulin binding. Insulin binding to milk fat globule membranes increased with supplemental dietary fat up to 4% added fat in the diet dry matter. Milk fat globule membranes may provide a useful model for assessing insulin receptor regulation in the mammary gland. Sources of variation in insulin binding to mammary membranes remain to be identified.