Abstract
OBJECTIVE
To determine relationships between measurements of total body, visceral, and ectopic (liver, skeletal muscle) fat with insulin sensitivity in pregnancy.
RESEARCH DESIGN AND METHODS
Pregnant women of varying prepregnancy weights were prospectively studied in early (n = 59) and late (n = 47) gestation. At each visit, participants underwent body composition measurements including fat mass (FM), fat-free mass (FFM), abdominal subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT), ectopic lipid amounts in liver (intrahepatic lipid [IHL]) and calf skeletal muscle (intramyocellular lipid [IMCL] and extramyocellular lipid), and hyperinsulinemia-euglycemic clamp to determine insulin sensitivity (Rd), endogenous glucose production (EGP), hepatic insulin sensitivity index (HISI), and free fatty acid (FFA) levels.
RESULTS
In early pregnancy, Rd ([mg/kg FFM/min]/µIU/mL) inversely correlated (P values <0.05) with BMI, FM, SAT, VAT, IHL, IMCL, and FFA. HISI inversely correlated (P values <0.05) with BMI, FM, SAT, VAT, IMCL, and FFA but not with IHL. In late pregnancy, however, neither EGP ([mg/kg FFM/min]/µIU/mL) nor Rd correlated with regional or ectopic fat measures, but HISI remained inversely correlated with BMI, FM, SAT, VAT, and IMCL. Early-pregnancy IHL levels did not predict late-pregnancy insulin sensitivity. Pregnant women with prepregnancy obesity were more insulin resistant but gained less gestational weight, VAT, and SAT, and experienced less decline in insulin sensitivity, than normal prepregnancy weight women.
CONCLUSIONS
Insulin resistance in early pregnancy is strongly associated with total, regional, and ectopic adiposity. However, in late pregnancy, factors other than regional and ectopic adiposity predominately influence insulin sensitivity. Prepregnancy weight categories proportionately alter gestational weight gain, adiposity distribution, and glucometabolic responses.
Graphical Abstract
Introduction
Obesity and insulin resistance are significant risk factors for adverse maternal and fetal pregnancy outcomes (1–3). Previous studies of small groups of women have shown that insulin sensitivity declines with pregnancy in women who are normal weight and those with obesity (4–6), both with and without gestational diabetes mellitus (7–9). Interestingly, the reduction in late-gestation insulin sensitivity was greater in the normal-weight women (5) than in those with obesity (6). These studies also demonstrate wide variability in insulin sensitivity among pregnant women with similar BMI, the cause of which remains poorly understood.
One explanation may be that BMI is not an accurate measure of regional and ectopic fat accumulation, collectively referred to as adiposity. One of the strongest links between adiposity and insulin resistance among nonpregnant adults is excess accumulation of fat in the abdominal visceral adipose tissue (VAT) depot, more so than abdominal subcutaneous adipose tissue (SAT) depots (10–13). Individuals with increased visceral adiposity also tend to accumulate excess liver fat (intrahepatic lipid [IHL]) and are at risk for hepatic steatosis (14–16). Studies have shown that both VAT and IHL accumulation are directly related to insulin resistance in nonpregnant adults (15,17,18), with newer data indicating that variations in IHL amounts are more strongly associated with both hepatic and whole-body insulin sensitivity than VAT, providing an explanation for insulin resistance variations in persons of similar visceral adiposity (15,18). While limited evidence suggests that pregnancy contributes to changes in central and visceral fat content (19–21), no studies to date have assessed whether changes in visceral, muscle, and liver adiposity are associated with insulin sensitivity parameters during gestation. Since regional and ectopic adiposity measures can vary greatly in people of the same BMI, this could explain the variations in insulin sensitivity in pregnant women of similar BMI noted above.
Therefore, we prospectively quantified abdominal, skeletal muscle, and hepatic lipid content using MRI and MRS and performed hyperinsulinemic-euglycemic clamps in early and late pregnancy to test associations of these body adiposity measures and insulin sensitivity in women ranging from those with normal prepregnancy BMIs to those with obesity. We hypothesized that changes in fat distribution would closely correlate with insulin sensitivity during gestation, and, specifically, that greater fat deposition in the visceral and ectopic liver depots would be associated with greater insulin resistance in early and late pregnancy. We also hypothesized that, of all the adiposity measures, early-pregnancy IHL would best predict changes in insulin sensitivity during gestation (15,18).
Research Design and Methods
Study Population
The methods for this prospective cohort study are published elsewhere (22,23). Briefly, pregnant women receiving care at Kaiser Permanente Northwest and Oregon Health & Science University were recruited between November 2014 and April 2017. Patients were eligible for the study if they were between 18 and 45 years of age, less than 12 weeks pregnant with a singleton gestation, had a BMI between 18.5 kg/m2 and 38 kg/m2 at time of enrolment, were fluent English speakers, were willing and able to undergo magnetic resonance measurements, and did not have any excluded conditions (22), including pre-existing diabetes or gestational diabetes mellitus. Potential participants underwent an explanatory visit at the Oregon Health & Science University Clinical Translational and Research Center, and those who consented to participate in the study returned for study visits in early (between 12 and 17 weeks of gestation, n = 59) and late (between 32 and 36 weeks of gestation, n = 47) pregnancy (Supplementary Fig. 1).
Demographic Measures
Demographic variables were extracted from the electronic health record, including parity and preconception BMI (22), and from the participant survey (race and ethnicity). Categories for normal weight, overweight, and obesity were defined using cutoffs of 18.5–24.9 kg/m2, 25–29.9 kg/m2, and ≥30 kg/m2(24), respectively.
Body Composition Measures
Height was measured at the first study visit to allow for the calculation of BMI. At each study visit, weight was measured using the same calibrated scale; fat mass (FM), fat-free mass (FFM), and percent body fat (% BF) were determined using air displacement plethysmography (BOD POD; COSMED USA, Inc., Concord, CA) with adjustments for gestational age using van Raaij equations (25); MRI and MRS (Siemens Magnetom Tim Trio 3 Tesla whole body system; Siemens Medical Solutions, Malvern, PA) were used to quantify total abdominal fat, SAT, VAT, liver volume, IHL, intramyocellular lipid (IMCL), and extramyocellular lipid (EMCL) (at the midsoleus muscle) as previously described (22).
Insulin Sensitivity
On a separate day and within a week of the body composition measures, participants underwent a hyperinsulinemic-euglycemic clamp to measure insulin sensitivity. For the 3 days prior to the clamp, participants were instructed to consume a standard diet consisting of 30% of total calories from fat sources, 15% from protein, and 55% from carbohydrates. The night before the study visit, participants were instructed to fast for 11 h overnight. The morning of the clamp, a primed constant infusion of [6,6-2H2] glucose (Cambridge Isotope Laboratories, Andover, MA) was started at 0.133 mL/min to achieve an intended enrichment of ∼1.0 mol percent excess and continued for the next 4 h. At the completion of the first 2 h of glucose isotope infusion, a primed constant infusion of regular insulin at 40 mU/m2/min was begun along with a variable infusion of D20 glucose (enriched with [6,6-2H2]) to achieve a target glucose level of 5.0 mmol/L (range 4.7–5.3 mmol/L) during the final 2 h of the study (Supplementary Fig. 2).
Blood samples were collected prior to insulin infusion start and every 5 min during the clamp to allow calculation of the rates of glucose disposal (clamp Rd) and endogenous glucose production (EGP) using non–steady-state equations based on plasma [6,6-2H2] glucose enrichment determined by gas chromatography–mass spectrometry (26,27). Because of changes in body composition, plasma volume, rate of clearance of insulin, and differences in achieved insulin levels during the clamp between early and late pregnancy (Supplementary Fig. 2), clamp Rd values were adjusted for FFM and insulin levels. FFM from early pregnancy was used in the adjustments at both study visits to avoid confounding from the influence of uterine contents and volume expansion on FFM in later pregnancy. Hepatic insulin sensitivity index (HISI) was calculated using the reciprocal of the product of the basal endogenous glucose production rate and the fasting plasma insulin concentration as previously described (16,28).
Insulin, Glucose, and Free Fatty Acid Levels
Blood insulin levels were measured by ELISA (Mercodia AB, Uppsala, Sweden); glucose levels were measured by hexokinase-based colorimetric assay (Stanbio Laboratory, Boerne, TX); and free fatty acid (FFA) levels were measured by an enzymatic colorimetric kit (Wako Chemicals USA, Inc., Richmond, VA) in the Laboratory Core of the Oregon Clinical and Translation Research Institute.
Statistical Analyses
We calculated means and SDs for body composition, insulin sensitivity, insulin, glucose, and FFA measures. Change scores between visits 1 and 2 (visit 2 measurement – visit 1 measurement) and their means and SDs were also calculated. Prior to analysis, we transformed variables that showed severe deviation from a normal distribution: for IHL at visit 1 and 2, we performed a negative reciprocal root transformation; for IMCL and EMCL at visit 1 and 2, we performed a natural log transformation. The winsorization method was used to address extreme outliers observed in basal EGP at visit 1 and adjusted clamp Rd at visit 2. Specifically, the most extreme value of each measure was replaced with the next less extreme value in the data; change scores were then calculated using the winsorized values. One-sample t tests and the Wilcoxon signed-rank test (for IHL) were performed on change scores to determine whether each variable changed significantly from visit 1 to visit 2. To assess relationships between body composition measurements and insulin sensitivity at baseline and during pregnancy, we calculated correlation coefficients between each fat variable (body fat, % BF, FM, FFM, IHL, IMCL, EMCL, VAT, SAT, and FFA levels) and insulin sensitivity (clamp Rd, EGP, and HISI) at visits 1 and 2. Correlation coefficients were also calculated between the change in each fat variable and change in insulin sensitivity between the early- and late-pregnancy visits. Spearman correlation coefficients are reported for the relationship between change in IHL and changes in insulin sensitivity; Pearson correlation coefficients are reported for the remaining bivariate relationships. One-way ANOVA and Kruskal-Wallis tests were performed on baseline body composition and insulin sensitivity measurements and change scores to evaluate differences in mean (and median for IHL) values by preconception BMI groups. Hierarchical multiple regression analysis was performed with HISI as the dependent variable; baseline BMI was entered as a covariate in the first block and baseline IHL, VAT, and FM were entered as independent variables in the second block to assess change in insulin sensitivity during pregnancy after controlling for preconception BMI. Standardized regression coefficients are reported to evaluate the relative importance of each variable in the model.
Results
Demographics
A total of 59 women underwent both body composition and insulin sensitivity measurements at an early-pregnancy visit, and most (n = 47) returned for repeat measurements at a late-pregnancy visit (Supplementary Tables 1 and 2). The average (minimum-maximum) age was 30.3 (19.0–39.0) years, and preconception BMI was 26.6 (17.1–37.7) kg/m2. Participants were predominantly White (84.7%) and non-Hispanic (88.1%), and just over half were nulliparous (57.6%). The average (SD) gestational age was 15.5 (0.9) weeks at the first early-pregnancy visit and 34.1 (1.4) weeks at the second late-pregnancy visit.
Body Composition Changes During Gestation
Body weight, BMI, and FM increased 15% from early to late gestation, while % BF did not significantly change (0.2%) (Fig. 1 and Supplementary Table 2). Other measures of body composition that showed increases during gestation included SAT (23%) and VAT (56%). In early pregnancy, mean (SD) IHL levels were low, at 1.1 (2.0%), with levels >5% (a threshold for hepatic steatosis [29]) found in only two participants. In late pregnancy, mean (SD) IHL increased slightly to 1.4 (2.4%), with >5% levels found in four participants, one of whom fell into this category at both study visits. Neither IMCL nor EMCL levels changed significantly between early and late gestation.
Figure 1.
Graphs of body composition and glucose metabolism by preconception BMI grouping in early pregnancy and late pregnancy and change during pregnancy. Box plot elements: box denotes 25th to 75th percentile; whiskers denote minimum and maximum observations below or above 1.5 (interquartile range) of the 25th and 75th percentile, respectively; vertical line within box denotes median; filled circle denotes mean; circle, triangle, “+,” and “×” symbols indicate outliers beyond 1.5 (interquartile range) of the 25th and 75th percentiles, respectively. Maroon bar and lines denote overall group. Light blue bar and lines denote prepregnancy normal-weight group (BMI 18.5–24.9 kg/m2). Medium blue bar and lines denote prepregnancy overweight group (BMI 25–29.9 kg/m2). Dark blue bar and lines denote prepregnancy obesity weight group (BMI ≥30 kg/m2).
Compared with women with normal or overweight prepregnancy BMI, women with prepregnancy obesity had the highest % BF, FM, SAT, VAT, and IMCL in early pregnancy, P < 0.001 (Fig. 1 and Supplementary Table 3). On the other hand, intrahepatic lipid content was not significantly different among the three preconception weight categories. Although not always reaching statistical significance, early- to late-pregnancy changes in % BF (P < 0.05), FM, SAT (P < 0.01), VAT, and IMCL (P < 0.05) were greatest in women in the normal prepregnancy BMI category and least in those with prepregnancy obesity (Fig. 1 and Supplementary Table 3). Notably, in the group of women with prepregnancy obesity, the mean gestational changes of several body composition parameters, including SAT and IMCL, were reductions.
Glucose Metabolism Changes During Gestation
In the entire cohort from early to late pregnancy, fasting glucose levels decreased (P < 0.001), insulin levels increased (P < 0.001), and FFA levels did not significantly change (Table 1). Glucose disposal rates (clamp Rd adjusted for FFM in early pregnancy and insulin levels at each respective pregnancy visit) significantly declined by 16%. Both unadjusted basal and clamp EGP significantly increased by 22% and 144%, but neither were significant after adjusting for FFM and insulin levels (Table 1), and mean percent suppression of EGP during the clamp did not significantly change. On the other hand, the mean HISI decreased by 41%, P < 0.001.
Table 1.
Measurements of glucose metabolism in the entire cohort
| Early gestation | Late gestation | Early to late gestation | ||||||
|---|---|---|---|---|---|---|---|---|
| n | Mean (SD) | n | Mean (SD) | n | Absolute change, mean (SD) | Percent change | P value | |
| Fasting glucose, mmol/L | 59 | 4.5 (0.3) | 47 | 4.2 (0.3) | 47 | −0.33 (0.37) | −7 | <0.001 |
| Fasting insulin, pmol/L | 59 | 29.7 (21.4) | 48 | 41.8 (22.8) | 48 | 17.5 (15.2) | 81 | <0.001 |
| Fasting FFA, µmol/L | 59 | 456 (151) | 48 | 477 (159) | 48 | 44.9 (195) | 21 | 0.12 |
| Rd, mg/min | 59 | 440 (118) | 47 | 383 (80) | 47 | −63.1 (96.0) | −11 | <0.001 |
| Rd adjusted for FFM in early pregnancy and insulin levels, (mg/kg/min)/µIU/mL | 59 | 0.17 (0.07) | 47 | 0.14 (0.05) | 47 | −0.04 (0.06) | −16 | <0.001 |
| Basal EGP, mg/min | 59 | 196 (57) | 46 | 219 (45) | 46 | 28.0 (59.6) | 22 | 0.003 |
| Basal EGP, (mg/kg/min)/µIU/mL | 59 | 0.08 (0.04) | 46 | 0.08 (0.06) | 46 | 0.01 (0.07) | 19 | 0.41 |
| Clamp EGP, mg/min | 59 | 79.2 (45.3) | 46 | 94.2 (33.1) | 46 | 20.7 (47.5) | 31* | 0.005 |
| Clamp EGP adjusted for FFM in early pregnancy and insulin levels, (mg/kg/min)/µIU/mL | 59 | 0.03 (0.02) | 46 | 0.03 (0.02) | 46 | 0.01 (0.03) | 25* | 0.05 |
| Percent suppression EGP | 59 | 61.1 (18.7) | 46 | 57.4 (10.9) | 46 | −5.3 (21.9) | -- | 0.11 |
| HISI | 59 | 12.7 (7.3) | 46 | 7.1 (3.8) | 46 | −7.0 (6.6) | −41 | <0.001 |
*Median percent change reported because of presence of extreme outlier.
While a limited number of participants in the prepregnancy obesity category allows only descriptive analysis of many variables, several trends are evident in the data comparing across pregnancy weight groups. For example, examining changes across prepregnancy by BMI strata, early-pregnancy glucose, insulin, and FFA levels were highest in the group with obesity compared with the normal group and the group with obesity (Fig. 1 and Table 2). All groups experienced reductions in fasting glucose levels and increases in fasting insulin levels. Changes in FFA levels among the weight groups were mixed and not significantly different between groups. Early-pregnancy clamp Rd was lowest in the group with obesity, and, although clamp Rd decreased between early and late pregnancy in all three weight category groups, the group with normal preconception BMI experienced the greatest proportional decline, followed by the overweight group, with the least proportional decline in the group with obesity. Likewise, early-pregnancy HISI was lower in participants with prepregnancy obesity, with all groups experiencing further reductions during pregnancy, the greatest of which occurred in women with normal prepregnancy weight and the smallest in the women in the prepregnancy obesity category (Fig. 1 and Table 2).
Table 2.
Measurements of glucose metabolism by prepregnancy BMI weight category
| All participants | Normal weight (<25 kg/m2) | Overweight (25.0–29.9 kg/m2) | Obesity (≥30.0 kg/m2) | P value* | |
|---|---|---|---|---|---|
| Fasting glucose (mmol/L) | |||||
| n | 47 | 24 | 17 | 6 | |
| Early pregnancy | 4.5 (4.4, 4.6) | 4.5 (4.4, 4.7) | 4.4 (4.2, 4.5) | 4.8 (4.5, 5.1) | 0.03 overweight-obesity |
| Absolute change (early to late) | −0.33 (−0.44, −0.23) | −0.34 (−0.49, −0.19) | −0.25 (−0.42, −0.071) | −0.55 (−0.85, −0.25) | 0.22 |
| Fasting insulin (pmol/L) | |||||
| n | 48 | 24 | 17 | 7 | |
| Early pregnancy | 24 (20, 28) | 19 (14, 24) | 25 (19, 31) | 40 (30, 49) | 0.001 normal weight–obesity, overweight-obesity |
| Absolute change (early to late) | 17 (13, 22) | 16 (9.5, 22) | 17 (9.8, 25) | 24 (12, 35) | 0.48 |
| Fasting FFA (µmol/L) | |||||
| n | 48 | 24 | 17 | 7 | |
| Early pregnancy | 433 (392, 474) | 356 (307, 405) | 491 (433, 549) | 552 (461, 642) | <0.001 normal weight–overweight, normal weight–obesity |
| Absolute change (early to late) | 45 (−12, 102) | 104 (25, 182) | −11 (−104, 81) | −20 (−164, 125) | 0.11 |
| Rd (mg/min) | |||||
| n | 47 | 24 | 17 | 6 | |
| Early pregnancy | 446 (412, 479) | 479 (434, 525) | 419 (364, 473) | 389 (298, 480) | 0.10 |
| Absolute change (early to late) | −63 (−91, −35) | −100 (−137, −63) | −31 (−75, 13) | −7.4 (−81, 67) | 0.02 |
| Rd, adjusted for FFM in early pregnancy and insulin levels at each pregnancy visit ([mg/kg/min]/µIU/mL) | |||||
| n | 47 | 24 | 17 | 6 | |
| Early pregnancy | 0.17 (0.15, 0.19) | 0.19 (0.17, 0.22) | 0.16 (0.13, 0.19) | 0.12 (0.075, 0.17) | 0.01 normal weight–obesity |
| Absolute change (early to late) | −0.035 (−0.053, −0.018) | −0.051 (−0.075, −0.026) | −0.024 (−0.053, 0.005) | −0.008 (−0.057, 0.041) | 0.19 |
| Basal EGP (mg/min) | |||||
| n | 46 | 23 | 17 | 6 | |
| Early pregnancy | 191 (175, 207) | 178 (156, 200) | 195 (169, 220) | 230 (187, 273) | 0.10 |
| Absolute change (early to late) | 28 (10, 46) | 38 (13, 62) | 27 (−2.0, 56) | −5.2 (−54, 44) | 0.30 |
| Basal EGP, adjusted for FFM in early pregnancy and insulin levels at each pregnancy visit ([mg/kg/min]/µIU/mL) | |||||
| n | 46 | 23 | 17 | 6 | |
| Early pregnancy | 0.076 (0.064, 0.087) | 0.072 (0.056, 0.088) | 0.074 (0.055, 0.092) | 0.095 (0.063, 0.13) | 0.42 |
| Absolute change (early to late) | 0.008 (−0.011, 0.027) | 0.008 (−0.020, 0.035) | 0.022 (−0.010, 0.054) | −0.030 (−0.084, 0.023) | 0.25 |
| Clamp EGP (mg/min) | |||||
| n | 46 | 23 | 17 | 6 | |
| Early pregnancy | 73 (62, 85) | 67 (51, 84) | 80 (61, 100) | 78 (45, 110) | 0.57 |
| Absolute change (early to late) | 21 (6.6, 35) | 23 (2.9, 44) | 16 (−7.9, 39) | 25 (−15, 65) | 0.87 |
| Clamp EGP, adjusted for FFM in early pregnancy and insulin levels at each pregnancy visit ([mg/kg/min]/µIU/mL) | |||||
| n | 46 | 23 | 17 | 6 | |
| Early pregnancy | 0.027 (0.023, 0.032) | 0.026 (0.020, 0.033) | 0.029 (0.022, 0.037) | 0.024 (0.012, 0.036) | 0.72 |
| Absolute change (early to late) | 0.008 (−0.000, 0.015) | 0.007 (−0.004, 0.018) | 0.010 (−0.003, 0.023) | 0.004 (−0.018, 0.026) | 0.84 |
| EGP suppression (%) | |||||
| n | 46 | 23 | 17 | 6 | |
| Early pregnancy | 63 (57, 68) | 64 (56, 71) | 58 (50, 67) | 71 (56, 85) | 0.31 |
| Absolute change (early to late) | −5.3 (−12, 1.2) | −6.0 (−15, 3.2) | −0.80 (−12, 9.9) | −16 (−34, 2.5) | 0.37 |
| HISI | |||||
| n | 46 | 23 | 17 | 6 | |
| Early pregnancy | 14 (12, 16) | 17 (15, 20) | 12 (9.1, 15) | 7.1 (1.7, 12) | 0.002 normal weight–obesity |
| Absolute change (early to late) | −7.0 (−8.9, −5.0) | −9.2 (−12, −6.5) | −5.5 (−8.5, −2.4) | −2.8 (−7.9, 2.4) | 0.05 |
Data are mean (95% CI) unless otherwise noted. *Results from one-way ANOVA analyses. Adjustments for FFM use only value from early-pregnancy visit (see RESEARCH DESIGN AND METHODS).
Correlations Between Adiposity Measures and Insulin Sensitivity in Early and Late Pregnancy
In early pregnancy, adjusted clamp Rd correlated inversely with most measures of body composition except EMCL and % BF, with the highest correlation with VAT (r = −0.52) (Table 3 and Supplementary Figs. 3 and 4). Unadjusted basal EGP was positively correlated with BMI, % BF, FM, and SAT but not with other body composition parameters or FFA levels (Table 3), whereas basal EGP adjusted for FFM and insulin was positively correlated only with % BF and FM. HISI was also inversely correlated with BMI, % BF, FM, SAT, VAT, and IMCL but not significantly correlated with IHL (Table 3 and Supplementary Figs. 3 and 4). Both clamp Rd and HISI were inversely correlated with fasting FFA levels (Table 3).
Table 3.
Pearson correlations between insulin sensitivity measures and body composition in early and late pregnancy
| BMI | % BF | FM | SAT | VAT | IHL† | EMCL† | IMCL† | FFA | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| n | Correlation | n | Correlation | n | Correlation | n | Correlation | n | Correlation | n | Correlation | n | Correlation | n | Correlation | n | Correlation | |
| Early pregnancy | ||||||||||||||||||
| Clamp Rd | 59 | −0.22 | 59 | −0.14 | 59 | −0.16 | 59 | −0.19 | 59 | −0.36** | 58 | −0.42** | 53 | 0.07 | 53 | −0.21 | 59 | −0.57** |
| Clamp Rd (adjusted for FFM in early pregnancy and insulin at each pregnancy visit) | 59 | −0.46** | 59 | −0.23 | 59 | −0.34** | 59 | −0.38** | 59 | −0.52** | 58 | −0.42** | 53 | 0.01 | 53 | −0.35** | 59 | −0.49** |
| Basal EGP | 59 | 0.44** | 59 | 0.48** | 59 | 0.47** | 59 | 0.41** | 59 | 0.24 | 58 | −0.14 | 53 | 0.26 | 53 | 0.14 | 59 | 0.14 |
| Basal EGP (adjusted for FFM in early pregnancy and insulin at each pregnancy visit) | 59 | 0.07 | 59 | 0.27* | 59 | 0.15 | 59 | 0.06 | 59 | −0.09 | 58 | −0.27* | 53 | 0.15 | 53 | −0.08 | 59 | 0.09 |
| Clamp EGP | 59 | 0.28* | 59 | 0.25 | 59 | 0.27* | 59 | 0.25 | 59 | 0.10 | 58 | −0.06 | 53 | 0.37** | 53 | 0.16 | 59 | −0.04 |
| Clamp EGP (adjusted for FFM in early pregnancy and insulin at each pregnancy visit) | 59 | 0.08 | 59 | 0.18 | 59 | 0.11 | 59 | 0.07 | 59 | −0.09 | 58 | −0.18 | 53 | 0.33* | 53 | 0.03 | 59 | −0.11 |
| Percent EGP suppression | 59 | −0.06 | 59 | −0.01 | 59 | −0.03 | 59 | −0.04 | 59 | 0.01 | 58 | 0.02 | 53 | −0.34* | 53 | −0.13 | 59 | 0.14 |
| HISI | 59 | −0.71** | 59 | −0.74** | 59 | −0.71** | 59 | −0.64** | 59 | −0.64** | 58 | −0.16 | 53 | −0.26 | 53 | −0.44** | 59 | −0.36** |
| Late pregnancy | ||||||||||||||||||
| Clamp Rd | 47 | 0.21 | 47 | 0.10 | 47 | 0.24 | 43 | 0.22 | 43 | 0.22 | 43 | −0.02 | 44 | 0.07 | 44 | 0.06 | 47 | −0.31* |
| Clamp Rd (adjusted for FFM in early pregnancy and insulin at each pregnancy visit) | 47 | −0.22 | 47 | −0.17 | 47 | −0.21 | 43 | −0.17 | 43 | −0.18 | 43 | −0.16 | 44 | 0.14 | 44 | −0.09 | 47 | −0.15 |
| Basal EGP | 46 | 0.33* | 46 | 0.20 | 46 | 0.36* | 42 | 0.29 | 42 | 0.31* | 42 | 0.03 | 43 | 0.15 | 43 | 0.01 | 46 | −0.29* |
| Basal EGP (adjusted for FFM in early pregnancy and insulin at each pregnancy visit) | 46 | −0.11 | 46 | 0.01 | 46 | −0.06 | 42 | −0.02 | 42 | −0.07 | 42 | −0.11 | 43 | 0.05 | 43 | −0.15 | 46 | −0.02 |
| Clamp EGP | 46 | 0.36* | 46 | 0.13 | 46 | 0.32* | 42 | 0.17 | 42 | 0.21 | 42 | −0.06 | 43 | 0.20 | 43 | −0.01 | 46 | −0.29* |
| Clamp EGP (adjusted for FFM in early pregnancy and insulin at each pregnancy visit) | 46 | −0.01 | 46 | −0.01 | 46 | −0.02 | 42 | −0.07 | 42 | −0.08 | 42 | −0.14 | 43 | 0.21 | 43 | −0.12 | 46 | −0.13 |
| Percent EGP suppression | 46 | −0.21 | 46 | 0.01 | 46 | −0.12 | 42 | 0.04 | 42 | −0.00 | 42 | 0.10 | 43 | −0.18 | 43 | 0.04 | 46 | 0.17 |
| HISI | 46 | −0.41** | 46 | −0.34* | 46 | −0.42** | 42 | −0.39* | 42 | −0.40** | 42 | −0.2 | 43 | −0.07 | 43 | −0.30* | 46 | −0.04 |
*P < 0.05;
**P < 0.01.
†IHL transformed using negative reciprocal root; EMCL and IMCL transformed using natural log.
In late pregnancy, adjusted clamp Rd was not significantly associated with any measure of body composition or fasting FFA levels (Table 3 and Supplementary Figs. 5 and 6). Unadjusted basal EGP in late gestation was positively associated with BMI, FM, VAT, and FFA, but the relationships were not significant after adjusting for FFM and insulin levels. On the other hand, HISI in late pregnancy correlated inversely with BMI, % BF, FM, SAT, VAT, and IMCL but not IHL, EMCL, or FFA (Table 3 and Supplementary Figs. 5 and 6).
Correlations Between Changes in Body Composition and Changes in Insulin Sensitivity
No significant correlations were found between changes in any measure of body composition from early to late gestation and changes in clamp Rd, EGP, or HISI (Supplementary Table 4). Only a single significant inverse relationship between changes in clamp Rd (adjusted for FFM and insulin) and fasting FFA was detected.
Correlations Between Early-Pregnancy Adiposity Measures and Changes in Insulin Sensitivity
We next tested whether early-pregnancy IHL correlated with gestational changes in insulin sensitivity and found no significant relationships (Supplementary Table 5). Additional exploratory correlational testing between early-pregnancy BMI, FM, and VAT and insulin sensitivity measure changes during pregnancy similarly found no relationships.
Conclusions
Previous studies of pregnant women examining relationships between body fat distributions and metabolic outcomes used techniques that have either not been validated for use during pregnancy (ultrasound) (30,31) or can only be used before and after pregnancy because of radiation risk, such as DEXA or computed tomography scans (32). By using MRI and MRS methodologies that are considered safe during gestation, this is the first study to explore relationships between changes in regional and ectopic lipid stores and insulin sensitivity during gestation in mothers with prepregnancy body weights ranging from normal to obesity.
As previously reported (4,6,8), we found that pregnant women with obesity are more insulin resistant than normal-weight pregnant women, and women in all pregestational weight categories (normal weight, overweight, obesity) become more insulin resistant during pregnancy, in regard to both clamp Rd and HISI. However, these relationships exhibit much interindividual variation. We hypothesized that known relationships between insulin sensitivity and regional fat distribution, independent of total body weight and including visceral and ectopic fat accumulation in liver and muscle, might account for some of this variability within and between individuals.
We found this to be true in early pregnancy, with clamp Rd and HISI correlating with measures of total, regional, and ectopic adiposity, including BMI, % BF, FM, SAT, VAT, IHL, and IMCL. We further hypothesized that early-pregnancy IHL levels would be the strongest predictor of later insulin sensitivity measures. Contrary to our hypotheses, however, by late pregnancy, none of the body composition measures were significantly correlated with glucose production (EGP) or disposal (clamp Rd), and none of the changes in regional and ectopic fat during gestation significantly correlated with changes in clamp Rd, EGP, or HISI, including early-pregnancy IHL. Interestingly, HISI remained strongly associated with FM, SAT, and VAT in late pregnancy, and, despite the increase in EGP in late pregnancy, suppression during hyperinsulinemic conditions was similar to early pregnancy. This may indicate that factors leading to insulin resistance late in pregnancy discussed below may supplant or supersede influences of regional and ectopic adiposity in muscle but not liver.
It is currently thought that placentally derived factors play major roles in expression of gestational insulin resistance, especially in later pregnancy (33). These include animal studies and the observation of rapid return of insulin sensitivity shortly after delivery of the placenta following birth (34–36), with hypothesized mediators including gestational increases in free cortisol, maternal cytokines, placental miRNAs, placental chorionic gonadotrophin, placental growth hormone, and placental lactogen (37,38). Our data support not only that this is likely but also that, in late pregnancy, these factors are more potent than expression of normal gestational adiposity changes in mediating this effect, especially for glucose disposal. Even FFA levels, which have experimental support to directly impair insulin sensitivity during pregnancy (39), demonstrated inverse relationships with clamp Rd and HISI in early pregnancy but not in late gestation; however, of all the tested factors correlating change during pregnancy, only change in FFA levels from early to late pregnancy correlated with the change in insulin sensitivity (clamp Rd). Further understanding of these factors and how they align (or not) with current models of insulin resistance involving altered adiponectin signaling or abnormal fatty acid metabolism in the liver and muscle merit further investigation.
In our study, women with prepregnancy obesity had the greatest amounts of total and regional fat measures and the highest FFA levels in early pregnancy. Predictably, they also had the lowest insulin sensitivity in early pregnancy. Interestingly, women in the normal prepregnancy BMI category experienced the greatest gestational weight gain, while the women with prepregnancy obesity experienced the least. In fact, many in the prepregnancy obesity group experienced gestational losses of FM, SAT, VAT, IMCL, and EMCL as well as declines in FFA levels. As a group, they also experienced the least proportional decline in insulin sensitivity measures during pregnancy compared with the other prepregnancy weight groups. The preservation of these expected proportional relationships between body composition (and FFA) and insulin sensitivity measures observed between the BMI weight categories suggests that adiposity changes may still play a role, albeit small, in influencing late-pregnancy glucose metabolism. Regardless, the greater gestational weight gain and decline in insulin sensitivity meant that women with prepregnancy normal weight “caught up” in late pregnancy compared with women with prepregnancy obesity and had similar measures of both clamp Rd and HISI. It is possible that, compared with pregestationally normal-weight women, women with obesity may lack the capacity to respond (or have reached a state of maximal responsiveness) to the physiologic forces governing energy balance, gestational weight gain, and insulin resistance in late pregnancy.
A major limitation to our study was the smaller numbers of pregnant women in the overweight and obesity categories compared with the normal-weight category, making our findings in the women with obesity more exploratory than conclusive. Nevertheless, the consistency in proportional and directional changes make these observations compelling. Another limitation is that independent contributions to insulin sensitivity from the mother and fetus were not quantified. This is especially relevant in late pregnancy when the enlarging fetoplacental unit significantly contributes to (noninsulin-mediated) glucose disposal (40), potentially confounding maternal glucose metabolism measurements at this time point. Strengths of our study were the use of advanced imaging techniques for quantifying regional and ectopic fat distribution paired with insulin clamp measurements of insulin sensitivity.
In summary, early-pregnancy insulin sensitivity measures demonstrate the same relationships to total, regional, ectopic fat measures, and FFA levels as reported in nonpregnant women. In late pregnancy, however, although regional and ectopic fat stores have increased during gestation, they are no longer as potently associated with insulin sensitivity measures, including VAT and IHL levels. The insulin resistance that occurs during later stages of pregnancy appears to be driven primarily by factors other than parameters of regional and ectopic adiposity accumulation, may differentially affect muscle and liver insulin responsiveness, and is relatively less impactful to women with prepregnancy obesity compared with mothers who are normal weight.
This article contains supplementary material online at https://doi.org/10.2337/figshare.31052392.
Article Information
Duality of Interest. No potential conflicts of interest relevant to this article were reported.
Author Contributions. J.Q.P. was involved in study conception, design, and conduct; data analysis and interpretation; writing the first draft; and editing. N.M. was involved in data collection and processing and in manuscript editing. M.F. conducted statistical analyses and drafted the statistical analysis section. M.L. conducted statistical analyses. W.D.R. was involved in data collection and processing and in manuscript editing. E.B. was involved in data collection and processing and in manuscript editing. A.F. was involved in study design, data collection, and processing. A.V. was involved in data collection and processing and in manuscript editing. P.C. contributed to study design, data interpretation, and manuscript editing. J.K. contributed to study design and data interpretation and to manuscript editing. K.K.V. was involved in study conception, design, and conduct; data analysis and interpretation; and manuscript editing. All authors approved the final version of the manuscript before submission. M.F. and M.L. are the guarantors of this work and, as such, had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Prior Presentation. Portions of this study were presented during an oral session at the Annual Meeting of The Obesity Society and the American Society for Metabolic and Bariatric Surgery, Nashville, TN, 11–15 November 2018.
Handling Editors. The journal editors responsible for overseeing the review of the manuscript were John B. Buse and David Simmons.
Funding Statement
This study was supported by the National Institute of Diabetes and Digestive and Kidney Diseases (R01DK098707) and Oregon Clinical & Translational Research Institute, which is supported by the National Center for Advancing Translational Sciences, National Institutes of Health, through grant UL1TR002369. N.M. received funding from the National Institute of Child Health and Human Development, grant K23HD069520-01A1. MRI/MRS was conducted at Oregon Health & Science University's Advanced Imaging Research Center, which has been supported by National Institutes of Health grants S10-OD018224 and S10-OD021701.
Supporting information
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