Background
Weight gain in pregnancy is an essential physiologic adaptation that supports growth and development of a fetus and is distributed among lean mass that includes total body water and fat mass gains. Although gestational weight gain provides a source of energy for the mother and fetus, excess gestational weight gain may underlie reported associations between parity and future metabolic disorders and is linked to postpartum weight retention and insulin resistance. Although weight gain often is proposed as a modifiable variable to mitigate adverse maternal and offspring health outcomes, our knowledge of specific maternal body composition changes with weight gain and the potential metabolic consequences is limited. Furthermore, although gestational weight gain alters maternal body composition, the impact of excess weight gain on fat and lean mass is not well-studied. Understanding the accrual of fat and lean body mass may improve our understanding of the role of excessive gestational weight gain and metabolic dysfunction.
Objective
The purpose of our study was to quantify the relationship between gestational weight gain and maternal fat and lean body mass accrual and to compare fat and lean body mass accrual according to the 2009 Institute of Medicine Guidelines for Gestational Weight Gain in Pregnancy adherence. We hypothesized that exceeding current weight gain guidelines would be associated with greater fat, compared with lean body, mass accrual.
Study Design
This is a secondary analysis of a randomized controlled trial of 49 overweight/obese women; all 49 are included in this secondary analysis. Maternal weight and body composition were collected in early (13 0/6 to 16 6/7 weeks gestation) and late (34 0/7 to 36 6/7 weeks gestation) pregnancy with the use of air densitometry. Correlations were drawn between gestational weight gain and change in fat and lean body mass. We compared change in fat and lean body mass by adherence to the 2009 Institute of Medicine Guidelines for Gestational Weight Gain in Pregnancy. Nonparametric tests and chi-square analyses were performed; a probability value of <.05 was significant.
Results
Early pregnancy body mass index was 30.3 kg/m 2 (interquartile range [IQR], 28.5–35.2 kg/m 2 ); women gained 9.0 kg (IQR, 5.3-13.2 kg). Overweight and obese women were equally likely to gain excess weight (48% vs 35%; P = .6). Weight gain correlated strongly with fat mass change ( r = 0.87; P < .001); women with excess vs adequate vs inadequate weight gain had greater fat mass change overall (5.2 [IQR, 4.2-8.1] vs 0.2 [IQR, –0.4-2.2] vs -2.7 [IQR, –5.2- –0.7] kg, respectively; P < .001) and in all pairwise comparisons. Weight gain also correlated with lean body mass change ( r = 0.52; P = .001), but women with excess vs adequate weight gain had similar lean body mass change (8.4 [IQR, 7.2–10.1] vs 7.8 [IQR, 6.0–8.7] kg; P = .1).
Conclusion
Excess gestational weight gain is associated primarily with maternal fat, but not with lean body mass accrual. Our results may help explain the reason that excess gestational weight gain or fat mass accrual is associated with long-term obesity, metabolic dysfunction, and cardiovascular disease risk.
Weight gain in pregnancy is an essential physiologic adaptation that supports growth and development of a fetus and is distributed among lean mass that includes total body water, lean mass, and fat mass gains. Approximately 35% of gestational weight gain (GWG) is products of conception, which includes the fetus, placenta, and amniotic fluid. The remaining GWG is maternal. Lean body mass accrual occurs throughout pregnancy and is primarily water; only up to approximately 1 kg of lean mass is protein and is accrued primarily in late pregnancy. Fat mass is accrued up to approximately 30 weeks gestation and may continue until delivery. With the use of the 1990 Institute of Medicine (IOM) Guidelines for Weight Gain in Pregnancy, fat mass accrual among women who gain within or above guidelines was greatest among underweight women, followed by normal weight and overweight, with obese women gaining the least fat mass in 1 study, but this trend is not reported consistently and has not been described with the use of the current IOM GWG guidelines.
Although GWG provides a source of energy for the mother and fetus, excess GWG may underlie reported associations between parity and future risk of obesity and other metabolic disorders. Excess GWG is associated with postpartum weight retention and insulin resistance and is routinely targeted as a modifiable outcome, aiming to mitigate the associated short- and long-term adverse perinatal outcomes. Different contributions of fat and lean mass accrual in GWG have not been considered in light of the current IOM GWG guidelines but should be a priority, because more than one-half of overweight/obese women exceed 2009 IOM GWG guidelines. Thus, specifically delineating the accrual of fat mass may improve our understanding of the role of excessive GWG and metabolic dysfunction. We measured the change in fat and lean body mass in a prospective pregnant cohort and hypothesized that exceeding current weight gain guidelines would be associated with greater fat mass, compared with lean body mass.
Materials and Methods
We conducted a secondary analysis of overweight/obese women who were enrolled in a randomized controlled trial that was conducted at a single university hospital from September 2009 to August 2011 and was designed to measure whether omega-3 (fish oil) supplements reduced inflammation in a pregnant population. The primary study measures were performed during 2 outpatient visits in the Clinical Research Unit at MetroHealth Medical Center/Case Western Reserve University. Visits 1 and 2 occurred in early (13 0/7 to 16 6/7 weeks gestation) and late (34 0/7 to 36 6/7 weeks gestation) pregnancy, respectively. MetroHealth Medical Center/Case Western Reserve University Institutional Review Board approval was obtained for this study, and written informed consent was obtained for each woman before any study procedures were performed.
Women were recruited for the original trial if they met inclusion criteria: singleton pregnancy from gestational age 8 0/7 to 16 6/7 weeks (determined by last menstrual period and confirmed or changed by ultrasound imaging at <20 0/7 weeks per standard clinical parameters) and planned to deliver at the clinical site. Women were excluded from the original trial if they had known fetal anomaly, used a nonsteroidal antiinflammatory agent daily, had a preexisting metabolic disorder (hypertension, diabetes mellitus, hyperthyroidism), were HIV positive, used tobacco in pregnancy, used illicit drugs or alcohol, or had a fish allergy or gluten intolerance (randomized controlled trial placebo contained wheat germ oil).
All women who completed primary study procedures remained eligible for the current analysis, and procedures that are specific to the current analysis are described. At visit 1, maternal demographic, medical, and pregnancy history data were collected prospectively. Maternal height without shoes was measured with a stadiometer to the nearest 1.0 mm and weight with a tared hospital gown was measured with a calibrated scale (Toledo, Inc, Toledo, OH) to the nearest 0.01 kg. Body mass index (BMI) was calculated as kg/m 2 . Body composition was estimated with air densitometry (BOD POD; COSMED USA, Inc, Concord, CA), reporting total lean and fat mass to the nearest 0.01 kg. Calculations of percent body fat were specific to early pregnancy and late gestation time points, with the use of previously developed formulas that use a hydration constant to account for the increased contribution of water to lean body mass in late pregnancy.
Our primary exposure was GWG that was calculated by subtracting weight at visit 1 from visit 2. We classified women as overweight (BMI, 25.0–29.9 kg/m 2 ) and obese (BMI, ≥30 kg/m 2 ) based on visit 1 BMI and as having adequate vs excess GWG, according to the 2009 IOM Guidelines for Gestational Weight Gain in Pregnancy. Outcome variables included the change in maternal lean body mass (ΔLBM) and fat mass (ΔFM), also calculated by subtracting visit 1 measurements from visit 2 measurements.
We described the cohort that used nonparametric tests and chi-square for continuous and categoric variables, respectively. We measured Spearman’s correlations between GWG and ΔFM and between GWG and ΔLBM for the full cohort and separately for overweight and obese women, reporting r value. We compared ΔFM and ΔLBM by adherence to 2009 IOM GWG guidelines (inadequate vs adequate vs excess GWG) using Kruskal-Wallis test then pairwise comparisons for significant overall findings. Multivariable regression evaluated the impact of potential maternal characteristics and randomization to omega-3 supplementation on results. Randomization to receive omega-3 supplementation was not a significant covariate in the relationship between GWG and either ΔFM or ΔLBM (data not shown); thus, all results are reported as 1 cohort. A probability value of <.05 was considered significant for all analyses.
Results
Forty-nine women completed all study procedures and were eligible for this analysis. Descriptive characteristics are shown in Table 1 . Overweight and obese women were equally likely to adhere to GWG guidelines ( Table 2 ). Maternal fat and lean body mass were similar in early pregnancy for overweight and obese women, regardless of subsequent adherence to GWG guidelines ( Table 3 ).
| Cohort characteristics | Measure |
|---|---|
| Median maternal age, y [interquartile range] | 26.0 [23–30] |
| Gestational diabetes mellitus, n (%) | 6 (12) |
| Race/ethnicity, n (%) | |
| White | 21 (43) |
| African-American | 17 (35) |
| Other | 11 (22) |
| Parity, n (%) | 37 (76) |
| Median gestational age at study visits, wk [interquartile range] | |
| Visit 1 | 14.6 [13–15.4] |
| Visit 2 | 35.3 [34.6–35.9] |
| Median body mass index: visit 1, kg/m 2 [interquartile range] | 30.3 [28.5–35.2] |
| Institute of Medicine adherence a | Class, n (%) | |
|---|---|---|
| Overweight (n = 23) | Obese (n = 26) | |
| Inadequate | 6 (26) | 10 (38) |
| Adequate | 6 (26) | 7 (27) |
| Excess | 11 (48) | 9 (35) |
| Body composition | Adherence, median [interquartile range] | ||
|---|---|---|---|
| Inadequate | Adequate | Excess | |
| Overweight | |||
| Fat mass | 27.9 [22.9–30.1] | 27.9 [23.4–34.5] | 27.4 [23.6–32.6] |
| Lean body mass | 44.5 [41.6–44.9] | 41.1 [38.9–48.0] | 45.7 [43.2–49.1] |
| Obese | |||
| Fat mass | 47.5 [38.6–58.4] | 39.4 [36.1–44.0] | 38.2 [31.9–42.6] |
| Lean body mass | 50.5 [45.7–56.0] | 56.1 [52.2–59.3] | 51.3 [50.0–55.7] |
Median ΔFM was 1.2 kg (interquartile range [IQR], –1.1 –4.6 kg). GWG was correlated strongly positively with ΔFM in the full cohort ( r = 0.87; P < .001) and among overweight ( r = 0.72; P < .001) and obese ( r = 0.93; P < .001) women ( Figure 1 , A). Median ΔFM was greatest among women with excess vs adequate vs inadequate GWG (5.2 [IQR, 4.2-8.1] vs 0.2 [IQR, –0.4-2.2] vs –2.7 [IQR, –5.2-0.7]; P < .001) and all pairwise comparisons remained significant ( Figure 1 , B).
