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For many women, pregnancy begets long-term weight gain. Modifiable behaviors that contribute to postpartum weight retention have not been well studied.
Prospective cohort study of 902 women enrolled in Project Viva, examining associations of postpartum television viewing, walking, and trans fat intake with weight retention ≥5kg at 12 months postpartum. Data were collected in 1999–2003 and analyzed in 2005–2006.
At 6 months postpartum, women reported a mean (SD) of 1.7 (1.3) hours of television viewing, 0.7 (0.7) hours of walking, and 1.1% (0.5) of energy intake from trans fat per day. At 1 year, participants retained a mean of 0.6 kg (range: −17.3 to 25.5), and 12% retained at least 5kg. In multivariate logistic regression models, adjusting for maternal sociodemographics, parity, pre-pregnancy body mass index, gestational weight gain, breastfeeding, and smoking, the odds ratio of retaining at least 5kg was 1.24 (95% confidence interval [CI]: 1.06–1.46) per daily hour of television viewing, 0.66 (95% CI: 0.46–0.94) per daily hour of walking, and 1.33 (95% CI: 1.09–1.62) per 0.5% increment in daily energy intake from trans fat. Women who watched less than 2 hours of television, walked at least 30 minutes, and consumed trans fat below the median had an odds ratio of 0.23 (95% CI: 0.08–0.66) of retaining at least 5 kg.
Postpartum television viewing, walking, and trans fat intake were associated with weight retention. Interventions to modify these behaviors may help reduce excess postpartum weight gain and prevent obesity among women.
The puerperal period may represent a critical window for long-term weight gain and the development of obesity.1,2 Compared with weight gain during other periods of life, excess weight retained after giving birth appears to be particularly harmful, as evidence suggests that it is deposited preferentially in central rather than peripheral sites.3,4
Postpartum weight retention is defined as the difference between weight at some time after delivery and weight prior to pregnancy. Previous studies indicate that mean postpartum weight retention is approximately 0.5 to 1 kg at 12 months, with a wide range of weight changes from more than 10 kg lost to more than 20 kg gained.5 Despite this modest mean change, a large subset of women have substantial weight retention.6 The reported proportion of women retaining 5kg or more 6 to 12 months postpartum has ranged from 14% to 25%.6,7
Gestational weight gain is the strongest predictor of weight retention following pregnancy.2,6 Other identified risk factors, including nonwhite race/ethnicity, primiparity, and high pre-pregnancy weight, are not modifiable or occur prior to pregnancy. Diet, physical activity, and inactivity provide three major behavioral targets for optimizing energy balance. Among nonpregnant adults, a large number of studies have examined modifiable lifestyle determinants of weight gain, yet few investigators have examined these factors during the postpartum period.2 In a recent analysis, Olson et al.8 reported that women who ate “much less food” 6 to 12 months postpartum than 0 to 6 months, and those who exercised “often,” were less likely to retain at least 4.55kg (10 pounds) at 1 year. Limitations of that and other studies 5,6,9,10 include behavior measures that are difficult to translate into specific recommendations, no assessment of inactivity, and use of diet quantity, which may be more difficult to modify than diet quality.11 The present study examines associations of television viewing, physical activity (especially walking), and dietary factors including intake of fiber, total fat, and trans fat in the early postpartum period with substantial weight retention at 1 year postpartum. These behaviors were selected a priori because they are associated with weight gain and obesity-related disease risk among men and nonpregnant women,12–16 and are likely to be modifiable.
Women attending their initial prenatal visit at one of eight urban and suburban obstetrical offices in a multi-specialty group practice in eastern Massachusetts were recruited.17,18 Eligibility criteria included fluency in English, gestational age <22 weeks, and singleton pregnancy; 65% eligible women were recruited. All women provided informed consent, and all procedures were approved by a human studies committee and in accordance with ethical standards for human experimentation.19 This study includes data collected in 1999–2003, and analyzed in 2005–2006.
Of the 2128 participating women who gave birth, 1585 enrolled for study continuation beyond 6 months postpartum. Information on both pre-pregnancy and 1-year postpartum weight as well as behaviors at 6 months postpartum was available from 1092 women. Women who became pregnant in the first year postpartum (n =102), missing information on height, race/ethnicity, parity, education, smoking, physical activity, marital, or work status (n =72), and who reported implausible amounts of physical activity (n =6) (walking ≥5 hrs/day, light/moderate activity ≥3 hrs/day, or vigorous activity ≥3 hrs/day) were excluded. The 902 women (57% of 1585) included in this analysis were somewhat more likely to be white (79% vs 70%) and had slight differences in mean pre-pregnancy body mass index (BMI) (24.3 vs 24.9 kg/m2), gestational weight gain (15.6 vs 15.4 kg), walking time (0.74 vs 0.80 hours daily), and daily trans fat intake (1.1 vs 1.2% of daily total energy), but did not differ in television viewing (1.7 vs 1.7 hours daily), compared with the 1585 women potentially eligible.
At 6 months postpartum, participants reported the average weekly hours they spent watching television or videos and in leisure-time physical activity. Physical activity was classified as walking (“for fun or exercise, including to or from work, but not at work”), light/moderate physical activity (“such as yoga, bowling, stretching classes, and skating, not including walking”), and vigorous recreational activities (“such as jogging, swimming, cycling, aerobic class, skiing, or other similar activities”). These questions were derived from the Physical Activity Scale for the Elderly (PASE),20 but instead of using the previous 7 days, women averaged their activity over the previous month, and reported average hours per week rather than both days per week and hours per day. Light and moderate activities were combined. The choice of examples of activities was influenced by the PASE, the Paffenbarger physical activity questionnaire,21 and knowledge of activities common to women in the northeastern U.S. Walking was of primary interest as a measure of physical activity, because of results from Project Viva that time spent walking did not decrease from pre-pregnancy to the postpartum period, whereas both light/moderate and vigorous physical activity did.22 Also at 6 months postpartum, participants completed a brief food frequency questionnaire called PrimeScreen, which included 21 questions about intake of foods and food groups since delivery, including questions quantifying intake of stick margarine, baked products, and deep-fried foods.23 PrimeScreen is reproducible and comparable to estimates of intake from a validated full-length food frequency questionnaire for a number of nutrients including energy-adjusted fiber (r =0.68 for reproducibility over time and r=0.58 for comparability with full length dietary questionnaire), trans fatty acids (r=0.80 and r=0.64), and saturated fat (r =0.76 and r =0.59).23 The Harvard nutrient database, which has been used for several large cohort studies,24 was used to calculate daily intake of nutrients. Intake of fats was adjusted for total energy intake using nutrient density, and nutrient residuals otherwise.11
Women reported their weight on a mailed questionnaire at 12 months postpartum. Weight retention was calculated as the difference between the self reported postpartum and pre-pregnancy weights. In a validation study, self-reported pre-pregnancy weight was compared with clinically measured weights among 170 participants who had a weight recorded in the medical record within 3 months prior to their last menstrual period. The association between self-reported and clinically measured weight was linear. Correlation coefficients (r= 0.99 overall) and mean underreporting of weight (approximately 1 kg overall) did not differ by maternal race/ethnicity, gestational age at enrollment into the study, or weight itself. The very high correlation indicates that ranking of individuals is well preserved. Although a similar validation was not performed postpartum, the magnitude of any underreporting is likely to have been similar at both time points.
Maternal age, race/ethnicity, parity, education, and household income were included in the analysis as covariates, all reported by the women at their first study visit. Pre-pregnancy BMI (kg/m2) was calculated from self-reported height and weight. Pregnancy weights were obtained from the medical record. Pregnancy weight gain was calculated as the difference between the last weight prior to delivery and the self-reported pre-pregnancy weight, and classified as inadequate, adequate, or excessive.25 At 6 months postpartum, women reported smoking habits and employment and completed an Edinburgh postpartum depression questionnaire.26
Associations of participant characteristics with postpartum weight retention were evaluated using t tests, Wilcoxon rank sum, and chi square analyses. Independent effects of 6 month postpartum television viewing, physical activity, and diet on weight retention at 12 months postpartum were studied using multivariable logistic regression. Individual nutrients studied were fiber, glycemic index, total fat, saturated fat, and trans fat. Glycemic index and saturated fat intake were not associated with weight retention. Because intake of fiber, total fat, and trans fat were strongly correlated with each other, each nutrient was evaluated individually and then in combination with the others. Trans fat was studied most extensively, as it was associated with weight retention independent of the other nutrients.
Separate analyses were performed for television viewing, walking, and trans fat intake, and then these three primary predictors were included together in the same model. Each primary exposure was examined as a continuous variable, and in a secondary analysis in quartiles. Covariates were considered based on the review of the literature and included if they were independent predictors of weight retention or confounded associations of behaviors with postpartum weight retention. Included covariates were maternal sociodemographic factors, parity, pre-pregnancy BMI, gestational weight gain, breastfeeding, employment, and smoking. Gestation length was not included as it did not influence results. Postpartum depression, light/moderate and vigorous activity and the time between delivery and reporting of 1-year postpartum weight were investigated to determine whether they altered effect estimates for exposures of interest. Effect modification by parity (0, ≥1), race/ethnicity (white, nonwhite), and pre-pregnancy BMI (< or ≥25 kg/m2) were investigated using interaction terms and stratification.
Additionally, beneficial exposure categories were defined as trans fat intake < median, watching <2 hours of television daily, and walking ≥30 minutes daily. Women reporting 3, 2 or 1 beneficial behaviors were compared with those reporting no beneficial behaviors, adjusting for the same covariates as in the main analysis. All analyses were performed using SAS version 8.2 (SAS institute, Cary NC).
Participating women were 21% nonwhite, including 8% black and 5% Hispanic; 76% had graduated from college. Mean (SD) age was 33.0 (4.7) years and pre-pregnancy BMI 24.3 (4.8) kg/m2. At 6 months postpartum, women reported a mean (SD) of 1.7 (1.3) hours of television viewing, 0.7 (0.7) hours of walking, 0.2 (0.3) hours of moderate physical activity, and 0.2 (0.3) hours of vigorous physical activity per day. Mean (SD) reported daily intake of total fat was 30.7% (7.2) of total energy, and intake of trans fat was 1.1% (0.5) of total energy.
Mean weight retention was 0.6 kg, with a range of −17.3 to 25.5 kg; 111 mothers (12%) retained at least 5 kg. Time between delivery and reporting of 1-year postpartum weight (mean 13.2 months) was not associated with the amount of weight retained. Mothers who retained at least 5 kg were younger, heavier before pregnancy, more likely to be nonwhite, unmarried, primiparous, and have lower income, and to have gained an excessive amount of weight during pregnancy. At 6 months postpartum they reported more hours viewing television and more intake of trans fat (Table 1). In these unadjusted results, postpartum walking, light/moderate activity, vigorous activity, return to work, breastfeeding, rates of depression, and smoking habits did not differ (Table 1). Compared with pre-pregnancy, television viewing did not change (mean [SD] difference 0.05 [1.23] hours/day).22
For each daily hour of television viewing, the adjusted odds ratio (OR) for retaining at least 5kg was 1.24 (95% confidence interval [CI] 1.06–1.46). Trans fat intake similarly increased risk (adjusted OR 1.33, 95% CI = 1.09, 1.62 for each 0.5% of energy from trans fat; Table 2). Adjustment for depression, light/moderate and vigorous physical activity did not alter the detrimental effects of television viewing or trans fat intake (Table 2).
Trans fat intake was correlated with intake of both fiber (Spearman r = −0.55) and total fat (Spearman r = 0.77). Daily intake of fiber and total fat were each associated with postpartum weight retention ≥5 kg, although neither association remained after including both nutrients. For example, the adjusted odds ratio for fiber (0.72, 95% CI = 0.56, 0.94 per 3g/day), was reduced (0.87, 95% CI = 0.63, 1.20) after adjustment for total fat intake. Similarly, the effect of total fat (OR 1.64, 95% CI = 1.18, 2.30 per 10% of energy from fat) was reduced (OR 1.46, 95% CI = 0.96, 2.23) after adjustment for fiber intake. Additional adjustment for physical activity did not alter estimates for fiber or total fat (data not shown). After adjustment for fiber intake, trans fat remained associated with substantial weight retention (OR 1.26, 95% CI = 1.02, 1.56 per 0.5% of energy from trans fat). Because of the high correlation between trans and total fat, including both nutrients in the same model resulted in substantially wider confidence intervals for both.
Walking did not appear to be associated with weight retention on bivariate analysis and after adjustment for age (Table 2). An association of walking with weight retention was found after adjustment for participant characteristics and television viewing (OR 0.66, 95% CI = 0.46–0.95), with little additional change after inclusion of trans fat intake (Table 2). Walking appeared even more beneficial after adjustment for light/moderate and vigorous activity (Model 5, Table 2). In this model neither light/moderate (OR 1.53, 95% CI = 0.77–3.03 per daily hour) nor vigorous activity (OR 1.06, 95% CI = 0.96–1.17 per daily hour) was associated with retaining ≥5kg, perhaps because only 353 (39%) women reported any light/moderate activity and 315 (35%) any vigorous activity. The effect of walking was also substantial when the analysis was restricted to the 441 participants who reported no postpartum light/moderate or vigorous activity (OR 0.47, 95% CI = 0.25–0.88 per daily hour of walking). There was no modification of the effects of television viewing, walking, and trans fat intake by race/ethnicity, parity, or pre-pregnancy BMI (all p values for interaction terms >0.15).
When additionally included, pre-pregnancy television viewing and physical activity were not associated with weight retention, and did not alter the observed significant effects of the three postpartum behaviors (data not shown). When postpartum behaviors were evaluated in categories rather than as continuous variables, associations with postpartum weight retention were generally linear. Adjusted odds ratios for the 4th through 2nd quartiles, each compared with the lowest quartile (OR 1.0) were 0.64 (95% CI = 0.35–1.17), 0.90 (95% CI = 0.47–1.73), and 1.02 (95% CI = 0.55–1.88) for walking; 2.21 (95% CI = 1.14–4.29), 1.57 (95% CI = 0.81–3.05), and 1.13 (95% CI = 0.50–2.52) for television viewing; and 1.92, (95% CI = 1.01–3.66), 1.25 (95% CI = 0.65–2.41), and 0.70 (95% CI = 0.34–1.43) for trans fat intake.
Beneficial categories of the three behaviors were defined as trans fat intake < median (1.06% of total energy), television viewing <2 hours/day (67% of participants), and walking ≥30 minutes/day (56% of participants). Sixty-one (7%) women reported none of the beneficial behaviors, 298 (33%) reported one, 368 (41%) reported two, and 175 (19%) reported all three. Compared with those who reported none, women reporting all three beneficial behaviors had a markedly reduced risk of retaining ≥5kg (OR 0.23, 95% CI = 0.08–0.66). Odds ratios decreased linearly across those reporting 0, 1, 2, or all 3 beneficial behaviors (p for trend <0.001, see Figure).
In this prospective study, television viewing and trans fat intake in the early postpartum period were directly associated, and walking inversely associated, with substantial weight retention at 1 year postpartum. Effects of these three behaviors were additive, as women who watched less than 2 hours of television, walked at least 30 minutes, and consumed below the median amount of trans fat daily had an estimated 77% reduced odds of retaining at least 5kg compared with women who reported none of the beneficial behaviors.
Postpartum weight retention may be more physiologically harmful than weight gained at other times in life. Excess weight retained after pregnancy appears to be preferentially deposited centrally,3,4 and central adiposity is tightly linked to insulin resistance and increased cardiovascular disease risk.27,28 These associations highlight the importance of identifying modifiable risk factors for postpartum weight retention.
Inactivity, primarily television viewing, and physical activity are important and independent determinants of body weight, weight gain, and risk of obesity-related diseases such as type 2 diabetes mellitus.12,13,29 However, few studies of postpartum weight retention have considered physical activity,7,9 and only one included a measure of inactivity.10 Small intervention studies among overweight postpartum women have shown that exercise is safe and promotes weight loss, but the extent to which new mothers are able to maintain rigorous exercise programs is unclear.1,30 In the postpartum period, many women can include walking in their daily routine (e.g., pushing the child in a stroller) and walking is efficacious in reducing disease risks.31–33 In contrast to walking, light/moderate and vigorous activity decreased from pre-pregnancy to postpartum in this cohort, and were not associated with decreased risk of substantial weight retention. It is not clear why more vigorous activities did not also protect against weight retention, as physical activity has been shown to promote postpartum weight loss in other studies.30 It is possible that women who were having more difficulty losing weight either undertook or reported more light/moderate and vigorous activities.
To our knowledge, no reports exist on associations of television viewing with postpartum weight retention. Previous studies among children and nonpregnant adults have related increased television viewing to overweight and obesity.34–38 Moreover, randomized controlled trials among children have shown that television viewing is modifiable and that reduction in television viewing leads to a reduction in overweight.39,40
The few prior studies of diet and weight retention in the postpartum period have focused on diet quantity or energy intake,8–10 but have not investigated individual foods and nutrients, including fats, fiber and glycemic index. Diet quality may be easier to modify than energy intake,11,41 particularly among breastfeeding mothers as concerns remain that energy restriction may adversely influence milk production and infant growth.42 In the present study, higher trans fat intake was associated with increased risk of substantial weight retention. While evidence linking intake of trans fat with adverse blood cholesterol profiles and risk of coronary heart disease is well known, 43,44 recent studies suggest that trans fat intake also is linked with weight, weight gain, and increasing waist circumference in nonpregnant adults,15,16 perhaps by means of increasing systemic inflammation.45–47 Women may now be more informed about their trans fat intake, since as of January 1, 2006, the U.S. Food and Drug Administration requires that all food labels must list the content of trans fat.
The relatively large sample size, prospective data collection, and inclusion of multiple potential confounding variables are strengths of this study. By examining television viewing, walking, and diet simultaneously, this study has demonstrated that each behavior yields incremental benefit. However, although 21% of mothers were nonwhite, their educational and income levels were relatively high, thus results may not be generalizable to socioeconomically disadvantaged populations. In this population, women had somewhat lower weight retention than has been reported elsewhere. Just over half of the eligible study population provided enough information to allow their inclusion in this analysis. Included participants did not substantially differ from the overall population in measured characteristics, but may have differed in their weight retention. Both pre-pregnancy and post partum weight were by self-report, but any underreporting bias would probably have been similar for both pre-pregnancy and postpartum weight. The physical activity questions used here have not been validated in other pregnant populations. Although we analyzed walking according to average daily activity, women reported their physical activity in hours per week; it is not possible to determine whether the most important exposure is by daily or cumulative weekly walking. Associations of television viewing with weight retention may differ among women viewing more television, as is common in other populations. Trans fat intake may be a marker for other unhealthy dietary or other lifestyle behaviors rather than causally associated with weight gain. Because PrimeScreen is a brief screening tool, it may not assess intake of total fat as well as for individual types of fat. Further evaluation of associations of postpartum weight retention with other nutrients such as fiber and total fat is warranted in future studies.
Improved understanding of common and modifiable determinants of weight gain is critical to the design of interventions to prevent obesity. The puerperal period is a time when women are particularly receptive to behavior change recommendations,48 and thus may provide an excellent opportunity to improve weight-related behaviors. Future interventions should test the potential benefits of limited television viewing, reduced trans fat intake, and frequent walking to reduce the risk of substantial weight retention and promote long-term maternal health.
We thank the participants and staff of Project Viva as well as Sheryl Rifas-Shiman for programming support. We appreciate the helpful suggestions regarding study design and manuscript revisions from Dr. Erica Gunderson, Division of Research, Kaiser Permanente of Northern California; and Dr. Walter Willett, Department of Nutrition, Harvard School of Public Health.
This study was supported by grants from the U.S. National Institutes of Health (HD 34568, HL 64925, HL 68041, HD 44807), the Robert Wood Johnson Foundation, Harvard Medical School (the Division of Nutrition and the Robert H. Ebert Fellowship), and the Harvard Pilgrim Health Care Foundation.
No financial conflict of interest was reported by the authors of this paper.
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