This study examined transitions to overweight and obesity among reproductive-aged women. Several predictors of adverse transitions over a 2-year follow-up period were identified. Among women who were in the normal BMI category at baseline, physical activity levels lower than those recommended by the guidelines25,26
more than doubled the odds of becoming overweight, as did experiencing a live birth in the follow-up interval. Among women who were overweight at baseline, only lower educational level and younger age were associated with transition to the obese category. These findings suggest that the determinants of transitioning from normal weight to overweight and from overweight to obesity in the short term among women of reproductive age are not the same, and they illustrate the need to identify the important behavioral and psychological factors that increase women's risk of transitioning to obesity.
The finding that younger women are more likely than older women to transition from overweight to obesity is consistent with previous longitudinal studies of weight gain. For example, Williamson et al.31
examined weight gain and incidence of overweight in a 10-year follow-up of the population-representative cohort of US adults in the First National Health and Nutrition Examination Survey Epidemiologic Follow-up Study. They found that the absolute gain in BMI declined with increasing age, and the incidence of major weight gain was greatest in women and men 25–34 years of age. Younger women in their study who were already overweight at baseline had the highest incidence of major weight gain. In a similar study conducted in Finland, Rissanen et al.32
found that the incidence of substantial weight gain over approximately 5 years was highest among those 20–29 years of age at the baseline assessment. Lewis et al.33
analyzed weight trends among black and white participants in the Coronary Artery Risk Development in Young Adults (CARDIA) study, using linear, mixed-model regression to partition weight gain into gain associated with age and gain associated with secular trends. The largest age-related increases in weight occurred in the youngest age groups for both women and men. The authors hypothesized that physical activity and energy expenditure levels tend to decrease sharply after the teenage years, although caloric intake remains steady, which results in weight gain. Recognition of increased body weight over the early adult years may eventually lead people to cut back on their energy intake, resulting in a return to energy balance by their 30s.33
Whereas some previous research has examined changes in weight or BMI scores
this study examined transition in BMI categories. This may be more clinically relevant, as clinical guidelines use BMI categories to determine who should be screened for diabetes,36
how optimal cardiovascular health is defined,37
and whom and how intensively to counsel about weight loss interventions.38
Moreover, transition into a less favorable BMI category is associated with excess disease risk for hypertension, high cholesterol, diabetes, and obesity-related cancers,6–8
as well as subsequent declines in cognitive functioning.6,9
In terms of preconception health, women who are overweight or obese have increased risk for a myriad of pregnancy complications and adverse outcomes and should be counseled accordingly. Overweight and obese women who become pregnant also need to be counseled about the newest IOM guidelines for the BMI-specific gestational weight gain goals because excessive prenatal weight gain elevates the risk for numerous maternal and infant complications, and it independently predicts short-term and long-term postpartum weight retention.11
Preventing such transitions in BMI categories is, therefore, an important goal,39
and identifying the predictors of such transitions can be used to identify those at risk of a transition to worse BMI status.
Although this study of adverse transition in BMI category has many strengths, including its prospective design, several limitations should be noted. One limitation is reliance on self-reported height and weight for the computation of BMI. To the extent that self-report is biased, misclassification of some respondents' BMI category could occur. In the general U.S. population, there is evidence that self-report underestimates obesity prevalence.40
Among U.S. women, studies show they tend to overreport their height from 0.04 to 2.53
cm and to underreport their weight from 0.56 to <2
These findings suggest that women may be misclassified into lower BMI categories based on self-reported height and weight; one international estimate is that 8.9% of North American females are categorized too low based on self-report.43
Among women of reproductive-age, however, the target population for this analysis, self-reported height and weight have been found to accurately represent BMI abstracted from medical records.24
Thus, the degree of bias, if any, in BMI classifications in this study is unknown, and future studies based on measured BMI category would be helpful. Future research examining short-term weight change might also consider additional analyses using as the definition of change a defined amount of change in weight, as this would include women who began at the lower end of their weight category and moved to the upper limit of their weight category in the changed category, as well as those who moved into the next category if they had a large increase in weight. Another limitation of the present study is that the dataset did not include weight history or sufficient information about weight gain during past and interim pregnancies to account for these factors in analyses. Also, both the sample and target populations are predominantly white, so finding only marginally significant effects of race/ethnicity for one transition group may reflect insufficient numbers of minority women in the sample. Finally, we have follow-up data over a 2-year time span but cannot examine longer term weight gain.
Our study has several important implications. First, we found that normal weight, reproductive-aged women who are not engaging in recommended levels of physical activity are at risk for becoming overweight or obese. Current U.S. Preventive Services Task Force guidelines recommend that after screening all adults for obesity, intensive counseling should be offered to obese patients about lifestyle modifications, including increasing physical activity.38
However, our findings suggest that engaging in recommended levels of physical activity may offer women protection against transitioning from normal weight to overweight status. Thus, an important public health recommendation may be that this counseling should be extended to nonobese women as a strategy for preventing obesity development among women of childbearing age. Second, adverse BMI transitions are common among normal weight women who experience a live birth. It is important that these women receive counseling about appropriate pregnancy weight gain, strategies for postpartum weight loss, and adopting and maintaining physical activity levels consistent with recommended guidelines. Third, overweight reproductive-aged women with lower educational levels require monitoring for obesity risk. Lower educational levels could be associated with less awareness of the importance of weight status for future health and of the health behaviors that are likely to prevent obesity, such as meeting physical activity guidelines. Fourth, because younger overweight women are at higher risk for transition to obesity in this study and other research,31–33,44
it should not be assumed that younger age is protective. Rather, counseling interventions to promote healthy weight-related behaviors should also target younger women of reproductive age.