Previous systematic reviews and meta‐analyses have investigated the relation between childhood abuse and obesity in adult populations ( 19 - 22 ). These have shown that adults who were (sexually) abused in childhood had an increased risk of obesity. The relation between childhood abuse and obesity was not identified in children ( 20 ). Accumulation of adverse life events in childhood, such as abuse, separation of parents, and mental illness or addiction of a household member, also seems to be related to overweight and obesity in adulthood ( 23 , 24 ). Adverse life events often co‐occur, and studying the impact of separate childhood events on overweight status can therefore lead to overestimation of the effect of specific events ( 25 - 27 ). Whether accumulation of adverse life events influences overweight measures already in childhood or whether changes in weight status emerge later in life is still unclear. Therefore, our systematic review investigates whether accumulation of adverse life events is related to measures of overweight in children.

Another category of psychosocial stressors that could be of importance in relation to obesity development is adverse childhood events ( 17 ). Adverse childhood events are defined in this study as events (not caused by medication) arising during childhood (<18 years of age) that can threaten the child's mental or physical well‐being ( 9 , 13 , 18 ). Examples of adverse childhood events are parental divorce, disease or death of a family member, parental incarceration, and childhood maltreatment. A distinctive feature of these adverse childhood events in comparison with the aforementioned psychosocial stressors is that they bring about a change in the child's life instead of being a (rather) stable aspect of it. To underline that this study is not about (rather) stable aspects but about relatively abrupt change in the child's life, we have chosen to use the term “adverse childhood events” instead of the term “adverse childhood experiences.” Although the term adverse childhood experiences is sometimes used to refer to adverse childhood events, it is also used to describe chronic exposures to adversity ( 18 ).

A growing body of literature acknowledges the role psychosocial stressors, external events or conditions that threaten an individual's wellbeing, may play in the development of obesity ( 9 , 10 ). Factors such as problems in the household and poor family functioning (e.g., the division of roles between family members and the level of connectedness, conflict, and the quality of communication within the family) have been found to be related to childhood overweight/obesity ( 9 , 11 ). Low socioeconomic status (SES) has also been identified as being related to (central) adiposity, overweight, and obesity in children ( 12 , 13 ). Psychosocial stressors are suggested to influence children's weight status via children's health behaviors, such as their physical activity level, dietary intake, and sedentary behavior, or via biological processes in the body, such as the stress response or inflammatory processes ( 14 - 16 ).

At the beginning of this century, an estimated 10% of children in the world had overweight or obesity ( 1 ). Children with overweight or obesity are likely to have overweight or obesity as adults ( 2 , 3 ). Overweight is related to diseases such as type 2 diabetes, cardiovascular disease, and cancer and is now one of the leading risk factors to the global burden of disease ( 4 - 6 ). Although the worldwide increase in childhood overweight and obesity seems to have stabilized, prevalence rates remain high, and therefore the threat childhood overweight poses to child as well as adult health has far from disappeared ( 7 , 8 ). Prevention of childhood overweight is thus warranted.

The meta‐analysis was performed using Stata (Stata, College Station, Texas). A random‐effects model was used to pool the risk estimates because of variations in study population, exposure assessment, and adjustment for covariates between studies ( 31 ). Estimates of longitudinal and of cross‐sectional and case‐control studies were pooled separately. Heterogeneity was assumed to be present if the I 2 ‐statistic exceeded 50% or when the chi‐square test was statistically significant ( P < 0.10) ( 31 ). Heterogeneity was explored using subgroup analyses and univariate meta‐regressions ( 31 ). Subgroup analyses were performed for ( 1 ) type of outcome measurement (i.e., overweight/obesity status vs. BMI), ( 2 ) type of exposure measurement (i.e., continuous vs. categorical), ( 3 ) mean age at outcome assessment (0‐6 years, >6‐12 years, and >12‐18 years), ( 4 ) time period included in adverse childhood events assessment (i.e., child's lifetime vs. ≤2 years), and ( 5 ) whether the estimate was adjusted for gender, SES, ethnicity, and/or age (yes/no). Meta‐regressions were performed if ≥10 studies could be included ( 31 ). A significance level of P < 0.05 was used. To investigate the presence of publication bias, a funnel plot of the log odds ratio against its standard error was produced. Egger's test was used to test funnel plot asymmetry if ≥10 studies could be included ( 36 , 37 ). A level of significance of P < 0.10 was used.

If other risk estimates than ORs were reported, such as correlation coefficients or (un)standardized regression coefficients, these were converted to ORs (95% CI) when possible. This was done using information on the exact point estimate, standard error, P value/significance level, t value, standard deviation of the exposure or outcome, and/or sample size of the study ( 31 - 35 ). When the estimate(s) could not be converted due to insufficient information, authors were contacted to provide the necessary information. If multiple risk estimates within one study could be converted, we selected the estimate (in order of importance) ( 1 ) adjusted for gender, SES, ethnicity, and/or age; ( 2 ) presenting the relation between exposure and outcome most compatible with the relation investigated in other studies; ( 3 ) based on the largest sample size; and, finally, ( 4 ) measured at the latest point in time. When ORs were reported for boys and girls separately, both were included in the meta‐analysis.

For the meta‐analyses, odds ratios (ORs) along with their 95% confidence intervals (95% CI) were used. The ORs represent the odds of children with a higher adverse childhood events score to have overweight/obesity or have a higher BMI score compared to children with a lower adverse childhood events score. In case multiple ORs were reported in one article (because several cut points for dichotomization of the adverse childhood events score were used), the OR comparing the smallest difference in adverse childhood events score (e.g., comparing a score of 1 vs. 0 instead of 2 vs. 0) was used to facilitate comparison with studies using a continuous exposure measure.

There is currently no gold standard for quality assessment of observational studies ( 28 ), but the Newcastle‐Ottawa Scale (NOS) ( 29 ) is considered suitable for systematic reviews of these studies ( 30 ). In this systematic review, the NOS, adjusted to study‐specific requirements, was used to assess the methodological quality of the included studies (see Supporting Information S3). Studies could achieve a maximum of nine stars. For cross‐sectional studies, the NOS for cohort studies was used. Because three questions did not apply to cross‐sectional studies, these could maximally be awarded six stars. Quality assessment was performed by two authors (LKE and KJEvW) independently. Discrepancies were resolved by consensus. In case of persistent disagreement, a third author (NS) made the final decision. The interrater agreement on the items of the methodological quality assessment was evaluated by calculating percentage agreement between the raters and Cohen's kappa.

Two authors (LKE and KJEvW) independently determined eligibility of identified articles in a two‐step procedure. First, titles and abstracts were screened. Second, the full text of all potentially eligible articles was screened. Finally, references of included articles were screened for eligibility. Discrepancies between the two authors were resolved by discussion. If consensus could not be reached, a decision was made by a third author (NS). If information needed to determine eligibility of the articles was missing, authors of potentially eligible articles were contacted. Duplicates were removed.

Articles were eligible for this systematic review if they met the following criteria: ( 1 ) Study design: cohort, cross‐sectional, or case‐control study. ( 2 ) Study population: at least 90% consisted of children <18 years from the general population. Studies enrolling children with illnesses or conditions other than overweight/obesity and studies including only children with overweight/obesity and no control group were excluded. ( 3 ) Exposure: A measure containing at least two adverse childhood events in at least two domains (e.g., not only events in one specific domain such as school achievements, housing issues, or maternal health). Only exposure measures containing events in at least two domains were included in order to ensure that no studies assessing the relation between just one specific type of adversity and overweight were included this systematic review. Adverse childhood events are defined in this study as events (not caused by medication) arising during childhood (<18 years of age) that can threaten the child's mental or physical well‐being ( 9 , 13 , 18 ). Studies not focusing on events, but solely assessing (rather) stable aspects of children's lives, such as SES, were not part of the scope of this systematic review. Studies using an adverse childhood events measure that included multiple adverse childhood events as well as (rather) stable aspects of children's lives were included. ( 4 ) Outcome: A measure of childhood overweight/obesity. ( 5 ) Data presentation: The relation between accumulated adverse life events and childhood overweight measures is presented in a quantitative way. ( 6 ) Other: Articles must be full text, peer reviewed, and written in English (see Supporting Information S2).

Visual inspection of the funnel plot of longitudinal studies and the funnel plot of cross‐sectional and case‐control studies included in the meta‐analysis indicated that publication bias was unlikely (Figure 3 ). The result of the Egger's test for cross‐sectional and case‐control studies was in line with the visual inspection (P = 0.63). Fewer than 10 longitudinal studies were included; therefore, funnel plot asymmetry was not statistically tested for longitudinal studies ( 37 ).

Pooling the results of the five longitudinal studies showed a positive association between accumulated adverse life events and overweight measures in childhood (OR [95% CI] = 1.12 [1.01‐1.25]; I 2 =45.2%, P = 0.10; Figure 2 ). The results of four cross‐sectional and case‐control studies could not be included in the meta‐analysis due to insufficient information to recalculate reported estimates into ORs ( 44 , 45 , 53 , 54 ). The results of the remaining cross‐sectional and case‐control studies were heterogeneous (I 2 = 52.0%, P = 0.027; Figure 2 ). Subgroup analyses showed that the study results of five subgroups were homogeneous according to our heterogeneity criteria (Table 4 ). These were the results of studies using a continuous overweight measure (pooled OR [95% CI]: 1.34 [1.11‐1.62]), studies using a continuous adverse childhood events measure (pooled OR [95% CI]: 1.31 [1.01‐1.70]), studies in children >6‐12 years of age (pooled OR [95% CI]: 1.34 [1.11‐1.62]), studies assessing adverse childhood events in the past 0‐2 years (pooled OR [95% CI]: 1.23 [0.95‐1.58]), and studies that did not adjust for gender, SES, ethnicity, and/or age (pooled OR [95% CI]: 1.59 [0.98‐2.57]). Meta‐regressions of the cross‐sectional and case‐control studies were only possible for type of outcome measurement, type of exposure measurement, the mean age of the study population, and whether studies were adjusted for gender, SES, ethnicity, and/or age. Based on the univariate meta‐regression analyses, we could not explain the heterogeneity in the study results (Table 4 ).

The methodological quality of the included studies is presented in Tables 2 and 3 . The methodological quality varied from four to six stars for longitudinal studies, one to three stars for cross‐sectional studies, and three to four stars for case‐control studies. Most methodological flaws (>75% of the studies did not score positively on this item) in longitudinal and cross‐sectional studies were found for ( 1 ) (description of) the representativeness of the exposed cohort of the population under investigation (93% of the studies); ( 3 ) ascertainment of exposure (often based on written self‐report instead of an interview, 93% of the studies); ( 6 ) adjustment for SES, ethnicity, and age (80% of the studies); and ( 9 ) (description of) the adequacy of follow‐up of cohorts (all of the studies). For case‐control studies, most methodological flaws (>75% of the studies did not score positively on this item) were found for ( 2 ) (description of) the representativeness of cases of the population under investigation; ( 6 ) adjustment for SES, ethnicity, and age; ( 7 ) ascertainment of exposure (often based on written self‐report instead of an interview); and ( 9 ) (description of) nonresponse rate of cases and controls (all of the studies for all four items). The interrater agreement on the items of the methodological quality assessment was very good (overall agreement 89% [118/132], Cohen's kappa 0.86).

There was homogeneity with regard to the outcome. All studies used overweight/obesity status ( 38 , 39 , 41 , 46 , 52 , 53 ) or BMI ( 40 , 42 - 45 , 47 - 52 , 54 , 55 ) as a measure of overweight. In most studies, overweight was objectively determined ( 38 - 40 , 42 , 44 - 55 ). One study used self‐reported overweight data ( 41 ), and one study did not clearly describe the assessment method used ( 43 ). In addition to reporting on the relation between accumulation of adverse childhood events and BMI, six studies also reported on the association between accumulation of adverse childhood events and other overweight measures, such as waist circumference, waist‐to‐hip ratio, waist‐to‐height ratio, body fat percentage, and/or skinfold thickness ( 42 , 44 , 45 , 47 , 50 , 51 ). The magnitude and direction of each of these estimates was comparable to the magnitude and direction of the estimates of the relation with BMI.

The study characteristics of the included articles are summarized in Table 1 . Three articles reported both cross‐sectional and longitudinal results ( 49 , 50 , 52 ). Most studies were performed in children between 7 and 13 years ( 40 - 44 , 47 - 55 ), with the exception of two studies in early childhood (i.e., ≤ 5 years) ( 39 , 46 ) and two studies in late adolescence (i.e., ≥ 15 years) ( 38 , 45 ). The percentage of females in 13 of the included studies ranged between 45% and 57% ( 38 - 44 , 46 - 52 , 55 ). Two studies only included boys ( 45 , 53 ), and in one study the gender distribution of the study population was not described ( 54 ). We observed heterogeneity in the measurement of adverse childhood events. Different measurement tools were used, measuring different numbers of adverse childhood events over different time periods. Furthermore, different informants (e.g., the child, mother, or parent) reported on the adverse life events that had occurred to either the child, the family of the child, and/or the mother of the child. Although all studies generated a sum score reflecting the level of adversity the children had experienced, the way in which these scores were created (e.g., as simple sum scores, weighted sum scores, combined with stable aspects such as SES, marital status, and housing issues, or not) differed between studies. Despite the heterogeneity, there was overlap in the types of adverse childhood events assessed in the different studies. The assessed events in the different studies can roughly be divided into 34 types of adverse childhood events that, except for 5 events, were all assessed by more than one study (see Supporting Information S4).

The search identified 5,402 articles, of which 332 were screened full‐text and, finally, 18 were included in the systematic review. These 18 articles contained information on 16 distinct studies, including two studies with longitudinal results ( 38 , 39 ), eight studies with cross‐sectional results ( 40 - 47 ), three studies (five articles) with longitudinal as well as cross‐sectional results ( 48 - 52 ), and three case‐control studies (Figure 1 ) ( 53 - 55 ).

Discussion

In this systematic review and meta‐analysis, we investigated the available evidence for a relation between accumulation of adverse life events and overweight measures in childhood. In longitudinal studies, we found a positive relation between accumulation of adverse childhood events and overweight measures. The results of cross‐sectional and case‐control studies were heterogeneous, but with the exception of one study all showed either a positive or a neutral association. Furthermore, the majority of homogeneous subgroups showed a positive pooled estimate. Thus, both types of studies indicated the existence of a positive relation between accumulation of adverse life events and overweight measures: the more adverse life events children experience, the higher their relative weight.

Subgroup analyses revealed that study results of cross‐sectional and case‐control studies (1) using a continuous overweight measure, (2) using a continuous adverse childhood events measure, and (3) assessing the relation between adverse childhood events and overweight measures in children >6‐12 years old were homogeneous and generated a positive pooled estimate. This shows that accumulation of adverse childhood events is related to higher measures of BMI, that the number of adverse childhood events is positively associated with higher overweight measures, and that the positive relation between accumulation of adverse childhood events and overweight measures is present in primary school children. There was one cross‐sectional study that was performed in children <6 years, and this was the only study to show a negative association between accumulation of adverse childhood events and overweight, suggesting that the relation might change from negative to positive over time (46).

The subgroup analyses further revealed that studies (1) assessing adverse childhood events in the past 0‐2 years and (2) not adjusting for gender, SES, ethnicity, and/or age were homogeneous and generated a nonsignificant pooled estimate. Differences in overweight (measures) in response to adverse childhood events might thus take longer than 0‐2 years to develop. This seems to be confirmed by the fact that the pooled estimate of longitudinal studies, 80% of which have follow‐up periods of ≥2 years, is positive. The subgroup of studies not adjusting for any of the specified covariates included only three studies, of which one study was of fair methodological quality (3 stars) and two studies were of low quality (1 star), limiting our ability to draw firm conclusions about the association within this subgroup.

The positive association identified in this study indicates that children experiencing more acute adversity are more prone to have a higher BMI and develop overweight and obesity than children experiencing less acute adversity. This is in line with the positive association between chronic adversity, e.g., low SES and poor family functioning, and higher BMI and risk of overweight and obesity identified in previous research (11-13). Children who are exposed to more adversity have a lower chance of a healthy weight. Additionally, they are probably more likely to have other adverse health conditions as well (41, 52). Therefore, it is very important to monitor accumulation of events and changes in the social situation of children and their family when children visit the well‐child clinic or regular medical care facilities. Early detection of such accumulation can facilitate proper referral and is thus likely to result in health gains.

Past research and future recommendations In addition to the identification of a positive relation between accumulation of adverse life events and overweight measures in childhood, another important finding of this systematic review is that there is heterogeneity in the measurement of adverse life events between the included studies. Different studies used different measurement tools to assess adverse childhood events. These included different types of adverse childhood events, different numbers of adverse childhood events, and different recall periods. Furthermore, they addressed different people (e.g., the child or parent) and asked them to report on adverse life events in the lives of different actors (e.g., the child or his/her family members). Finally, the adverse childhood events score was constructed in different ways in different studies and sometimes included other, more stable, stressors as well. Researchers investigating the relation between accumulation of adverse life events and overweight measures in childhood are therefore recommended to consider three aspects carefully in future research. The first aspect to consider is which measurement tool to use or, alternatively, which and how many adverse childhood events to include in the study. Currently, the most widely applied tool in this research area for measuring adverse life events in children is the Coddington Life Events Scale, and the most widely applied tool for measuring adverse life events in mothers is the List of Threatening Experiences (56, 57). Both tools are easily retrievable and found to be valid (50, 58). The use of these tools is therefore encouraged. Alternatively, when researchers consider existing tools to be unsuitable for their research, they can construct their own list of adverse childhood events. Researchers are encouraged to look at past research and existing scales to make sure they include all relevant types of adverse childhood events in their list. Further, the number of adverse childhood events in their list should be sufficient (>10 events) for the resulting score to truly reflect which children experienced more (severe) and which children experienced less (severe) adverse life events. Finally, in choosing a measurement tool, it is important to consider the implications of measuring adverse events the child experienced versus adverse events other family members experienced and the implications of the informant used to report on these events. A child only reports on events of which he or she is aware, whereas other informants may also report on events of which the child is unaware. This is especially true when family members are asked to report on events in their family's life instead of the child's life. Thus, events reported by children (and their accumulation) may be more strongly related to weight issues than events that occurred to or were reported by family members. The second aspect that should receive careful consideration is the time period over which participants are asked to recall adverse childhood events. Obviously, the longer the included time period, the less accurate recall is likely to be. However, it is important to keep in mind that weight changes in response to adverse events do not take place overnight. Therefore, a recall period of ≤1 year is strongly discouraged and periods of ≥2 years are advised. Including the child's entire lifetime can also be sensible to get a complete picture of all the adverse life events the child experienced, especially if the informant is the child's mother or parent. The third recommendation is to not combine adverse life events that occurred to participants within a given time frame with more stable stressors, such as low SES and living in a single‐parent household, into one exposure measure. These are very different types of stressors; therefore, testing the influence of these stressors separately is highly recommended. Future research is further recommended to create sum scores of different types of adverse events (such as health events, relationship events, or events of abuse) in addition to sum scores of all experienced events and to test the association between these domain‐specific sum scores and overweight measures. In this way, future research can shed light on whether it truly is accumulation of all adverse events, rather than accumulation of specific types of events, that is related to overweight measures in children. Previous research in this area has shown that experiencing many adverse family health events was related to overweight in adolescents, whereas experiencing many adverse relationship events was not (38). Future research is encouraged to examine whether these findings can be replicated in other populations.