Childhood predictors of adult obesity: a systematic review

OBJECTIVE: To identify factors in childhood which might influence the devel opment of obesity in adulthood. BACKGROUND: The prevalence of obesity is increasing in the UK and other dev eloped countries, in adults and children. The adverse health consequences o f adult obesity are well documented, but are less certain for childhood obe sity. An association between fatness in adolescence and undesirable socio-e conomic consequences, such as lower educational attainment and income, has been observed, particularly for women. Childhood factors implicated in the development of adult obesity therefore have far-reaching implications for c osts to the health-services and economy. SEARCH STRATEGY: In order to identify relevant studies, electronic database s-Medline Embase, CAB abstracts, Psyclit and Sport Discus-were searched fro m the start date of the database to Spring 1998. The general search structu re for electronic databases was (childhood or synonyms) AND (fatness or syn onyms) AND (longitudinal or synonyms). Further studies were identified by c itations in retrieved papers and by consultation with experts. INCLUSION CRITERIA: Longitudinal observational studies of healthy children which included measurement of a risk factor in childhood (< 18y), and outco me measure at least ly later. Any measure of fatness, leanness or change in fatness or leanness was accepted. Measures of fat distribution were not in cluded. Only studies with participants from an industrialized country were considered, and those concerning minority or special groups, e.g. Pima Indi ans or children born preterm, were excluded. FINDINGS: Risk factors for obesity included parental fatness, social factors, birth w eight, timing or rate of maturation, physical activity, dietary factors and other behavioural or psychological factors. Offspring of obese parent(s) were consistently seen to be at increased risk of fatness, although few studies have looked at this relationship over lon ger periods of childhood and into adulthood. The relative contributions of genes and inherited lifestyle factors to the parent-child fatness associati on remain largely unknown. No clear relationship is reported between socio- economic status (SES) in e arly life and childhood fatness. However, a strong consistent relationship is observed between low SES in early life and increased fatness in adulthoo d. Studies investigating SES were generally large but very few considered c onfounding by parental fatness. Women who change social class (social mobil ity) show the prevalence of obesity of the class they join, an association which is not present in men. The influence of other social factors such as family size, number of parents at home and childcare have been little resea rched. There is good evidence from large and reasonably long- term studies for an apparently clear relationship for increased fatness with higher birth weigh t, but in studies which attempted to address potential confounding by gesta tional age, parental fatness, or social group, the relationship was less co nsistent. The relationship between earlier maturation and greater subsequent fatness was investigated in predominantly smaller, but also a few large studies. Ag ain, this relationship appeared to be consistent, but in general, the studi es had not investigated whether there was confounding by other factors, inc luding parental fatness, SES, earlier fatness in childhood, or dietary or a ctivity behaviours. Studies investigating the role of diet or activity were generally small, and included diverse methods of risk factor measurement. There was almost no evidence for an influence of activity in infancy on lat er fatness, and inconsistent but suggestive evidence for a protective effec t of activity in childhood on later fatness. No clear evidence for an effec t of infant feeding on later fatness emerged, but follow-up to adulthood wa s rare, with only one study measuring fatness after 7y. Studies investigati ng diet in childhood were limited and inconclusive. Again, confounding vari ables were seldom accounted for. A few, diverse studies investigated associations between behaviour or psych ological factors and fatness, but mechanisms through which energy balance m ight be influenced were rarely addressed. CONCLUSIONS AND RESEARCH PRIORITIES: The major research gap identified by t he current review is the lack of long-term follow-up data spanning the chil dhood to adulthood period. This gap could in part be filled by: (i) follow- up of existing groups on whom good quality baseline data have already been collected; and (ii) further exploitation of existing longitudinal datasets. Many of the risk factors investigated are related, and may operate on the s ame causal pathways. Inherent problems in defining and measuring these risk factors make controlling for confounding, and attempts to disentangle rela tionships more difficult. A given risk factor may modify the effect of anot her, and cumulative effects on the development of obesity are likely, both over time for specific risks, or at any particular time over a range of ris k factors. An additional approach to addressing these issues may be to use large samples on whom more basic measures of risk factors have been collect ed. Specific issues that remain unresolved include: the mechanism by which SES in early life influences obesity in adulthood; whether the relationships between birth weight and maturation and later obe sity persist after accounting for confounding factors; whether any relationships between dietary factors and activity and later fa tness are due to a direct effect, or to tracking in dietary or activity beh aviour; how psychological factors and behaviours influence energy balance, and ther efore fatness. A further neglected area of research is the identification of factors predi cting the maintenance of a healthy relative weight, which may or may not be the opposite of predictors of obesity. The challenge to future research re mains to discern which are the important and modifiable factors, or cluster ing of factors, and effects over time, on the causal pathway to the develop ment of obesity.