Obesity Epidemiology

428 EPIDEMIOLOGIC STUDIES OF DETERMINANTS OF OBESITYindividuals may be classified as overweight based on their BMI, but have low body fat.Many studies, however, rely on BMI to classify participant weight since more accuratemethods, such as underwater weighing and skinfold measurements, are expensive andlabor intensive, which make them cost prohibitive for many studies. Second, amongchildren, unlike adults, weight gain is not necessarily unhealthy. In fact, most childrenshould be gaining some weight. Therefore, the challenge is to separate healthy fromunhealthy or excessive weight gain. One reason for the lack of consistent findings inpredictors of weight gain and the development of obesity is the fact that dietary intakeand physical activity may be predictive of both healthy and unhealthy weight gain. Furthermore,due to gender-specific changes in fat mass during adolescence, studies thatdo not account for maturational stage may be misclassifying participants if they includeboth males and females in their analyses since body fat increases among females butdecreases among males during adolescence.Dietary DataAn important issue to consider is the difficulty in assessing dietary intake among children.Children must have the ability to think abstractly to provide information on averagedietary intake; thus children younger than approximately 9 to 10 years of age mayprovide information that is not sufficiently accurate for the purpose of identifying predictorsof weight gain. Moreover, although dietary methods such as 24-hour recalls donot require children to provide information on average intake, there is a consensus thatyoung children are not reliable reporters on intake even when it is framed in terms ofconcrete meals consumed recently. 132 Therefore, regardless of the method used to accuratelycapture the dietary patterns of young children, information from multiple informants(i.e., parents, teacher, and child) is necessary. Given that relatively few studies ofearly childhood diet include dietary assessments completed by multiple informants, it isnot surprising that the literature on dietary intake and weight gain is inconsistent; theremay be too much measurement error to identify true predictors of excessive weightgain.Another measurement issue that is important to consider is the fact that the dietaryassessment tools used in most studies assessing childhood dietary intake were not developedto assess long-term dietary patterns that predict weight gain. The most accuratemethods, such as 24-hour recalls, might do a good job at capturing diet on any given day,but since there is considerable day-to-day variation in dietary intake, 133,134 a considerablenumber of nonconsecutive days including both weekdays and weekends would benecessary to assess long-term dietary patterns as predictors of weight gain. Owing tologistical and financial reasons, very few studies are able to collect an adequate numberof 24-hour recalls to capture average dietary intake with sufficient precision to predictsubsequent weight gain. Methods such as food-frequency questionnaires do a better jobat capturing average diet, but the methodology is appropriate only for children who areat least 9-10 years of age. Nevertheless, food-frequency questionnaires may not be sensitiveenough to capture the small imbalances between energy intake and expenditure thatresult in weight gain. Another important methodological issue is that overweight childrenhave been found to underreport their dietary intake 135 and children in overweightprevention interventions have been found to have a tendency to report a desirable dietrather than their true diet at follow-up. 136 Thus, it is likely that some true associationsbetween dietary intake and weight gain are missed due to the dietary methods that areused and that those that have been consistently identified most likely underestimate thetrue associations.