Some studies conducted in adults indicated that consuming larger amounts of food in the evening hours was associated with higher body weight. In particular, researchers found that consumption of a late dinner was associated with larger overall energy intake throughout the day; conversely, individuals with larger energy intake in the morning hours tended to have smaller total energy intake . Therefore, circadian variations in energy intake can be assumed to be an important modifiable dietary intake factor in obesity prevention.
In an effort to improve the understanding of a potential relationship between the time of energy consumption and the current childhood obesity epidemic in the U.S., we examined the association of the relative amount of childrens' total daily energy intake consumed in the latter part of the day and their body weight status in a nationally representative sample of 2–18 year old children. As the results of this study indicated, total energy intake varied by age and sex of the children. The proportion of total daily energy consumed in the latter part of the day was associated positively with being classified as overweight in the 6–11 year olds but negatively in the overweight 12–18 year olds. No significant differences were found between the proportion of energy consumed between 8 pm and midnight and body weight status. This lack of association however, may have been due to the limited number of individuals consuming food so late in the day (n = 3,802), which may have affected the statistical power to detect significant differences in the proportion of energy consumed between the time periods.
As reported previously, healthy weight children consume larger amounts of energy during breakfast whereas overweight and especially obese children eat less during that meal [11, 12]. Our findings add to these results in that overweight school-age children also consume higher levels of energy later in the day compared to the healthy weight children. As children get older, they become more independent and not only consume larger amounts of energy but may also change the frequency of eating throughout the day. This phenomenon reflects the structure of children's life. Young children spend a considerable amount of their day in school with structured meal and snack times. It is only when children are in junior high school and high school that they can purchase food more freely at will and may opt not to eat at all during the morning and early afternoon but focus their consumption patterns to the latter part of the day.
Our study indicates a significant association between body weight status and meal or snack consumption in the latter part of the day only in school-age children and adolescents. Surprisingly, children ages 6–11 years old who were classified as overweight consumed larger proportions of total energy in the later in the day whereas overweight adolescents consumed less energy in the same time period. This apparent paradox is not easily explained. The positive association between food consumption later in the day and higher body weight status in the 6–11 year olds is more intuitive and confirm results from studies on the Night Eating Syndrome (NES), which indicate that adults who consume food during the night have higher body weights than individuals who do not eat during night time . One of the underlying mechanisms for this observation could be the lack of physical activity in the evening hours or overeating due to distractions such as watching TV. The energy in meals or snacks consumed later in the day might be stored in the body fat stores, rather than being oxidated to energy for physical movement during the more active hours of the day.
A recent study investigating the characteristics of individuals with NES elucidated that the increased risk for higher body weight status may not be attributable to the NES but to the underlying psychological factors leading to NES . For instance, in a study using body weight matched samples, individuals with NES were found to have altered distribution of food intake throughout the day but also reported higher prevalence of depressed mood, sleep disturbance, disordered eating, perceived stress, decreased quality of life and increased anxiety.
Results of this study are limited by the use of only one-day dietary intake estimates rather than multiple-day food records to estimate average energy intake. However, since the focus of this paper was to examine dietary intake patterns and report on the association between the time of energy consumption and body weight status in the U.S. pediatric population, the use of a single 24-hour recall was appropriate. In addition, the dietary intake data of the sample is limited by the use of a proxy for the diets of young children, which were reported by their caretakers due to the cognitive limitations in individuals less than six years old. Although this procedure to obtain dietary intake data on young children is widely accepted as the most appropriate method, it is limited by the potential for under- or over-reporting of intakes based on the caretakers' characteristics and the likelihood that the caretaker did not personally observe all food intake occasions of the young children.
This study was designed to increase the understanding of the potential importance of the circadian rhythm of food intake on childhood obesity. Heretofore, no data was available on the effect of the time of energy consumption on childhood obesity in sex, age, and ethnic subgroups of the U.S. pediatric population. Although this study was of purely observational nature, results aid in the generation of hypothesis for future studies on childhood obesity treatment and prevention efforts.