This study describes the tracking of overweight and obesity using longitudinal data from a child cohort followed from 4 to 9 years of age. The high increase in GO and AO stands out, with prevalences three-fold and two-fold higher at 9 years old than 4 years old. This increase is due to the large number of incident cases compared to the low remission percentage and is observed for all socioeconomic levels, although the prevalence is twice as high in those with low household affluence. It is very important to detect obesity early, since its appearance at 4 or 6 years determines obesity at 9 years. The two indicators of obesity (GO and AO) are complementary, and their combined use in young children maximizes the detection of obesity.
Most studies show that GO increases until 9–10 years and in adolescence decreases until the beginning of adulthood1,2. In our study, the evolution of the prevalence of overweight and GO from 4 to 9 years of age were of similar magnitude and trend with respect to other cross-sectional studies of our environment5,24. In the measurements at 6 and 9 years, we estimated a higher prevalence of GO in boys than in girls. This difference may be due not so much to environmental or behavioural factors but instead to hormonal changes, adiposity rebound, and distribution of body and muscle fat, which vary with age and sex25,26.
In our study, the prevalence of AO increased with age and was similar to that observed by Schröeder et al.27 and lower than the recent data by Aranceta-Bartrina, which estimated a prevalence of 21.5%24. These differences could be explained by the different measurement criteria and cut-off points of the WC used to define AO; we chose the IDF consensus that allows for international comparisons21. International organizations should make an effort to have international reference populations given their importance in the detection of cardiometabolic risk from an early age.
Regarding socioeconomic status, in Western countries, childhood obesity follows a clear gradient, with a higher prevalence in children with lower socioeconomic status28. However, it is not clear that these differences are maintained or continue to increase, because while some studies describe increasing inequality, others note a fairly uniform evolution29. We found higher prevalences in children with low household affluence, though the age 4–9 growth trends of both GO and AO were similar in low-, medium-, and high-affluence children. In Spain, great disparity in numbers has been observed. The study by Albaladejo-Vicente et al.30 based on the national health survey from 1997 to 2017 examined the population aged 5–15 years, showing a decreasing trend of obesity in children of parents of low socioeconomic status, while in girls, the trend was increasing. In a study from Andalusia, the social gradient of childhood obesity by socioeconomic indicators observed in 2011–2012 disappeared by 2015-201631. However, a study conducted in Catalonia based on primary care electronic medical records noted an increase in inequality (estimated ecologically) between 2006 and 201632.
On the other hand, our estimates are consistent with studies suggesting that obesity frequently begins early on and that children with GO and AO tend to stay obese33,34. Children with or without GO at 4 or 6 years in our study tended to remain in the same category at 9 years. We also found that the risk of obesity at 9 years was greater if they had been obese at 4 and 6 years than if they had been obese at only one of the previous measurements. Similar results were reported by other studies, such as the one carried out in Norway, where children with excess weight at 2–4 years had an 11 times higher risk (odds ratio) of being overweight at 5–7 years than children with normal weight35. In Australia, 5-year-old children who were obese were 25 times more likely to be obese 3 years later than those who were not obese36.
To compare the variations and risk of AO, little information is available for this age group in Spain or in other countries. Vogelezang et al.34 found that the persistence of AO was very high at early ages. A recent study by Ochiai et al.37 estimated that half of the children classified with AO remained in the same category in adolescence. We observed even greater persistence of AO; 60–80% of obese children at 4 or 6 years old remained in the same category at 9 (see supplementary material), and the risk of AO at 9 years was 10 times higher if the child had been obese at both 4 and 6 years. These results are in line with other studies, but comparisons should be made with caution given the differences in follow-up time, ages, and classification methods used38,39.
A relevant aspect of important public health implications is the greater probability of having GO at 6 or 9 years in those children who at 4 had AO, but not GO, which represented 3.4% of our analysed children. Considering that children with a normal weight and BMI but with AO have a high cardiometabolic risk, the combined measurement of weight, height, and WC will improve the predictive power to detect this health problem40.
To correctly interpret the results of the study, several limitations must be taken into account. On the one hand, we do not have information on the weight status between birth and 4 years, which prevents the completion of the children’s obesity trajectories. In the constitution of the ELOIN cohort, a moderate bias was observed in the selection of the sample that would affect its population representativeness: children with low educational level and foreign parents had a lower response rate at the baseline measurement20. However, those who participated in the three follow-up measurements maintained their similarity with respect to the sociodemographic variables of the baseline cohort.
As strengths, we highlight the longitudinal design of the study. In addition, the sample is representative of the population of Community of Madrid, even with the selection bias mentioned above. Anthropometric measurements are based on objective criteria and were performed in a standardized way in all measurements, so they are subject to fewer validity errors, such as self-reported measures or those provided by parents41.
In conclusion, GO and AO begin at an early age, are associated with low socioeconomic status, and increase rapidly with age. Obesity at 9 years is closely correlated with having previously been obese, either stably or intermittently, so prevention and management interventions should be established very early. The combined use of GO and AO indicators maximizes the detection of this health problem.