The roots of the obesity epidemic need to be traced back as early in life as possible in order to develop effective means for preventing obesity and its health consequences in the future. The aim of this paper is to examine a broad range of factors that may simultaneously contribute to childhood overweight in a population-based cohort of children followed from birth to 4.5 years, to determine which factors exert the most influence in early life.
The analyses were performed using data from the Quebec Longitudinal Study of Child Development 1998–2002 (QLSCD).
The study follows a representative sample (n=2103) of children born in 1998 in the Canadian province of Quebec.
Measured height and weight were available for 1550 children aged 4.5 years. At 4.5 years, BMI was analyzed using the US CDC sex- and age-specific growth charts. In order to study children at their highest weights at various ages, odds ratios were presented for high birth weight, weight-for-stature at or above the 95th percentile at 5 months, and BMI at or above the 95th percentile at 4.5 years. Monthly weight gain between birth and five months has been analyzed. Children were also evaluated by the Z-score obtained from the standardized weight divided by height. Factors potentially related to children's weight include sex, gestational age and birth rank, breastfeeding, mothers' smoking status during pregnancy, family type at child's birth, and family income before pregnancy and when the children were 5 months and 4.5 years old. Other parental factors such as height and overweight/obesity (based on BMI) and other maternal factors (age, education, immigrant status) were also part of the analysis.
Being in the highest quintiles of weight gain between birth and 5 months, as well as maternal smoking during pregnancy, almost double the odds of being overweight at 4.5 years. Parental overweight or obesity also increased the odds of being overweight at this age, as well as being raised in middle-income or in poor families. A greater proportion of children born to nonsmoking mothers with higher weights (more than 4000 g) were overweight at 4.5 years, the percentage being greatest for those in the highest weight-gain categories from birth to 5 months. The pattern was different for children born to smoking mothers. The greatest proportion of 4.5-year-old overweight children was seen for children born in the normal weight-range category (3000–4000 g) who were in the highest quintiles of weight gain from birth to 5 months, and for children with high birth weights (more than 4000 g) who were in the lowest quintiles of birth-to-5-months weight gain. Children who were overweight at 4.5 years and who had been born to smoking mothers started life with a birth weight around that for the population means, but they gained more weight in the first 5 months of life than did the children of nonsmoking mothers.
This study indicates that behavioral and social factors exert critical influences on the onset of childhood overweight in preschool years. From a population–health perspective, interventions aimed at preventing childhood obesity would do well to target smoking pregnant women, as well as nonsmoking pregnant women at risk for giving birth to high-birth-weight children, paying particular attention to rapid weight gain in the first months of life.
The increase in the prevalence of obesity in recent years has brought this condition to the forefront of the public health agenda in many countries. No simple treatment for this condition is known, nor is it known at what age it is best to intervene. It is thus important to investigate why adults, adolescents and children are getting fatter. The roots of the obesity epidemic need to be traced back as early in life as possible in order to develop effective means for preventing this condition and its health consequences in the future.
Various factors at different periods in early life have been associated with obesity later in life. Birth weight, a crude indicator of growth in utero, is positively related with adult fatness and obesity,1 but it is less clear how birth weight relates to body weight in childhood. Several confounding factors influence this relationship. For example, maternal smoking during pregnancy is related to low birth weight, which has been associated with catch-up growth in the first months of life, which in turn has been associated with overweight and obesity in childhood and later in life.2, 3, 4 Conversely, breast-feeding has been associated with lower weight gain in infancy and less obesity in childhood and adolescence,5, 6, 7, 8 but research on the subject remains inconclusive.9, 10 Similarly, numerous studies performed in various developed countries have demonstrated a negative relationship between socioeconomic status and obesity in adults and adolescents, but a less consistent relationship for children.1 Maternal smoking during pregnancy is more prevalent among mothers of lower socioeconomic status, who give birth to smaller children and breast-feed less. These relationships are complex, and little is known about their overall influence on body-weight changes and overweight development in the preschool years.
The aim of this paper is to examine simultaneously a broad range of factors related to preschoolers' overweight using a population-based cohort of children followed from birth to 4.5 years, to determine which factors exert the most influence in early life.
The analyses were performed using data from the Quebec Longitudinal Study of Child Development 1998–2002 (QLSCD), conducted by Santé Québec, a division of the Institut de la Statistique du Québec (ISQ).8, 11 The study analyzes the role of familial and social factors on child health, cognitive and behavioral outcomes. The study follows a representative sample (n=2103) of children born in 1998 in the Canadian province of Quebec (total population over 7 million, with approximately 70 000 newborns per year). The representative sample was chosen by randomly selecting children born in each public health region in Quebec during 1998 such that the seasonality effect was minimized. Twins, children with major diseases or handicaps at birth, and those who died before the age of 5 months were not part of the study. The children were first seen at 5 months (gestational age), and once a year thereafter. The study is based on face-to-face interviews and involves a set of questionnaires addressed to children's mothers and fathers. Children's questionnaires are answered by the most knowledgeable person, generally the mother. Of the 2103 babies included in the first round of data collection, 1944 remained in the sample by age 4 and 5 years (in 2002) (ages of the children ranged from 44 to 56 months).
Measured height and weight were available for 1550 children aged 4.5 years. Data were weighted by a factor based on the inverse of the selection probability, the probability of nonresponse, the poststratification rate, and the attrition rate, to ensure that the data were longitudinally representative of the same-age children in the population.12 Statistical analyses were based on individuals with no missing values for any of the studied variables. Among the 1550 babies, 1450 (94% of the sample) were included in the sample. We analyzed the impact of missing data by conducting with-and-without analyses. Missing data were excluded from the analyses since they had no impact on the results.
Birth weight was analyzed in four categories: under 2500 g (low birth weight), 2500–2999 g, 3000–4000 g, and over 4000 g (high birth weight). At 5 months, weight-for-stature was defined as weight adjusted for height based on the US Centre for Disease Control (CDC) sex-specific Growth Chart. At 4.5 years, BMI was analyzed using the CDC sex- and age-specific growth charts.13 Monthly weight gain from birth to 5 months (i.e., the ratio of the weights at both ages divided by number of months between them) was analyzed in quintiles. Analysis performed on monthly weight gain between birth and 5 months, 5 months and 1.5 years, 1.5 and 2.5 years, 2.5 and 3.5 years, and 3.5 and 4.5 years indicates that there is a significant statistical association between monthly weight gain between birth and 5 months and overweight at 4.5 years, but not between monthly weight gain for each of the other studied ages above 5 months and overweight at 4.5 years. In order to study the children at the highest weight levels at different ages, odds ratios (OR) were presented for high birth weight, weight-for-stature at or above the 95th percentile at 5 months, and BMI at or above the 95th percentile at 4.5 years. Children were also evaluated by the Z-score obtained from the standardized weight divided by height.
Factors potentially related to children's weight are presented in Table 1, along with their proportion of the sample studied. Sex, gestational age and birth rank were collected from medical records. At 5 months, mothers were asked about their smoking status during pregnancy, family type at child's birth, and family income before pregnancy. Mothers were also asked for family income when the children were 5 months and 4.5 years old. Parental overweight/obesity was based on BMI, calculated from reported parental height and weight (BMI=weight in kg/height2 in meters), and was defined as having a BMI of ‘25 or above’. The duration of breast-feeding was obtained from the 5-month questionnaire and complemented with the 18-month questionnaire, and was broken into two categories: ‘breast-fed at least 3 months’ and ‘breast-fed less than 3 months or not breast-fed’.
Statistical analyses were performed with SAS (version 8.2). All variables were treated as categorical variables. Preliminary associations between the independent variables and weights were verified through a χ2 test on contingency tables. Significant independent variables were included in multivariate analyses. However, some variables were removed because of an inter-correlation with household income. These variables were no longer associated in multivariate analyses, and in these cases, intermediate statistical analyses indicated that the association with household income and the studied variables was stronger than it was with these variables. Crude and adjusted OR estimates, as well as their confidence intervals, were made using logistic regressions. Weighted data were used in the analysis, and the significance level was set at 5%.
Table 1 presents the unadjusted OR for the factors examined according to whether birth weight was over 4000 g, at or above the 95th percentile for weight-for-stature at 5 months, and at or above the 95th percentile for BMI at 4.5 years.
At birth, the odds of weighing more than 4000 g were higher for boys, whereas the odds were lower for first-born children and for children born to smoking mothers. At 5 months, the odds of being at or over the 95th percentile for weight-for-stature were higher for boys, for children born to single mothers, for the poorest children, for children in the highest quintiles of weight gain between birth and 5 months, and for children who were not breast-fed for at least 3 months.
At 4.5 years, the odds of being at the 95th percentile or higher for BMI were lower for children born with weights between 2500 and 2999 g. The odds were higher for children with birth weights of more than 4000 g, for boys, for children born to mothers who smoked during pregnancy, and for children living in poor families before pregnancy and at 4.5 years. Overweight at 4.5 years was also positively related to high-weight-gain in the first 5 months of life, and to parental overweight or obesity. Mother's age, education and immigrant status, as well as mother's and father's height, were not statistically associated with baby weights at the ages studied.
Table 2 presents the adjusted OR for being overweight at 4.5 years. The multivariate analysis was performed for weight gain between birth and 5 months, maternal smoking during pregnancy, parental overweight or obesity, and household income. Data were adjusted for gestational age at birth, child sex and birth weight.
Being in the highest quintiles of monthly weight gain between birth and 5 months (OR 1.8 quintile 4; OR 3.9 quintile 5), as well as maternal smoking during pregnancy (OR 1.8), increased the odds of being overweight at 4.5 years. Parental overweight or obesity also increased the odds of being overweight at this age. The odds doubled for children with one overweight or obese parent, and tripled for children with two overweight or obese parents. Being raised in middle-income (OR 1.6) or in poor (OR 2.5) families also increased the odds of being overweight at 4.5 years.
We performed further analyses to better understand the relationship between maternal smoking during pregnancy, weight gain in the first months of life and overweight at 4.5 years. Figures 1 and 2 present the proportion of 4.5-year-old overweight children by birth-weight and weight-gain categories for those born to nonsmoking mothers and for those born to smoking mothers. For children born to nonsmoking mothers (Figure 1), the data reveal a regular pattern. A greater proportion of children born with higher weights (more than 4000 g) were overweight at 4.5 years, the percentage being greatest for those in the highest weight-gain categories from birth to 5 months. This represents 31% of the 4.5-year-old overweight children born to nonsmoking mothers.
The pattern is different for children born to smoking mothers (Figure 2). The greatest proportion of 4.5-year-old overweight children was seen for children born in the normal weight-range category (3000–4000 g), who were in the highest quintiles of weight gain from birth to 5 months (24% of the 4.5-year-old children with smoking mothers), and for children with high birth weights (more than 4000 g), who were in the lowest quintiles of birth-to-5-months weight gain (23%).
Figure 3 presents the weight-for-height functions between birth and 4.5 years for children at different percentiles of BMI at 4.5 years, according to the population means for these values at each age. For each 4.5-year BMI category, children born to smoking mothers had birth weights lower than the mean, and lower than the one for children born to non-smoking mothers. After birth, in each BMI category, children of smoking mothers gained more weight than children of nonsmoking mothers. Children who were overweight at 4.5 years and who were born to smoking mothers started their life with a birth weight around that for the population means, but they gained more weight than children from nonsmoking mothers in the first 5 months of life. By comparison, children who were overweight at 4.5 years but who were born to nonsmoking mothers began life with higher weights than the population mean, did not gain more weight than the population mean between birth and 5 months, and had lower weights at 4.5 years than did those born to smoking mothers.
This study indicates that when a broad range of factors were taken into consideration simultaneously, maternal smoking during pregnancy, low socioeconomic status at 4. 5 years, and parental overweight or obesity had the largest effect on childhood overweight in the studied population. Parental overweight or obesity, which may indicate a genetic predisposition toward overweight in preschool years, could act in concert with behavioral and social factors to exert a critical influence on the onset of childhood overweight at various stages of child development.
Parental height has previously been reported as being an important factor in offspring overweight.1 This factor, however, was not related to childhood overweight in our sample. It may be that the parents of our children had reached their genetic potential for height because of better socioeconomic conditions while they were growing up, compared with conditions for parents from studies performed in other countries or at other times.
Breast-feeding has also been reported by different authors as being a protective factor against overweight and obesity.5, 6, 7 We did not find this relationship in our study, however. Univariate analysis indicates that breast-feeding reduces the odds of having high weight-for-stature at 5 months, but not the odds of being overweight at 4.5 years. These results are in accordance with those of other authors who have reported no relationship between breast-feeding and overweight in childhood or adolescence.9, 10 Our results are similar to those of Parsons et al.9 which were also based on data from a birth cohort using multiple confounding factors such as maternal smoking during pregnancy, maternal BMI, social class, etc., and which followed children from pregnancy on. These results may be more accurate than those from studies that employed cross-sectional studies, from research that used retrospectively reported data on breast-feeding and on weight and height, and from studies that did not take into consideration maternal smoking during pregnancy or maternal BMI. Breast-feeding clearly should be promoted because it has many benefits for children and their mothers, but from a public health perspective, it is less evident that it can be cited as a protective factor for obesity. Mothers who breast-fed should thus not avoid taking other measures to prevent obesity as their children grow up.
Low birth weight followed by a rapid weight gain in the first months of life, high birth weight, and high weight gain in the first months of life have been associated with obesity later in life.2, 3, 14 Research on low birth weight and high birth weight is generally performed separately, rather than on the whole spectrum of birth weight in a population. In our study, a higher rate of weight gain in the first 5 months of life was related to overweight at 4.5 years, independently of other factors in our multivariate analysis. This result is similar to that of Stettler et al.2 Our study goes further by analyzing in more detail the interrelationships of all these elements at the population level, and it adds the dimension of maternal smoking to the analysis.
Maternal smoking during pregnancy, which is related to lower birth weights, is associated with overweight at 4.5 years. This relationship between maternal smoking during pregnancy and childhood overweight or obesity has been reported in other studies performed in various countries.15, 16, 17 Other research has indicated that maternal smoking during pregnancy may influence obesity through its contribution to lower birth weights, which are followed by periods of rapid weight gain – or catch-up growth – in the first months of life.18 The rate of weight gain between birth and 4 months in the USA has also been associated with overweight at 7 years,2 but this study did not take into account maternal smoking during pregnancy.
In our study, univariate analysis indicates that low birth weight is not per se a risk factor for overweight, since low-birth-weight children generally remain smaller in preschool years. Only a small proportion of these children – the proportion being similar for children born to smoking and non-smoking pregnant women – are overweight at 4.5 years, even when they are in the highest quintiles of weight gain in their first months of life. This finding accords with other studies indicating that children born with lower weights tend to stay smaller in early childhood,19, 20 given that high birth weights are more consistently associated with later obesity than low birth weights.18
With reference to maternal smoking during pregnancy, birth weight, and weight gain in the first months of life, our analysis indicates that three groups of children were the most prone to being overweight at 4.5 years. The first group is made up of high-birth-weight children with nonsmoking mothers, especially children with the greatest weight gain in the first months of life. This group could have been subject to conditions favouring high weight gain during pregnancy that also continue to exert their influence after birth. The second group is made up of normal birth weight children with smoking mothers who are in the highest categories of weight gain in the first months of life. It may be that even though these children belong to the normal-birth-weight category, their growth was initially restricted because of maternal smoking but then caught up after birth. The third group is made up of high-birth-weight children of smoking mothers with the least weight gain in the first months of life. This growth pattern could indicate that even in the presence of maternal smoking during pregnancy, other factors were strong enough to counteract the effect of tobacco, such that body weight increased rapidly even in the absence of catch-up growth. As nearly half of the overweight 4.5-year-old children were in the first group, from a population health perspective, interventions aimed at preventing childhood obesity would do well to target smoking pregnant women, as well as nonsmoking pregnant women at risk for giving birth to high-birth-weight children, paying particular attention to weight gain in the first months of life.
When all BMI categories are considered at 4.5 years, the data show that compared with children born to nonsmoking mothers, those born to smoking mothers gain more weight than the mean in their first months of life. This could be the result of catch-up-growth episodes for these children, even when they are born at weights near the mean of the population. Depending on birth weight, these accelerated-growth episodes will not necessarily predispose them to childhood overweight, which is most often the case for children born at near-the-mean weights than for those born at lower-than-the-mean weights. Children born to nonsmoking mothers did not gain more weight than the means, which could indicate that their weight gains were not the result of episodes of catch-up growth. This finding is important given that the 1958 British Birth Cohort Study concluded that the relationship between maternal smoking and obesity could hold until adulthood.17 Moreover, in the USA, 60% of overweight 5–10-year-old children already have associated cardiovascular disease risk factors.21 In rich countries like Canada, where mean birth weight is increasing,22, 23, 24 we should pay special attention to high-birth-weight children in general and to those who gain more weight after birth in particular.
With reference to the ‘foetal origin hypothesis’, even if a greater proportion of children with high birth weights are at risk of being overweight at 4.5 years, special attention should be devoted to low-birth-weight/high-postnatal-weight-gain children because they may be at risk for various clinical conditions as children and adults.25, 26
Our study included children who could well become, depending on their circumstances, obese in the future. They were born and will be raised in a materially well-off society with a universal health care system. Public health interventions for preventing childhood overweight should prioritize the reduction of maternal smoking during pregnancy, which is related to lower birth weights but also increases the odds of being overweight at 4.5 years. Public health policy should also be directed at preventing high birth weights. After birth, children from families of lower socioeconomic status and those with overweight or obese parents should receive particular attention. Children with these risk factors may be predisposed to lifestyles such as high-caloric diets and physical inactivity.
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L Dubois's research work is supported by the Canada Research Chair Program. This study has been partly financed by the Canadian Institutes of Health Research, Population Health Initiative. This study is funded in part by the Ministère de la Santé et des Services sociaux du Québec (Quebec Ministry of Health and Social Services).
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Dubois, L., Girard, M. Early determinants of overweight at 4.5 years in a population-based longitudinal study. Int J Obes 30, 610–617 (2006). https://doi.org/10.1038/sj.ijo.0803141
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