Our findings extend those found by Stunkard and colleagues,9 who examined a cohort of children during their first 2 y of life and found no statistical relationship between the body weights of parents and offspring. Their results challenged what seems to be a widely accepted belief that genetic influences on obesity are exerted throughout the life cycle.
We found the emergence of statistically significant correlations between the BMI of children beginning at the age of 7 and their parents. This finding indicates that familial factors, rather than influencing children and adults independently, find expression in later childhood.
We also found that children with two overweight parents had a significantly different BMI growth pattern from birth to age 8 compared to children with no or only one overweight parent. While a number of studies show that having an obese parent increases the risk of obesity in adulthood,7,16,17,18 less is known about the emergence of this relationship in early childhood.
In addition to examining correlations between the BMI of parents and children, another way to explore the influence of familial factors on BMI is by examining longitudinal patterns of BMI changes in children over time. Klesges and colleagues2 found parental adiposity status to be a predictor for accelerated rates of growth in children aged 3-5. Our findings extend those of Klesges and colleagues by including children from birth through age 8.
A review of the literature reporting familial relationships (mother-child, father-child, parent-child) for the BMI shows correlation coefficients that consistently range between 0.20-0.30.8 These values, obtained largely through cross-sectional studies3,19 in which data on children of various ages were included, are in general agreement with those reported in this study for children at least 7 or 8. Given the consistency of a familial BMI correlation across studies, the question is raised as to why our prospective study demonstrated these coefficients only beginning with age 7. In other words, since there are familial effects on a child's relative growth rate, why are they not evidenced earlier?
One way of understanding the generally low parent-child BMI correlations before children reach the age of 7 is to take into account the phenomenon of 'adiposity rebound.' As described by Rolland-Cachera and colleagues,20 BMI increases over the first year of life and decreases thereafter until between the ages of 5½ and 7, when it begins to climb again. Figure 1 reveals that the BMI of the children with two overweight parents experience this rebound earlier (eg begin to increase at age 3-4), whereas the mean BMI of the children with no overweight parents does not exhibit adiposity rebound until at least age 6. With the BMI of some children increasing and some, over the same span of years decreasing, a 'washout' effect could obscure the detection of significant correlations.
Another possible reason for the low correlations between parents and their children over the first 6 y of life is that there may be a certain time span necessary before the effects of the environment, such as eating and activity habits, are observed in individuals carrying a particular genetic make-up. The emergence of diabetes type 2 may be an example of this pattern of genetic expression mediated via interactions with the environment.21
Finally, the low correlations between parents and their children over the first 6 y of life may be because the BMI is not an accurate enough measure of fatness; other indexes of adiposity might correlate more highly. However, despite some controversy about the utility of the BMI vs other measures of adiposity, the consensus (including conclusions from the International Obesity Task Force22) is that the BMI is a reasonable measure with which to assess fatness in children and adults.22,23,24,25 In this study, the correlations for parents and children using other anthropometric measures of adiposity were almost always lower than BMI correlations and showed no consistent pattern over more than one time period. Correlations for midarm circumference between fathers and sons beginning at age 5 provided the only exception to this. By contrast there was an absence of significant correlations between father-daughter midarm circumference at these same ages.
The finding of no overall significant parent-child correlations for the adiposity measures of triceps skinfold and subscapular skinfold is puzzling. Other researchers have demonstrated moderately high correlations between the BMI and both triceps and subscapular skinfolds.23,26 However, especially with young children, skinfolds can be difficult to measure.26,27 Also, the great deal of variation in fat distribution in both children and adults further limits the likelihood of any particular adiposity measure accurately reflecting the totality of an individual's adiposity status.
One unexpected finding was that children with only an overweight mother had higher mean BMI over time and exhibited a growth pattern more similar to that of two overweight parents than the group with fathers only overweight, although these differences did not reach statistical significance. Theoretically, there should be no difference between the effects of genes for overweight from one's mother and from one's father, as each parent contributes half of his/her genes to the child. While the effects of heredity and the environment cannot be distinguished in this study, this greater similarity may be due to children's food environments usually being shaped more by mothers than fathers. Perhaps mothers are more often involved in food preparation and share more meals with their children than fathers do. Interestingly, in the study of adopted children aged 7-13 y by Sorenson and colleagues,28 the BMI of the adoptees showed a weak correlation with their adoptive mothers but no correlation at all with their adoptive fathers.
This study has several limitations. First, while we have demonstrated the relationship between parental and childrens' BMI, we cannot draw any conclusions regarding how much of the relationship is due to genetic or environmental factors, or an interaction between them. In other words, parental adiposity represents a cumulative influence of not only genetic make-up but also lifestyle and behavior. It is beyond the scope of this current study to include measures of lifestyle and behavior in these analyses.
An additional limitation stems from the fact that data are only available for children through age 8. Also, because the sample was predominantly well-educated, white and middle class, generalizability to other socioeconomic and ethnic classes is unclear. Finally, the sample size may have been too small to detect some significant parent-child correlations before age 7. In particular, there were not enough children with two overweight parents to explore whether parent-child BMI correlations in this group would be higher than when based on mean BMI for the entire sample.
In summary, this study extends the findings of Stunkard and colleagues9 by prospectively measuring parent-offspring BMI correlations over nine yearly time periods and showing that parental influences on the child's BMI first emerge at age 7. Our findings also suggest that adiposity status of both parents significantly affects their child's BMI growth patterns. Interestingly, this two-parent influence also begins at age 7.
These findings support the hypothesis that familial factors affecting the development of adiposity emerge at specific ages. Longitudinal studies designed to measure environmental influences on adiposity are needed to help design programs to prevent or limit the expression of these familial factors.
Our findings extend those found by Stunkard and colleagues,9 who examined a cohort of children during their first 2 y of life and found no statistical relationship between the body weights of parents and offspring. Their results challenged what seems to be a widely accepted belief that genetic influences on obesity are exerted throughout the life cycle.
We found the emergence of statistically significant correlations between the BMI of children beginning at the age of 7 and their parents. This finding indicates that familial factors, rather than influencing children and adults independently, find expression in later childhood.
We also found that children with two overweight parents had a significantly different BMI growth pattern from birth to age 8 compared to children with no or only one overweight parent. While a number of studies show that having an obese parent increases the risk of obesity in adulthood,7,16,17,18 less is known about the emergence of this relationship in early childhood.
In addition to examining correlations between the BMI of parents and children, another way to explore the influence of familial factors on BMI is by examining longitudinal patterns of BMI changes in children over time. Klesges and colleagues2 found parental adiposity status to be a predictor for accelerated rates of growth in children aged 3-5. Our findings extend those of Klesges and colleagues by including children from birth through age 8.
A review of the literature reporting familial relationships (mother-child, father-child, parent-child) for the BMI shows correlation coefficients that consistently range between 0.20-0.30.8 These values, obtained largely through cross-sectional studies3,19 in which data on children of various ages were included, are in general agreement with those reported in this study for children at least 7 or 8. Given the consistency of a familial BMI correlation across studies, the question is raised as to why our prospective study demonstrated these coefficients only beginning with age 7. In other words, since there are familial effects on a child's relative growth rate, why are they not evidenced earlier?
One way of understanding the generally low parent-child BMI correlations before children reach the age of 7 is to take into account the phenomenon of 'adiposity rebound.' As described by Rolland-Cachera and colleagues,20 BMI increases over the first year of life and decreases thereafter until between the ages of 5½ and 7, when it begins to climb again. Figure 1 reveals that the BMI of the children with two overweight parents experience this rebound earlier (eg begin to increase at age 3-4), whereas the mean BMI of the children with no overweight parents does not exhibit adiposity rebound until at least age 6. With the BMI of some children increasing and some, over the same span of years decreasing, a 'washout' effect could obscure the detection of significant correlations.
Another possible reason for the low correlations between parents and their children over the first 6 y of life is that there may be a certain time span necessary before the effects of the environment, such as eating and activity habits, are observed in individuals carrying a particular genetic make-up. The emergence of diabetes type 2 may be an example of this pattern of genetic expression mediated via interactions with the environment.21
Finally, the low correlations between parents and their children over the first 6 y of life may be because the BMI is not an accurate enough measure of fatness; other indexes of adiposity might correlate more highly. However, despite some controversy about the utility of the BMI vs other measures of adiposity, the consensus (including conclusions from the International Obesity Task Force22) is that the BMI is a reasonable measure with which to assess fatness in children and adults.22,23,24,25 In this study, the correlations for parents and children using other anthropometric measures of adiposity were almost always lower than BMI correlations and showed no consistent pattern over more than one time period. Correlations for midarm circumference between fathers and sons beginning at age 5 provided the only exception to this. By contrast there was an absence of significant correlations between father-daughter midarm circumference at these same ages.
The finding of no overall significant parent-child correlations for the adiposity measures of triceps skinfold and subscapular skinfold is puzzling. Other researchers have demonstrated moderately high correlations between the BMI and both triceps and subscapular skinfolds.23,26 However, especially with young children, skinfolds can be difficult to measure.26,27 Also, the great deal of variation in fat distribution in both children and adults further limits the likelihood of any particular adiposity measure accurately reflecting the totality of an individual's adiposity status.
One unexpected finding was that children with only an overweight mother had higher mean BMI over time and exhibited a growth pattern more similar to that of two overweight parents than the group with fathers only overweight, although these differences did not reach statistical significance. Theoretically, there should be no difference between the effects of genes for overweight from one's mother and from one's father, as each parent contributes half of his/her genes to the child. While the effects of heredity and the environment cannot be distinguished in this study, this greater similarity may be due to children's food environments usually being shaped more by mothers than fathers. Perhaps mothers are more often involved in food preparation and share more meals with their children than fathers do. Interestingly, in the study of adopted children aged 7-13 y by Sorenson and colleagues,28 the BMI of the adoptees showed a weak correlation with their adoptive mothers but no correlation at all with their adoptive fathers.
This study has several limitations. First, while we have demonstrated the relationship between parental and childrens' BMI, we cannot draw any conclusions regarding how much of the relationship is due to genetic or environmental factors, or an interaction between them. In other words, parental adiposity represents a cumulative influence of not only genetic make-up but also lifestyle and behavior. It is beyond the scope of this current study to include measures of lifestyle and behavior in these analyses.
An additional limitation stems from the fact that data are only available for children through age 8. Also, because the sample was predominantly well-educated, white and middle class, generalizability to other socioeconomic and ethnic classes is unclear. Finally, the sample size may have been too small to detect some significant parent-child correlations before age 7. In particular, there were not enough children with two overweight parents to explore whether parent-child BMI correlations in this group would be higher than when based on mean BMI for the entire sample.
In summary, this study extends the findings of Stunkard and colleagues9 by prospectively measuring parent-offspring BMI correlations over nine yearly time periods and showing that parental influences on the child's BMI first emerge at age 7. Our findings also suggest that adiposity status of both parents significantly affects their child's BMI growth patterns. Interestingly, this two-parent influence also begins at age 7.
These findings support the hypothesis that familial factors affecting the development of adiposity emerge at specific ages. Longitudinal studies designed to measure environmental influences on adiposity are needed to help design programs to prevent or limit the expression of these familial factors.
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