Contributing Factors

Many factors are involved in the development of childhood obesity with two of the strongest being genetics and the childŐs environment. A study of 504 youth found that the degree of obesity in the family (parents and grandparents) and the degree of overweight in puberty were the most important factors for weight level in adulthood. (Mossberg 1989) The risk of obesity is greatest when both parents are obese. Parental obesity increases risk of obesity by 2- to 3-fold in both overweight and non-overweight children at all ages. (Strauss 1999) Obesity in children under age 3 does not appear to predict future obesity, unless at least one parent is obese. (Moran 1999).

The odds for obesity continuing into adulthood increase with the number of parents who are overweight. Twin studies and adoption studies are frequently used in determining the influence of genetics versus that of environmental factors on the development of obesity. These studies have shown that children with obese biological parents tended to become obese even if adoptive parents were slim (Laitinen 1998). Studies have shown genetic influences on fat mass during childhood and adolescence may contribute 70% to individual differences in BMI. (Faith 1999, Yanovski 1999).

However, genetics alone do not determine adiposity. A child who is genetically inclined to become overweight will develop varying levels of adiposity when exposed to different environments. For instance, a child who is predisposed to obesity can be expected to become very overweight in an environment that limits physical activity and exposes the child to many high fat, high calorie foods. Conversely, if this same child is encouraged to be physically active and is offered a diet high in fruits, vegetables and whole grains, weight control may not be a problem. Thus, parental modeling and other environmental factors may determine the degree of adiposity in youth who are genetically predisposed to obesity. Studies of parental factors on child weight show that these can have a profound effect on decreasing a childŐs ability to control food intake. (See table 3).

Table 3. Parental factors that affect food intake and physical activity among young children

Although the role of resting metabolic rate (RMR) in the development of obesity remains controversial, there is accumulating evidence that obesity more likely develops in individuals with lower basal metabolic rates (BMR). Studies of Pima Indians have shown that low resting energy expenditure predicts the development of weight gain over a 2-year period. (Strauss, 1999)

Other studies involving infants have revealed higher weight gains with low-resting energy expenditure. (Strauss 1999) Other factors that may lead to childhood obesity include maternal famine, low birth weight and fetal hyperglycemia. (Strauss 1999) Despite the fact that there is only a very small likelihood that overweight in infancy will persist into adulthood, some studies suggest that overfeeding in infancy may permanently enhance fat storage (Strauss 1999) Additionally, breastfeeding and duration of breastfeeding may protect against development of excess adiposity in early life. (Strauss 1999).

In addition to eating patterns, children learn from their parents and others in their environment about physical activity. The belief that young children are always naturally active is no longer true. Children watch more television daily, physical education has been markedly reduced in schools, many neighborhoods lack safe areas for walking, household chores are assisted by labor saving machines, and automobile travel has almost exclusively replaced walking or biking. Surveillance of preschool children at play shows that only 11% of play is spent in vigorous activity, with most of the other time spent in sedentary activity. (Strauss 1999) Studies have shown that children who watch the most television were more than 8 times more likely to become obese (Gortmaker 1996). Each factor contributes to the degree of childhood obesity.

Reference:

1. Johnson SJ, Birch LL. Parents' and children's adiposity and eating style. Pediatr 1994;94(5):653-661.
2. Fisher JO, Mitchell DC, Smicikilas-Wright H, Birch LL. Maternal milk consumption predicts the tradeoff between milk and soft drinks in young girls' diets. J Nutr 2000;131:246-250.
3. Cutting TM, Fisher JO, Grimm-Thomas K, Birch LL. Like mother, like daughter: familiar patterns of overweight are mediated by mothers' dietary disinhibition. Am J Clin Nutr 1999;69:608-613.
4. Fisher JO, Birch LL. Restricting access to palatable foods affects childrens' behavioral response, food selection, and intake. Am J Clin Nutr 1999;69:1264-1272.
5. Birch LL, Fisher JO. Development of eating behaviors among children and adolescents. Pediatr 1998;101:539-549.
6. Fisher JO, Birch LL. Fat preferences and fat consumption of 3- to 5-year-old children are related to parental adiposity. J Am Diet Assoc 1995;95:759-764.