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Childhood origins of adult obesity
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Dr Tracy McCaffrey and Prof. Barbara Livingston
10.1 - March 2009
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The global prevalence of overweight and obesity continues to increase and recent estimates suggest that if current trends are sustained, approximately half of the UK adult population and over two thirds of children will be overweight or obese by 2050(1). Obesity in childhood has far reaching consequences. Approximately one in five obese 4-year-olds and four in five obese adolescents will become obese adults(2) and diseases typically observed in obese adults (eg impaired glucose tolerance) are now manifesting in children as young as 4 years(3). It is widely acknowledged that obesity is a complex multi-faceted disorder, which requires an integrated approach that addresses not only the physiology of energy balance (food intake and energy expenditure), but also the influences of the micro- and macro-environment; including the areas of individual psychology and societal influences; individual activity and the activity environment; biology; food consumption and food production. To date, feasible and sustainable approaches to prevent further increases in overweight and obesity, let alone attenuate it, have remained largely elusive.
Childhood risk factors for obesity in adulthood…a few of them
There are a number of critical periods in childhood that may act as a trigger for adult obesity, and include birth weight, growth in childhood and dietary and physical activity habits.
Birth weight and child growth
High birth weight is associated with higher body mass index (BMI, kg/m2) in adulthood. However, several studies reviewed by Druet & Ong reported positive associations between birth weight and lean mass in later life(4). Thus associations between high birth weight and high BMI may actually reflect the associations between lean mass rather than adiposity, given that BMI cannot distinguish between fat and lean mass. However, children born to mothers with gestational diabetes (due to high maternal body mass) were more likely to have higher birth weights and were at increased risk of type 2 diabetes in later life(5).
In contrast, a large study in Finland demonstrated that people with low birth weight (<3kg), low BMI at age 2y, but higher BMI at age 11y had the highest risk of coronary heart disease in adulthood(5). In “adiposity rebound”, babies typically acquire more muscle and fat from birth to 1 year and thus BMI increases. In the second year of life, they grow taller and slimmer, until age 6y when they begin to get fatter again(5). The important factor is the age at which the rebound occurs, if it occurs too early (age 4 or 5y) the rates of obesity tend to be much higher compared to children where it occurs at age 7 or 8y(5).
Dietary Habits
Parental food choices and modelling of eating behaviours play a pivotal role in shaping children’s food preferences before and during the critical period of adiposity rebound. At the outset, decisions made by parents whether to breastfeed or formula feed are complex and the implications for future body weight gain cannot be extracted from the myriad of other parental and infant factors. Humans are born with an innate preference for sweet and salty foods (often foods that tend to be higher in energy per gram or energy dense (ED, kJ/g)), and an initial aversion to foods that tend to taste sour and bitter (including vegetables that are low ED). However, these preferences may be altered as a result of early experiences with food. From birth the strongest influence in shaping food habits is the family, and in particular those family members who have responsibility for preparing the meals. It appears that this modelling of food behaviours is more important in the younger pre–school years (2–4y) than later years (4–8y)(6). During the weaning phase, children initially tend to reject new foods, but with repeated exposure this neophobia can be overcome. It is suggested that 8 to 15 exposures to new foods may need to occur before an unfamiliar food becomes fully accepted by children(7).
Recent work has demonstrated that those children who have the highest ED diet at 6-8 years, are twice as likely to have greater fat mass in adolescence than children who consume lower ED diets(8). However advocating a low ED diet for young children may not be practical, as they have smaller stomach capacity and therefore may be unable to consume the amounts of low energy density foods needed to achieve an appropriate energy intake (EI). In conjunction with the types of foods chosen by children, the amount eaten or portion size is also important. Infants are effectively able to self-regulate EI on a milk-based diet. However, self-regulation of food intake by internal satiety cues may start to be over-ridden by environmental cues (including pressure from parents to “clean up their plate”) once a more varied diet is introduced. Data from the USA have demonstrated that the frequency of eating occasions may also influence the magnitude of portion sizes selected. In 4-24 month old children, those children who ate less frequently during the day consumed larger–than–average portions than their counterparts who ate more frequently(9).
Getting the right balance between restricting access to foods and allowing children to respond to appetite cues is a delicate one. As children become more independent, the dietary habits formed in early life tend to prevail, and the use of foods which are normally restricted as a reward for good or bad behaviour (ie “junk foods”) may paradoxically serve to reinforce consumption preferences when away from their parents. Some studies have reported that children and adolescents who regularly eat dinner with family members are significantly less likely to be overweight, and more likely to have healthier eating habits, compared to those who eat less regularly with the family. Studying these various factors is currently difficult. Assessing food intake in free-living situations by self-reporting is known to be inaccurate. But the accuracy obtained by rigid experimental control distances the eating situations from normal life, where environmental cues play an important role.
Physical Activity
In addition to dietary habits being formed in early life, physical activity patterns are also shaped by the family environment, as well as influences from the neighbourhood with regards to safety and opportunities for play. Sedentary behaviours have been consistently linked to a higher risk of obesity in both prospective observational studies and intervention studies. Indeed, randomised controlled trials have reported significant, if modest, decreases in fat mass gain with reductions in TV viewing and have demonstrated better success in reducing the risk of obesity than interventions that have concentrated on either increasing activity levels or changing dietary habits alone(10). These associations prompted the American Academy of Pediatrics to introduce recommendations to limit TV viewing to a maximum of 2 hours per day for children. This may lead to replacing TV viewing with more active pursuits and thus increasing physical activity. However, this may be an over simplistic cause-effect explanation since reducing TV viewing may also have an impact on dietary intake. Some studies have shown that high levels of TV viewing have been associated with greater consumption of high ED foods, sugar-sweetened beverages and lower fruit and vegetable consumption in children. However, at present there is insufficient evidence to conclude whether these consumption patterns are linked directly to TV viewing by providing children with more opportunities to snack and/or whether the consumption patterns of more ED foods are in response to the influence of food prompts, including advertisements, on television. Ofcom has considered the latter possibility sufficiently likely that it has introduced restrictions on the advertising of high fat, sugar and salt (HFSS) foods and drinks to children age 4-15 years. In January 2009 the final phase of the recommendations were enforced, whereby advertising of all HFSS products are banned from children’s channels.
Magic bullet…..
The interactions between dietary factors cannot be separated from each other as well as the influence of physical activity and the wider environment. Therefore no one course of action is likely to solve, let alone arrest, the obesity crisis. This article has highlighted a few factors that are amenable to intervention that may prevent obesity in childhood and subsequently in adulthood. It is crucial that obesity prevention programmes address the influences of the micro- and macro- environment outlined at the beginning of this article. Learning from the successes and failures of past interventions can provide valuable information for future studies on the most effective areas to intervene, provided interventions are appropriately monitored. What is evident from the extensive amount of literature is that it is never too early to intervene and the promotion of healthy behaviours will not be detrimental to children whether the risk of obesity is high or low.
References
- Butland B (2007). Tackling obesities: future choices. Project report. [London]: Dept. of Innovation, Universities and Skills. 161.
- Whitaker RC et al (1997). Predicting obesity in young adulthood from childhood and parental obesity. N Engl J Med; 337(13): 869-73.
- Cali AMG and Caprio S (2008). Obesity in Children and Adolescents. J Clin Endocrinol Metab; 93(11 Suppl 1): s31-36.
- Druet C and Ong KK (2008). Early childhood predictors of adult body composition. Best Pract Res Clin Endocrinol Metab; 22(3): 489-502.
- Barker DJ (2007). Obesity and early life. Obes Rev; 8 (Suppl 1): 45-9.
- Skinner JD et al (2002). Children’s food preferences: a longitudinal analysis. J Am Diet Assoc; 102(11): 1638-47.
- Carruth BR et al (2004). Prevalence of picky eaters among infants and toddlers and their caregivers’ decisions about offering a new food. J Am Diet Assoc; 104(Suppl 1): 57-64.
- McCaffrey TA et al (2008). Energy density of the diet and change in body fatness from childhood to adolescence; is there a relationship? Am J Clin Nutr; 87: 1230-7.
- Fox MK et al (2006). Relationship between portion size and energy intake among infants and toddlers: evidence of self-regulation. J Am Diet Assoc; 106(1 Suppl 1): S77-83.
- Doak CM et al (2006). The prevention of overweight and obesity in children and adolescents: a review of interventions and programmes. Obes Rev; 7(1): 111-36.
Dr Tracy McCaffrey, Research Associate in Human Nutrition
Prof. Barbara Livingstone, Professor of Human Nutrition
Northern Ireland Centre for Food and Health, Biomedical Sciences, University of Ulster
