Challenging Dogma - Fall 2007

...Using the social and behavioral sciences to improve the practice of public health.

Monday, December 10, 2007

BMI Report Cards: Bafflingly Misguided and Irresponsible – Alicia Fraser

The public health community has declared an obesity epidemic, both in the United States and abroad (1, 2). Between 1980 and 2000, the prevalence of obesity in the US more than doubled, rising from 15% to 31% of the population (3). In children, the prevalence tripled during that same 20-year period (4). Similar trends have occurred across the world (2) prompting the World Health Organization (WHO) to convene a consultation on obesity to “review current epidemiological information, contributing factors and associated consequences” (5). The WHO review identified a number of adverse health outcomes associated with obesity including coronary heart disease, hypertension, stroke, certain types of cancer, osteoarthritis, sleep apnea, and type 2 diabetes (5).

As the results of the WHO review confirm, the sudden rise in obesity prevalence is certainly cause for concern. Accordingly, there has been a surge of obesity research as well as treatment and prevention measures aimed at curbing this epidemic. Research demonstrates that the etiology of obesity is complex and involves genetic, metabolic, environmental, social, and behavioral factors (6). To date, we know neither how these factors work to cause obesity nor how best to measure, treat or prevent it. Nevertheless, the public health and medical communities have been measuring obesity and have been working to treat and prevent it. Unfortunately, the majority of this work has relied on body mass index (BMI), a poor measure of obesity, and it has focused on just one aspect of the disease’s etiology, individual diet and exercise behaviors. One such intervention, undoubtedly doomed to failure, is the BMI Report Card.

In 2004, Arkansas initiated state legislation to mandate public elementary and secondary schools to include children’s BMI measurements on their report cards (7, 8). Since then, several states have followed suit, establishing programs to measure BMI in schools. If a child's BMI falls in the range considered to be “at risk for overweight” or “overweight” (according to the Centers for Disease Control BMI-for-age charts) then educational materials are sent home with the report card to help the family modify the child's diet and exercise (7). Additionally, parents are urged to seek advice from their family doctor about their child’s weight gain risks. Emphasizing that the BMI measurement is simply a screening test, similar to a vision or hearing test, health officials stress that families need to follow up with a doctor (7). Some criticism has been raised against the program, but there exists a severe lack of understanding regarding the full extent of the BMI Report Card's weaknesses and potential to cause harm.

BMI as a Measurement
To understand this issue, it is first necessary to explore the meaning of obesity and the history of BMI as a measurement. Obesity, which is defined as an excess accumulation of body fat, can be measured in a number of ways (9-11). The simplest and most common measure is BMI, which is calculated as an individual’s weight divided by the square of their height. This measure was first described by Adolphe Quitelet in the 19th century, and was later reinvented and dubbed BMI by Keys in the 1950s (12). It was most recently proposed for use in children by Cole in 1979 (12). In adults, BMI is divided into 5 categories: underweight, normal, overweight, obese, and morbidly obese. In children the raw BMI score is calculated the same as in adults (weight/height2), but then that score is ranked as a percentile in comparison to other children of the same sex and age. The CDC developed BMI-for-age charts based on data from the 2000 National Health and Nutrition Examination Survey (13). The charts divide BMI percentiles into four categories: underweight (<5th>95th percentile).

Though BMI has proved a useful tool for tracking obesity prevalence and incidence trends in populations, it is not a good measure of individual health risk. It has numerous limitations that make it a poor choice for screening individuals (10). For example, it is only a crude proxy for body fat and does not take into consideration muscle mass, bone structure or differences in body fat by ethnicity (14, 15). Additionally, in adults it does not account for sex or age. In children, the complications are even greater since body mass and fat mass change throughout development. These changes make it difficult to attribute a change in BMI to an increase in lean body mass or in fat mass (12). Athletes and others with a high percentage of muscle mass will have a high BMI. On the other hand, those with a normal BMI may still have excessive body fat, particularly if they have a small bone structure and low lean mass percentage. Using BMI as a screening tool means that some children may be falsely identified as overweight while many other children, those truly at risk, may be ignored (16). On a population level these misclassifications even out; on an individual level, they mean missed opportunities to treat children at risk for obesity. real mistakes in targeting children for treatment.

Another important factor to keep in mind when using BMI as a screening tool for obesity is that not all fat is created equally (17). According to a recent study in the European Journal of Endocrinology, “obesity does not necessarily imply disease and similarly obese individuals may manifest obesity-related morbidity or seemingly be in reasonably good health” (17). There is increasing evidence that adverse health outcomes are far more closely associated with central adiposity, or weight accumulated around the waist, than overall body fat (18). For example, cardiovascular disease risk is most associated with abdominal fat and the majority of current studies agree that waist circumference (WC) is probably a better indicator of abdominal fatness and cardiovascular disease risk than either BMI or waist-to-hip ratio (WHR) (19). This is because abdominal fat cells are metabolically different than other fat cells, releasing different amounts of hormones such as leptin and resistin (20). The more we learn about the connection between obesity and health, the more we realize how much there is yet to be understood. It is clear, though, that a simple measurement of weight adjusted by height is extremely limited in assessing individual risk of disease.

Relying on the Health Belief Model
Even if BMI were a perfect measure of health risk in individuals, the BMI Report Card would still fail because it relies on the Health Belief Model (HBM) to change individual behaviors. Though consistently ineffective, the HBM has been used time and again by the public health community since its development in the 1950s as a model to predict individual human behavior based on rational decision making (21). The HBM assumes that behavior is determined by an objective comparison of the perceived costs and benefits of a healthy behavior (21, 22). Following this model, an individual who is convinced of the severity of a disease and of their susceptibility to its health effects will adopt behaviors in order to avoid the disease (21, 22). An important aspect of this model is the assumption that one’s intention to do a behavior leads directly to that behavior being carried out.

The problem with the HBM is that it ignores the fact that people do not always make rational decisions, that they do not always carry out their intentions, and that there are real barriers to healthy behaviors beyond individual perception. In the case of the BMI Report Card, overweight and at risk children are targeted in order to improve their diets and increase their physical activity. Minimal assistance is given in helping children and families make these changes. Simply educating people about the need to exercise and eat healthily will not overcome barriers such as the availability of healthy foods, which varies between poor and affluent neighborhoods (23) or the lack of access to play grounds and organized sporting activities. It also does not address insufficient self efficacy, which is likely needed to make the changes.

Ignoring Social and Environmental Factors
Much of the blame for the obesity epidemic has been placed on an increasingly sedentary lifestyle and the ubiquity of fatty, processed foods that lack nutritional value (5, 11). While these two factors are largely societal in nature, the BMI Report Card and other interventions insist on focusing on individual behavior change. The very presence of an obesity epidemic, however, implies that there are important group level factors contributing to the problem. These factors may be lowered food quality across the population, reduced free time in a society where people routinely work multiple jobs, less physical activity in schools, the pervasiveness of televisions and computers, or even the presence of environmental contaminants capable of inducing obesity.

Obesity is extremely difficult, some would say impossible, to treat on an individual level. A review, published in 2007, of interventions to treat overweight adolescents found no evidence of clinically significant benefits (24). Another review of the same year studied treatments that involved parents and children together, that controlled exercise and diet and that promoted lifestyle changes. This review similarly found that there is limited quality data on the effects of obesity treatment programs and that “no generalisable conclusions can be drawn with confidence” (25). The BMI Report Card ignores this paucity of data and assumes that once overweight children are correctly identified, effective interventions are just a pamphlet away. Prevention of obesity is the best method to reduce current trends precisely because treatment is so difficult. In addition, treatment must be addressed through social and environmental factors, rather than individual behaviors. A 2005 review of prevention strategies for obesity in children reports that “practitioners need to consider the issues impacting on sustainability and environmental change whilst simultaneously addressing behavior change. The interventions identified in this review rarely consider the impact of parents and families’ increasingly complex working and living arrangements, yet the potential for change at the family level in the absence of addressing supportive strategies is likely to be diminished” (26). Unfortunately, the BMI
Report Card initiative ignores this advice.

Many scientists have come to the conclusion that, while social factors such as food quality and lifestyle are important, they are insufficient to fully explain the immense rise in obesity prevalence (6, 27). One interesting hypothesis is that exposure to certain compounds during critical periods of fetal development changes the pattern of gene expression within the DNA resulting in permanent alterations in appetite control, metabolic balance and fuel utilization (28, 29). In this regard, obesity may be less a cause of health outcomes and more like one symptom of a disease that also results in the long list of obesity-associated illnesses.

It is well established that patterns of gene expression are developed early in life, partly through a process of methylation of the DNA code, which essentially silences some genes and not others (28). This process is tightly regulated and orchestrated by minute chemical signals within the body (28, 29). Interference of these signals by compounds such as organotins (30) and bisphenol A (31) have been found to occur in mice, resulting in increased adipose tissue mass and serum cholesterol levels (31). Bisphenol A is a chemical known to leach from the lining of food cans and from polycarbonate plastic, which is used to make consumer products such as baby bottles. Research in this field is just beginning, but early results are strongly suggestive that these chemicals may play an important role in explaining the obesity epidemic.

Causing More Harm than Good
Considering the weaknesses inherent in using BMI as a measurement of obesity, as well as the indication that overall fat mass may not be an important determinant of health outcomes, it is clear that the BMI Report Card initiative is short sighted. While it’s true that having a high BMI as a child is associated with having a high BMI as an adult, screening for this characteristic may be meaningless if having a high BMI as an adult is not indicative of health risk. More importantly, the idea of basing an obesity intervention on changing individual diet and physical activity is completely misguided. Even if we were to assume that diet and exercise are the two most important risk factors for obesity—although, there is no evidence to confirm this—targeting individuals to change those behaviors, particularly through use of the Health Belief Model, will likely fail. It will fail because it’s not targeting the root cause(s) of the problem, which did not originate with individuals. Only an intervention targeting social and environmental factors is likely to be successful.

In light of these arguments, the BMI Report Card is bound to be a tremendous failure. One still might presume, though, that a little more education on diet and exercise and a little nudge in the right direction for overweight children and their parents can’t hurt. Unfortunately, this is far from true. Given that there is a longstanding epidemic of unhealthy body image among adolescents, increased pressure to lose weight and increased awareness of body size is likely to promote eating disorders and depression.
Cultural norms regarding acceptable body size result in body dissatisfaction for large numbers of women and men in the US. Psychological distress as well as discrimination in areas such as employment and healthcare are experienced daily by overweight and obese individuals (32, 33). In response to several studies documenting such mistreatment, Massachusetts is currently considering the addition of overweight and obese as categories in its employment anti-discrimination legislation (32).

Negative attitudes regarding fatness are adopted by children as young as five years old and studies report that preschoolers rank fat peers as the least desirable playmates (34). This negativity of young children towards fat playmates has increased markedly since the 1960s and research shows this rise to be due in part to a rise in antifat attitudes (34, 35). Antifat attitudes refer to the belief that overweight and obese individuals are responsible for their weight (36). Contrary to this popular notion, research strongly suggests that an individual’s weight is biologically programmed to stay within a certain 10-20 pound range. Moving outside of that range is extremely difficult; and maintaining a weight outside that range for an extended period of time is impossible for most people (32, 37, 38). These are biologic phenomena. And yet, antifat attitudes have risen, and with them, increased discrimination towards overweight and obese individuals.

Concerns about weight and body dissatisfaction are highly prevalent among adolescent girls and boys. In a California study, as much as 26% of boys and 35% of girls in the third grade reported wanting to lose weight (39). A troubling side-effect of such body dissatisfaction is actually poorer self-care. A study of 376 teen girls who were at or above the 85th percentile for BMI found that body satisfaction was protective against increased BMI (40). A previous study by the same authors found that lower body satisfaction predicted less physical activity and more unhealthy eating behaviors (40).

Interventions such as the BMI Report Card, in which children are measured and ranked, do not fare well for improving adolescent body image. Such interventions may in fact lead to worse diet and exercise behavior and, subsequently, poorer health outcomes for overweight children. Such programs also bolster the myth that if you’re fat, it’s your own fault. In a culture already obsessed with thinness, where fat people are shunned and discriminated against, the public health community has a responsibility to think carefully about interventions targeting obesity, particularly in sensitive populations such as children. The BMI Report Card fails in this regard, as well.

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