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B eginning in the 1980s, the average weight of Americans began to soar, and by 2002, 31 percent were classi-

fi ed as obese. Although the reasons for this epidemic remain controversial, researchers have implicated at least four developments: The fi rst is the decrease in energy expendi-ture throughout the 20th century, following the introduction of automobiles and the re-placement of high-energy blue-collar work by low-energy offi ce occupations. The sec-ond is the growing affl uence of Americans, who could now afford more and better food. The third is the tech-nological transformation of the food industry in the past 30 years that has made available cheap, convenient, high-calorie and tasty foods. The last is the decline through much of the 20th century in extended breast-feeding, which tends to reduce an infant’s chanc-es of obesity in later life.

As a consequence of these shifts, the average weight of Americans increased throughout the century. But because many were underweight in the earlier years, the epidemic of obesity did not become apparent until the 1980s, when these historical de-velopments apparently reached critical mass.Dietary fat has not played a major role in the epidemic. Although clinical trials have shown that reducing dietary fat leads to weight reduction, the average amount con-sumed has declined in recent decades. Heavy television viewing by children is linked to obesity, yet there is no defi nitive proof of a causal relation. The decline in smoking and the popularity of eating out—restaurants typically serve extra-large por-tions—have also been blamed, but the evi-dence remains contradictory.

Susceptibility to obesity tends to decline as income rises, with well-to-do Americans more likely to follow a goal of modish thin-ness. Women tend to be more susceptible than men because they are less physically

active and because many are unable to re-turn to their prepregnancy weight after childbirth. Income disparity may explain why African- and Mexican-Americans have a higher prevalence of obesity than white Americans, although genetic and cultural factors may also play a role. For instance, some data suggest that obesity may be more socially acceptable among blacks than among whites.

The U.S. has been the trendsetter in a worldwide epidemic of obesity. Other devel-

oped countries have followed a similar tra-jectory, although none of them has yet achieved the levels seen in the U.S. Average weights are climbing at a fast rate in devel-oping countries. Accompanying the rising prevalence of obesity is the prospect for a rapidly growing global epidemic of chronic disease, including, most particularly, type 2 diabetes. According to a theory advanced by economist Tomas J. Philipson of the Uni-versity of Chicago and jurist Richard A. Posner of the U.S. Court of Appeals for the Seventh Circuit, the rise in obesity will only come to an end as income and education increase.

Rodger Doyle can be reached at rodgerpdoyle@verizon.net

Sizing UpROOTS OF OBESITY EPIDEMIC LIE IN THE MID-20TH CENTURY BY RODGER DOYLE

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FURTHER READING

Why Have Americans Become More Obese? David M. Cutler,

Edward L. Glaeser and Jesse M. Shapiro in Journal of Economic

Perspectives, Vol. 17, No. 3, pages 93–118; Summer 2003.

The Long-Run Growth in Obesity as a Function of

Technological Change. Tomas J. Philipson and Richard A. Posner in

Perspectives in Biology and Medicine, Vol. 46, No. 3,

Supplement, pages S87–S107; 2003.

The Pediatric Obesity Epidemic: Causes and

Controversies. Arnold H. Slyper in Journal of Clinical

Endocrinology & Metabolism, Vol. 89, No. 6, pages 2540–2547;

June 2004.

DEFINING OBESITY

The National Institutes of Health defi nes obesity as a body mass

index (BMI) of 30 or greater, using the formula of weight in kilograms divided by the square of height in

meters. Underweight is a BMI of less than 18.5; overweight is a

BMI of 25 to 29.9.

Percent of American children who are obese

(data from 1999 to 2000):

Age Range Total Boys Girls

12 to 19 15.5 15.5 15.5

6 to 11 15.3 16 14.5

2 to 5 10.4 9.9 11

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