Obesity now threatens more people in poor countries than undernourishment. Over 115 million adults in the developing world suffer from obesity-related problems, and these numbers are rising more rapidly than those of the 170 million undernourished, according to the World Health Organization. Many developing countries—including China, Mexico, Brazil, and Togo—suffer from higher rates of obesity than undernourishment. Obesity is rising three times faster in Mexico and Egypt than it is in the United States.

The rapid spread of Western foods and of labor-reducing technology is partly to blame, according to Barry Popkin, a professor of nutrition at the University of North Carolina. However, Popkin also attributes the shift to more sedentary lifestyles fostered by rapid urbanization and typified by unprecedented rates of television viewing in developing countries.

Industrialized countries have had decades to fight obesity and encourage healthier lifestyles, yet they are still struggling to contain the problem. Developing countries lack this experience and thus face the problem chronically unprepared; prevention is therefore crucial, argues Popkin. He urges governments to intervene. Brazil has experienced some success by aggressively promoting active lifestyles through such measures as Agita Mundo (Shake the World) days, where millions of people walk, run, and exercise en masse. But Popkin encourages countries to go further—to place a punitive tax on high-calorie foods and to more strictly regulate advertising of those...