EUREKA: Insights into Human Energy and Weight Regulation from Simple—Bathtub-Like—Models

Introduction: The Obesity Problem

The results of the latest National Health and Nutrition Examination survey reveals that two of every three American adults older than 20 (or 65%) are overweight, with a body mass index (BMI) of more than 25—calculated as weight in kilograms divided by the square of the height in meters. This means that, currently, there are more than 130 million Americans who are overweight enough to begin experiencing health problems as a direct result of that weight. Even more concerning, close to half of them (approximately 60 million Americans) are heavy enough to count as clinically obese (with a BMI greater than 30)—that’s being so overweight that their lives will likely be cut seriously shorter by excess fat (Critser, 2003; Hamid, 2009).

While the complications of obesity may not be as dramatic as those of more direful diseases, such as HIV, its burden—as suggested by the above statistics—is affecting more people and is a source of far more deaths. Indeed, according to the Centers for Disease Control (C.D.C.), obesity now kills five times as many Americans as “microbial agents” (Easterbrook, 2004). And experts predict that if current trends continue—with Americans smoking less but continuing to get fatter—obesity will soon overtake smoking as the primary preventable cause of death (Manson & Bassuk, 2003).

The problem is not just an American problem. The situation is nearly as dismal around the globe, as country after country follows the American lead and grows heavier. According to Dr. Stephan Roessner, a past president of the International Association for the Study of Obesity, “There is no country in the world where obesity is not increasing. Even in developing countries we thought were immune… the epidemic is coming on very fast... In some areas of Africa, overweight children outnumber malnourished children three to one” (Brownell & Horgen, 2004).

In her annual message, Dr. Gro Harlem Brundtland, the Director-General of the World Health Organization, was clearly alarmed:

An Unbalanced Act

The human body, like all systems—whether technological or biological—obeys the laws of thermodynamics, which define the immutable principle of energy conservation. Body weight can increase only if energy intake exceeds expenditure. The upward trend in the population’s weight, thus, suggests that the amount of calories consumed is exceeding those burned-off by a growing proportion of the American population. The obvious question: Is the imbalance caused by eating too much food, not expending enough energy, or both?

There is growing evidence to confirm what many of us don't want to admit: “We're fat because we eat a lot—a whole lot more than we used to” (Nielsen & Popkin, 2004). Analysis of the national food supply data from the U.S. Department of Agriculture (USDA) indicates that per capita food consumption remained relatively steady between 1900 and 1965, albeit with occasional moderate declines—for example, during World War I and the Great Depression. Since 1965, however, the food supply markedly increased, and particularly so in the last three decades. USDA data indicate that between 1980 and 2000, the number of calories consumed per person per day in the United States increased by 20%, from 2,200 to slightly more than 2,700 calories. (The 500 calories increase is the equivalent of one Big Mac per person per day.)

The data on the growth in America’s appetite point to two things: (1) after remaining roughly constant, the number of calories consumed has risen markedly and in lockstep with the recent escalation in obesity (which started to spike up in the U.S. in the early 1980s); and (2) the growth in energy intake is substantial enough to account for a significant chunk of the inflation we are witnessing in our aggregate “fat reserves.”