30-3 What factors predispose some people to become and remain obese?

Obesity can be socially toxic, by affecting both how you are treated and how you feel about yourself. Obesity has been associated with lower psychological well-being, especially among women, and increased depression (de Wit et al., 2010; Luppino et al., 2010; Riffkin, 2014). Obese 6- to 9-year-olds are 60 percent more likely to suffer bullying (Lumeng et al., 2010). And obesity has physical health risks as well. Yet few overweight people win the battle of the bulge. Why? And why do some people gain weight while others eat the same amount and don’t?

Our bodies store fat for good reason. Fat is an ideal form of stored energy—a high-calorie fuel reserve to carry the body through periods when food is scarce—a common occurrence in our prehistoric ancestors’ world. No wonder people in impoverished places have often found heavier bodies attractive. Where food is scarce, plumpness may signal affluence and status (Swami, 2015; Swami et al., 2011).

In parts of the world where food and sweets are now abundantly available, the rule that once served our hungry distant ancestors—When you find energy-rich fat or sugar, eat it!—has become dysfunctional. Pretty much everywhere this book is being read, people have a growing problem. A worldwide study of 188 countries (Ng et al., 2014) revealed that

According to the World Health Organization (WHO), an overweight person has a body mass index (BMI) of 25 or more; someone obese has a BMI of 30 or more. In the United States, the adult obesity rate has more than doubled in the last 40 years, reaching 36 percent, and child-teen obesity has quadrupled (Flegal et al., 2010, 2012). In 1990, no U.S. state had an obesity rate greater than 15 percent. By 2010, no state had an obesity rate of less than 20 percent (CDC, 2012).

In one digest of 97 studies of 2.9 million people, being simply overweight was not a health risk, while being obese was (Flegal et al., 2013). Fitness matters more than being a little overweight. But significant obesity increases the risk of diabetes, high blood pressure, heart disease, gallstones, arthritis, and certain types of cancer, thus increasing health care costs and shortening life expectancy (de Gonzales et al., 2010; Jarrett et al., 2010; Sun et al., 2009). Research also has linked women’s obesity to their risk of late-life cognitive decline, including Alzheimer’s disease and brain tissue loss (Bruce-Keller et al., 2009; Whitmer et al., 2008). One experiment found improved memory performance 12 weeks after severely obese people had weight-loss surgery and lost significant weight. Those not having the surgery showed some further cognitive decline (Gunstad et al., 2011).

SET POINT AND METABOLISM Research on the physiology of obesity challenges the stereotype of severely overweight people being weak-willed gluttons. Once we become fat, we require less food to maintain our weight than we did to attain it. Fat has a lower metabolic rate than does muscle—it takes less food energy to maintain. When an overweight person’s body drops below its previous set (or settling) point, the brain triggers increased hunger and decreased metabolism. The body adapts to what it perceives as starvation by burning off fewer calories and seeking to restore lost weight. Blame your brain for weight regain (Cornier, 2011).

Lean people also seem naturally disposed to move about. They burn more calories than do energy-conserving overweight people, who tend to sit still longer (Levine et al., 2005). Individual differences in resting metabolism help explain why two people of the same height, age, and activity level can maintain the same weight, even if one of them eats much less than does the other.

THE GENETIC FACTOR Do our genes predispose us to eat more or less? To burn more calories by fidgeting or fewer by sitting still? Studies confirm a genetic influence on body weight. Consider two examples:

As with other behavior traits (such as intelligence, sexual orientation, and personality) there is no one “weight gene.” Rather, many genes—including nearly 100 genes identified from a recent DNA analysis of 340,000 people—each contribute small effects (Locke et al., 2015).

THE FOOD AND ACTIVITY FACTORS Genes tell an important part of the obesity story. But environmental factors are mighty important, too.

Studies in Europe, Japan, and the United States show that children and adults who suffer from sleep loss are more vulnerable to obesity (Keith et al., 2006; Nedeltcheva et al., 2010; Taheri, 2004a, b). With sleep deprivation, the levels of leptin (which reports body fat to the brain) fall, and ghrelin (the appetite-stimulating stomach hormone) rise.

384

Social influence is another factor. One 32-year study of 12,067 people found them most likely to become obese when a friend became obese (Christakis & Fowler, 2007). If the obese friend was a close one, the odds of likewise becoming obese almost tripled. Moreover, the correlation among friends’ weights was not simply a matter of seeking out similar people as friends. Friends matter.

The strongest evidence that environment influences weight comes from our fattening world (FIGURE 30.5). What explains this growing problem? Changing food consumption and decreased work-related physical activity. We are eating more and moving less, with lifestyles sometimes approaching those of animal feedlots (where farmers fatten inactive animals). Worldwide, 31 percent of adults (including 43 percent of Americans and 25 percent of Europeans) are now sedentary, which means they average less than 20 minutes per day of moderate activity such as walking (Hallal et al., 2012).

The “bottom” line: New stadiums, theaters, and subway cars—but not airplanes—are widening seats to accommodate the girth growth (Hampson, 2000; Kim & Tong, 2010). Washington State Ferries abandoned a 50-year-old standard: “Eighteen-inch butts are a thing of the past” (Shepherd, 1999). New York City, facing a large problem with Big Apple bottoms, has mostly replaced 17.5-inch bucket-style subway seats with bucketless seats (Hampson, 2000). In the end, today’s people need more room.

These findings reinforce an important finding from psychology’s study of intelligence: There can be high levels of heritability (genetic influence on individual differences) without heredity explaining group differences. Genes mostly determine why one person today is heavier than another. Environment mostly determines why people today are heavier than their counterparts 50 years ago. Our eating behavior also demonstrates the now-familiar interaction among biological, psychological, and social-cultural factors. For tips on shedding unwanted weight, see TABLE 10.2.

385