The power of physiological needs was vividly demonstrated when Ancel Keys and his research team (1950) conducted a now-classic study of semistarvation. They first fed 36 male volunteers (all wartime conscientious objectors) just enough to maintain their initial weight. Then, for six months, they cut this food level in half. The effects soon became visible. Without thinking about it, the men began conserving energy. They appeared sluggish and dull. After dropping rapidly, their body weights stabilized at about 25 percent below their starting point.
As Maslow might have guessed, the men became food obsessed. They talked food . They daydreamed food. They collected recipes, read cookbooks, and feasted their eyes on delectable forbidden food. Preoccupied with their unmet basic need, they lost interest in sex and social activities. As one man reported, "If we see a show, the most interesting part of it is contained in scenes where people are eating. I couldn't laugh at the funniest picture in the world, and love scenes are completely dull."
The semistarved men's preoccupations illustrate how activated motives can hijack our consciousness, As journalist Dorothy Dix (1861-1951) observed, "Nobody wants to kiss when they are hungry." When we're hungry, thirsty, fatigued, or sexually aroused, little else seems to matter. When we're not, food, water, sleep, or sex just don't seem like such big things in life, now or ever.
Deprived of a normal food supply, Keys' semistarved volunteers were clearly hungry. But what precisely triggers hunger? Are the pangs of an empty stomach the source of hunger? So it seemed to A. L. Washburn. Working with Walter Cannon (Cannon & Washburn, 1912), Washburn agreed to swallow a balloon attached to a recording device (FIGURE 38.1). When inflated to fill his stomach, the balloon transmitted his stomach contractions. Washburn supplied information about his feelings of hunger by pressing a key each time he felt a hunger pang. The discovery: Washburn was indeed having stomach contractions whenever he felt hungry.
Can hunger exist without stomach pangs? To answer that question, researchers removed some rats' stomachs and created a direct path to their small intestines (Tsang, 1938). Did the rats continue to eat? Indeed they did. Some hunger persists similarly in humans whose stomachs have been removed as a treatment for ulcers or cancer. So the pangs of an empty stomach are not the only source of hunger. What else might trigger hunger?
Somehow, somewhere, your body is keeping tabs on the energy it takes in and the energy it uses. If this weren't true, you would be unable to maintain a stable body weight. A major source of energy in your body is the blood sugar glucose. If your blood glucose level drops, you won't consciously feel this change, but your stomach, intestines, and liver will signal your brain to motivate eating. Your brain, which is automatically monitoring your blood chemistry and your body's internal state, will then trigger hunger.
How does the brain integrate these messages and sound the alarm? The work is done by several neural areas, some housed deep in the brain within the hypothalamus (FIGURE 38.2). This neural traffic intersection includes areas that influence eating. For example, one neural arc (called the arcuate nucleus) has a center that secretes appetite-stimulating hormones, and another center that secretes appetite-suppressing hormones. Explorations of this neural area and others reveal that when an appetit - enhancing center is stimulated electrically, well-fed artimals begin to eat. If the area is destroyed, even starving animals have no interest in food. The opposite occurs when an appetite-suppressing area is stimulated: Animals will stop eating. Destroy this area and animals will eat and eat, and become extremely fat (Duggan & Booth, 1986; Hoebel & Teitelbaum, 1966) (FIGURE 38.3).
Blood vessels supply the hypothalamus, enabling it to respond to our current blood chemistlY as well as to incoming neural information about the body's state. One of its tasks is monitoring levels of appetite hormones, such as ghrelin, a hunger-arousing hormone secreted by an empty stomach. During bypass surgery for severe obesity, surgeons seal off part of the stomach. The remaining stomach then produces much less ghrelin, and the person's appetite lessens (Lemonick, 2002). Other appetite hormones include insulin, leptin, orexin, and PYY; FIGURES 38.3 and 38.4 illustrate and describe how they influence your feelings of hunger.
The interaction of appetite hormones and brain activity suggests that the body has some sort of "weight thermostat." When semistarved rats fall below their normal weight, this system signals the body to restore the lost weight. The rats' hunger increases and their energy output decreases. If body weight rises – as happens when rats are force fed – hunger decreases and energy expenditure increases. In this way, rats (and humans) tend to hover around a stable weight, or set point, influenced in part by heredity (Keesey & Corbett, 1983).
We humans (and other species, too) vary in our basal metabolic rate, a measure of how much energy we use to maintain basic body functions when our body is at rest. But we share a common response to decreased food intake: Our basal metabolic rate drops, as it did for participants in Keys' experiment. After 24 weeks of sernistarvation, they stabilized at three-quarters of their normal weight, although they were taking in only half their previous calories. How did their bodies achieve this dieter's nightmare? They reduced their energy expenditure, partly by being less active, but partly by dropping their basal metabolic rate by 29 percent.
Some researchers have suggested that the idea of a biologically fixed set point is too rigid to explain some things. One thing it doesn't address is that slow, sustained changes in body weight can alter a person's set point (Assanand et al., 1998). Another is that when we have unlimited access to a wide variety of tasty foods, we tend to overeat and gain weight (Raynor & Epstein, 2001). And set points don't explain why psychological factors influence hunger. For all these reasons, some prefer the looser term settling point or set range to indicate the level at which a person's weight settles in response to caloric intake and energy use. As we will see next, these factors are influenced by environment as well as biology.
Body chemistry and environmental factors together influence not only the when of hunger, but also the what-our taste preferences. When feeling tense or depressed, do you crave starchy, carbohydrate-laden foods? Carbohydrates such as pasta, chips, and sweets help boost levels of the neurotransmitter serotonin, which has calming effects. When stressed, even rats find it extra rewarding to scarf Oreos (Artiga et al., 2007; Boggiano et al., 2005).
Our preferences for sweet and salty tastes are genetic and universal, but conditioning can intensify or alter those preferences. People given highly salted foods may develop a liking for excess salt (Beauchamp, 1987). People sickened by a food may develop an aversion to it. (The frequency of children's illnesses provides many chances for them to learn to avoid certain foods.)
Our culture teaches us that some foods are acceptable but others are not. Bedouins enjoy eating the eye of a camel, which most North Americans would find repulsive. North Americans and Europeans also shun horse, dog, and rat meat, all of which are prized elsewhere.
But there is biological wisdom to many of our taste preferences. Environments can influence the human genetics that affect diet and taste. In places where agriculture has produced milk, for example, survival patterns have favored people with lactose tolerance (Arjamaa & Vuorisalo, 2010). And in hot climates (where foods spoil more quickly) recipes often include spices that inhibit the growth of bacteria (FIGURE 38.5). India averages nearly 10 spices per meat recipe; Finland, 2 spices. Pregnant women's food dislikes-and the nausea associated with them-peak about the tenth week, when the developing embryo is most vulnerable to toxins.
Rats tend to avoid unfamiliar foods (Sclafani, 1995). So do we, especially Mean annual temperature those that are animal based. This neophobia (dislike of things unfamiliar) surely was adaptive for our ancestors by protecting them from potentially toxic substances. In time, though, most people who repeatedly sample an initially novel fruit drink or
To a surprising extent, situations also control our eating-a phenomenon psychologists have called the ecology of eating. Here are three situations you may have noticed but underestimated:
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 risk of depression (de Wit et al., 2010; Luppino et al., 2010; Mendes, 2010a) . Obese 6-to 9-year-olds are 60 percent more likely to suffer bullying (Lumeng et al., 2010). And, as we will see, 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 seldom add a pound?
Our bodies store fat for good reasons. 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 that in many developing societies today (as in Europe in earlier centuries) people find heavier bodies attractive: Obesity signals affluence and social status (Furnham & Baguma, 1994; 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. The World Health Organization (WHO) (2007) has estimated that more than 1 billion people worldwide are overweight, and 300 million of them are clinically obese, defined by the WHO as a body mass index (BMI) of 30 or more. (See www.cdc.gov/ healthyweight/assessing/bmi to calculate your BMI.) In the United States, the adult obesity rate has more than doubled in the last 40 years, reaching 34 percent, and child-teen obesity has quadrupled (Flegal et al., 2010).
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). Recent 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).
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 person’s hunger increases and metabolism decreases. Thus, the body adapts to starvation by burning off fewer calories.
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). These 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 the other does.
Do our genes predispose us to fidget or sit still? Studies do reveal a genetic influence on body weight. Consider two examples:
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 a1., 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.
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 38.6). What explains this growing problem? Changing food consumption and activity levels are at work. We are eating more and moving less, with lifestyles approaching those of animal feedlots (where farmers fatten inactive animals). In the United States, jobs requiring moderate physical activity declined from about 50 percent in 1960 to 20 percent in 2011 (Church et al., 2011).
The “bottom” line: New stadiums, theaters, and subway cars-but not airlplanes-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.
We will revisit this lesson in Unit XI’s study of individual differences. There can be high levels of heritability (genetic influence on individual differences in such things as intelligence) 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 pounds, see Close-up: Waist Management.
ASK YOURSELF: Do you feel in touch with your body’s hunger signals? Do you eat when your body needs food? Or do you tend to be more externally influenced by enticing foods even when you’re full?
TEST YOURSELF: You’ve skipped lunch to meet with your guidance counselor so you haven’t eaten anything in eight hours. As your favorite dish is placed in front of you, your mouth waters. Even imagining this may set your mouth to watering. What triggers this anticipatory salivation?
