Understanding what drives us to eat and what affects food choice will help you understand the complexity of factors that influence eating, especially the effects of ethnicity and societal change. You can then see why foods may have different meanings to different people. In turn, this allows for greater appreciation of food habits that differ from yours.
Two drives, hunger and appetite, influence our desire to eat. These differ dramatically. Hunger, our primarily physical biological drive to eat, is controlled by internal body mechanisms. For example, as nutrients are processed by the stomach and small intestine, these organs send signals to the liver and brain to reduce further food intake. Appetite, our primarily psychological drive to eat, is affected by external food choice mechanisms, such as seeing a tempting dessert or smelling popcorn popping at the movie theater. Fulfilling either or both drives by eating sufficient food normally brings a state of satiety, temporarily halting our desire to continue eating.
The Hypothalamus Contributes to Satiety
Certain chemicals, surgery, and some cancers can destroy either center in the hypothalamus. Without satiety center activity, laboratory animals (and humans) eat their way to obesity. Without feeding center activity, the opposite happens and weight is lost. Admittedly, this concept of a tug-of-war between the feeding and satiety centers is an oversimplification of a complex process. Overall, the entire system depends on the hypothalamus to process the signals generated by nerves throughout the body and influenced in various ways by recent food intake. In reality, the hypothalamus has numerous sites crisscrossed with a network of nerves constantly receiving and passing on information about the body’s nutritional state. In addition, the brain’s cortex controls conscious thought and can overcome signals from the hypothalamic hunger and satiety centers.
Meal Size and Composition Affect Satiety
The effects of stomach expansion from food intake combined with the later intestinal absorption of nutrients during a meal likely reduce our desire to eat more food. These actions of the gastrointestinal (GI) tract contribute to a feeling of satiety. In fact, a meal generally ends before significant amounts of nutrients are made available for metabolism and storage. Putting this information into practice, researchers have recently shown that bulky meals (high in fiber and water) produce much more satiety than do more concentrated meals. As the fiber and water content of the foods increase, humans experience increased satiety and thus do not seek another meal as quickly. Consider how satisfied you would feel if you ate five pieces of whole fruit versus a small serving of French fries (each yielding about 380 kcals).
Hormones Affect Satiety
Hormones and hormone-like compounds in our body influence whether we eat. Hormones that increase hunger include endorphins, ghrelin, and neuropeptide Y. Those that cause satiety include leptin (working in conjunction with the hormone insulin), serotonin, and cholecystokinin (CCK). New research has shown how leptin, discovered in 1995, and ghrelin, discovered in 1999, work together to balance our hunger and satiety. Leptin is made by the fat cells, also known as adipose tissue, travels to the brain and stimulates the satiety center and turns off the hunger center.
Ghrelin is made by the stomach and travels to the brain where it stimulates the hunger center and turns off the satiety center. High leptin levels also turn off the production of ghrelin. Problems arise when leptin levels are high over a long period, such as in obesity, making the satiety center insensitive to leptin. To add to the confusion, low ghrelin levels make the hunger center sensitive to ghrelin. This situation helps explain why high leptin levels in obese individuals do not decrease hunger. As you can imagine, these hormones and their mechanisms are potential targets for new weight loss drugs.
Does Appetite Affect What We Eat?
Various feeding and satiety messages from body cells do not single-handedly determine what we eat. Almost everyone has encountered a mouthwatering dessert and devoured it, even on a full stomach. Appetite can be affected by a wide variety of external forces, such as environmental and psychological factors as well as social customs (Figure 1-4). We often eat because food confronts us. It smells good, tastes good, and looks good. We might eat because it is the right time of day, we are celebrating, or we are seeking emotional comfort to overcome the blues. After a meal, memories of pleasant tastes and feelings reinforce appetite. If stress or depression sends you to the refrigerator, you are mostly seeking comfort, not food calories. Appetite may not be a biological process, but it does influence food intake.
Food means so much more to us than nourishment—it refl ects much of what we think about ourselves. In the course of our lives, we spend the equivalent of 13 to 15 years eating. Beyond simple hunger and emotions, why do we choose what we eat?
• Flavor, texture, and appearance are the most important factors determining our food choices. Creating more flavorful foods that are both healthy and profi table is a major focus of the food industry (referred to as “better for you” products).
• Early infl uences that expose us to various people, places, and events have a continuing impact on our food choices. Many ethnic diet patterns begin as we are introduced to foods during childhood.
• Routines and habits are tied to some food choices. Food habits, food availability, and convenience strongly influence choices. Most of us eat from a core group of foods: About 100 basic items account for 75% of an individual’s total food intake.
• Nutrition, or what we think of as “healthy foods,” directs our food purchases. North Americans who tend to make health-related food choices are often well-educated, middle-class professionals. These same people are generally health-oriented, have active lifestyles, and focus on weight control.
• Advertising is a major media tool for capturing the food interest of the consumer. The food industry in the United States spends well over $33 billion on advertising. Some of this advertising is helpful, as it promotes the importance of food components such as calcium and fiber in our diets. However, the food industry also advertises highly sweetened cereals, cookies, cakes, and soft drinks because they bring in the greatest profits.
• Restaurants have long been a growing industry in North America. Today, about 45% of food dollars is spent on meals outside the home. Restaurant food is often calorie-dense and of poorer nutritional quality compared to foods made at home. However, in response to consumer demands, restaurants have placed healthier items on their menus and many are listing nutritional content on their menus.
• Social changes are leading to a general time shortage for many of us. This creates the need for convenience. Supermarkets now supply prepared meals, microwavable entrees, and various quick-prep frozen products.
• Economics play a minor role in our food choices. The average North American spends only about 12% of after-tax income on food (greater for low-income people). However, as income increases, so do meals eaten away from home.
Overall, daily food intake is a complicated mix of biological and social influences. The first Rate Your Plate activity in Chapter 1 asks you to keep track of what infl uences your food intake on a daily basis. This assessment is an important part of developing a plan to improve your diet as necessary. How do you compare to the typical North American?
Putting Hunger and Appetite into Perspective
By Gordon M. Wardlaw and Anne M. Smith in the book 'Contemporary Nutrition', McGraw-Hill, New York, 2009, p.19-22. Adapted to be posted by Leopoldo Costa.
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