Understanding energy expenditure or output

To truly understand energy balance, we have to start by understanding energy expenditure or output. Your body's natural way of maintaining balance is to replenish the calories it uses. It does not, however, automatically use whatever energy you put into it. Energy expenditure has three major components.

1. The energy you need in order to maintain normal functions, such as breathing, circulating blood, and maintaining tissues (e.g., liver, brain, muscles). The number of calories you use for these functions is called your resting metabolic rate, which is influenced by a variety of factors (see figure 26.2).
2. The energy you need to ingest and digest food. This process starts with chewing, continues through swallowing, and ends with moving food through your intestines.
3. The energy you use in movement and physical activity.

Your body uses most of its energy (60 percent to 70 percent) to maintain normal functions of survival, about 10 percent for ingestion and digestion of food, and the remainder for movement and physical activity (see figure 26.3).

Some stimulants (e.g., caffeine) are marketed as tools for increasing your metabolism - thus helping you burn (use) more calories - but their effect is minimal, and they can cause damaging side effects. A better solution is exercise. The more movement and activity you do, the more your energy expenditure increases. In addition, movement is the source of energy expenditure that you can control directly through your own actions. In fact, an extremely active person might double his or her energy expenditure through activity. In addition, active bodies often consist of more muscle tissue, which naturally raises the metabolic rate, thus using more calories.

Your energy intake, or the number of calories you consume each day, makes up the other half of the energy balance equation. You need to balance the number of calories you eat with the number you use in order to remain in energy balance. Recall that calories come from the carbohydrate, fat, and protein you eat. Teenage girls from ages 14 to 18 who are not active need to eat about 1,800 calories each day. Teenage boys from ages 14 to 18 who are not active need to eat about 2,200 calories each day. As just explained, your energy needs will rise as physical activity and exercise levels increase. How much you choose to eat can also be influenced by our hunger, satiety, and appetite. Each of these factors is explained in this lesson.

Your body regulates your food consumption through a series of internal and external cues and stimuli. Hunger is your physiological drive to eat. When it's time to eat, your body tells you so through a set of internal changes, including a drop in blood sugar and the onset of stomach contractions. Other parts of your body also play a role in establishing your hunger - specifically, your brain, central nervous system (CNS), endocrine system, and digestive system. When you eat, your blood sugar and nutrient levels rise, and your hormone and neurotransmitter levels change, all of which eventually tells your brain and your body that you're full and therefore that it's time to stop eating.

When your body is well balanced and you're comfortable between meals, you're in a state of satiety, or fullness. The systems that contribute to hunger and satiety are complicated and very intricate. Studies have shown that certain conditions - for example, imbalances in hormones and neurotransmitters - can make it more difficult for some individuals to feel full. Figure 26.4 shows the relationship between hunger, satiation, and satiety.

Physiological hunger isn't the only thing that affects your food intake. Have you ever eaten popcorn at a movie or a hot dog at a ball game even though you didn't feel hungry? As these examples illustrate, our choices about what - and when - to eat can be influenced by our psychological needs. Whereas your hunger is your physiological drive to eat, your appetite is your psychological drive to eat. Thus it's different from your hunger. Appetite has to do with the pleasure you derive from food. It can be triggered by the sight and smell of food or even the sound of food cooking. On the other hand, if you're sick with a cold or the flu, the sight or smell of food can make you feel nauseated.

Appetite is influenced by many factors, including traditions. For example, people often associate eating with celebrations, holidays, particular family gatherings, and religious traditions. We may also associate eating with particular circumstances. Some people snack when they watch TV or when they get home from work or school - even if they aren't hungry. Others eat dessert after dinner even if they just ate a very large and satisfying meal.

Emotions also play a particularly important role in appetite. Is there a food you tend to eat when you feel sad or lonely? How about when you feel happy? Do you eat a lot, or nothing at all, when you feel stressed about a major exam? Most people let emotions affect their eating habits in consistent ways; therefore, understanding what circumstances and emotions trigger you to eat can prevent you from taking in unnecessary calories. Getting control of your appetite is a critical part of establishing and maintaining healthy eating habits.

If hunger is not the problem, then eating is not the solution.

- Anonymous

When energy balance is not maintained, weight loss or weight gain may occur. In American society, many individuals seem to have a chronic (long-term) positive energy balance that results in gradual weight gain during the adult years. This weight gain may result in obesity and may bring many health risks. As a result, many people take various measures to try and lose weight. Other individuals struggle with trying to gain weight even when they maintain a positive energy balance.