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3.1: Food Is Fuel

Difficulty Level: At Grade Created by: CK-12

How do you measure food energy?

Remember that the food you see on the table in front of you is of little use to your body. It must be digested-broken down into chemical components-so the body can take advantage of the nutrients available. First, we will take a look at how to measure calories. Then you will explore the major food groups from which you get the six nutrients needed for survival-carbohydrates, fat, protein, Vitamins, minerals, and water.

How do we know how much energy your food provides? Sometimes we say that our cells “burn” the food we eat as fuel to get energy. Of course, our cells are not little furnaces with flames inside! What happens in a furnace is a rapid chemical reaction that combines molecules of oxygen with molecules of fuel. That reaction converts the energy stored in the fuel such as oil or natural gas to heat.

In cells, oxygen is also combined with fuel molecules to release the stored energy in the fuel. However, this is a slower and more controlled process in cells. The process is slower so the cell can use the energy that is released to do work. Just remember that there are similarities but important differences in the way furnaces and cells “burn” fuel. In the furnace the process is called combustion. In the cell the process is called cellular respiration.

In both combustion and cellular respiration, heat is produced. We can measure heat energy as calories or kilocalories. A calorie is the amount of heat needed to raise the temperature of one milliliter of water one degree Celsius \begin{align*}(^\circ C)\end{align*}(C). That is a very small amount of energy compared to the amount of energy our bodies use every day. Usually we think in terms of bigger units of heat energy when we are dealing with how our bodies use energy. A convenient big unit of energy is a kilocalorie, which is 1,000 calories-just like a kilogram is 1,000 grams. People seemed to find kilocalorie a big word, however, and started to use the word calorie instead. So the word calorie, abbreviated as Cal with a capital C, is used to mean kilocalorie.

Did You Know?

The word calorie can be abbreviated in two different ways to mean two different things. One abbreviation, Cal, refers to big units of energy in food. The other abbreviation, cal, refers to smaller units of energy. This smaller type of calorie is usually not used when referring to food.

Using calories (Cal or kilocalories) as a way of measuring is useful to compare the energy available in food. For example, the calorie charts on food labels refer to big calories (abbreviated with a capital C) or kilocalories. How many calories (Cal) do you need each day? Well, this depends on how big and how active you are. If you eat more calories than your body needs, the excess calories will be stored as fat. To make one pound of body fat, you must eat over 4,000 more calories (Cal) than your body needs. To lose one pound of body fat, you must eat at least 4,000 fewer calories (Cal) than your body needs.

1. Maintain weight

calories in = calories used

2. Lose weight

Eat fewer calories than needed.

Burn more calories with exercise.

3. Gain weight

Ear more calories than needed.

Burn fewer calories with less activity.

Figure 2.1 The scales represent the amount you need to eat to (1) maintain weight, (2) lose weight, and (3) gain weight.

Activity 2-1: Calories: In a Nutshell


How can we measure food energy? In this activity you measure the energy (heat) in half a peanut.

One calorie (cal) is the amount of heat needed to raise the temperature of 1 gram \begin{align*}(1 \ ml)\end{align*}(1 ml) of water 1 degree Celsius.

One calorie (Cal) is the amount of heat needed to raise the temperature of 1 liter (1,000 cc) of water 1 degree Celsius. The energy in food is usually measured in these larger calories (Cal).

  • If a burning peanut heats \begin{align*}10 \ ml\end{align*}10 ml of water from 20-60 degrees Celsius \begin{align*}(20-60^\circ C)\end{align*}(2060C), then how many calories (cal) of heat were used?
  • If 1,000 small calories make up one large calorie, then how many large calories (Cal) were measured in this example?


  • Safety goggles
  • Resource
  • Data Sheet
  • Activity Report
  • Test tube
  • Test-tube holder
  • Calorimeter (a can adapted for this purpose)
  • Thermometer
  • Graduated cylinder
  • Cork
  • Needle
  • Matches
  • Peanuts

CAUTION: You should wear goggles in all experimental laboratory situations. Make sure you are wearing goggles when working with tire. Also, be very careful with the matches and the flame in this activity.


Step 1 Put \begin{align*}10 \ ml\end{align*}10 ml (10 grams) of water into a test tube.

Step 2 Measure the temperature of the water and record the temperature on the Data Sheet.

Step 3 Carefully stick the blunt end of the needle in the smaller, foil-covered end of a cork.

Step 4 Carefully place the peanut on the sharp end of the needle.

Step 5 Light the peanut half.

Step 6 Place the calorimeter/can with the test tube of water supported in it over the flame and heat the water with the burning peanut until it stops burning.

Step 7 Measure the temperature of the water and record the temperature on the Data Sheet.

Step 8 Repeat Steps 1-7 two more times using fresh water and a new peanut half.

Step 9 Calculate the average.

Do you think different nuts have different amounts of calories?

How much energy do you need?

Here is a table of average daily requirements for calories (Cal) at different ages.

Calories (kilocalories) Needed
Age (in years) male female
1-3 1,300 1,300
4-6 1,800 1,800
7-10 2,000 2,000
11-14 2,500 2,200
15-18 3,000 2,200
19-24 2,900 2,200
25-50 2,900 2,200
51+ 2,300 1,900

\begin{align*}^*\end{align*} Recommended fat intake is 30% or less of total calories in a person's diet. If the calorie requirement is 2,500, then 30% of 2,500 calories is 750 calories from fat, or about 83 grams of fat. A possible healthy range for the intake of fat for ages 11 -14 is 65- 85 grams.

What can you learn from these numbers? Do these numbers tell us that all males between 11 and 14 years of age need 2,500 calories (Cal) a day to live? No. Why? Because these numbers are averages. You don't know how the average was determined. You don't know how many males you are talking about or what they do. You know some “couch potatoes” need fewer calories than future Olympic champs. How many couch potatoes and how many champion athletes were in the sample? You don't know. What these types of numbers say is that the person who created the table of information believes that somewhere in the neighborhood of 2,500 calories is adequate to power most 11- to 14-year-old males. But individuals may need fewer or more calories. So what can you learn from the table? One thing you can see is that, on the average, females need fewer calories each day than males after age 11. But, again, individuals can differ widely from these averages. Think about a 15-year-old female who runs three miles each day. How many calories do you think she needs?

You are the advertising director for a campaign to encourage teens to exercise. What specific aspects of the lifestyles of different teenagers would you target in your campaign? What are some slogans you would use in your advertisements? Why do you think they would be effective?

Activity 2-2: Calories: How Much Energy Do You Use?


How much energy do you use? One way to find out the number of calories (Cal) used each day is to estimate the number of calories (Cal) used each hour. In this activity you get an idea of how much energy you use in a day. Below is a sample list of activities and their energy requirements per hour.

Activity Calories Burned Per Hour
Walking 210
Swimming 300
Jogging 500
Bowling 270
Skating 350
Biking 660
Reading 100
Soccer 405
Sleeping/ resting 60
Sitting 100
Light Activity 150
Moderate Activity 230
Strenuous Activity 420


  • Data Sheet
  • Activity Report


Step 1 Chart your activities and the calories (Cal) you use in two 24-hour periods. One of the days should be a weekday and one should be a Saturday or Sunday. Use the Data Sheet to record the data.

Step 2 Calculate the average amount of energy in calories you use in a day. Divide the total energy used in 2 days by 2. Record this information in the Activity Report.

Step 3 Record the average energy use by 10 males and 10 females on the Activity Report. Calculate the average energy use for those 10 males and for those 10 females during a 24-hour period.

The Food Groups

How do you make careful food choices? There are many things to take into consideration. You need to consider such things as calories, vitamins, amino acids, fiber, kinds of fats, and amounts of fat. It can be confusing. There are two things to help you-the food pyramid and the five food groups.

Recently, the U.S. government adopted this food pyramid to educate people about eating right. The building blocks of most of the pyramid are the five food groups. At the very top of the pyramid is a collection of things we like to eat but shouldn't eat much of. This collection of foods includes things such as candy and foods high in fat. The top of the pyramid is not a food group. These things are at the top of the pyramid, but they are not the most important foods nor are they good for you!

To learn more about the meaning of the pyramid, you have to think about calories. The pyramid represents the total amount of calories (Cal) your body needs. The food group building blocks represent the proportion of your calorie needs that you should get from different kinds of foods. Thus, the pyramid tells you that you should get most of your calories from grains, fruits, and vegetables (complex carbohydrates). You should get very few of your calories from fats and simple carbohydrates-for example, junk food.

Figure 2.2 Food pyramid.

Reading Food labels Find some food labels from the foods you eat and analyze them. Every food label provides some information about the food inside that container. Some labels tell you much more than others. Federal law requires that nutritional information appear on a label only if a nutrient has been added to the product. For example, if a drink has been “vitamin enriched,” you can look at the nutritional information to see what vitamins have been added.

Can you guess the importance of the food groups? Each food group contains foods that are similar. Therefore, the foods in each group have a similar combination of the five nutrients-carbohydrates, proteins, fats, vitamins, and minerals. (Remember that water is considered a sixth nutrient.) When you eat the five nutrients in the proportions shown in the pyramid, you do not eat too much fat and cholesterol or too little fiber.

The food pyramid can help you balance both the calories and the nutrients in your diet.

The Five Food Groups

Now, let's study the five food groups in more detail. You will also get an idea of the calories in different foods. Then you will explore what the body must do to burn calories.

1. Bread/Cereal/Rice/Pasta Group

These foods are composed of complex carbohydrates. They provide fiber and bulk for your food tube to squeeze and keep things moving. Cereals also give you B vitamins, minerals, and protein. Six to eleven servings from the bread/cereal/rice/pasta group are recommended each day.

Figure 2.3 Bread/cereal/rice/pasta group

Food Serving Amount Calories Fat(g)
Whole wheat bread 1 slice 60 1
Enriched white bread 1 slice 70 1
Corn bread 1 piece 190 5
Pancake 1 60 2
Rice \begin{align*}\frac{1}{2}\end{align*}12 cup 110 -
Hamburger bun \begin{align*}\frac{1}{2}\end{align*}12 bun 120 2
Corn tortilla 1 60 1
Oatmeal \begin{align*}\frac{1}{2}\end{align*}12 cup 66 1
Pasta 1 cup 200 1

What's the matter with the amount “1 pancake” or “1 piece of corn bread”? Remember the top of the food pyramid? That is where the butter and syrup that you might put on the pancake or corn bread come from. The table above does not include the “goodies” you add to the bread or pancake.

2. Vegetable Group

Vegetables are a good source of vitamins A and C, minerals, and fiber. Vegetables are low in calories, but they are high in nutrients. Three to five servings of vegetables are recommended each day.

Figure 2.4 Vegetable group

Food Serving Amount Calories Fat(g)
Green beans \begin{align*}\frac{1}{2}\end{align*}12 cup 16 -
Carrots \begin{align*}\frac{1}{2}\end{align*}12 cup 22 -
Lettuce 1 cup 5 -
Baked potato 1 130 -
Spinach (cooked) \begin{align*}\frac{1}{2}\end{align*}12 cup 20 .2

Once again, remember that the baked potato in this table does not include a big pat of butter or scoop of sour cream that comes from the top of the pyramid. Foods from the top of the pyramid add lots of calories and fat.

3. Fruit Group

Fruits are a good source of minerals and vitamins. Two to four servings of fruit per day are recommended.

Figure 2.5 Fruit group

Food Serving Amount Calories Fat(g)
Orange juice (frozen) 1 cup 90 -
Canned peaches \begin{align*}\frac{1}{2}\end{align*}12 cup 100 -
Banana 1 medium 100 -
Grapefruit \begin{align*}\frac{1}{2}\end{align*}12 medium 45 -
Avocado \begin{align*}\frac{1}{2}\end{align*}12 185 18
Apple 1 medium 70 -
Strawberries 1 cup 55 -
Tomato 1 20 -
Tomato juice 1 60 -

4. Protein/Meat/Fish/Dry Beans/Eggs/Nuts Group

This group is mostly protein. These foods also give you iron and some B vitamins. You should have two or three servings of food high in protein each day. However, you should remember that many foods in this group could contain a lot of fat. Here are some examples of the protein group.

Figure 2.6 Protein/meat/fish/dry beans/eggs/nuts group

Food Serving Amount Calories Fat(g)
Lima beans \begin{align*}\frac{1}{2}\end{align*}12 cup 94 -

Fried chicken

(no skin)

(with skin)

\begin{align*}85 \ g\end{align*}85 g (3 oz)

\begin{align*}85 \ g\end{align*}85 g (3 oz)





Broiled chicken

(no skin)

(with skin)

\begin{align*}85 \ g\end{align*}85 g (3 oz)

\begin{align*}85 \ g\end{align*}85 g (3 oz)





Boiled egg 1 79 6
Halibut (broiled) \begin{align*}85 \ g\end{align*}85 g (3 oz) 144 3
Lean hamburger \begin{align*}85 \ g\end{align*}85 g (3 oz) 235 15
Tofu (soy curd) \begin{align*}120 \ g\end{align*}120 g (4.3 oz) 86 5
Tuna (in water) \begin{align*}92 \ g\end{align*}92 g (3.3 oz) 117 -

What Do You think?

Why do you think it is recommended that you take the skin off chicken before you eat it?

5. Milk/Yogurt/Cheese Group

You should have two to three servings of food from this group each day. Milk is a good food, especially low-fat and non-fat milk. You do not need the fat in whole milk, but you do need the calcium, riboflavin (vitamin B2), protein, and vitamins A and D you get from milk. Here are some examples of foods in the milk group.

Figure 2.7 Milk/yogurt/cheese group

Food Serving Amount Calories Fat(g)
American cheese \begin{align*}28 \ g\end{align*}28 g (1 oz) 99 3
Cheddar cheese \begin{align*}28 \ g\end{align*}28 g (1 oz) 115 9
Cottage cheese 1 cup 114 9
Low-fat milk (1% fat) 1 cup 100 3
Low-fat milk (2% fat) 1 cup 121 5
Skim milk (nonfat) 1 cup 85 -
Whole milk 1 cup 150 8
Low-fat yogurt with fruit 1 cup 230 3
Low-fat plain yogurt 1 cup 145 4

What Do You Think?

Why do you think fruit yogurt has more calories (Cal) than plain yogurt?

The Top of the Food Pyramid

This collection of foods contains added fat and sugar. The recommendation is to control your intake of these kinds of foods. Some foods contain a lot of fat and sugar. For example, a cheeseburger, chocolate shake, and French fries from a fast food restaurant together contain about 18 teaspoons of fat! Yuck! While these foods can provide half of your daily calories, they don't come even close to providing half of your daily nutrients.

Figure 2.8 Top of the pyramid: these foods are not considered a food group.

Food Serving Amount Calories Fat(g)
Soft drink 1 can 145 -
Chocolate cake 1 piece 235 9
Candy bar, milk chocolate \begin{align*}58 \ g\end{align*}58 g 295 9
Chocolate chip cookie medium 50 3
Ice cream 1 cup 270 7
Jelly 1 tbs. 50 -
Potato chips 10 chips 115 8
Apple pie 1 piece 345 15
Brownie 1 small 85 4
Mayonnaise 1 tbs. 101 11

A healthy amount of fat for 11- 14-year-olds to consume ranges from 65 to 85 grams per day. Suppose that in one day you eat 2 chocolate chip cookies, 4 servings of potato chips, an ice cream cone, and 2 tablespoons of mayonnaise on your sandwich. How many grams of fat have you consumed?

What Do You Think?

Considering the list of nutritional guides shown here, which do you think would be the easiest for you to follow and why? Which would be the hardest for you to follow and why?

Look again at the foods you recorded in Your Food Diary, Activity 1-1: Are You What You Eat? You might have some new thoughts about how to choose foods now. Learn about which foods are good for you.

The following are some nutritional guides to keep in mind.

  • Limit fats to 30% or less of your total calories \begin{align*}(65-85 \ g)\end{align*}(6585 g).
  • Protein should be about 15% of your total calories (about \begin{align*}45 \ g\end{align*}45 g).
  • Include complex carbohydrates for energy as well as vitamins, minerals, and fiber.
  • Limit foods high in saturated fats (animal fat and palm and coconut oil).
  • Choose foods that help you grow, look, and feel your best instead of choosing foods with “empty calories.” Limit foods high in sugar and salt.
  • Eat a variety of foods from all five food groups.

Review Questions

  1. What is the relationship between a calorie (Cal), a piece of bread, and the term energy?
  2. Explain why the five food groups are displayed in the shape of a pyramid instead of a square.
  3. Design and complete a table that includes the food groups, major nutrients in each group, and examples of food in each group.

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