Reading the ingredients lists on food labels can help you choose the healthiest foods. At the top of the list, look for ingredients such as whole grains, vegetables, milk, and fruits. These are the ingredients you need in the greatest amounts for balanced eating. Avoid foods that list fats, oils, sugar, or salt at the top of the list. For good health, you should avoid getting too much of these ingredients. Be aware that ingredients such as corn syrup are sugars.
You should also use moderation when eating foods that contain ingredients such as white flour or white rice. These ingredients have been processed, and processing removes nutrients. The word "enriched" is a clue that an ingredient has been processed. Ingredients are enriched with added nutrients to replace those lost during processing. However, enriched ingredients are still likely to have fewer nutrients than unprocessed ingredients.
Balancing Food with Exercise
Look at MyPyramid in Figure below. Note the person walking up the side of the pyramid. This shows that exercise is important for balanced eating. Exercise helps you use any extra energy in the foods you eat. The more active you are, the more energy you use. You should try to get at least an hour of physical activity just about every day. Figure below shows some activities that can help you use extra energy.
Figure 17.9
All of these activities are good ways to exercise and use extra energy. The Calories given for each activity are the number of Calories used in an hour by a person that weighs 100 pounds. Which of these activities uses the most Calories? Which of the activities do you enjoy?
Any unused energy in food is stored in the body as fat. This is true whether the extra energy comes from carbohydrates, proteins, or lipids. What happens if you take in more energy than you use, day after day? You will store more and more fat and become overweight. Eventually, you may become obese. Obesity is having a very high percentage of body fat. Obese people are at least 20 percent heavier than their healthy weight range. The excess body fat of obesity is linked to many diseases. Obese people often have serious health problems, such as diabetes, high blood pressure, and high cholesterol. They are also more likely to develop arthritis and some types of cancer. People that remain obese throughout adulthood usually do not live as long as people that stay within a healthy weight range.
The current generation of children and teens is the first generation in our history that may have a shorter life than their parents. The reason is their high rate of obesity and the health problems associated with obesity.
You can avoid gaining weight and becoming obese. The choice is yours. Choose healthy foods by using MyPyramid and reading food labels. Then get plenty of exercise to balance the energy in the foods you eat.
Lesson Summary
MyPyramid shows how much you should eat each day of foods from six different food groups.
Reading food labels can help you choose the healthiest foods.
Regular exercise helps you use extra energy and avoid unhealthy weight gain.
Review Questions
List the six food groups represented by MyPyramid.
Which food group contains soybeans, kidney beans, and fish?
What guideline should you follow in choosing foods from the grains food group?
Which ingredient is always listed first on a food label?
What happens if you take in more energy than you use, day after day?
Explain how you can use MyPyramid to choose foods that provide the proper balance of nutrients.
Why should you limit foods like ice cream and potato chips in a healthy eating plan?
Explain how you can use food labels to choose foods that are high in fiber.
Why should you try to avoid foods with processed ingredients? What are some examples of processed ingredients?
How does physical activity help prevent obesity?
Further Reading / Supplemental Links
Eric Schlosser and Charles Wilson. Chew on This: Everything You Don’t Want to Know about Fast Food. Houghton Mifflin, 2006.
John Burstein. The Shape of Good Nutrition: The Food Pyramid. Crabtree Publishing Company, 2008.
Rose McCarthy. Food Labels: Using Nutrition Information to Create a Healthy Diet. Rosen Publishing Group, 2008.
Sandra Giddens. Making Smart Choices about Food, Nutrition, and Lifestyle. Rosen Central, 2008.
CK–12. High School Biology. Chapter 38, Lesson 1.
http://www.cfsan.fda.gov/~acrobat/nutfacts.pdf
http://www.cfsan.fda.gov/~dms/foodlab.html
http://www.fns.usda.gov/tn/parents/nutritionlabel.html
http://www.health.gov/dietaryguidelines/dga2005/document/pdf/DGA2005.pdf
http://www.mypyramid.gov
http://www.newswise.com/articles/view/537296
http://www.nlm.nih.gov/medlineplus/ency/article/002459.htm
http://www.nlm.nih.gov/medlineplus/exerciseforchildren.html
http://www.prb.org/Articles/2005/WillRisingChildhoodObesityDecreaseUSLifeExpectancy.aspx
http://www.sciencemag.org/cgi/content/summary/307/5716/1716
http://en.wikipedia.org/wiki
Vocabulary
enriched
Term used for an ingredient that has been processed; ingredients are enriched with added nutrients to replace those lost during processing; likely to have fewer nutrients than unprocessed ingredients.
ingredient
A specific item that a food contains.
main ingredient
The ingredient that is present in the food in the greatest amount.
MyPyramid
Diagram that shows how much you should eat each day of foods from six different food groups.
nutrition facts label
The label on packaged food that shows the nutrients in the food.
obesity
Having a very high percentage of body fat; obese people are at least 20 percent heavier than their healthy weight range.
serving size
Tells you how much of the food you should eat to get the nutrients listed on the label.
Points to Consider
Discuss how foods may be broken down into nutrients that your body can use? For example, how do you think an apple becomes simple sugars that your body can use for energy? Or how might a piece of cheese become proteins that your body can use for building materials?
Lesson 17.3: The Digestive System
Lesson Objectives
State the functions of the digestive system.
Explain the role of enzymes in digestion.
Describe the digestive organs and their functions.
Explain the roles of helpful bacteria in the digestive system.
List ways to help keep your digestive system healthy.
Check Your Understanding
What is a chemical reaction?
What is an enzyme?
What are bacteria?
Introduction
Nutrients in the foods you eat are needed by the cells of your body. How do the nutrients in foods get to your body cells? What organs and processes break down the foods and make the nutrients available to cells? The organs are those of the digestive system. The processes are digestion and absorption.
What Does the Digestive System Do?
The digestive system is the body system that breaks down food and absorbs nutrients. It also gets rid of solid food waste. The main organs of the digestive system are shown in Figure below.
Figure 17.10
This drawing shows the major organs of the digestive system. Trace the path of food through the organs of the digestive system as you read about them in this lesson.
Digestion is the process of breaking down food into nutrients. There are two types of digestion: mechanical digestion and chemical digestion. In mechanical digestion, large chunks of food are broken down into small pieces. This is a physical process. In chemical digestion, large food molecules are broken down into small nutrient molecules. This is a chemical process.
Absorption is the process i
n which substances are taken up by the blood. After food is broken down into small nutrient molecules, the molecules are absorbed by the blood. Then the nutrient molecules travel in the bloodstream to cells throughout the body.
Some substances in food cannot be broken down into nutrients. They remain behind in the digestive system after the nutrients are absorbed. Any substances in food that cannot be digested and absorbed pass out of the body as solid waste. The process of passing solid food waste out of the body is called elimination.
The Role of Enzymes in Digestion
Chemical digestion could not take place without the help of digestive enzymes. An enzyme is a protein that speeds up chemical reactions in the body. Digestive enzymes speed up chemical reactions that break down large food molecules into small nutrient molecules.
Did you ever use a wrench, like the one in Figure below, to tighten a bolt? You could tighten a bolt with your fingers, but it would be difficult and slow. If you use a wrench, you can tighten a bolt much more easily and quickly. Enzymes are like wrenches. They make it much easier and quicker for chemical reactions to take place. Like a wrench, enzymes can also be used over and over again. But you need the appropriate size and shape of the wrench to efficiently tighten the bolt, just like each enzyme is specific for the reaction it helps.
Figure 17.11
Turning a bolt with a wrench is easier and quicker than trying to turn a bolt with your fingers. How is a wrench like an enzyme?
Digestive enzymes are secreted by the organs of the digestive system. Examples of digestive enzymes are:
Amylase is produced by the mouth. It helps break down large starches molecules into smaller sugar molecules.
Pepsin is produced by the stomach. Pepsin is a protease; it helps break down proteins into amino acids.
Trypsin is produced in the pancreas. Trypsin is a protease; it cleaves peptide chains.
Pancreatic lipase is secreted by the pancreas. It is a lipase, used to break apart fats.
Deoxyribonuclease and ribonuclease are nucleases secreted by the pancreas. They are enzymes that break bonds in nucleic acid backbones.
Bile salts are bile acids whose main function is to facilitate the processing of dietary fat. Bile acids are made in the liver. Upon eating a meal, the contents of the gallbladder are secreted into the intestine, where bile acids break down dietary fats. Bile acids serve other functions, including eliminating cholesterol from the body.
Hormones and Digestion
If you are a typical teenager, you like to eat. Chances are, if you could, you would be eating in class right now. For your body to break down, absorb and distribute the nutrients, and maintain homeostasis, requires both the digestive system and endocrine system to work together. Digestive hormones are made by cells lining the stomach and small intestine. These hormones cross into the blood where they can affect other parts of the digestive system. Some of these hormones are listed below.
Gastrin-Stimulates gastric acid secretion.
Cholecystokinin-Stimulates secretion of pancreatic enzymes, and contraction and emptying of the gall bladder.
Secretin-Stimulates secretion of water and bicarbonate from the pancreas and bile ducts.
Ghrelin-A strong stimulant for appetite and feeding.
Gastric inhibitory polypeptide-Inhibits gastric secretion; also causes the release of insulin in response to elevated blood glucose concentration.
Digestive Organs and Their Roles
The mouth and stomach are just two of the organs of the digestive system. Other digestive system organs are the esophagus, small intestine, and large intestine. From Figure below, you can see that the digestive organs form a long tube. In adults, this tube is about 9 meters (30 feet) long! At one end of the tube is the mouth. At the other end is the anus. Food enters the mouth and then passes through the rest of the digestive system. Food waste leaves the body through the anus.
The organs of the digestive system are lined with muscles. The muscles contract, or tighten, to push food through the system. This is shown in Figure below. The muscles contract in waves. The waves pass through the digestive system like waves through a Slinky®. This movement of muscle contractions is called peristalsis. Without peristalsis, food would not be able to move through the digestive system. Peristalsis is an involuntary process, which means that it occurs without your conscious control.
Figure 17.12
This diagram shows how muscles push food through the digestive system. Muscle contractions travel through the system in waves, pushing the food ahead of them. This is called peristalsis.
The liver, gall bladder, and pancreas are also organs of the digestive system. They are shown in Figure below. Food does not pass through these three organs. However, these organs are important for digestion. They secrete or store enzymes or other chemicals that are needed to help digest food chemically.
Figure 17.13
This drawing shows the liver, gall bladder, and pancreas. These organs are part of the digestive system. Food does not pass through them, but they secrete substances needed for chemical digestion.
Mouth, Esophagus, and Stomach
The mouth is the first organ that food enters. However, digestion may start even before you put the first bite of food into your mouth. Just seeing or smelling food can cause the release of saliva and digestive enzymes in your mouth. Once you start eating, saliva wets the food, which makes it easier to break up and swallow. Digestive enzymes, including amylase, start breaking down starches into sugars. Your tongue helps mix the food with the saliva and enzymes.
Your teeth also help digest food. Your front teeth are sharp. They cut and tear food when you bite into it. Your back teeth are broad and flat. They grind food into smaller pieces when you chew. Chewing is part of mechanical digestion. Your tongue pushes the food to the back of your mouth so you can swallow it. When you swallow, the lump of chewed food passes down your throat to your esophagus.
The esophagus is a narrow tube that carries food from the throat to the stomach. Food moves through the esophagus because of peristalsis. At the lower end of the esophagus, a circular muscle controls the opening to the stomach. The muscle relaxes to let food pass into the stomach. Then the muscle contracts again to prevent food from passing back into the esophagus. Some people think that gravity moves food through the esophagus. If that were true, food would move through the esophagus only when you are sitting or standing upright. In fact, because of peristalsis, food can move through the esophagus no matter what position you are in—even upside down. Just don’t try to swallow food when you upside down! You could choke if you try to swallow when you are not upright.
The stomach is a sac-like organ at the end of the esophagus. It has thick muscular walls. The muscles alternately contract and relax. This churns the food and helps break it into smaller pieces. The churning also mixes the food with the enzyme pepsin and other chemicals that are secreted by the stomach. The pepsin and other chemicals help digest proteins chemically.
Water, salt, and simple sugars can be absorbed into the blood from the stomach. Most other substances are broken down further in the small intestine before they are absorbed. The stomach stores food until the small intestine is ready to receive it. A circular muscle controls the opening between the stomach and small intestine. When the small intestine is empty, the muscle relaxes. This lets food pass from the stomach into the small intestine.
Small Intestine
The small intestine is narrow tube that starts at the stomach and ends at the large intestine (see Figure below). In adults, the small intestine is about 7 meters (23 feet) long. It is made up of three parts: the duodenum, jejunum, and ileum. Each part has different functions.
The duodenum is the first part of the small intestine. This is where most chemical digestion takes place. Many enzymes and other chemicals are secreted here. Some are secreted by the duodenum itself. Others are secreted by the pancreas or liver.
The jejunum is the second part of the small intestine. This
is where most nutrients are absorbed into the blood. The jejunum is lined with tiny “fingers” called villi. A magnified picture of villi is shown in Figure below. Villi contain microscopic blood vessels. Nutrients are absorbed into the blood through these tiny vessels. There are millions of villi, so altogether there is a very large area for absorption to take place. In fact, villi make the inner surface area of the small intestine 1,000 times larger than it would be without them. The entire inner surface area of the small intestine is about as big as a basketball court!
Figure 17.14
This is what the villi lining the small intestine look like when magnified. Each one is actually only about 1 millimeter long. Villi are just barely visible with the unaided eye.
The ileum is the third part of the small intestine. Like the jejunum, the ileum is covered with villi. A few remaining nutrients are absorbed in the ileum. From the ileum, any remaining food waste passes into the large intestine.
The small intestine is much longer than the large intestine. So why is it called “small”? If you compare the small and large intestines in Figure below, you will see why. The small intestine is smaller in width that the large intestine.
Large Intestine
The large intestine is a relatively wide tube that connects the small intestine with the anus. In adults, it is about 1.5 meters (5 feet) long. Waste enters the large intestine from the small intestine in a liquid state. As the waste moves through the large intestine, excess water is absorbed from it. After the excess water is absorbed, the remaining solid waste is called feces. Circular muscles control the anus. They relax to let the feces pass out of the body through the anus. After feces pass out of the body, they are called stool. The excretion of stool is referred to as a bowel movement.
CK-12 Life Science Page 44
