CK-12 Biology I - Honors

by CK-12 Foundation

  Regulating the salt-water and acid-base balance in body fluids.

  Transporting nutrients and other vital substances in the blood.

  Dietary proteins are broken down during digestion to provide the amino acids that cells need to make proteins for the body. Twenty different amino acids are needed for this purpose. Ten of these amino acids can be synthesized by cells from simple components. The other ten cannot be synthesized and must be obtained from foods. They are called essential amino acids because they are essential in the diet.

  Proteins that contain all ten essential amino acids are referred to as complete proteins. They are found in animal foods such as milk and meat. Proteins that are missing one or more essential amino acids are referred to as incomplete proteins. They are found in plant foods such as legumes and rice. By eating a variety of different plant foods containing incomplete proteins, you can include all ten essential amino acids in your diet.

  If you eat more protein than needed for the synthesis of new proteins by cells, the excess is used for energy or stored as fat. One gram of protein provides four kilocalories of energy. This is the same amount of energy that one gram of carbohydrate provides.

  Lipids

  Lipids, or fatty acids, are organic compounds that consist of repeating units of carbon, hydrogen, and oxygen. They provide the body with energy. The heart and skeletal muscles rely mainly on lipids for fuel. One gram of lipids provides nine kilocalories of energy, more than twice the amount provided by carbohydrates or proteins. Lipids have several other functions as well. Lipids form an insulating sheath around nerve cells that helps nerve messages travel more quickly. Lipids also help form substances that regulate blood pressure, blood clotting, and blood lipid levels. In addition, lipids make up the membranes that surround cells.

  The term fat is often used interchangeably with the term lipid, but fats are actually a particular type of lipid, called triglycerides, in which three fatty acids are bound to a compound called glycerol. Fats are important in the body. They are the main form in which the body stores energy. Stored body fat is called adipose tissue. Stored fat not only provides an energy reserve but also cushions and protects internal organs. In addition, stored fat insulates the body and helps prevent heat loss in cold weather.

  Although lipids and fats are necessary for life, they may be harmful if they are present in the blood at high levels. Both triglycerides and the lipid called cholesterol are known to damage blood vessels if their concentrations in the blood are too high. By damaging blood vessels, triglycerides and cholesterol also increase the risk of heart disease.

  Lipids are classified as either saturated fatty acids or unsaturated fatty acids. This classification is based on the number of chemical bonds between carbon atoms in lipid molecules.

  Saturated fatty acids have only single bonds between carbon atoms. This gives them properties that make them unhealthful. Their amount in the diet should be kept as low as possible. If consumed in excess, they contribute to high blood levels of cholesterol and triglycerides. Saturated fatty acids are found in animal foods, such as meat, whole milk, and eggs.

  Unsaturated fatty acids have at least one double bond between carbon atoms. This gives them properties that make them more healthful. Eaten in appropriate amounts, they may help lower blood levels of cholesterol and triglycerides and decrease the risk of cardiovascular disease. They are found mainly in plant foods.

  The human body can synthesize all but two of the fatty acids it needs: omega-3 fatty acids and omega-6 fatty acids. Both are unsaturated fatty acids. They are called essential fatty acids because they must be present in the diet. They are found in salmon, vegetable oil, flaxseed, eggs, and whole grains. Small amounts of these two fatty acids may help lower blood pressure as well as blood levels of harmful lipids.

  Unsaturated fatty acids known as trans fatty acids (or trans fats), are manufactured from plant oils and do not occur naturally. They are added to foods to extend their shelf life. Trans fats have properties like saturated fats and may increase risk of cardiovascular disease. They should be avoided in balanced eating. Many manufacturers no longer add trans fats to food products, and their use in restaurants has been banned in some cities.

  Water

  You may not think of water as a food, but it is a nutrient. Water is essential to life because it is the substance within which all the chemical reactions of life take place. An adult can survive only a few days without water. Table 1, above, shows water requirements for young people.

  Water is lost from the body in exhaled air, sweat, and urine. Dehydration occurs when a person does not take in enough water to replace the water that is lost. Symptoms of dehydration include headaches, low blood pressure, and dizziness. If dehydration continues, it can quickly lead to unconsciousness and even death. When you are very active, particularly in the heat, you can lose a great deal of water in sweat. To avoid dehydration, you should drink extra fluids before, during, and after exercise.

  Taking in too much water—especially without consuming extra salts—can lead to a condition called hyponatremia. In this condition, the brain swells with water, causing symptoms such as nausea, vomiting, headache, and coma. Hyponatremia can be fatal, so it requires emergency medical care.

  Balanced Eating

  Balanced eating is a way of eating that promotes good health. It includes eating several medium-sized meals regularly throughout the day. It also includes eating the right balance of different foods to provide the body with all the nutrients it needs. Table 1, above, lists macronutrient needs for young people, and you just read about foods that provide each of these macronutrients. How much of these foods should you eat to get the right balance of nutrients? Two tools for choosing foods that provide balanced nutrition are MyPyramid and nutrition labels on food packages.

  MyPyramid

  MyPyramid was developed by the U.S. Food and Drug Administration. It shows how much you should eat each day of foods in different food groups. MyPyramid is shown in Figure below. You can visit the MyPyramid.gov website for more details or to customize MyPyramid for your gender, age, activity level, and other factors.

  Figure 23.1

  MyPyramid is visual representation of how much you should eat each day of foods in different food groups.

  Guidelines for Using MyPyramid

  1. The six colored bands represent six food groups:

  Brown = Grains—At least half should be whole grains.

  Green = Vegetables—Choose a variety of vegetables, including dark green and orange vegetables, dry beans and peas.

  Red = Fruits—Include a variety of fruits, and consume whole fruits instead of fruit juices.

  Yellow = Oils—Choose mainly unsaturated nut and vegetable oils.

  Blue = Milk—Dairy products should be low-fat or fat-free choices.

  Purple = Meat and Beans—Choose fish and low-fat meats, as well as beans, peas, nuts, and seeds.

  2. The width of each colored band shows the proportion of food that should come from each food group.

  3. The figure climbing stairs reminds you to balance food with exercise: 30–60 min/day of moderate-to-vigorous activity is recommended for most people.

  Each food group represented by a colored band in MyPyramid is a good source of nutrients. The wider the band, the more you should eat from that food group. For example, the brown band is widest, so the largest proportion of foods should come from the grains group. The white tip of MyPyramid represents foods that should be eaten only in very small amounts or very infrequently. They include foods such as ice cream and potato chips that contain few nutrients and may contribute excess kilocalories to the diet.

  The figure “walking” up the side of MyPyramid in Figure above represents the role of exercise in balanced eating. Daily exercise helps you burn any extra energy that you consume in foods. The more active you are, the more energy you use. Light activities, such as golfing, typically use only a few hundred kilocalories per hour. Strenuous activities, such as run
ning, may use over 900 kilocalories per hour.

  Harvard University recently developed an alternative healthy eating pyramid, which is shown in Figure below. It differs from MyPyramid in placing more emphasis on exercise and a greater focus on eating fruits, vegetables, and healthy plant oils. It moves red meats and starchy, low-nutrient foods, such as white bread and white rice, to the category of foods to eat in very limited amounts. Some experts think that the Harvard pyramid is less confusing than MyPyramid and represents an even healthier way of eating.

  Figure 23.2

  Healthy eating pyramid.

  Food Labels

  Packaged foods are required by law to carry a nutrition facts label, like the one in Figure below, showing the nutrient content and ingredients in the food.

  Figure 23.3

  Nutrition facts label.

  Reading nutrition facts labels can help you choose foods that are high in nutrients such as protein and low in nutrients such as fat. Nutrition facts labels can also help you choose foods that are nutrient dense. Nutrient density is the ratio of nutrient content, measured in grams, to total energy content in kilocalories.

  Consider the following two foods: 15g/300 kcal = 0.05 g/kcal

  Nutrient Density:

  Energy: 300 kcal

  Protein: 15 g

  Food A

  10g/120 kcal = 0.08 g/kcal

  Nutrient Density:

  Energy: 120 kcal

  Protein: 10 g

  Food B

  In terms of protein, Food B is more nutrient dense than Food A, because it provides more protein per kilocalorie. Eating nutrient-dense foods helps you to get enough of each nutrient without taking in too many kilocalories.

  Reading the ingredients list on food labels can also help you choose healthful foods for balanced eating. At the top of the list, look for ingredients such as whole grains, vegetables, and fruits. These are foods you need the most of in a balanced diet. Avoid foods that contain processed ingredients, such as white flour or white rice. Processing removes nutrients. As a result, processed foods generally supply fewer nutrients than whole foods, even when they have been enriched or fortified with added nutrients.

  Weight Gain and Obesity

  Any unneeded energy in food, whether it comes from carbohydrates, proteins, or lipids, is stored in the body as fat. An extra 3,500 kilocalories of energy results in the storage of one pound (0.45 kg) of fat. People who consistently consume more food energy then they need gain weight. People who continue to store fat and gain weight may eventually become obese.

  Obesity occurs when the body mass index is 30.0 kg/m2 or greater. Body mass index (BMI) is a simple way to estimate the percentage of fat in the body. It is calculated by dividing an individual’s weight (in kilograms) by the square of the individual’s height (in meters). For example, a man who weighs 88 kilograms and is 1.7 meters tall has a BMI of:

  88 kg ÷ (1.7 m)2 = 30.4 kg/m2.

  Compare this BMI with the BMI values in Table below. The man’s BMI is greater than 29.9 kg/m2, so he would be considered obese.

  Body Mass Index and Weight Status BMI Value (kg/m2) Weight Status

  <18.5 Underweight

  18.5–24.9 Normal weight

  25.0–29.9 Overweight

  >29.9 Obese

  People who are obese are at greater risk of many serious health problems, including metabolic syndrome. Metabolic syndrome is a cluster of conditions that together greatly increase the risk of cardiovascular disease. The conditions include type 2 diabetes, high blood pressure, and high blood levels of LDL cholesterol and triglycerides. A wide range of other disorders may also be related to obesity, including menstrual disorders in females, certain types of cancer, osteoarthritis, and depression. In addition, people who are obese have a lower life expectancy.

  From 1980 to 2002, the number of obese adults in the U.S. doubled. By 2004, almost one-third of U.S. adults aged 20 years or older were obese. The prevalence of obesity in the U.S. is the highest in the developed world. Given its prevalence and serious health risks, obesity is now a leading public health problem in this country.

  The combination of eating too much and moving too little generally causes obesity. The best way to lose weight and avoid obesity is to eat less and exercise more. However, many factors may play a role in obesity, making it difficult for most people to eat wisely and lose weight. These factors may be genetic or environmental.

  Several genes have been identified that control appetite and may contribute to some cases of obesity. An important environmental factor that contributes to obesity is the availability of high-fat, high-Calorie fast foods. Other environmental factors that may influence eating habits and contribute to obesity include stress, cultural traditions, and food advertisements. Some people who are obese have an eating disorder called binge eating. Eating disorders are discussed below.

  Vitamins and Minerals

  Unlike the major macronutrients, micronutrients—including vitamins and minerals—do not provide energy. Nonetheless, adequate amounts of micronutrients are essential for good health. The needed amounts generally can be met with balanced eating. However, many people do not eat enough of the right foods to meet their requirements. They may need vitamin or mineral supplements to increase their intake of micronutrients.

  Vitamins

  Vitamins are organic compounds that are needed by the body to function properly. There are 13 vitamins that humans need. They are described in Table below, which also includes recommended daily vitamin intakes for teens.

  Vitamins play many roles in good health, ranging from helping maintain vision to helping form red blood cells. Many vitamins are components of enzymes. For example, vitamin K is a component of enzymes involved in blood clotting. Several vitamins, including vitamins C and E, act as antioxidants. An antioxidant is a compound that neutralizes chemicals called free radicals. Free radicals are produced naturally during cellular activities and may cause some types of cancer. Neutralizing free radicals makes them harmless.

  Some vitamins, including vitamin B6, are produced by bacteria that normally live in the intestines, where they help digest food. Vitamin D is synthesized in the skin when it is exposed to UV radiation in sunlight. Most other vitamins must be obtained from foods because the body is unable to synthesize them. Good food sources of vitamins are listed in the table below. They include whole grains, vegetables, fruits, milk, and nuts.

  Consuming inadequate amounts of vitamins can cause deficiency diseases. For example, consuming inadequate amounts of vitamin D causes soft bones. In children this is called rickets. It can cause permanent bone deformities. Consuming too much of some vitamins can also be dangerous. Overdoses of vitamins can cause problems ranging from diarrhea to birth defects and even death.

  Vitamins are either fat-soluble or water-soluble. This determines whether they can accumulate in the body and lead to overdoses.

  Vitamins A, D, E, and K are fat soluble. Excess intakes of these vitamins are stored in fatty tissues of the body. Because they are stored in the body, they can build up to toxic levels, especially if they are taken improperly in supplements.

  Vitamin C and all the B vitamins are water soluble. Excess amounts of these vitamins are excreted in the urine, so they are unlikely to reach toxic levels in the body.

  Vitamins Vitamin (Chemical Name) Functions in the Body Good Food Sources Recommended Daily Intakes f or Ages 14–18 yr

  (Retinoids)

  Vitamin A

  Needed for good vision, reproduction, and fetal development Carrots, spinach, milk, eggs Females: 700 μg

  Males: 900 μg

  (Thiamine)

  Vitamin B1

  Helps break down macronutrients; essential for proper functioning of nerves Whole wheat, peas, beans, fish, peanuts, meats Females: 1.0 mg

  Males: 1.2 mg

  (Riboflavin)

  Vitamin B2

  Helps the body process amino acids and fats; acts as antioxidant Milk, liver, green leafy vegetables, almo
nds, soybeans Females: 1.0 mg

  Males: 1.3 mg

  (Niacin)

  Vitamin B3

  Helps release energy from macronutrients; needed for healthy skin and nerves Beets, beef liver, pork, turkey, fish, sunflower seeds, peanuts Females: 14 mg

  Males: 16 mg

  (Pantothenic Acid)

  Vitamin B5,

  Helps form critical enzymes for synthesis of macronutrients Whole grains, legumes, eggs, meat Females: 5mg*

  Males: 5 mg*

  (Pyridoxine)

  Vitamin B6

  Forms enzymes needed for amino acid synthesis and energy storage Cereals, yeast, liver, fish, avocadoes, nuts, green beans Females: 1.2 mg

  Males: 1.3 mg

  (Biotin)

  Vitamin B7

  Enables synthesis of fatty acids; helps store energy; keeps level of blood sugar stable None Females: 25 μg*

  Males: 25 μg*

  (Folate)

  Vitamin B9

  Needed to make red blood cells Liver, green leafy vegetables, dried beans and peas Females: 400 μg

  Males: 400 μg

  (Cyanocobalamin)

  Vitamin B12

  Needed for normal functioning of nervous system and formation of blood Meat, liver, milk, shellfish, eggs Females: 2.4 μg

  Males: 2.4 μg