Do carbohydrates in our diets cause many of our health problems? Is a high protein diet always safe and foolproof? Are some foods unsafe? Do you need to take a balanced multivitamin and mineral supplement? Should you become a vegetarian? If you have asked yourself any of these questions and are confused about what you should eat, you are not alone. As you begin this study of nutrition, keep this in mind. Research over the last 40 years has shown that a healthy diet—especially one rich in fruits, vegetables, and whole grains—coupled with regular prolonged, vigorous exercise and some strength-building exercise—can both prevent and treat many age-related diseases. Overall, it is clear that the nutrition aspect of the lifestyles of some (but not all) North Americans are out of balance with their metabolism and physiology. We live longer than our ancestors did, so preventing the diseases that develop during aging is a more important focus today than in the past. By making optimal dietary choices, we can strive to bring the goal of a long, healthy life within reach.
Good Health: The Nutrition Connection
In your lifetime, you will eat about 70,000 meals and 60 tons of food. Chapter 1 will take a close look at the general classes of nutrients supplied by this food, the role research plays in sorting out which food components are essential for the maintenance of health, and the powerful effect of dietary habits in determining overall health.
What Is Nutrition?
Nutrition is the science that links foods to health and disease. It includes the processes by which the human organism ingests, digests, absorbs, transports, and excretes food substances.
What is the difference between food and nutrients? Food provides the energy (in the form of calories) as well as the materials needed to build and maintain all body cells. Nutrients are the substances obtained from food that are vital for growth and maintenance of a healthy body throughout life. For a substance to be considered an essential nutrient, three characteristics are needed:
• First, at least one specific biological function of the nutrient in the body must be identified.
• Second, omission of the nutrient from the diet must lead to a decline in certain biological functions, such as production of blood cells.
• Third, replacing the omitted nutrient in the diet before permanent damage occurs, will restore those normal biological functions.
Why Study Nutrition?
Nutrition is a lifestyle factor that is a key to developing and maintaining an optimal state of health for you. A poor diet and a sedentary lifestyle are known to be risk factors for life-threatening chronic diseases such as cardiovascular (heart) disease, hypertension, diabetes, and some forms of cancer. Together, these and related disorders account for two-thirds of all deaths in North America. Not meeting nutrient needs in younger years makes us more likely to suffer health consequences, such as bone fractures from the disease osteoporosis , in later years. At the same time, taking too much of a nutrient—such as vitamin A supplement—can be harmful. Another dietary problem, drinking too much alcohol, is associated with many health problems. U.S. government scientists have calculated that a poor diet combined with a lack of sufficient physical activity contributes to up to 3 0,000 fatal cases of cardiovascular disease, cancer, and diabetes each year among adults in the United States. Thus, the combination of poor diet and too little physical activity may be the second leading cause of death in the United States. In addition, obesity is considered the second leading cause of preventable death in North America (smoking is the first). Put together, obesity and smoking cause even more health problems. And as you will learn in Chapter 7, surgery to help treat obesity costs about $12,000 to $ 0,000. Compare that to the low cost of prevention. Obesity and chronic diseases are often preventable. Age fast or age slowly: It is partly your choice. As you gain understanding about your nutritional habits and increase your knowledge about optimal nutrition, you will have the opportunity to dramatically reduce your risk for many common health problems.
Classes and Sources of Nutrients
To begin the study of nutrition, let’s start with an overview of the six classes of nutrients. You are probably already familiar with the terms carbohydrates, lipids (fats and oils), proteins, vitamins, and minerals. These, plus water, make up the six classes of nutrients found in food. Nutrients can then be assigned to three functional categories: (1) those that primarily provide us with calories to meet energy needs (expressed in kilocalories [kcal]); (2) those important for growth, development, and maintenance; and (3) those that act to keep body functions running smoothly. Some function overlap exists among these categories. The energy-yielding nutrients make up a major portion of most foods Let’s now look more closely at these six classes of nutrients.
Carbohydrates
Chemically, carbohydrates are composed mainly of the elements carbon, hydrogen, and oxygen. Carbohydrates provide a major source of calories for the body, on average 4 kcal per gram. Carbohydrates can exist as simple sugars and complex carbohydrates. Simple sugars, frequently referred to as sugars, are relatively small molecules. The smallest simple sugars consist of a single sugar unit and are called monosaccharides. The sugar in your blood, glucose (also known as blood sugar or dextrose), is an example of a monosaccharide. Other simple sugars are made by joining two monosaccharides to form a disaccharide. Table sugar, sucrose, is an example of a disaccharide because it is formed from fructose and glucose (both monosaccharides). Joining many monosaccharides—often found as repeating units—forms polysaccharides, also known as complex carbohydrates. For example, plants store carbohydrates in the form of starch, a polysaccharide made up of hundreds of repeating glucose units. Aside from enjoying their taste, we need sugars and other carbohydrates in our diets primarily to help satisfy the calorie needs of our body cells. Glucose, a simple sugar that the body can derive from most carbohydrates, is a major source of calories for most cells. When not enough carbohydrate is consumed to supply suffcient glucose, the body is forced to make glucose from proteins—not a healthy change. During digestion, complex carbohydrates are broken down into single sugar molecules (such as glucose), and absorbed via cells lining the small intestine into the bloodstream. However, the bonds between the sugar molecules in certain complex carbohydrates, called fiber, cannot be broken down by human digestive processes. Fiber passes through the small intestine undigested to provide bulk for the stool (feces) formed in the large intestine (colon).
Lipids
Lipids (mostly fats and oils) are composed primarily of the elements carbon and hydrogen; they contain fewer oxygen atoms than do carbohydrates. Lipids yield more calories per gram than do carbohydrates—on the average, 9 kcal per gram—because of this difference in composition. Lipids dissolve in certain solvents (e.g., ether and benzene) but not in water. The basic structure of most lipids is the triglyceride. Triglycerides provide a key calorie source (e.g., fatty acids) for the body and are the major form of fat in foods. They are also the main form for energy storage in the body. In this book, the more familiar terms fats or fats and oils will generally be used, rather than lipids or triglycerides. Generally, fats are lipids that are solid at room temperature and oils are lipids that are liquid at room temperature. Most lipids can be separated into two basic types—saturated fat and unsaturated fat—based on the chemical structure of their fatty acids. Saturated fats are rich in saturated fatty acids. These fatty acids do not contain carbon-carbon double bonds. Unsaturated fats are rich in unsaturated fatty acids. These fatty acids contain one or more of carbon-carbon double bonds. The presence of carbon-carbon double bonds determines whether the lipid is solid or liquid at room temperature. Think of a double bond as a “kink” somewhere along the carbon chain of a fatty acid. Having one or more kinks limits the extent to which fatty acids can pack tightly together, and therefore how solid a mass of fatty acids can be. Plant oils, such as corn oil, tend to contain many unsaturated fatty acids—this makes them liquid at room temperature. Animal fats, such as butter or lard, are often rich in saturated fatty acids—this makes them solid at room temperature. Almost all foods contain a variety of saturated and unsaturated fatty acids. Saturated fat should be limited in our diet because it can raise blood cholesterol. High blood cholesterol leads to clogged arteries and can eventually lead to cardiovascular disease. Certain unsaturated fatty acids are essential nutrients that must come from our diet. These key fatty acids that the body can’t produce, called essential fatty acids, perform several important functions in the body: they help regulate blood pressure and play a role in the synthesis and repair of vital cell parts. However, we need only about four tablespoons of a common plant oil (such as the canola or soybean oil) each day to supply these essential fatty acids. A serving of fatty fish, such as salmon or tuna, at least twice a week is another healthy source of essential fatty acids. The unique fatty acids in these fish complement the healthy aspects of common vegetable oils.
Proteins
Like carbohydrates and fats, proteins are composed of the elements carbon, oxygen, and hydrogen. But, unlike the other energy-yielding nutrients, all proteins also contain nitrogen. Proteins are the main structural material in the body. For example, proteins constitute a major part of bone and muscle; they are also important components in blood, body cells, enzymes , and immune factors. Proteins can also provide calories for the body—on average, kcal per gram. Typically, however, the body uses little protein for the purpose of meeting daily calorie needs. Proteins are formed when amino acids are bonded together. Twenty or so common aminoacids are found in food; nine of these are essential nutrients for adults, and one additional amino acid is essential for infants. Most North Americans eat about one and a half to two times as much protein as the body needs to maintain health. In a person with no evidence of cardiovascular disease, kidney disease, diabetes, or family history of colon cancer or kidney stones, this amount of extra protein in the diet is generally not harmful—it refi ects the standard of living and the dietary habits of most North Americans. The excess is used for calorie needs but ultimately can contribute to storage of fat and carbohydrate production.
Vitamins
Vitamins have a variety of chemical structures and can contain the elements carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, and others. The main function of vitamins is to enable many chemical reactions to occur in the body. Some of these reactions help release the energy trapped in carbohydrates, lipids, and proteins. Remember, however, that vitamins themselves provide no usable calories for the body.
The 13 vitamins are divided into two groups: four are fat-soluble in that they dissolve in fat (vitamins A, D, E, and K); nine are water-soluble in that they dissolve in water (vitamin C and the B vitamins). The two groups of vitamins often act differently. For example, cooking destroys water-soluble vitamins much more readily than it does fat-soluble vitamins. Water-soluble vitamins are also excreted from the body much more readily than are fat-soluble vitamins. Thus, the fat-soluble vitamins, especially vitamin A, are much more likely to accumulate in excessive amounts in the body, which then can lead to toxicity.
Minerals
Minerals are structurally simple, inorganic substances, which exist as groups of one or more of the same atoms. All of the nutrients discussed so far are organic compounds. These terms, inorganic and organic, have nothing to do with agriculture but are based on simple chemistry concepts. Inorganic substances for the most part do not contain carbon atoms. Minerals such as sodium and potassium typically function independently in the body, whereas minerals such as calcium and phosphorus function as parts of simple mineral combinations, such as bone mineral. Because of their simple structure, minerals are not destroyed during cooking, but they can still be lost if they dissolve in the water used for cooking and that water is then discarded. Minerals are critical players in nervous system functioning, other cellular processes, water balance, and structural (e.g., skeletal) systems, but yield no calories as such for the body.
The amounts of the 16 or more essential minerals required in the diet for good health vary enormously. Thus, they are divided into two groups: major minerals and trace minerals, based on dietary needs. If daily needs are less than 100 milligrams, the mineral is classified as a trace mineral, otherwise, it is a major mineral. The dietary requirement for some trace minerals has yet to be determined. Minerals that conduct electricity when dissolved in water are also called electrolytes; these include sodium, potassium, and chloride.
Water
Water makes up the sixth class of nutrients. Although sometimes overlooked as a nutrient, water (chemically, H2O) has numerous vital functions in the body. It acts as a solvent and lubricant, as a vehicle for transporting nutrients and waste, and as a medium for temperature regulation and chemical processes. For these reasons, and because the human body is approximately 60% water, the average man should consume about 3 liters—equivalent to 3000 grams or about 13 cups—of water and/or other fiuids containing water every day. Women need closer to 2200 grams or about 9 cups per day. Water is not only available from the obvious sources, but it is also the major component in some foods, such as many fruits and vegetables (e.g., lettuce, grapes, and melons). The body even makes some water as a by-product of metabolism.
Other Important Components in Food
Another group of compounds in foods, especially within the fruit and vegetable groups, is what scientists call phytochemicals. These plant components are not considered essential nutrients in the diet. Still, many of these substances provide significant health benifits. Considerable research attention is focused on various phytochemicals in reducing the risk for certain diseases (e.g., cancer). You can’t just buy a bottle of phytochemicals—they are generally available only within whole foods.
Nutrient Composition of Diets and the Human Body
The quantities of the various nutrients that people consume vary widely, and the nutrient amounts present in different foods also vary a great deal. On a daily basis we consume about 500 grams, or about 1 pound, of protein, fat, and carbohydrate. In contrast, the typical daily mineral intake totals about 20 grams (about 4 teaspoons), and the daily vitamin intake totals less than 300 milligrams (1/15 of a teaspoon). Although we require a gram or so of some minerals, such as calcium and phosphorus, we need only a few milligrams or less of other minerals each day. For example, we need about 10 milligrams of zinc per day, just a few specks of the mineral. This is because growth, development, and later maintenance of the human body are directed by the genetic material (DNA) inside body cells. This genetic blueprint determines how each cell uses the essential nutrients to perform body functions. These nutrients can come from a variety of sources. Cells are not concerned about whether available amino acids come from animal or plant sources. The carbohydrate glucose can come from sugars or starches. Thus, you really aren’t what you eat. Rather, the food that you eat provides cells with basic materials to function according to the directions supplied by the genetic material (genes) housed in body cells.
Energy Sources and Uses
Humans obtain the energy we need for body functions and to do work from various calorie sources: carbohydrates, fats, and proteins. Foods generally provide more than one calorie source. Plant oils are one exception; these are 100% fat. Alcohol is also a source of calories for some of us, supplying about 7 kcal per gram. It is not considered an essential nutrient, however, because it has no required function. Still, alcoholic beverages, such as beer—also rich in carbohydrate—are a contributor of calories to the diet of many adults.
The body releases the energy from the chemical bonds in carbohydrate, protein, and fat (and alcohol) into other forms of energy in order to:
• Build new compounds.
• Perform muscular movements.
• Promote nerve transmissions.
• Maintain ion balance within cells.
You have likely noticed on food labels that the energy in food is often expressed in terms of calories. As defined earlier, a calorie is the amount of heat energy it takes to raise the temperature of 1 gram of water 1 degree Celsius (1°C, centigrade scale). (Chapter 7 has a diagram of the instrument that can be used to measure calories in foods [bomb calorimeter].) A calorie is a tiny measure of heat, so food energy is more conveniently expressed in terms of the kilocalorie (kcal), which equals 1000 calories. (If the “c” in calories is capitalized, this also signifies kilocalories.) A kcal is the amount of heat energy it takes to raise the temperature of 1000 grams (1 liter) of water 1°C. The abbreviation kcal is used throughout this book. On food labels, the word calorie (without a capital “C”) is also used loosely to mean kilocalorie. Any values given on food labels in calories are actually in kilocalories. A suggested intake of 2000 calories per day on a food label is technically 2000 kcal.
Current State of the North American Diet and overall Health
The Food and Nutrition Board (FNB) of the National Academy of Sciences advocates that 10% to 3 % of calories come from protein, % to 6 % from carbohydrate, and 20% to 3 % from fat. (These standards apply to both people in the United States and Canada.) If we ignore alcohol, North American adults consume about 16% of their calorie intake as proteins, 0% as carbohydrates, and 33% as fats. These percentages are estimates and vary slightly from year to year and from person to person and fall within the FNB recommendations. Animal sources supply about two-thirds of protein intake for most North Americans; plant sources supply only about one-third. In many other parts of the world, it is just the opposite: plant proteins—from rice, beans, corn, and other vegetables—dominate protein intake. About half the carbohydrate in North American diets comes from simple sugars; the other half comes from starches (such as in pastas, breads, and potatoes). About 60% of dietary fat comes from animal sources and 0% from plant sources.
Assessing the Current North American Diet
With the aim of fi nding out what and when North Americans eat, federal agencies conduct surveys to collect data about food and nutrient consumption, as well as connections between diet and health. In the United States, the U.S. Department of Health and Human Services monitors food consumption with the National Health and Nutrition Examination Survey (NHANES). In Canada, this information is gathered by Health Canada in conjunction with Agriculture and Agrifood Canada. Results from national nutrition surveys and other studies show that North Americans consume a wide variety of foods, but often do not choose the right balance of foods to meet their nutrient needs. Chapter 2 will look at this situation in more detail. For now, more attention should be focused on foods rich in iron, calcium, potassium, magnesium, various B vitamins, vitamin C (especially smokers), vitamin D, vitamin E, and fiber. Daily intake of a balanced multivitamin and mineral supplement is one strategy to help meet nutrient needs, but does not make up for a poor diet, particularly for calcium, potassium, and fiber intake. Also keep in mind that taking numerous dietary supplements can lead to health problems. Routinely, experts also recommend that we pay more attention to balancing calorie intake with needs. An excess intake of calories is usually tied to overindulgence in chance of developing hypertension than do other ethnic groups in North America and therefore may need to decrease the amount of salt (sodium chloride) and alcohol in their diets. These substances are two of the many factors linked to hypertension. Moderation of salt and alcohol intake—along with saturated fat, trans fat, cholesterol, and total calorie intake— is a recommended practice for all adults. Many North Americans would benefit from a healthier balance of food in their diets. Moderation is the key for some foods, such as sugared soft drinks and fried foods. For other foods, such as fruits and vegetables, increased quantity and variety is warranted. Few adults currently meet the “5 to 9 a-day” recommendation promoted for total servings of vegetables and fruits.
Health Objectives for the United States Include Numerous Nutrition Objectives
Health promotion and disease prevention have been public health strategies in North America since the late 1970s. One part of this strategy is Healthy People 2010, a report issued in 2000 by the U.S. Department of Health and Human Services’ Public Health Service. This report outlines health promotion and disease prevention objectives for the nation for the year 2010 and assigns each of the objectives to appropriate U.S. federal agencies to address. Many nutrition-related objectives are part of the overall plan. The overarching goals of Healthy People 2010 are to increase quality and years of healthy life and to eliminate health disparities.
Improving Our Diets
Greater efforts are needed by the general public to lower intake of saturated fat, trans fat, and cholesterol intake and to improve variety in our diets. However, our cultural diversity, varied cuisines, and generally high nutritional status should be points of pride for North Americans. Today we can choose from a tremendous variety of food products, the result of continual innovation by food manufacturers.
We are eating more breakfast cereals, pizza, pasta entrees, stir-fried meats and vegetables served on rice, salads, tacos, burritos, and fajitas than ever before. Sales of whole milk are down, while in the same period sales of fat-free and 1% low-fat milk have increased. Consumption of frozen vegetables, rather than canned vegetables, is also on the rise. A dietary objective that deserves more attention is to try to eat with others more often. Mealtime is a key social time of the day. The Japanese are ahead of us in recognizing that food’s powers go beyond the realm of nutrition. Their national dietary guidelines stress the importance of eating a variety of foods, maintaining healthy weight, and moderating fat in the diet, but also advise people to make all activities pertaining to food and eating pleasurable.
Today, North Americans live longer than ever and enjoy better general health. Many also have more money and more diverse food and lifestyle choices to consider. The nutritional consequences of these trends are not fully known. Deaths from various forms of cardiovascular disease, for example, have dropped dramatically since the late 1960s, partly because of better medical care and diets. Still, if affluence leads to sedentary lifestyles and high intakes of saturated fat, trans fat, cholesterol, salt, and alcohol, this lifestyle pattern can lead to problems such as cardiovascular disease, hypertension, and obesity. With better technology and greater choices, we can have a much better diet today than ever before—if we know what choices to make.
The goal of this book is to help you find the best path to good nutrition. Nutrition experts generally agree that there are no “good” or “bad” (i.e., “junk”) foods, but some foods provide relatively few nutrients in comparison to calorie content. One’s total diet is the proper focus in a nutritional evaluation. Chapter 2 will emphasize this point and show you how to balance your diet. As you reexamine your nutritional habits, remember your health is largely your responsibility. Your body has a natural ability to heal itself. Offer it what it needs, and it will serve you well. Confusing and conflicting health messages hinder diet change. Nutrition science does not have all the answers, but as you will see, enough is known to help you (1) set a path to good health and (2) put diet-related recommendations you hear in the future into perspective.
Why Am I So Hungry ?
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
The hypothalamus, a region of the brain, helps regulate satiety. Imagine a game of tug-of-war in the brain: two sites in the hypothalamus—the feeding center and the satiety center—work in opposite ways to promote adequate availability of nutrients at all times. When stimulated, cells in the feeding center of the hypothalamus signal us to eat. As we eat, cells in the satiety center are stimulated, and we stop eating. What stimulates these two centers of the hypothalamus? The amounts of macronutrients in the blood probably stimulate both the satiety and feeding centers. For example, when we haven’t eaten for a while, stimulation of the feeding center signals us to eat. When the macronutrient content in the blood rises after a meal, we no longer have a strong desire to seek food. (Exactly how the two are interrelated is under study.) 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.
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. 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.
Putting Hunger and Appetite into Perspective
The next time you pick up a candy bar or ask for second helpings, remember the internal and external infl uences on eating behavior. Body cells (brain, stomach, small intestine, liver, and other organs), macronutrients in the blood, hormones (like leptin and ghrelin), brain chemicals (like serotonin and neuropeptide Y), and social customs all influence food intake. When food is abundant, appetite—not hunger—most likely triggers eating. Keep track of what triggers your eating for a few days. Is it primarily hunger or appetite? Also, satiety regulation is not perfect; body weight fluctuate.
By Gordon M. Wardlaw & Anne M. Smith in the book ' Contemporary Nutrition', McGraw-Hill, New York, 2009, p-3-22. Edited, adapted and illustrated to be posted by Leopoldo Costa.
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