1. Identify diet and lifestyle factors that contribute to the 10 leading causes of death in North America.
2. Define the terms nutrition, carbohydrate, protein, lipid (fat), alcohol, vitamin, mineral, water, kilocalorie (kcal) , and fiber .
3. Determine the total calories (kcal) of a food or diet using the weight and calorie content of the energy-yielding nutrients.
4. Use the basic units of the metric system to calculate percentages, such as percent of calories from fat in a diet.
5. List the major characteristics of the North American diet and the food habits that often need improvement.
6. Describe how our food habits are affected by physiological processes, meal size and composition, early experiences, ethnic customs, health concerns, advertising, social class, and economics.
7. Identify food and nutrition issues relevant to college students.
ARE YOU EATING TOO MUCH SATURATED FAT, TRANS FAT, AND CHOLESTEROL?
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. We 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. Many factors influence our food choices and will also be discussed.
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.
Nutrients Come from Food
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.
Some nutrients that perform important functions can be produced by the body if they are missing from the diet. Whether these nutrients are essential is not clear-cut. For example, humans require vitamin D, but the body is capable of synthesizing its own vitamin D upon exposure of the skin to sunlight. This reduces the need for vitamin D from dietary sources among people who experience regular sun exposure.
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 (Table 1-1). Together, these and related disorders account for two-thirds of all deaths in North America ( Table 1-2 ).
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 later 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.
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 (Table 1-3).
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 fi sh, such as salmon or tuna, at least twice a week is another healthy source of essential fatty acids. The unique fatty acids in these fi sh complement the healthy aspects of common vegetable oils. This will be explained in greater detail, which focuses on lipids.
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 amino acids 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 reflects 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. Table 1-5 lists some noteworthy phytochemicals with their common food sources.
Written by Gordon M. Wardlaw and Anne M. Smith in "Contemporary Nutrition", McGraw-Hill Higher Education, USA, 2009, excerps chapter I, pp-3-12. Digitized, adapted and illustrated to be posted by Leopoldo Costa.