Introduction to Nutrition:
Nutrition is a science. Compared with some other sciences, such as chemistry, that have been studied for thousands of years, nutrition is a young science. Many nutritional facts revolve around nutrients, such as carbohydrates. Nutrients are the nourishing substances in food that provide energy and promote the growth and maintenance of the body. In addition, nutrients aid in regulating body processes such as heart rate and digestion and in supporting the body’s optimum health.
Nutrition researchers look at how nutrients relate to health and disease. Almost daily we are bombarded with news reports that something in the food we eat, such as fat, is not good for us—that it may indeed cause or complicate conditions such as heart disease and cancer. Researchers look closely at the relationships between nutrients and disease, as well as the processes by which you choose what to eat and the balance of foods and nutrients in your diet. In summary, nutrition is a science that studies nutrients and other substances in foods and in the body and the way those nutrients relate to health and disease. Nutrition also explores why you choose the foods you do and the type of diet you eat. Diet is a word that has several meanings. Anyone who has tried to lose weight has no doubt been on a diet. In this sense, diet means weight-reducing diet and is often thought of in a negative way. But a more general definition of diet is the foods and beverages you normally eat and drink.
Type of Nutrients:
As stated, nutrients provide energy or kcalories, promote the growth and maintenance of the body, and/or regulate body processes. There are about 50 nutrients that can be arranged into six classes, as follows:
Each nutrient class performs different functions in the body.
Foods rarely contain just one nutrient. Most foods provide a mix of nutrients. For example, bread often is thought of as providing primarily carbohydrates, but it is also an important source of certain vitamins and minerals. Food contains more than just nutrients. Depending on the food, it may contain colorings, flavorings, caffeine, phytochemicals (minute substances in plants that may protect health), and other substances.
Carbohydrates, lipids, and protein are called energy-yielding nutrients because they can be burned as fuel to provide energy for the body. They provide kcalories as follows:
Carbohydrates: 4 kcalories per gram
Lipids: 9 kcalories per gram
Protein: 4 kcalories per gram
(A gram is a unit of weight in the metric system; there are about 28 grams in 1 ounce.) Vitamins, minerals, and water do not provide energy or calories.
The body needs vitamins and minerals in small amounts, and so these nutrients are called micronutrients (micro means small). In contrast, the body needs large amounts of carbohydrates, lipids, and protein, and so they are called macronutrients (macro means large).
Another way to group the classes of nutrients is to look at them from a chemical point of view. In chemistry, any compound that contains carbon is called organic. If a compound does not contain carbon, it is called inorganic. Carbohydrates, lipids, proteins, and vitamins are all organic. Minerals and water are inorganic.
Carbohydrates are a large class of nutrients, including sugars, starches, and fibers, that function as the body’s primary source of energy. Sugar is most familiar in its refined forms, such as table sugar and high-fructose corn syrup, and is used in soft drinks, cookies, cakes, pies, candies, jams, jellies, and other sweetened foods. Sugar is also present naturally in fruits and milk (even though milk does not taste sweet). Starch is found in breads, breakfast cereals, pastas, potatoes, and beans. Both sugar and starch are important sources of energy for the body. Fiber can’t be broken down or digested in the body, and so it is excreted. It therefore does not provide energy for the body. Fiber does a number of good things in the body, such as improve the health of the digestive tract. Good sources of fiber include legumes (dried beans and peas), fruits, vegetables, whole-grain foods such as whole-wheat bread and cereal, nuts, and seeds.
Lipids are a group of fatty substances, including triglycerides and cholesterol, that are soluble in fat, not water, and that provide a rich source of energy and structure to cells. The most familiar lipids are fats and oils, which are found in butter, margarine, vegetable oils, mayonnaise, and salad dressings. Lipids are also found in the fatty streaks in meat, the fat under the skin of poultry, the fat in milk and cheese (except skim milk and products made with it), baked goods such as cakes, fried foods, nuts, and many processed foods, such as canned soups and frozen dinners. Most breads, cereals, pasta, fruits, and vegetables have little or no fat. Triglycerides are the major form of lipids. They provide energy for the body as well as a way to store energy as fat.
Protein major structural component of the body’s cells that is made of nitrogen containing amino acids assembled in chains, particularly rich in animal foods. Most of the kcalories we eat come from carbohydrates or fats. Only about 15 percent of total kcalories come from protein. This doesn’t mean that protein is less important. On the contrary, protein is the main structural component of all the body’s cells. It is made of units called amino acids, which are unique in that they contain nitrogen. Besides its role as an important part of cells, protein regulates body processes and can be burned to provide energy (although the body prefers to burn carbohydrates and lipids). Protein is present in significant amounts in foods from animal sources, such as beef, pork, chicken, fish, eggs, milk, and cheese. Protein appears in plant foods, such as grains, beans, and vegetables, in smaller quantities. Fruits contain only very small amounts of protein.
Vitamins Noncaloric, organic nutrients found in a wide variety of foods that are essential in small quantities to regulate body processes, maintain the body, and allow growth and reproduction. There are 13 different vitamins in food. Vitamins are noncaloric, organic nutrients found in a wide variety of foods. They are essential in small quantities to regulate body processes, maintain the body, and allow growth and reproduction. Instead of being burned to provide energy for the body, vitamins work as helpers. They assist in the processes of the body that keep you healthy. For example, vitamin A is needed by the eyes for vision in dim light. Vitamins are found in fruits, vegetables, grains, meat, dairy products, and other foods. Unlike other nutrients, many vitamins are susceptible to being destroyed by heat, light, and other agents.
Minerals Noncaloric, inorganic chemical substances found in a wide variety of foods; needed to regulate body processes, maintain the body, and allow growth and reproduction. Minerals are also required by the body in small amounts and do not provide energy. Like vitamins, they work as helpers in the body and are found in a variety of foods. Some minerals, such as calcium and phosphorus, become part of the body’s structure by building bones and teeth. Unlike vitamins, minerals are indestructible and inorganic.
Nutrient Density A measure of the nutrients provided in a food per kcalorie of that food.
All foods were not created equal in terms of the kcalories and nutrients they provide. Some foods, such as milk, contribute much calcium to your diet, especially when you compare them with other beverages, such as soft drinks. The typical can of cola (12 fluid ounces) contributes large amounts of sugar (40 grams, or about 10 teaspoons), no vitamins, and virtually no minerals. When you compare calories, you will find that skim milk (at 86 kcalories per cup) packs fewer calories than does cola (at 97 kcalories per cup). Therefore, we can say that milk is more “nutrient-dense” than cola, meaning that milk contains more nutrients per kcalorie than colas do.
The nutrient density of a food depends on the amount of nutrients it contains and the comparison of that to its caloric content. In other words, nutrient density is a measure of the nutrients provided per kcalorie of a food. As Figure 1-6 shows, broccoli offers many nutrients for its few calories. Broccoli is considered to have a high nutrient density because it is high in nutrients relative to its caloric value. Vegetables and fruits are examples of nutrient-dense foods. In comparison, a cupcake contains many more kcalories and few nutrients. By now, you no doubt recognize that some foods, such as candy bars, have a low nutrient density, meaning that they are low in nutrients and high in kcalories. These foods are called empty-kcalorie foods because the kcalories they provide are “empty” (that is, they deliver few nutrients). The next section will tell you more about what a nutritious diet is.
Characteristics of A Nutritious Diet:
A nutritious diet has four characteristics. It is:
Your diet must provide enough nutrients, but not too many. This is where adequate and moderate diets fit in. An adequate diet provides enough kcalories, essential nutrients, and fiber to keep you healthy, whereas a moderate diet avoids taking in excessive amounts of kcalories or eating more of one food or food group than is recommended. In the case of kcalories, for example, consuming too many leads to obesity. The concept of moderation allows you to choose appropriate portion sizes of any food as well as to indulge occasionally in high-kcalorie, high-fat foods such as french fries and premium ice cream.
Although it may sound simple to eat enough, but not too much, of the necessary nutrients, surveys show that most adult Americans find this hard to do. One of the best ways to overcome this problem is to select nutrient-dense foods. As stated earlier, nutrient-dense foods contain many nutrients for the kcalories they provide.
Next, you need a balanced diet. Eating a balanced diet means eating more servings of nutrient-dense foods such as whole grains, fruits, and vegetables and fewer servings of foods such as cakes, cookies, and chips, which supply few nutrients. For example, if you drink a lot of soft drinks, you will be getting too much sugar and possibly not enough calcium, a mineral found in milk. This is a particular concern for children, whose bones are growing and who are more likely than ever before to be obese. The typical American diet is unbalanced. We eat more fried foods and fatty meats than we need, and we drink too much soda. At the same time we eat too few fruits, vegetables, and whole grains. A balanced diet is also likely to be adequate and moderate.
Last, you need a varied diet—in other words, you need to eat a wide selection of foods to get the necessary nutrients. If you imagine everything you eat for one week piled in a grocery cart, how much variety is in that cart from week to week? Do you eat the same bread, the same brand of cereal, the same types of fresh fruit, and so on, every week? Do you constantly eat favorite foods? Do you try new foods? A varied diet is important because it makes it more likely that you will get the essential nutrients in the right amounts. Our next topic, the Dietary Reference Intakes, gets specific about the amounts we need of most nutrients.
Nutrient Recommendations: Dietary Reference Intakes:
The Dietary Reference Intakes (DRIs) expand and replace what you may have known as the Recommended Dietary Allowances in the United States and the Recommended Nutrient Intakes in Canada. (The Recommended Dietary Allowance (RDA) is the amount of a nutrient that meets the known nutrient needs of practically all healthy persons.) The DRIs are developed by the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes of the Food and Nutrition Board (a unit of the Institute of Medicine, part of the National Academy of Sciences), with involvement by Canadian scientists.
DRIs are estimates of nutrient intakes to be used for planning and evaluating diets. The DRIs are greatly expanded from the original RDAs and include the original RDAs as well as three new values.
The DRIs vary depending on age and gender, and there are DRIs for pregnant and lactating women. The DRIs are meant to help healthy people maintain health and prevent disease. They are not designed for seriously ill people, whose nutrient needs may be much different.
The 2002 Dietary Reference Intake report established an Estimated Energy Requirement (EER) for healthy individuals. EER is the dietary energy intake measured in kcalories that is needed to maintain energy balance in a healthy adult so that he or she does not gain or lose weight. Your actual EER depends on your age, gender, weight, height, and level of physical activity. There is no RDA or UL for kcalories because these concepts do not apply to energy and would lead to weight gain.
The 2002 Dietary Reference Intake report also established Acceptable Macronutrient Distribution Ranges (AMDR) for carbohydrate, fat, and protein. AMDR is defined as the percent of total kilocalories coming from carbohydrate, fat, or protein that is associated with a reduced risk of chronic disease while providing an adequate intake. For example, adults (and children over 1 year old) should obtain 45 to 65 percent of their total kcalories from carbohydrates. The AMDR for adults is 20 to 35 percent of total kcalories from fat and 10 to 35 percent of total kcalories from protein. The wide range allows for more flexibility in dietary planning for healthy people.
The RDA and AI are useful in planning diets for individuals. The EAR can be used to plan diets for groups to ensure that most people get enough nutrients and also to assess the number of people with inadequate intakes within a group.
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