Good nutrition results from eating foods in the proper quantities and with the needed distribution of nutrients to maintain good health in the present and in the future. Malnutrition, on the other hand, is the outcome of a diet in which there is underconsumption, overconsumption, or unbalanced consumption of nutrients that leads to disease or increased susceptibility to disease. These definitions implicitly state that proper nutrition is essential to good health. A history of poor nutritional choices eventually leads to health consequences. Poor nutritional choices have been linked to chronic conditions including CVD and cancer.
The public is bombarded with messages about nutrition, and it is often difficult for the layperson to distinguish good information from bad. Fitness professionals can play an important role in conveying basic nutritional information. However, a registered dietitian is the appropriate health care professional to perform detailed dietary analysis and to counsel individuals with special nutrition needs.
The body requires many nutrients for the maintenance, growth, and repair of tissues. Nutrients can be divided into six classes: carbohydrate, fat, protein, vitamins, minerals, and water. The Food and Nutrition Board of the Institute of Medicine has established dietary reference intakes (DRIs) to help people achieve a healthy intake of nutrients (12). The DRIs consist of recommended intakes for nutrients based on age and sex, including the recommended dietary allowances (RDAs), or the amounts found to be adequate for approximately 97% of the population; adequate intakes (AIs), or the amounts considered adequate although insufficient data exist to establish the appropriate RDA; and tolerable upper intake levels (UL), or the highest intakes believed to pose no health risk. Additionally, acceptable macronutrient distribution ranges (AMDRs) have been established for fat, carbohydrate, and protein. The Institute of Medicine reports can be accessed at the National Academies Press website (www.nap.edu).
Carbohydrate is a nutrient composed of carbon, hydrogen, and oxygen and is an essential source of energy in the body. It can be divided into three categories: monosaccharides, disaccharides, and polysaccharides. Examples of monosaccharides are glucose and fructose. Lactose and sucrose are two of the disaccharides, which are carbohydrate that forms when monosaccharides combine. The monosaccharides and disaccharides are sometimes called simple sugars. Simple sugars contribute significantly to the caloric content of foods such as fruit juices, soft drinks, and candy. The most important simple sugar in the human body is glucose. The molecular formula for glucose is C6H12O6. Polysaccharides are complex carbohydrate formed by combining three or more sugar molecules. Starches and fiber are polysaccharides found in plants. Rice, pasta, and whole-grain breads are just a few examples of foods that are high in complex carbohydrate. When carbohydrate is stored in the body, glucose molecules join together to form large molecules called glycogen. Glycogen is stored in the liver and skeletal muscle.
Grains, vegetables, and fruits are excellent sources of carbohydrate. It is recommended that 45% to 65% of a person’s daily calories come from carbohydrate (14) (see figure 5.1). The RDA for carbohydrate is 130 g · day−1 (14), but the average carbohydrate intake of Americans is well beyond this RDA. The majority of carbohydrate calories should come from complex carbohydrate; foods with added sugar should be limited (22). The reason for eating complex rather than simple carbohydrate is the higher nutrient density of complex carbohydrate. Nutrient density refers to the amount of essential nutrients in a food compared with the calories it contains. For example, a candy bar (containing simple sugars) has a low nutrient density, whereas a slice of whole-grain bread (containing complex carbohydrate) has a high nutrient density.
One of the benefits of consuming foods that are high in complex carbohydrate is that they also typically contain dietary fiber, a nonstarch polysaccharide that is found in plants and cannot be broken down by the human digestive system. Although fiber cannot be digested, it helps prevent hemorrhoids, constipation, and cancers of the digestive system because it helps food move quickly and easily through the digestive system. In addition, consuming water-soluble fiber has been shown to lower cholesterol levels (7). Unfortunately, the typical American diet is low in fiber, with the average intake being approximately 15 g · day−1 (22). The fiber AI for men and women aged 50 and younger is 38 g and 25 g · day−1, respectively (14). For older men and women with lower calorie consumption, the daily recommended levels are 30 g and 21 g, respectively (14). Dietary Guidelines for Americans, 2010 (22) recommends that adults consume approximately 14 g of fiber for every 1,000 kcal consumed. Excellent sources of dietary fiber are grains, vegetables, legumes, and fruits.
As mentioned earlier, carbohydrate is a vital source of energy in the human body. During high-intensity exercise, carbohydrate is the primary fuel source for ATP production. When carbohydrate is broken down in the human body, it yields approximately 4 kcal of energy per gram. This means that a person who eats 10 g of carbohydrate gains approximately 40 kcal of energy to use or store.
The six classes of nutrients are carbohydrate, fat, protein, vitamins, minerals, and water. The metabolism of 1 g of carbohydrate yields 4 kcal of energy. Carbohydrate should contribute 45% to 65% of one’s daily calories, with limited calories coming from simple sugars.
Fat is essential to a healthy diet and contributes to vital functions in the human body. Among the functions performed by fat are temperature regulation, protection of vital organs, distribution of some vitamins, energy production, and formation of cell membranes. Similar to carbohydrate, fat is composed of carbon, hydrogen, and oxygen; however, the chemical structure is different. Triglycerides are the primary storage form of fat in the body. These large molecules are composed of three fatty acid chains connected to a glycerol backbone. The majority of triglycerides are stored in adipose cells (i.e., fat cells), but these stored fats can also be found in other tissues (e.g., skeletal muscle). The aerobic metabolism of triglycerides provides much of the energy needed during rest and low-intensity exercise. When metabolized, 1 g of fat yields 9 kcal of energy. Phospholipids are another type of fat found in the body. As the name implies, these fats have phosphate groups attached to them. Phospholipids are important constituents of cell membranes. Lipoproteins are large molecules that allow fat to travel through the bloodstream. Cholesterol is a sterol, or a fatty substance in which carbon, hydrogen, and oxygen atoms are arranged in rings. In addition to the cholesterol we consume in our diet, the body constantly produces cholesterol, which is used in forming cell membranes and making steroidal hormones. Meat and eggs are the major sources of cholesterol in the typical American diet, and it is recommended that people consume no more than 300 mg of cholesterol each day (22). However, for those attempting to lower blood lipids, limiting daily cholesterol intake to 200 mg is recommended (11).
Sources of dietary fat come from both animals and plants. The AMDR for fat is 20% to 35% (14) (figure 5.1). Saturated fat comes primarily from animal sources and is typically solid at room temperature. Plant sources of saturated fat include palm oil, coconut oil, and cocoa butter. The chemical structure of saturated fat contains no double bonds between carbon atoms; in other words, the fat is saturated with hydrogen atoms. A high intake of saturated fat directly relates to increased CVD. Therefore, one should limit consumption of saturated fat to no more than 10% of total calories (22). Unsaturated fat contains fewer hydrogen atoms because there are some double bonds between the carbon atoms. This type of fat is typically liquid at room temperature. Corn, peanut, canola, and soybean oil are sources of unsaturated fat. Trans fat is unsaturated fat that is common in many processed baked goods such as cookies and cakes. The intake of these fatty acids should be as low as possible because they are linked with negative health outcomes; in fact, various public health initiatives have focused on reducing the level of trans fat in the American diet. The effects of various kinds of fat on health risk are discussed later in the chapter (see Diet, Exercise, and the Blood Lipid Profile).
Monounsaturated fatty acids, found in olive and canola oil, have a single double bond between carbon atoms in the fatty acid chain. Polyunsaturated fatty acids (e.g., fish, corn, soybean, and peanut oils) have two or more double bonds between carbon atoms. Two polyunsaturated fatty acids, alpha-linolenic acid (a type of omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid) cannot be made by the body and must be consumed in the diet. The AI for alpha-linolenic acid is 1.6 g · day−1 for men and 1.1 g · day−1 for women (14). Fish, walnuts, and canola oil are sources of this fatty acid. The AI for linolenic acid is 17 g and 12 g · day−1 for men and women, respectively (14). Sources include vegetable oils, nuts, avocados, and soybeans.
The AMDR for fat is 20% to 35%, and no more than 10% of calories should come from saturated fat. Cholesterol intake should be limited to 300 mg · day−1. Intake of trans fat should be as low as possible. The breakdown of 1 g of fat yields 9 kcal of energy.
Protein is a substance composed of carbon, hydrogen, oxygen, and nitrogen. All forms of protein are combinations of amino acids. Amino acids are molecules composed of an amino group (NH3), a carboxyl group (COO), a hydrogen atom, a central carbon atom, and a side chain. The differences in the side chains give unique characteristics to each amino acid. Amino acids can combine in innumerable ways to form proteins, and it is estimated that tens of thousands of protein types exist in the body. The order of the linked amino acids provides the unique structure and function of protein that allow it to serve many functions in the body. Some of the most common functions are the following:
- Carry oxygen (hemoglobin)
- Fight disease (antibodies)
- Catalyze reactions (enzymes)
- Allow muscle contraction (actin, myosin, and troponin)
- Act as connective tissue (collagen)
- Clot blood (prothrombin)
- Act as a messenger (protein hormones such as growth hormone)
Of the 20 amino acids required by the human body, most can be constructed from other substances in the body; however, there are eight essential amino acids (nine for children and some older adults) that the body cannot synthesize and so must be a part of one’s regular diet. Eating a variety of protein-containing foods typically meets this need. Protein is found in both meat and plant products. Animal sources of protein such as meat, milk, and eggs contain the essential amino acids; however, plant sources of protein such as beans, starchy vegetables, nuts, and grains do not always contain all essential amino acids. Because of this, vegetarians must consume a variety of protein-containing foods. Examples of foods that contain complementary proteins are legumes and grains, vegetables and nuts, and legumes and seeds (9). For more information on planning vegetarian diets, see the summary of the vegetarian food guide established by the American Dietetic Association (ADA) and Dietitians of Canada (18).
The AMDR for protein is 10% to 35% (figure 5.1). This amount ensures adequate protein for the growth, maintenance, and repair of cells. The adult RDA is 0.8 g of protein for each kilogram of body weight (14). As discussed later in this chapter, individuals who are training intensely may have higher protein requirements. Additionally, children need more protein to support their continually growing bodies.
In addition to the functions listed previously, protein can be metabolized for energy production. The breakdown of 1 g of protein yields approximately 4 kcal of energy for the body. The contribution of protein to energy needs during exercise is usually quite small (1 hr) or when a person is not well nourished, protein may supply up to 15% of the body’s energy needs.
Protein is made of amino acids and serves numerous functions. To ensure that all needed amino acids are adequately available, people should eat a variety of protein-containing foods each day. The AMDR for protein is 10% to 35%. The breakdown of 1 g of protein yields 4 kcal of energy.
Vitamins are organic substances that are essential to the normal functioning of the human body. Although vitamins do not contain energy for the body, they are essential in the metabolism of fat, carbohydrate, and protein. The body needs 14 vitamins for numerous processes, including blood clotting, protein synthesis, and bone formation. Because of the critical role vitamins play, they need to exist in proper quantities in the body. Major functions, important dietary sources, and recommended intakes of vitamins are listed in table 5.1. There are two major classifications of vitamins: fat soluble and water soluble.
The chemical structure of fat-soluble vitamins causes them to be transported and stored with lipids. The four fat-soluble vitamins are A, D, E, and K. Because these vitamins are stored in the body, it is not necessary to continually ingest large amounts of them; however, a small daily intake of each is recommended.
The B vitamins and vitamin C are water-soluble vitamins. These vitamins are not stored in large quantities in the body and therefore must be consumed daily. Deficiencies related to water-soluble vitamins such as scurvy (vitamin C deficiency) and beriberi (thiamin deficiency) may occur rather quickly. Overconsuming either fat-soluble or water-soluble vitamins can lead to toxic effects; however, because fat-soluble vitamins are stored in the body, the potential for overdose with these substances is greater (9).
Minerals are inorganic elements that serve a variety of functions in the human body. The minerals that appear in the largest quantities (calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium) are often called macrominerals or major minerals. Other minerals are also essential to normal functioning of the body, but because they exist in smaller quantities, they are called microminerals ortrace elements. Functions, dietary sources, and recommended intakes of minerals are listed in table 5.2.
Calcium is often inadequately consumed by Americans. It is important in the mineralization of bone, in muscle contraction, and in transmission of nervous impulses. Osteoporosis is a disease characterized by a decrease in the total amount of bone mineral in the body and in the strength of the remaining bone. This condition is most common in the elderly but also may exist in younger people who have diets inadequate in calcium, vitamin D, or both. It is estimated that in the United States, osteoporosis results in approximately 1.5 million fractures per year with resultant health care costs of nearly $20 billion (23). Maximal bone density is achieved during the early adult years, and during older adult years, bone density declines in everyone. Those who achieve the highest bone density and maintain adequate intakes of calcium and vitamin D are most protected from osteoporosis. Recently, RDAs were released for calcium (see table 5.3) (16). The DRIs for vitamin D are contained in table 5.1. The calcium RDA for adults aged 19 to 50 is 1,000 mg · day−1, with higher values for older adults and adolescents. Milk, dark green vegetables, and nuts are excellent sources of calcium. One cup (8 oz or 237 ml) of 1% milk has approximately 300 mg of calcium, which is almost one-third of the daily recommendation for a young adult.
Iron is another mineral that is often underconsumed by Americans, particularly women and children. In fact, the most prevalent nutrient deficiency in the United States is iron deficiency (9). In addition to being a critical component of hemoglobin and myoglobin, iron is necessary for the functioning of the immune system, the formation of brain neurotransmitters, and the functioning of the electron transport chain (9). The oxygen-carrying properties of hemoglobin depend on iron. There is a continual turnover of red blood cells in the body, and much of the iron used to form new hemoglobin comes from old red blood cells. However, there is a daily need for iron, and if iron reserves (liver, spleen, bone marrow) and intake are inadequate, hemoglobin cannot be formed and iron-deficiency anemia results. In this condition, the amount of hemoglobin in red blood cells falls, which decreases the capacity of the blood to transport oxygen. The recommended intake of iron for males and postmenopausal women is 8 mg · day−1 (13). For females during the childbearing years, the recommended daily intake is 18 mg · day−1 (13). Red meat and eggs are excellent sources of iron. Additionally, spinach, lima and navy beans, and prune juice are excellent vegetarian sources of iron. Consuming vitamin C with meals increases the ability to absorb iron.