KEY TERMS

calorie: a measure of the energy in food or of the energy used by the body

carbohydrate: one of three macronutrients; foods that provide carbohydrates are starches, sugars, fruit, vegetables, and milk products

fat: one of three macronutrients; foods that provide fat are oils, margarine, butter, meat, and dairy

macronutrient: carbohydrate, protein, or fat

minerals: inorganic substances that are essential for body processes; the major minerals include calcium, phosphorus, magnesium, sodium, chloride, and potassium

protein: one of three macronutrients; foods that provide protein are meat and dairy, with smaller amounts of protein found in starches

vitamins: organic (carbon-containing) substances found in plants and animals that are essential for body processes; examples include vitamins A, C, and D, and the B vitamins

STRUCTURE AND FUNCTIONS

Nutrients are necessary for all aspects of living, including cellular metabolism, individual organ function, and multiple organ systems function. Breathing, moving, thinking, playing, and working all rely on the availability of nutrients. The study of nutrition has revolved around either healthy growth and development or nutrition concerning the prevention and treatment of disease. Periods of noticeable growth, such as pregnancy, infancy, childhood, and adolescence, are particular areas of study in nutrition because nutrient needs change during these periods.

The number of calories required to maintain a healthy weight during each life cycle stage depends upon the amount of energy expended. Because men generally have a larger body mass than women, they usually have a larger caloric requirement. Higher caloric requirements are found when body mass is relatively large, and energy output is relatively high, as seen in later adolescence and young adulthood.

Macronutrients. Carbohydrates are an important source of energy. The recommended intake range is 45 to 65 percent of the total caloric intake. Each gram of carbohydrate contributes four calories to the diet. Carbohydrates are found in starchy foods such as potatoes or corn, vegetables and fruits, and milk and yogurt. Carbohydrates are not found in meats or fats unless the food is a mixed dish, such as a hamburger casserole or a candy bar. Simple carbohydrates require little digestion, such as sucrose or sugar. Complex carbohydrates require more digestion, such as starches and fiber. General dietary guidelines suggest an increase in the higher fiber foods. The recommendation is to consume 25 to 35 grams of fiber daily; Americans generally consume 5 to 10 grams. In addition to fruits and vegetables, nuts and seeds, whole wheat bread, and cereal are high-fiber foods.

Dietary protein is required to supply essential amino acids so the body can synthesize new proteins such as enzymes, hormones, or structural proteins to build muscle. Meats (including pork, beef, chicken, or turkey), fish, eggs, and nuts contain substantial protein. Protein is also found in dairy products such as milk, cheese, and yogurt. Some protein can be found in most foods, including starches and vegetables, except those foods that are all fat, such as oil, or all simple carbohydrates, such as sugar. Each gram of protein contributes four calories to the diet. Protein requirements are closely related to caloric intake. With adequate or excess calories, protein is pared, meaning less can be consumed while still meeting all body demands for protein. In these cases, protein does not need to be used for energy. However, inadequate caloric intake requires higher protein levels to meet the body’s needs. Some protein will also be converted to calories for energy needs. The recommended intake assumes that adequate calorie needs are consumed. Protein requirements may be higher than the recommended levels in cases of stress. Although both psychological and physical stress can increase protein requirements, physical stress (including surgery and burns) usually causes a more substantial increase in requirements.

Dietary fat is a risk factor in the development of atherosclerosis or heart disease. Because of this, dietary fat intake recommendations are restricted in total intake and type of fat ingested. Saturated fat is solid at room temperature and is derived from animals. Lard, shortening, and bacon fat are examples. Unsaturated fat can either have many unsaturated bonds (polyunsaturated) in the structure or one (monounsaturated). Polyunsaturated fat is liquid at room temperature and derived from plants such as corn or soybeans. Monounsaturated fat is derived from plants such as canola or olive oil. Whereas recommendations had previously specified levels of intake for both polyunsaturated and monounsaturated fats, current recommendations reflect only a limited total fat intake with a restriction on trans fats. Trans fat is unsaturated fat that has been partially hydrogenated. This process causes a liquid fat to become more solid and is sometimes desirable in baked products. Trans fats are linked to cardiovascular disease and should be limited. Foods that have higher values of trans fat should be labeled as such and are most often processed baked goods, such as cookies, cakes, or pies, or snack foods, such as chips. Regardless of the type of fat, each gram of fat contributes nine calories to the diet.

Minerals. The major minerals include calcium, phosphorus, magnesium, sodium, chloride, and potassium. These minerals are often called “electrolytes,” meaning they can have a negative or positive charge, thus conducting electricity. In the body, these anions (negatively charged) and cations (positively charged) are important for the action potentials of cells, nerve conduction, and the excitation of muscles. The trace minerals are so-called because only very small amounts are needed daily. One of the most common trace minerals is iron. The minerals in a particular food often vary depending on the soil in which a plant is grown or the feed an animal consumes. Minerals are inorganic and cannot be destroyed with cooking or processing.

Calcium is required for normal growth and development of bone as well as nervous and muscular activity, enzyme regulation, and blood clotting. Poor calcium intake is associated with developing porous bones or osteoporosis. Food labels may designate a food as an excellent source (at least 200 milligrams of calcium) or a good source (100 to 199 milligrams of calcium).

Most phosphorus is in the bone as hydroxyapatite. However, phosphorus also occurs as phospholipids in most cell membranes. It is a component of nucleic acids—phosphorus functions as an acid-base buffer in enzymatic reactions and energy transfer. Phosphorus is found in nearly all foods, but good sources include meat, milk products, eggs, grains and legumes, and soft drinks.

Magnesium is essential for hundreds of enzymatic reactions and muscle contractions. About half of the body’s magnesium is found in bone. Green leafy vegetables, fruits, grains, nuts, milk, meat, shellfish, and eggs are good sources of magnesium.

Potassium is found in many foods, including milk, meat, fruit, and vegetables. Together with sodium, potassium is involved in maintaining fluid balance. A diet high in sodium and low in potassium may be involved in developing high blood pressure or hypertension. The major source of sodium in the diet is salt, which is sodium chloride. Foods high in sodium include any food with visible salt (such as crackers and snack foods), pickled foods, processed foods such as lunch meat, canned soup, canned meat, and cured foods such as bacon and ham. A diet high in potassium and calcium and low in sodium is recommended to prevent hypertension.

Most of the body’s iron is found in hemoglobin in red blood cells, which transport oxygen in the blood. Food sources of iron are either heme (from meat) or nonheme (from plant sources or iron-fortified foods). Very little iron is excreted from the body, with most of the iron from degraded hemoglobin being reabsorbed in the gastrointestinal tract. A deficiency of iron occurs gradually with chronic poor intake of iron-rich foods. Other causes of iron deficiency include excessive blood loss and malabsorption. Chronic iron deficiency will cause anemia.

Vitamins. Vitamin A food sources include both animal sources (retinoids) and plant sources (carotenoids). Good animal food sources of vitamin A include liver, egg yolks, milk fat, and fish oils. Carotenoids can be converted to retinol in the intestinal mucosa. They will then have the same metabolic role as retinoids from animal sources. The most common of these is beta carotene, but there are more than five hundred carotenoids. Vitamin A is required for optimal vision, with most of its effects found in maintaining night vision. Vitamin A also has a role in maintaining epithelial tissues, mucus production, and bone health. Vitamin A appears to have a role in fertility and maintaining immune function.

Vitamin C is an important antioxidant with the biochemical ability to neutralize free radicals. Free radicals are metabolites of oxygen used in the cell and are believed to promote aging and several chronic diseases. Good sources of vitamin C include citrus fruits, broccoli, kiwi, potatoes, strawberries, and tomatoes, as well as most other fruits and vegetables. Heat, alkalinity, and exposure to air will destroy vitamin C. Therefore, certain cooking, processing, and storage practices can greatly reduce the vitamin C content of food.

Another antioxidant is vitamin E. Vitamin E is a group of compounds, the most common of which is α-tocopherol. Good sources of vitamin E include vegetable oils, margarine, and nuts. Vitamin E is not destroyed by exposure to air, primarily because it is protected by dietary fat. Vitamin E can be destroyed by high temperatures, such as in frying.

Vitamin D food sources are very limited. While milk is fortified with vitamin D, other dairy products, such as cheese and yogurt, generally are not. Some new products are fortified with calcium and vitamin D, such as yogurt, margarine, and juice. Exposure of the skin to sunlight converts a pre-vitamin D compound to vitamin D₃ (cholecalciferol). Cholecalciferol will be hydroxylated in the liver and the kidney before it becomes active vitamin D. Vitamin D is required for calcium regulation and bone health. Still, emerging research areas suggest that vitamin D may also have a role in autoimmune diseases.

The B vitamins are water-soluble and include thiamin, niacin, riboflavin, pantothenic acid, vitamin B₆, biotin, folate, and vitamin B₁₂. As a group, the B vitamins are essential for the metabolism of macronutrients, cell growth, division, and all organ functions. The B vitamins are found in various foods, although vitamin B₁₂ is primarily found in animal products. Once a concern for vegetarians, vitamin B₁₂ is fortified in many kinds of cereal and nonmeat breakfast foods.

DISORDERS AND DISEASES

Most chronic diseases result from a complex interaction between genetics and environmental factors. Diet is an important environmental factor that is potentially modifiable, and it has received much attention in preventing chronic disease. Most chronic disease prevention or treatment includes a nutritional component. The most prevalent chronic diseases in the United States are cancer, cardiovascular disease, diabetes, obesity, and osteoporosis.

Cancer. Although overall rates are declining in the United States, cancer continues to be a major cause of mortality. Cancer generally involves three phases: initiation, promotion, and progression. During the initiation step, a genetic alteration may remain quiescent or continue through the second step of promotion. During promotion, cellular proliferation is stimulated, and the abnormal cells begin to grow without regulation. The third phase is progression, when the neoplastic cells become invasive and spread or metastasize to other body parts. Dietary components may be involved in initiating and promoting certain cancers and their inhibition. The dietary components that have been linked to the development of cancer include dietary fat, total calories, and alcohol, as well as salted, cured foods and molds that may grow in certain foods.

Antioxidants have been investigated as inhibitors of cancer. Fruits and vegetables are rich sources of antioxidants, and high fruit and vegetable intake has been linked to a lower incidence of certain cancers. The results of many studies, however, are inconclusive. Although a high intake of dietary fat and red meat has been associated with an increased risk of colon cancer, lower-fat diets have not proved to be an effective intervention in decreasing colon cancer incidence. Nevertheless, a diet high in fruits and vegetables, at least five servings each day, and lower in fat and alcohol is recommended as a preventive measure against cancer. Some of the benefits of high fruit and vegetable intake may be attributable to the fiber content of these foods. Higher fiber diets increase fecal bulk, diluting carcinogens that enter the gastrointestinal tract. By increasing intestinal motility, fiber also decreases the amount of time that fecal material is in the gastrointestinal tract, thereby limiting exposure of the mucosa to potential toxins.

Cardiovascular disease. Cardiovascular diseases are the leading cause of death in the United States. They include arrhythmias, congestive heart failure, and valvular diseases. Still, most of the morbidity and mortality are related to coronary heart disease or atherosclerosis. Hyperlipidemia is a risk factor for atherosclerosis, and dietary fat influences the level of blood lipids. According to the Center for Disease Control (CDC), the recommended fat intake is 25 to 35 percent of total calories per day. Of that dietary fat, less than 7 percent of total calories should be saturated fat, and less than 1 percent should be trans fat. Lower fat meats and dairy are recommended, as well as replacement of some meat with vegetable alternatives. Sources of trans fat should be limited. The effects of various levels of polyunsaturated and monounsaturated fats are debated. While limited cholesterol intake is recommended, cholesterol intake has had less effect on blood lipids than total fat and trans fat.

Eating fish, especially oily fish, is recommended as a source of omega-3 fatty acids, which are long-chain polyunsaturated fatty acids associated with a decreased risk of certain heart diseases. The two omega-3 fatty acids are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Although fish may contain contaminants known to be hazardous to health, the benefits of eating it are believed to outweigh the risks for adults. Restricted intake may be recommended for children and pregnant women. Supplements of DHA and EPA are not recommended for the prevention of heart disease. However, they may be prescribed as treatment under a physician’s supervision.

Higher intakes of fruits, vegetables, and whole grains are recommended to prevent heart disease. In addition to fiber, these foods may contain antioxidants or other bioactive compounds that are beneficial to health. In addition, these foods may displace other, higher-calorie foods from the diet, thus promoting a healthy weight. Limiting foods high in added sugars is recommended because of the association of these foods with weight gain and obesity. Obesity is a significant risk factor for cardiovascular disease, and achieving a healthy weight through diet and physical activity is important.

A healthy weight is also significant in the maintenance of optimal blood pressure. Because sodium intake is associated with increased blood pressure on average, limiting sodium intake is also recommended for heart health. Limited amounts of alcohol, if alcohol is consumed at all, is also included as a healthy lifestyle measure for the prevention of heart disease. Moderate alcohol intake is generally considered two drinks for men and one for women each day.

Foods being investigated concerning their role in the prevention of cardiovascular disease include soy and plant stanols. Supplements of antioxidants and fish oils for their DHA and EPA are generally not recommended. Still, foods containing these compounds may be beneficial.

Diabetes. The incidence of diabetes continues to grow in parallel to the incidence of obesity. Diabetes mellitus has been categorized as either type 1 diabetes (formerly known as insulin-dependent diabetes mellitus, IDDM) or type 2 diabetes (formerly known as non-insulin-dependent diabetes mellitus, NIDDM). Nutrition is an important component of both the prevention and treatment of diabetes, regardless of type.

Obesity enhances insulin resistance. Therefore, the main goal of type 2 diabetes management is to prevent or reduce obesity. Weight loss in obese persons with type 2 diabetes improves glycemic control and blood lipid profile. Because carbohydrates are the main determinant of postprandial plasma glucose, the number of carbohydrates and timing of foods eaten may need to be regulated. The total amount of carbohydrates in the diet or meal is more important than the type of carbohydrate, with certain exceptions. Liquid carbohydrates are more easily digested and absorbed than those from solid foods. Beverages such as milk and orange juice may cause a more rapid rise in blood glucose. Sucrose and sucrose-containing foods do not need to be eliminated. Still, these foods need to be included in the total carbohydrates and calories consumed for meal planning and coverage with medication. Restriction of sucrose and sucrose-containing foods usually relates to the restriction of total calories. The glycemic response to carbohydrates depends on many components, including the type of carbohydrate, the cooking or processing, prior food intake, other macronutrients in the food, and glycemic control of the individual. Because dietary modifications need to be individualized, people with diabetes should receive individualized medical nutrition therapy, preferably by a registered dietitian or certified diabetes educator.

Obesity. Obesity occurs when caloric intake exceeds the needs of the individual and is therefore stored in adipose tissue. Although normal weight varies with age, gender, and height, for each group, there are indicators of obesity. Usual indicators of obesity are based on the assumption that variations in weight at various heights are attributable to body fat and are often calculated as the body mass index (BMI). According to the CDC, a BMI between 25 and 30 is considered overweight, and above 30 is considered obese. The optimal macronutrient distribution to facilitate weight loss is not known. Without clear conclusions, higher and lower amounts of protein, fat, and carbohydrates have been investigated. Consuming fewer calories while increasing the number of calories used through physical activity remains the cornerstone of obesity prevention and treatment.

Osteoporosis. As with other chronic diseases, the incidence of osteoporosis continues to rise. Osteoporosis is asymptomatic until the condition produces deformity or contributes to fractures. While genetics play an important role in the development of osteoporosis, modifying risk factors include diet and physical activity. Optimal calcium levels are beneficial in maintaining high bone mineral density, which is critical in preventing osteoporosis. Most calcium is obtained from dairy products, although grain-based foods and juices are increasingly being fortified with calcium. Vitamin D plays a critical role in regulating calcium balance. Therefore, adequate vitamin D status is important in preventing osteoporosis. Vitamin D deficiency can contribute to osteoporosis in older individuals secondary to poor skin synthesis, lower hydroxylation of vitamin D in the kidneys, and inadequate nutritional intake. As with calcium, more food products are being fortified with vitamin D, increasing awareness of osteoporosis.

PERSPECTIVE AND PROSPECTS

Although the science of nutrition began as a branch of biochemistry, early discoveries of the health properties of food date to the eighteenth century and the discovery that limes could prevent the painful bleeding disorder scurvy. Since then, the knowledge of nutrition has progressed beyond identifying deficiency diseases to understanding and appreciating the complexity of nutrition in optimal health. In addition to further investigations into macronutrients, vitamins, and minerals, many bioactive substances in foods are being identified, such as bioflavonoids and probiotics. The interactions of these and more traditional nutrients are being investigated as potential modifiers of chronic disease and promoters of longevity.

Further Reading