What is nutrition?

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 in relation to 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 amount of calories required to maintain a healthy weight during each stage of the life cycle depends on the amount of energy expended. 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. Because men generally have a larger body mass than women, they usually have a larger caloric requirement.

Macronutrients.
Carbohydrates are an important source of energy. The recommended amount of intake is 45 to 65 percent of the total caloric intake, according to dietary guidelines published by the US Department of Agriculture and the US Department of Health and Human Services. Each gram of carbohydrates contributes four calories to the diet. Carbohydrates are found in starchy foods such as potatoes or corn, in vegetables and fruits, and in milk and yogurt. They 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 are those that require little digestion, such as sucrose or sugar. Complex carbohydrates include those that require more digestion, such as starches and fiber. General dietary guidelines suggest an increase in the higher-fiber foods. The recommendation for adults over eighteen is to consume between twenty-two and thirty-four grams of fiber each day, depending on age and sex; Americans generally consume five to ten grams. In addition to fruits and vegetables, nuts and seeds, whole-wheat bread, and cereal are all higher-fiber foods.

Dietary protein is required to supply essential amino acids so that the body can synthesize new proteins such as enzymes or hormones, or structural proteins to build muscle. Meats (including pork, beef, chicken, or turkey), fish, eggs, and nuts contain substantial amounts of 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, the exception being 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 that less can be consumed while still meeting all body demands for protein. In these cases, protein does not need to be used for energy. With inadequate caloric intake, however, higher levels of protein are required to meet the body’s needs because 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 recommend 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, recommended dietary fat intake is restricted in both total intake and type of fat ingested. Saturated fat, such as lard, shortening, and bacon fat, is solid at room temperature and is derived from animals. Unsaturated fat can either have many unsaturated bonds (polyunsaturated) in its structure or just 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 previously specified levels of intake for both polyunsaturated and monounsaturated fats, current recommendations reflect only a limited total fat intake, with a recommendation to emphasize monounsaturated and polyunsaturated fats and avoid trans fats in particular. Trans fat is an 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 intake should be limited. Foods that have higher values of trans fat should be labeled as such; these 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 at times referred to as electrolytes, meaning that they can have a negative or positive charge and thus are able to conduct electricity. In the body, these anions (negatively charged ions) and cations (positively charged ions) 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 on a daily basis. One of the most common trace minerals is iron. The amounts of minerals in a particular food often vary depending on the soil in which a plant is grown or the feed that 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. Recommended daily intake of calcium is 1,000 milligrams for people older than three years of age, increasing to 1,300 milligrams during adolescence and 1,200 milligrams over age fifty. Poor intake of calcium is associated with the development of porous bones, or osteoporosis.

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

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

Potassium is found in many foods, including milk, meat, fruit, and vegetables. Together with sodium, potassium plays an important role in maintaining fluid balance. A diet high in sodium and low in potassium may contribute to the development of 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, where its function is to 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 excess 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 and 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 the maintenance of night vision. It also has a role in maintaining epithelial tissues, mucus production, and bone health. Vitamin A appears to have a role in fertility and in maintaining immune function as well.

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 really a group of several different compounds, the most common of which is α-tocopherol. Good sources of vitamin E include vegetable oils, margarines, and nuts. Vitamin E is not destroyed by exposure to air, primarily because it is protected in dietary fat. However, it 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 being fortified with both calcium and vitamin D, such as yogurt, margarine, and juice. Exposure of the skin to sunlight converts a pre-vitamin D compounds to vitamin D3 (cholecalciferol), which 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, and emerging areas of research suggest that it may have a role in autoimmune diseases as well.

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, as well as cell growth and division and all organ functions. The B vitamins are found in a variety of foods, although vitamin B12 is primarily found in animal products. Once a concern for vegetarians, vitamin B12 is now added to many cereals and nonmeat breakfast foods.

Disorders and Diseases

Most chronic diseases result as 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 the prevention of 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. In general, cancer involves three phases: initiation, promotion, and progression. During the initiation step, there is a genetic alteration that may remain quiescent or continue though 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 parts of the body. Dietary components may be involved in the initiation and promotion of certain cancers as well as 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 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 benefit of high fruit and vegetable intake may be attributable to the fiber content of these foods. Higher-fiber diets increase fecal bulk, thereby diluting any 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, but most of the morbidity and mortality is related to coronary heart disease, or atherosclerosis. Hyperlipidemia is a risk factor for atherosclerosis, and dietary fat has influence on the level of blood lipids. The recommended intake of fat is 25 to 35 percent of total calories per day for people over three years old, with less than 10 percent of total calories being saturated 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 intake of cholesterol is recommended, cholesterol intake has had less of an effect on blood lipids than have 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, although 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 increases in blood pressure on average, limiting sodium intake is also recommended for heart health. Limiting one's alcohol intake, if alcohol is consumed at all, is also considered a healthy lifestyle measure for the prevention of heart disease. US guidelines define moderate alcohol intake as up to two drinks for men and one drink for women per day.

Foods that are 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, but 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 insulin-dependent diabetes mellitus (IDDM), which is also called type 1 diabetes, and non-insulin-dependent diabetes mellitus (NIDDM), also known as type 2 diabetes. Nutrition is an important component of both the prevention and the treatment of diabetes, regardless of type.

Obesity enhances insulin resistance. Therefore, a main goal in the treatment of type 2 diabetes 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 amount of carbohydrates consumed and the timing of food intake 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, so 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, but they do 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, how it has been cooked or processed, 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 Centers for Disease Control and Prevention (CDC), a BMI between 25 and 29.9 is considered overweight, while a BMI above 30 is considered obese. The optimal macronutrient distribution to facilitate weight loss is not known. Higher and lower amounts of protein, fat, and carbohydrates have been investigated, without clear conclusions. Consuming fewer calories while increasing the amount 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 levels of calcium have been shown to be beneficial in maintaining high bone-mineral density, which is critical in preventing osteoporosis. Most calcium is obtained from dairy products, although increasingly grain-based foods and juices are being fortified with calcium. Vitamin D plays a critical role in regulating calcium balance, so adequate vitamin D status is important in preventing osteoporosis. Vitamin D deficiency can be a contributing factor 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 with an increasing awareness of osteoporosis.

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