Diet and Nutrition
Adequate nutrition and a well-balanced diet in every phase of life are essential requirements for normal development and growth, health maintenance, and disease prevention, as well as for the recovery from illness or injury. The human organism is a dynamic system, constantly using stored energy to perform physiologic functions such as blood circulation, respiration, immune surveillance and defense against infections, synthesis of proteins, hormones, and neurotransmitters necessary for muscle activity, sensory perception, thought processing, digestion of food and elimination of body wastes, cell and tissue detoxification, and DNA repair. Food is the main source of the micronutrients the organism utilizes to perform these vital functions, thus keeping the many physiologic systems in a state of homeostasis, or dynamic functional balance.
Micronutrients are substances the body extracts from food through digestion, the process of breaking down large and complex molecules of food into more simple and smaller ones. Micronutrients are then absorbed through the walls of the small intestine into the blood vessels to be distributed to and processed by different organs and tissues. Different classes of micronutrients are used for several different purposes. For instance, some micronutrients such as vitamins are essential for cellular protection against naturally occurring metabolic toxins formed as a byproduct of cellular activity, or against toxins derived from the environment, such as pollution, chemicals, solar radiation, or drugs. Micronutrients are divided in the following categories: amino acids, fatty acids, sugars or carbohydrates, vitamins, and minerals.
Amino acids are the building blocks of all types of proteins that constitute cells, organs, tissues, and muscles. Some proteins are mediators of signals between cells of different organs, regulating intracellular physiology and growth. Although approximately 300 amino acids are known in nature, the human body only utilizes about 20 of them. The body itself manufactures half of the amino acids required by humans to make proteins. However, 10 of these are called essential amino acids because humans depend on their presence in food, since the body cannot adequately manufacture them. Eight of the 10 essential amino acids must be present in the diet throughout life, whereas two are necessary during development and growth, or when tissue repair is needed.
Some amino acids are created in the brain and play an important role in the regulation of mood, cognitive function, attention, and sleep pattern. The synthesis of neurotransmitters, chemical messengers in the brain that regulate neural activity, is also dependent on adequate dietary intake of essential amino acids. Examples of neurotransmitters are acetylcholine, gamma-aminobutyric acid (GABA), dopamine, and serotonin. The main source of essential amino acids is animal protein such as fish, meat, milk, and eggs. Plants are also a source of amino acids, although none contain all of the essential amino acids. It is important, therefore, to combine different types of plants within the same meal, such as nuts, beans, grains, fruits, especially in vegetarian diets.
Enzymes are another important type of protein that regulates all metabolic events. Some enzymes are responsible for the detoxification of cells and tissues, and
Deficiency in digestive enzymes causes slow and incomplete digestion of larger nutrients, thus reducing the availability of micronutrients to the body and resulting in a nutritional deficit. Although the body manufactures some digestive enzymes, a diet rich in fruits and vegetables provides a reliable source for digestive enzymes. Papaya, pineapple, cucumber (eaten with the skin), tomatoes, and green leafy vegetables are especially good sources for digestive enzymes.
Another frequent cause of nutritional deficiency is malabsorption of nutrients in the intestinal tract due to parasite infestation, infections, or disruption of the normal intestinal microorganism balance by some medications. Normally, a mixed population of bacteria permanently lives in the intestinal mucosa, helping to break down some larger molecules such as complex carbohydrates. When this balance is disrupted, even though the daily diet contains the correct amounts of all necessary nutrients, nutritional deficiencies may occur due to the inability of the intestinal tract to absorb molecules that are not broken down by the beneficial bacteria.
Fatty acids are the components of lipids or fats that may be combined with proteins and/or sugars to form a variety of functional and structural molecules such as cholesterol, hormones, and enzymes. Fatty acids are also an important source of body energy and are stored in the adipose tissue (i.e., fat cells). Lipoproteins (such as cholesterol) are present in the structure of cell membranes and in blood plasma, and have a variety of other functions. For example, cholesterol is a precursor of bile acid and of steroid hormones such as testosterone, progesterone, and estrogen. Myelin, the white substance that involves nerve fibers as a multi-layered sheath, is constituted of lipids and proteins, and is essential for normal neural signal transmission, and muscle control and coordination. Fatty acids are present in whole milk, butter, fish, seafood, lard, meat, vegetable oils, margarine, nuts, olives, corn, soybean, and grains.
Carbohydrates encompass a variety of sugar molecules that play a multitude of roles in body physiology and are also a structural component of the cell membrane. Carbohydrates supply and store energy, aid in intercellular communication, and regulate many metabolic events in the body. The digestive process transforms carbohydrates into glucose, the main source of energy used by cells. Glucose, a simple sugar, is a component of many proteins known as glycoproteins, and is also present in the molecular structure of DNA as pentose. The central and peripheral nervous systems demand a constant supply of glucose in the blood, as does the muscular system. The body stores glucose in the form of glycogen that can be promptly mobilized when the level of glucose in the blood falls. Glycogen is mainly stored in skeletal muscles and in the liver, but it is also present in small amounts in virtually every cell of the body. Carbohydrates are present in milk, fruits, potatoes, cereals, sugar, and honey. Whole grains, lettuce, and fruits also contain a type of fibrous carbohydrate humans cannot digest, known as cellulose. Nevertheless, cellulose helps digestion because these fibers stimulate movement of the intestinal tract, preventing constipation and removing pathogenic germs.
The body needs to protect its cells and DNA from the damage oxygen and free radicals can do. Free radicals are highly reactive substances that form when oxygen interacts with other molecules during digestion or other cellular processes. To combat this damage, the body uses a defense system of antioxidant molecules that react safely with the free radicals. Some antioxidant molecules are naturally occurring enzymes. Vitamins are another important source of antioxidants.
Vitamins neutralize free radicals and protect tissue integrity and function. They are also essential for a number of other cellular functions such as tissue renewal and healing, red blood cell production, body resistance to infections, brain and muscle activity, DNA replication during cell cycle, adequate regulation of several metabolic events, recovery from disease, and prevention of chronic disease. Vitamins are divided in two categories according to their solubility: water-soluble vitamins and fat-soluble vitamins.
Vitamin C (ascorbic acid) and B-complex vitamins (thiamine, niacin, riboflavin, biotin, folic acid, cobalamin, pyridoxine, and pantothenic acid) are water-soluble vitamins. Since kidneys easily eliminate water-soluble vitamins through the urine, they must be present in the daily diet because only trace amounts are stored in the organism. The main dietary sources of vitamin C are tomatoes, green leafy vegetables, and citrus fruits such as oranges, although other fruits and vegetables do contain smaller amounts of vitamin C. Raw meat and fish also contain vitamin C that is lost in the cooking process. Vitamin C protects cells against oxidation, helps collagen formation, and the transformation of cholesterol into bile acids. The detoxification properties of vitamin C help in the elimination of the toxins and free radicals that build up in the extracellular fluids and in cells during infections.
B-complex vitamins participate as co-factors in a vast number of enzyme activities and act as co-antioxidants as well. Some B vitamins are required for red blood cell formation, while others are required for regulation of plasma cholesterol levels, energy release in tissues, amino acid synthesis, embryo development, brain development and neuronal activity, bone marrow formation, and infection resistance. Additionally, some B vitamins promote myelin sheath formation around nerve fibers and neurons during brain development in the fetus and during child growth as well. The main dietary sources of B-complex vitamins are whole milk, chicken, pork, egg, seafood, meat, liver, corn, wheat and whole grains, green leaves, and legumes. As not all B vitamins are present in each of these foods, it is important to keep a well-balanced and varied diet. Strict vegetarians, especially vegans, need supplementation of some B vitamins such as biotin and cobalamin as animal products are eliminated as a dietary source.
The fat-soluble vitamins are vitamins A, D, E, and K. The precursors of these vitamins are present in food, and are transformed by the body into the active vitamin form. Dietary precursors of vitamin A are beta-carotene and other carotenes found in carrots, yellow fruits and seeds, as well as in dark green vegetables. Retinol, found in animal products such as meat, fish, egg yolk, whole milk, and butter, is vitamin A itself. Vitamin A is essential for normal fetal development, child growth, tissue repair, healing, and renewal, vision, cell protection against free radicals, and reproduction. Beta-carotene shows several benefits of its own, independently of being converted into vitamin A by the body. Some scientific evidence shows that adequate levels of beta-carotene in the diet help to prevent chronic and degenerative diseases such as skin cancer, cardiac diseases, and cataracts. This vegetable precursor of vitamin A also has its own antioxidant activity, and enhances immune system function. Whereas excessive intake of retinol may cause liver and nerve cell toxicity, beta-carotene does not offer such a risk.
Vitamin D is, in fact, a group of molecules that function as hormones. The dietary precursor of vitamin D in plants is known as ergocalciferol. Animal products contain some preformed active molecules of vitamin D. However, the main source of vitamin D in the organism is in the form of an intermediate molecule of cholesterol that is converted into calcitriol in the skin through the action of solar radiation. Long winter months in the northern hemisphere or little exposure to sunlight sometimes lead to deficiency of vitamin D, thus requiring greater dietary intakes of animal products such as fatty fish and egg yolk. Calcitriol, one active form of vitamin D, regulates the synthesis of proteins responsible for calcium and phosphate absorption in the intestinal tract. Vitamin D also regulates the levels of calcium in blood plasma, and helps the mineralization of bones. This micronutrient is essential for normal skeletal development of infants and children, and to prevent osteoporosis in adults, especially women and elderly men.
Tocopherols are different forms of vitamin E, such as alpha and beta tocopherols, and are important antioxidants that protect cholesterol and fatty acids against peroxidation, the chemical process that transforms lipids into rancid fat. Peroxidation of circulating cholesterol causes progressive vascular obstruction, which may lead to heart attack or stroke. Vitamin E also protects fatty acids and lipids that are components of cell membrane structure, thus maintaining the cell's normal functionality. The best dietary sources of vitamin E are vegetable oils.
Vitamin K occurs as phylloquinone in plants, and as menaquinone in bacteria of the intestinal flora. It is essential for the right formation of clotting factors, the proteins responsible for normal blood coagulation. Dietary sources are spinach, cabbage, egg yolk, and liver, although the normal intestinal bacterial flora constitutes a regular source of the vitamin as well.
Discrete (trace) amounts of some minerals are also vital for cell metabolism, neural and muscle activity, bone development and maintenance, electro-chemical reactions, and transport of nutrients and metabolic waste through the cell membrane. The most important minerals are calcium, phosphorus, potassium, magnesium, sodium, and iron.
Calcium and phosphorus are required by a variety of body functions such as bone formation and maintenance, neural signal transmission or synapses, smooth muscle contraction, and skeletal muscle activity. They also regulate glandular and enzymatic activity. Major sources of these nutrients are milk and dairy products. Magnesium works together with calcium, regulating calcium transport into cells and to and from bones. Magnesium controls the levels of calcium transported to heart tissue, maintaining the heartbeat in a steady pace. Magnesium is also important in cells of the immune system such as lymphocytes, in skeletal muscles, and as a facilitator of oxygen delivery. Magnesium participates in the production of ATP (adenosine triphosphate), the source of energy utilized by cells.
Sodium and potassium regulate levels of fluids entering and leaving the cells, and moving between blood vessels and the lymphatic system, and are, therefore, important agents in the regulation of blood pressure. Iron is an essential component of red blood cells (hemoglobin), which transport oxygen to all tissues. Iron is stored in the plasma in proteins known as ferritin. Adequate plasma levels of ferritin are required for hematopoiesis, or blood formation. However, excess ferritin in plasma increases cholesterol peroxidation, leading to cardiovascular disease. Trace amounts of minerals are present in fruits and other vegetables, as well as in animal products such as seafood, fish, liver, milk, meat, eggs, and poultry.
Dieticians are the best advisors when a specific diet is important, such as during pregnancy, or in infancy and early childhood development, in order to prevent nutritional deficits. Physicians can refer patients to trusted dieticians. Elderly citizens and ill people also need professional nutritional guidance to meet deficiencies associated with the aging process or disease. The same is true for professional athletes and individuals working in strenuous physical and/or mental conditions. For the general population, the United States Department of Agriculture has designed the Food Guide Pyramid, illustrating the groups of foods and the daily-required variety of foods for optimum nutrition and health maintenance.
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American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, IL 60606-6995. (800) 877-1600. firstname.lastname@example.org. <http://www.eatright.org>.