Articles: Science

Human Nutrition

Science that deals with nutrients and other food substances and how the body assimilates them is referred to as Human Nutrition. The extremely complex processes that nutrients undergo in the body - how they affect one another, how they are broken down and released as energy, and how they are transported and used to rebuild countless specialized tissues and sustain the overall health of the individual - are understood only in part.

Nevertheless, important nutrition decisions must be made for the health of individuals, groups such as the young and the elderly, and entire populations who suffer from malnutrition.

Essential Nutrients

Nutrients are classified into five major groups: proteins, carbohydrates, fats, vitamins, and minerals. These groups comprise between 45 and 50 substances that scientists have established, mostly through experiments with animals, as essential for maintaining normal growth and health.

Besides water and oxygen, they include about eight amino acids from proteins, four fat-soluble and ten water-soluble vitamins, about ten minerals, and three electrolytes (the ions required by cells to regulate the electrical charge and flow of water molecules across cell membranes). Although carbohydrates are needed for the body's energy, they are not considered absolutely essential because protein can be converted for this purpose.

Energy

The body uses energy to conduct vital functions and maintain itself at a constant temperature. By using a calorimeter (a device for measuring the heat generated by a chemical reaction or change in a physical state), scientists have established the amounts of energy in the body's fuels - carbohydrates, fats, and proteins.

About 4 calories each are yielded by 1g (0.035oz) of pure carbohydrate and 1g of pure protein; 1 gram of pure fat yields about 9 calories. (A kilogram calorie, in nutrition, is defined as the heat energy needed to raise the temperature of 1 kilogram of water from 14.5 to 15.5 C (58.1 to 59.9 F).

Carbohydrates are the most abundant foods in the world, and fats are the most concentrated and easily stored fuels. If the body exhausts available carbohydrates and fats, it can use proteins directly from the diet or break down its own protein tissue for energy.

Alcohol is also a source of energy, yielding 7 calories per gram. Alcohol cannot be oxidized by the body cells but must be processed by the liver into fat, which is then stored by the liver or the adipose tissue (a layer of fat just beneath the skin and around various internal organs).

Functions of Nutrients

The functions of the various categories of nutrients are described below.

Proteins

The primary function of protein is to build body tissue and synthesize enzymes, some hormones, such as insulin, that regulate physiological activity, and other complex substances that govern body processes. Animal and plant proteins are not used in the form in which they are ingested but are broken down by digestive enzymes called proteases into nitrogen-containing amino acids. Proteases disrupt the peptide bonds by which ingested amino acids are linked, so the amino acids can be absorbed through the intestine into the blood and recombined into the particular tissue needed.

Proteins are usually readily available from both animal and plant sources. Of the 20 amino acids that make up protein, 8 are considered essential - that is, because the body cannot synthesize them, they must be supplied ready-made in foods. If these essential amino acids are not all present at the same time and in specific proportions, the other amino acids, in whole or in part, cannot be used for metabolizing human protein. Therefore, a diet containing these essential amino acids is vital for sustaining growth and health. When any of the essential amino acids is lacking, the remaining ones are converted into energy-yielding compounds, and their nitrogen is excreted.

When an excess of protein is eaten, which is often the case with heavy meat diets in the United States, the extra protein is similarly broken down into energy-yielding compounds. Because protein is far scarcer than carbohydrates and yields the same 4 calories per gram, the eating of meat beyond the tissue-building demands of the body becomes an inefficient way to procure energy. Foods from animal sources contain complete proteins because they include all the essential amino acids. In most diets, a combination of plant and animal protein is recommended: 0.8 grams of protein per kilogram of body weight is considered a safe daily allowance for normal adults.

Many illnesses and infections lead to an increased loss of nitrogen from the body, which needs to be replaced by a higher consumption of dietary protein. Infants and young children also require more protein per kilogram of body weight. A protein deficiency accompanied by energy deficits results in a form of protein-energy malnutrition called kwashiorkor, which is characterized by loss of body fat and wasting of muscle.

Minerals

Inorganic mineral nutrients are required in the structural composition of hard and soft body tissues; they also participate in such processes as the action of enzymes, the contraction of muscles, nerve reactions, and the clotting of blood. These mineral nutrients, all of which must be supplied in the diet, are of two classes: major elements such as calcium, phosphorus, magnesium, iron, iodine, and potassium; and trace elements such as copper, cobalt, manganese, fluorine, and zinc.

Calcium is needed for developing and maintaining the rigidity of bones. It also contributes to the formation of intracellular cement and the cell membranes, and regulation of nervous excitability and muscular contraction. About 90 percent of calcium is stored in bone, where it can be reabsorbed by blood and tissue. Milk and milk products are the chief source of calcium.

Phosphorus, also present in many foods and especially in milk, combines with calcium in the bones and teeth. It plays an important role in energy metabolism of the cells, affecting carbohydrates, lipids (fatty acids in the blood that also include cholesterol and triglycerides), and proteins.

Magnesium, which is present in most foods, is essential for human metabolism and is important for maintaining the electrical potential in nerve and muscle cells. A deficiency in magnesium among malnourished people, especially alcoholics, leads to tremors and convulsions.

Sodium, which is present in small and usually sufficient quantities in most natural foods, is found in liberal amounts in salted prepared and cooked foods. It is present in extracellular fluid (the fluid between cells or cell layers), which sodium helps regulate. Too much sodium causes edema, an over-accumulation of extracellular fluid. Evidence now exists that excessive dietary salt contributes to high blood pressure.

Iron is needed to form hemoglobin (the pigment in red blood cells responsible for transporting oxygen), but the mineral is not readily absorbed by the digestive system. It exists in sufficient amounts in men, but women of menstrual age, who need nearly twice as much iron because of blood loss, often have deficiencies and must take absorbable iron supplements. Iodine is needed to synthesize hormones of the thyroid gland. A deficiency leads to goiter (swelling of the thyroid gland in the lower neck). Goiter, which used to be common in the U.S. population, remains prevalent in certain parts of Asia, Africa, and South America. Low iodine intakes during pregnancy may result in the birth of cretinous or mentally retarded infants. It is estimated that worldwide more than 150 million people suffer from iodine deficiency diseases.