Human Nutrition
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.
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 carry on vital activities and to maintain itself at a constant temperature. By using a calorimeter, scientists have been able to establish the energy amounts of the body's fuels—carbohydrates, fats, and protein. About 4 calories each are yielded by 1 g (0.035 oz) of pure carbohydrate and 1 gram of pure protein; 1 gram of pure fat yields about 9 calories.Carbohydrates are the most abundant foods in the world, and fats are the most concentrated and easily stored fuel. If the body exhausts its available carbohydrates and fats, it can use proteins directly from the diet or break down its own protein tissue to make fuel. Alcohol is also a source of energy and yields 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 in the adipose tissue.
Functions of Nutrients
Proteins
The primary function of protein is to build body tissue and to synthesize enzymes, some hormones such as insulin that regulate communication among organs and cells, 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 (proteases) into nitrogen-containing amino acids. Proteases disrupt the peptide bonds by which the ingested amino acids are linked, so that they 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, eight 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 very important 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 in countries with heavy meat diets, the extra protein is similarly broken down into energy-yielding compounds.
Many illnesses and infections lead to an increased loss of nitrogen from the body. This needs to be replaced by a higher consumption of dietary protein. Infants and young children also require more protein per kilogram of body weight.
Carbohydrates
Carbohydrates provide a great part of the energy in most human diets. Foods rich in carbohydrates are usually the most abundant and cheapest, when compared with foods high in protein and fat content. Carbohydrates are burned during metabolism to produce energy, liberating carbon dioxide and water. Humans also get energy less efficiently from fats and proteins in the diet.
The two kinds of carbohydrates are starches, which are found mainly in grains, legumes, and tubers, and sugars, which are found in plants and fruits. Carbohydrates are used by the cells in the form of glucose, the body's main fuel. After absorption from the small intestine, glucose is processed in the liver, which stores some as glycogen, a starchlike substance, and passes the rest into the bloodstream. In combination with fatty acids, glucose forms triglycerides, fat compounds that can easily be broken down into combustible ketones. Glucose and triglycerides are carried by the bloodstream to the muscles and organs to be oxidized, and excess quantities are stored as fat in the adipose and other tissues, to be retrieved and burned at times of low carbohydrate intake.
The carbohydrates containing the most nutrients are the complex carbohydrates, such as unrefined grains, tubers, vegetables, and fruit, which also provide protein, vitamins, minerals, and fats. A less beneficial source is foods made from refined sugar, such as confectionery and soft drinks, which are high in calories but low in nutrients and fill the body with what nutritionists call empty calories and it will be stored as fat!
Fats
Although scarcer than carbohydrates, fats produce more than twice as much energy. Being a compact fuel, fat is efficiently stored in the body for later use when carbohydrates are in short supply. Animals obviously need stored fat to tide them over dry or cold seasons, as do humans during times of scarce food supply.
In industrial nations, however, with food always available and with machines replacing human labour, the accumulation of body fat has become a serious health concern.
Dietary fats are broken down into fatty acids that pass into the blood to form the body's own triglycerides. The fatty acids that contain as many hydrogen atoms as possible on the carbon chain are called saturated fatty acids and are derived mostly from animal sources. Unsaturated fatty acids are those that have some of the hydrogen atoms missing; this group includes monounsaturated fatty acids, which have a single pair of hydrogens missing, and polyunsaturated fatty acids, which have more than one pair missing. Polyunsaturated fats are found mostly in seed oils. Saturated fats in the bloodstream have been found to raise the level of cholesterol, and polyunsaturated fat tends to lower it. Saturated fats are generally solid at room temperature; polyunsaturated fats are liquid. 1
1 ."Nutrition, Human," Microsoft® Encarta® 99 Encyclopedia. © 1993-1998 Microsoft Corporation.
