Classes of Nutrients

Since almost all foods are of animal or plant origin, their composition will include (except when destroyed by processing) all the various components of life cells. Among these, the nucleic acids have not in the past been considered of great significance in nutrition. Dietary requirements of nucleic acids for humans are not known, although claims have been made that an increased intake may slow down the ageing process and increase lifespan. 

The remaining types of biopolymer constitute three of six main classes of nutrients  

The phosphorus content or requirement in dietetics is often assessed separately and classed as a mineral under (11.5). The element is, however, usually present not only in inorganic mineral form (e.g. Ca or Na phosphates), but also in the biopolymers (11.1) through (11.3). In addition, the element is present with some vitamins (see below) and in phosphate energy-carrying molecules, which are vital components of all living cells (Section 11.3). Although the quantities of these energy carriers in food are very small indeed, their presence is essential for both the synthesis and breakdown of the biopolymer nutrients which form part of the metabolism of plants and animals. 

Whereas the phosphorus in foods may be associated with all the nutrient classes (11.1) through (11.5), in the case of nitrogen, nearly all of it is present as protein. 

It is impossible to list aU the phosphorus compounds present in foods or in the human body, many of which are present in only trace amounts. The more important ones dealt with in this book are listed in Table 11.12. On entering the body most of the food P compounds undergo various catabolic and anabolic processes in which they are broken down and reassanbled or at least modified in some way. 

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JOHNSON I, WILLIAMSON G and MUSK s (1994) Anticarcinogenic factors in plant foods a new class of nutrients , Nutrition Research Reviews, 7 175-204. 

Nutrients are the constituents of food necessary to sustain the normal functions of the body. All energy is provided by three classes of nutrients fefe, carbohydrates, protein, and in some diets, ethanol (Figure 27.1). The intake of these energy-rich molecules is larger than that of the other dietary nutrients. Therefore, they are called the macronutrients. This chapter focuses on the kinds and amounts of macronutrients that are needed to maintain optimal health and prevent chronic disease in adults. Those nutrients needed in lesser amounts, such as vitamins and minerals, are called the micronutrients, and are considered in Chapter 28. 

Koch points out the reasonableness of the model from the point of view of evolution since a membrane barrier capable of retaining important metabolites but of selectively allowing the passage of certain classes of nutrients would be an early development. The addition of the specific permease and energy-coupling features to the primitive mechanism illustrates the manner in which a complex transport process may evolve. 

While there are no epidemiologic data available on lipotropic factors and colon cancer in human populations, results of animal studies suggest a possible role for this class of nutrients (choline, methionine, vitamin and folate) in colon carcinogenesis (69). Table XVI lists results typical of those observed when rats are fed a diet high in fat, low in lipotropes and exposed to a colon carcinogen. 

Specific chemical reactions and catalysts are required for digestion of the three major classes of nutrients — proteins, fats (lipids), and carbohydrates. Others are required for the absorption of digesbve products through the wall of the small intestine, Hydrolysis, phosphorolysis, and the condensabon of COj with water are important biochemical mechanisms in digestion and absorption. 

Several diseases, called malabsorption syndromes, result in the lack of absorption of a number of nutrients. Their consequent loss in the feces dearly is not desirable. Malabsorption of lipids, which occurs more frequently than malabsorption of any other class of nutrient, can result in diarrhea and steatorrhea. Steatorrhea, production of feces with a high fat content, derives its name from "stearic add," a fatt> acid, With severe steatorrhea, over 30 g fat may appear in the feces per day. The problem can be elicited by liver diseases that piev ent the pn>duction of bile salts and thus impair the absorption of fats. 

How are lipids involved in the generation and storage of energy We have already seen how carbohydrates are processed catabolically and anabolically. Lipids are another class of nutrient. The catabolic oxidation of lipids releases lai e quantities of energy, whereas the anabolic formation of lipids represents an efficient way of storing chemical energy. 

Nutrition, then, is concerned with the chemical requirements of the body—the nutrients and the chemical energy we get from them. Nutrients are chemical substances in foods that provide the energy and raw materials required by biochemical reactions. The Jive classes of nutrients are carbohydrates, fats, proteins, vitamins, and minerals. In addition, an average adult needs about 2.5 L of water each day. 

The majority of foodstuffs contain measurable amounts of all six classes of nutrients already mentioned. Relatively high concentrations of the following are associated with particular foods ... 

Lipids are another important class of nutrients in our diet. They are found in dairy products, grains, meats, and oils. A lipid is a type of biochemical that does not dissolve in water, has a high percentage of C and H atoms, and is soluble in nonpolar solvents. As a class, lipids are not nearly as similar to each other as carbohydrates are. Long-chain fatty acids, phospholipids, steroids, and cholesterol are lipids. 

For centuries and until the end of the 19 century, food was considered important as a source of protein, energy and ash . Raising the concept that low intakes of specific nutrients can cause certain diseases was considered revolutionary. The term vitamin , which appears at the very end of the 19 century, was bom from this new revolutionary concept that diet and health were closely interrelated, concept which allows emergence of nutrition as a science (Combs, 1998). Unlike other classes of nutrients except minerals, vitamins have no structural functions. Indeed, as B-vitamins play highly specific roles, such as coenzymes/cofactors, they are requested only in small amounts. 

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