About 50 substances required for normal or optimal body function cannot be synthesized by the body or are synthesized in amounts inadequate to keep pace with the rates at which they are broken down or excreted. Such substances are known as essential nutrients (Table 4-9). Because they are all removed from the body at some finite rate, they must be continually supplied in the foods we eat.
It must be emphasized that the term "essential nutrient" is reserved for substances that fulfill two criteria: (1) they must be essential for health, and (2) they must not be synthesized by the body in adequate amounts. Thus, glucose, although "essential" for normal metabolism, is not classified as an essential nutrient because the body normally can synthesize all it needs, from amino acids, for example. Furthermore, the quantity of an essential nutrient that must be present in the diet in order to maintain health is not a criterion for determining if the substance is essential. Approximately 1500 g of water, 2 g of the amino acid methionine, but only about 1 mg of the vitamin thi-amine are required per day.
Water is an essential nutrient because far more of it is lost in the urine and from the skin and respiratory tract than can be synthesized by the body. (Recall that water is formed as an end product of oxidative phos-phorylation as well as from several other metabolic reactions.) Therefore, to maintain water balance, water intake is essential.
The mineral elements provide an example of substances that cannot be synthesized or broken down but are continually lost from the body in the urine, feces, and various secretions. The major minerals must be supplied in fairly large amounts, whereas only small quantities of the trace elements are required.
We have already noted that 9 of the 20 amino acids are essential. Two fatty acids, linoleic and linolenic
PART ONE Basic Cell Functions
TABLE 4-9 Essential Nutrients
7 major mineral elements (see Table 2-1) 13 trace elements (see Table 2-1)
Essential Amino Acids
Essential Fatty Acids
Water-soluble vitamins Bn: thiamine B2: riboflavin B6: pyridoxine B12: cobalamine Niacin
Pantothenic acid Folic acid Biotin Lipoic acid Vitamin C
Fat-soluble vitamins Vitamin A Vitamin D Vitamin E Vitamin K
Vitamin B complex
Inositol Choline Carnitine acid, which contain a number of double bonds and serve important roles in chemical messenger systems, are also essential nutrients. Three additional essential nutrients—inositol, choline, and carnitine—have functions that will be described in later chapters but do not fall into any common category other than being essential nutrients. Finally, the class of essential nutrients known as vitamins deserves special attention.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.