Besides being available for energy production, each of the three food types is used in anabolic reactions to synthesize necessary materials for cells and tissues.
Figure 17–4. Synthesis uses of foods. See text for description.
QUESTION: Excess amino acids can be used to synthesize carbohydrates or fats. Can any other food be used to synthesize proteins?
A simple summary of these reactions is shown in Fig. 17–4. The three food types and their end products of digestion are at the bottom of the picture, and the arrows going upward indicate synthesis and lead to the products formed. You may wish to refer to Fig. 17–4 as you read the next sections.
Glucose is the raw material for the synthesis of another important monosaccharide, the pentose sug-ars that are part of nucleic acids. Deoxyribose is the five-carbon sugar found in DNA, and ribose is found
in RNA. This function of glucose is very important, for without the pentose sugars our cells could neither produce new chromosomes for cell division nor carry out the process of protein synthesis.
Any glucose in excess of immediate energy needs or the need for pentose sugars is converted to glycogen in the liver and skeletal muscles. Glycogen is then an energy source during states of hypoglycemia or during exercise. If still more glucose is present, it will be changed to fat and stored in adipose tissue.
As mentioned previously, the primary uses for amino acids are the synthesis of the non-essential amino acids by the liver and the synthesis of new proteins in all tissues. By way of review, we can mention some proteins with which you are already familiar: keratin and melanin in the epidermis; collagen in the dermis, tendons, and ligaments; myosin, actin, and myoglobin in muscle cells; hemoglobin in RBCs; antibodies pro-duced by WBCs; prothrombin and fibrinogen for clotting; albumin to maintain blood volume; pepsin and amylase for digestion; growth hormone and insulin; and the thousands of enzymes needed to cat-alyze reactions within the body.
The amino acids we obtain from the proteins in our food are used by our cells to synthesize all of these proteins in the amounts needed by the body. Only when the body’s needs for new proteins have been met are amino acids used for energy production. But notice in Fig. 17–4 what happens to excess amino acids; they will be deaminated and converted to simple carbohydrates and contribute to glycogen storage or they may be changed to fat and stored in adipose tissue.
The end products of fat digestion that are not needed immediately for energy production may be stored as fat (triglycerides) inadipose tissue. Most adipose tis-sue is found subcutaneously and is potential energy for times when food intake decreases. Notice in Table 17–4 that insulin promotes fat synthesis and storage. One theory of weight gain proposes that a diet high in sugars and starches stimulates the secretion of so much insulin that fat can only be stored, not taken out of storage and used for energy.
Fatty acids and glycerol are also used for the syn-thesis of phospholipids, which are essential compo-nents of all cell membranes. Myelin, for example, is a phospholipid of the membranes of Schwann cells, which form the myelin sheath of peripheral neurons.
The liver can synthesize most of the fatty acids needed by the body. Two exceptions are linoleic acid and linolenic acid, which areessential fatty acids and must be obtained from the diet. Linoleic acid is part of lecithin, which in turn is part of all cell membranes. Vegetable oils are good sources of these essential fatty acids.
When fatty acids are broken down in the process of beta-oxidation, the resulting acetyl groups may also be used for the synthesis ofcholesterol, a steroid. This takes place primarily in the liver, although all cells are capable of synthesizing cholesterol for their cell mem-branes. The liver uses cholesterol to synthesize bile salts for the emulsification of fats in digestion. The steroid hormones are also synthesized from choles-terol. Cortisol and aldosterone are produced by the adrenal cortex, estrogen and progesterone by the ovaries, and testosterone by the testes.