Nutrition and Energy Production

Given the diversity of animal life on our planet, it is not surprising that the animal diet would also vary substantially. The animal diet is the source of materials needed for building DNA and other complex molecules needed for growth, maintenance, and reproduction; collectively these processes are called biosynthesis. The diet is also the source of materials for ATP production in the cells. The diet must be balanced to provide the minerals and vitamins that are required for cellular function.

Food Requirements

What are the fundamental requirements of the animal diet? The animal diet should be well balanced and provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability. These requirements for a human are illustrated graphically in Figure 15.14

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Figure 15.14. For humans, a balanced diet includes fruits, vegetables, grains, and protein. (credit: USDA)

The first step in ensuring that you are meeting the food requirements of your body is an awareness of the food groups and the nutrients they provide. To learn more about each food group and the recommended daily amounts, explore this interactive site by the United States Department of Agriculture.

Organic Precursors

The organic molecules required for building cellular material and tissues must come from food. Carbohydrates or sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy through metabolic pathways. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification; however, humans do not produce the enzyme cellulase and lack the ability to derive glucose from the polysaccharide cellulose. In humans, these molecules provide the fiber required for moving waste through the large intestine and a healthy colon. The intestinal flora in the human gut are able to extract some nutrition from these plant fibers. The excess sugars in the body are converted into glycogen and stored in the liver and muscles for later use. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Excess glycogen can be converted to fats, which are stored in the lower layer of the skin of mammals for insulation and energy storage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility.

Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must be excreted as it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories. Fats are required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones.

Essential Nutrients

While the animal body can synthesize many of the molecules required for function from the organic precursors, there are some nutrients that need to be consumed from food. These nutrients are termed essential nutrients, meaning they must be eaten, and the body cannot produce them.

The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids. Vitamins are another class of essential organic molecules that are required in small quantities for many enzymes to function and, for this reason, are considered to be co-enzymes. Absence or low levels of vitamins can have a dramatic effect on health, as outlined in Table 15.1 and Table 15.2. Both fat-soluble and water-soluble vitamins must be obtained from food. Minerals, listed in Table 15.3, are inorganic essential nutrients that must be obtained from food. Among their many functions, minerals help in structure and regulation and are considered co-factors. Certain amino acids also must be procured from food and cannot be synthesized by the body. These amino acids are the “essential” amino acids. The human body can synthesize only 11 of the 20 required amino acids; the rest must be obtained from food. The essential amino acids are listed in Table 15.4.

Table 15.1.
Water-soluble Essential Vitamins
VitaminFunctionDeficiencies Can Lead ToSources
Vitamin B1(Thiamine)Needed by the body to process lipids, proteins, and carbohydrates Coenzyme removes CO2 from organic compoundsMuscle weakness, Beriberi: reduced heart function, CNS problemsMilk, meat, dried beans, whole grains
Vitamin B2(Riboflavin)Takes an active role in metabolism, aiding in the conversion of food to energy (FAD and FMN)Cracks or sores on the outer surface of the lips (cheliosis); inflammation and redness of the tongue; moist, scaly skin inflammation (seborrheic dermatitis)Meat, eggs, enriched grains, vegetables
Vitamin B3(Niacin)Used by the body to release energy from carbohydrates and to process alcohol; required for the synthesis of sex hormones; component of coenzyme NAD+ and NADP+Pellagra, which can result in dermatitis, diarrhea, dementia, and deathMeat, eggs, grains, nuts, potatoes
Vitamin B5(Pantothenic acid)Assists in producing energy from foods (lipids, in particular); component of coenzyme AFatigue, poor coordination, retarded growth, numbness, tingling of hands and feetMeat, whole grains, milk, fruits, vegetables
Vitamin B6(Pyridoxine)The principal vitamin for processing amino acids and lipids; also helps convert nutrients into energyIrritability, depression, confusion, mouth sores or ulcers, anemia, muscular twitchingMeat, dairy products, whole grains, orange juice
Vitamin B7(Biotin)Used in energy and amino acid metabolism, fat synthesis, and fat breakdown; helps the body use blood sugarHair loss, dermatitis, depression, numbness and tingling in the extremities; neuromuscular disordersMeat, eggs, legumes and other vegetables
Vitamin B9(Folic acid)Assists the normal development of cells, especially during fetal development; helps metabolize nucleic and amino acidsDeficiency during pregnancy is associated with birth defects, such as neural tube defects and anemiaLeafy green vegetables, whole wheat, fruits, nuts, legumes
Vitamin B12(Cobalamin)Maintains healthy nervous system and assists with blood cell formation; coenzyme in nucleic acid metabolismAnemia, neurological disorders, numbness, loss of balanceMeat, eggs, animal products
Vitamin C (Ascorbic acid)Helps maintain connective tissue: bone, cartilage, and dentin; boosts the immune systemScurvy, which results in bleeding, hair and tooth loss; joint pain and swelling; delayed wound healingCitrus fruits, broccoli, tomatoes, red sweet bell peppers
Table 15.2.
Fat-soluble Essential Vitamins
VitaminFunctionDeficiencies Can Lead ToSources
Vitamin A (Retinol)Critical to the development of bones, teeth, and skin; helps maintain eyesight, enhances the immune system, fetal development, gene expressionNight-blindness, skin disorders, impaired immunityDark green leafy vegetables, yellow-orange vegetables fruits, milk, butter
Vitamin DCritical for calcium absorption for bone development and strength; maintains a stable nervous system; maintains a normal and strong heartbeat; helps in blood clottingRickets, osteomalacia, immunityCod liver oil, milk, egg yolk
Vitamin E (Tocopherol)Lessens oxidative damage of cells,and prevents lung damage from pollutants; vital to the immune systemDeficiency is rare; anemia, nervous system degenerationWheat germ oil, unrefined vegetable oils, nuts, seeds, grains
Vitamin K (Phylloquinone)Essential to blood clottingBleeding and easy bruisingLeafy green vegetables, tea
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Figure 15.15.  A healthy diet should include a variety of foods to ensure that needs for essential nutrients are met. (credit: Keith Weller, USDA ARS)

Table 15.3.
Minerals and Their Function in the Human Body
MineralFunctionDeficiencies Can Lead ToSources
*CalciumNeeded for muscle and neuron function; heart health; builds bone and supports synthesis and function of blood cells; nerve functionOsteoporosis, rickets, muscle spasms, impaired growthMilk, yogurt, fish, green leafy vegetables, legumes
*ChlorineNeeded for production of hydrochloric acid (HCl) in the stomach and nerve function; osmotic balanceMuscle cramps, mood disturbances, reduced appetiteTable salt
Copper (trace amounts)Required component of many redox enzymes, including cytochrome c oxidase; cofactor for hemoglobin synthesisCopper deficiency is rareLiver, oysters, cocoa, chocolate, sesame, nuts
IodineRequired for the synthesis of thyroid hormonesGoiterSeafood, iodized salt, dairy products
IronRequired for many proteins and enzymes, notably hemoglobin, to prevent anemiaAnemia, which causes poor concentration, fatigue, and poor immune functionRed meat, leafy green vegetables, fish (tuna, salmon), eggs, dried fruits, beans, whole grains
*MagnesiumRequired co-factor for ATP formation; bone formation; normal membrane functions; muscle functionMood disturbances, muscle spasmsWhole grains, leafy green vegetables
Manganese (trace amounts)A cofactor in enzyme functions; trace amounts are requiredManganese deficiency is rareCommon in most foods
Molybdenum (trace amounts)Acts as a cofactor for three essential enzymes in humans: sulfite oxidase, xanthine oxidase, and aldehyde oxidaseMolybdenum deficiency is rare 
*PhosphorusA component of bones and teeth; helps regulate acid-base balance; nucleotide synthesisWeakness, bone abnormalities, calcium lossMilk, hard cheese, whole grains, meats
*PotassiumVital for muscles, heart, and nerve functionCardiac rhythm disturbance, muscle weaknessLegumes, potato skin, tomatoes, bananas
Selenium (trace amounts)A cofactor essential to activity of antioxidant enzymes like glutathione peroxidase; trace amounts are requiredSelenium deficiency is rareCommon in most foods
*SodiumSystemic electrolyte required for many functions; acid-base balance; water balance; nerve functionMuscle cramps, fatigue, reduced appetiteTable salt
Zinc (trace amounts)Required for several enzymes such as carboxypeptidase, liver alcohol dehydrogenase, and carbonic anhydraseAnemia, poor wound healing, can lead to short statureCommon in most foods
*Greater than 200mg/day required
Table 15.4.
Essential Amino Acids
Amino acids that must be consumedAmino acids anabolized by the body
*The human body can synthesize histidine and arginine, but not in the quantities required, especially for growing children.

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