Biochemistry: Nutrition and Protein Quality

Nutrition

• Nutrition is the provision of needed energy and essential constituents that cannot be synthesized de novo.
• Sound nutrition depends on food intake that includes carbohydrates, proteins, fats, vitamins, minerals, and water.

Protein Quality

• Measured by comparing the proportions of essential amino acids in a food with production required for good nutrition.
• Egg and milk proteins are high-quality proteins used as a reference standard against which other proteins can be compared.

Energy Intake

• Energy derived from carbohydrates and fats affects protein requirements because it spares the use of proteins as an energy source.

Physical Activity

• Increases nitrogen retention from dietary protein.

Protein Energy Malnutrition

Marasmus

• Generalized wasting due to deficiency of both energy and protein.

Kwashiorkor

• Characterized by edema due to deficiency of both quantity and quality of protein although energy intake may be adequate.

Carbohydrate Requirements

• Glucose is needed by tissues, but does not have to be part of the diet, since other dietary carbohydrates are readily converted to glucose.

Fiber Requirement

• Denotes all plant cell wall components that cannot be digested by animal enzymes.
• Beneficial effects of high fiber include:
  • Aiding in water retention during passage of food along the gut, producing softer feces.
  • Reducing incidence of diverticulosis, cancer of the colon, cardiovascular disease, and diabetes mellitus.
  • Slowing stomach emptying and delaying the rise in blood glucose.

Lipid Requirements

• Increases the palatability of food.
• Produces a feeling of satiety.
• Acts as a dietary vehicle for fat-soluble vitamins.
• Supplies essential polyunsaturated fatty acids that the body cannot synthesize.
• High fat consumption, especially of saturated fat, correlates with coronary heart disease.

Vitamin Requirements

• Organic nutrients required in small quantities for normal metabolism that cannot be synthesized by the body in adequate amounts.

Fat-Soluble Vitamins

Vitamin A (Retinol)

• Provitamin is beta carotene from plants for maintenance of epithelial cells of skin, eye, and mucous membrane.
• Regenerates visual purple of the eye.
• Deficiency causes xerophthalmia, night blindness, and retardation of growth.
• Sources: butter, eggs, milk, carotene of plants, cod-liver oil.

Vitamin D (Calciferol)

• Provitamins are ergasterol from plants and yeasts and 7-dehydrocholesterol from skin.
• Regulates calcium phosphorus metabolism.
• Deficiency causes rickets in young.
• Sources: eggs, fish oil, beef fat, skin (exposure to UV rays).

Vitamin E (Alpha-Tocopherol)

• Necessary for nuclear growth and activity.
• Deficiency causes sterility in some animals and death of embryos.
• Sources: green leaves, spinach, soybean oil, egg yolk, liver.

Vitamin K (Anti-Hemorrhagic Vitamin)

• Synthesizes prothrombin in liver, essential for blood clotting.
• Deficiency results in failure of blood to clot.
• Sources: green leaves, spinach, soybean oil, egg yolk, liver.

Water-Soluble Vitamins

Vitamin B Complex

Thiamin (B1) Antineuretic

• Necessary for carbohydrate metabolism; acts as a coenzyme to carboxylase.
• Deficiency causes beriberi, loss of appetite, and cessation of growth.
• Sources: yeast, germ of cereals, egg yolk, nuts, lean protein.

Riboflavin (B2)

• Concerned with oxidative process and intermediate metabolism of food; hydrogen acceptor and donor for cellular synthesis.
• Deficiency causes stunted growth, dermatitis, and cheilosis (inflammation of mouth corners).
• Sources: green leaves, eggs, meat, cheese, milk, liver.

Niacin (Nicotinic Acid) - Antipellargic

• Essential to cellular functions, constituents of certain coenzymes.
• Deficiency causes pellagra in man and degeneration of nerve cells.
• Sources: green leaves, egg yolk, wheat germ, liver, yeast.

Pyridoxine (B6)

• Functions as coenzymes of some transmitting enzymes.
• Deficiency results in failure to grow, together with anemia and dermatitis.
• Sources: yeast, meat, eggs, nuts, cereals.

Folic Acid

• Essential for growth and formation of blood cells.
• Deficiency causes anemia and sprue in man.
• Sources: green leaves, soybeans, yeasts, egg yolks.

Pantothenic Acid

• Forms coenzyme A of Krebs cycle metabolism, necessary for nerve and skin.
• Sources: eggs, milk, meat, sweet potatoes, cane, molasses.

Biotin

• Forms coenzyme necessary for carbon dioxide utilization.
• Sources: egg yolk, meat, molasses, fresh fruits, and vegetables, yeast, cereal grain.

Cyanocobalamin (B12)

• Extrinsic factor of anti-anemic factor.
• Deficiency causes pernicious anemia.
• Sources: milk, egg yolk, liver, oysters.

Vitamin C (Ascorbic Acid)

• Formation of intercellular material.
• Deficiency causes scurvy in man.
• Sources: citrus fruits, tomatoes, cabbage, spinach.

Mineral Requirements

Macrominerals

• Required in amounts greater than 100 mg/d.

Calcium

• Constituents of bones, teeth; regulates nerve and muscle function.

Phosphorus

• Constituents of bones, teeth, ATP, phosphorylated metabolic intermediates, nucleic acids.

Sodium

• Principal cation in extracellular fluid, regulates plasma volume, acid-base balance, nerve and muscle function.

Potassium

• Principal cation in intracellular fluid, regulates nerve and muscle function.

Chloride

• For fluid and electrolyte balance; constituent of gastric fluid.

Magnesium

• Constituent of bones, teeth; enzyme cofactor.

Microminerals (Traced Elements)

• Required in amounts less than 100 mg/d.

Chromium

• Trivalent chromium, a constituent of "glucose tolerance factor."

Cobalt

• Constituent of vitamin B12.

Copper

• Constituent of oxidase enzymes like cytochrome oxidase, ferroxidase.

Iodine

• Constituent of thyroxin, triodothyroxi.

Iron

• Constituent of hemoglobin, cytochromes.

Manganese

• Constituent of hydrolase, decarboxylase, and transferase enzymes; needed for glycoprotein and proteoglycan synthesis.

Molybdenum

• Constituent of oxidase enzyme, xantine oxidase.

Selenium

• Constituent of glutathione peroxidase.

Zinc

• Cofactor of many enzymes like lactate dehydrogenase, alkaline phosphatase, carbonic anhydrase, etc.

Fluoride

• Increases hardness of bones and teeth.

Energy Requirement

• Nutrients provide the energy needed by the body.
• The energy unit is the kilocalorie, the amount of heat necessary to raise 1 kg of water from 15 to 16°C.

Expenditure of Energy

Basal Metabolic Rate

• The energy expenditure necessary to maintain basic physiologic functions under standardized conditions.
• Proportionate to mean body weight and surface area.
• Higher in males than in females, in children, in people with fever and hyperthyroidism.

Thermogenic Effect (Specific Dynamic of Action of Food)

• The extra heat associated with the consumption of food equivalent to about 5-10% of total energy expenditure.

Physical Activity

• The range is over 10-fold between resting and maximum athletic activity.

Environmental Temperature

• Low environmental temperature causes increased energy expenditure, while at temperatures above body heat, extra energy is expended in cooling.

Carbohydrates Metabolism

• Glucose is the focal point of carbohydrate metabolism.
• Commonly called blood sugar, glucose is supplied to the body via the circulatory system and, after being absorbed by a cell, can be either oxidized to yield energy or stored as glycogen for future use.

Digestion and Absorption of Glucose

• Digestion is the biochemical process by which food molecules, through hydrolysis, are broken down into simpler chemical units that can be used by cells for their metabolic needs.
• The digestion of carbohydrates begins in the mouth, where the enzyme salivary a-amylase catalyzes the hydrolysis of a-glycosidic linkages in starch from plants and glycogen from meats to produce smaller polysaccharides and the disaccharide maltose.