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.

Learn about nutrition, protein quality, and amino acid requirements. Understand the importance of essential amino acids in food for good nutrition.