a. The intake of food is necessary for life because the foods we eat provide essential nutrients.

Nutrients are substances necessary for growth and repair of tissue and for maintenance of normal body functioning. A “nutritionally adequate” diet will contain all the essential nutritive substances in the amounts and proportions required to maintain life and health.

These essential nutrients are carbohydrates, proteins, fats, minerals, vitamins, and water.

(1) Carbohydrates, proteins, and fats are the basic fuels for cellular activity.

(2) Minerals are inorganic substances that help to regulate body processes. Some work with the enzymes, some act as catalysts, and some work within the buffer systems.

(3) Vitamins are organic nutrients that function to regulate physiological processes such as growth and metabolism.

(4) Water is an important nutrient with many functions. It acts as a coolant, a lubricant, a suspending medium, and as a reactant in chemical processes.

b. Since the food we eat cannot be used for fuel in its consumed form, it must be broken down (digested) to the molecular level. In molecular form, the chemicals can be transported and absorbed through the cell membranes for utilization by the body cells. This process of digestion consists of both mechanical breakdown and chemical breakdown.

(1) Mechanical digestion includes chewing, swallowing, peristalsis, and defecation.

(2) Chemical digestion is the enzymatic breakdown of the food- stuffs into chemically simple molecules that can be absorbed and utilized by the cells.

c. Carbohydrates, also known as sugars and starches, are organic compounds that provide the most ready source of energy to the body. Carbohydrates are broken down to their simplest form, called a monosaccharide, to be absorbed from the digestive tract.

Carbohydrates consist of three major groups: monosaccharides, disaccharides, and polysaccharides.

(1) Monosaccharides are called the “simple sugars” because they cannot be further broken down into simpler molecules. The monosaccharide glucose is the major carbohydrate used for fuel by the cells.

(2) Disaccharides are two monosaccharides that are joined chemically.

(3) Polysaccharides are a group of five or more monosaccharides that are joined chemically.

d. Proteins are complex molecules of chemically linked chains of amino acids. Proteins are essential components of all cells in the body and have many functions within the human body. Some proteins function as enzymes, some as antibodies, and some are used for nutrition. The diet must contain sufficient protein to replace the protein broken down during normal body functions and growth. Proteins are broken down into their constituent amino acids to be absorbed from the digestive tract. These amino acids are transported to the body’s cells, where they are recombined to form (synthesize) new protein molecules.

(1) All proteins are synthesized from combinations of the naturally occurring amino acids. A great variety of proteins are made possible with only a limited number of amino acids because a different protein is created with each variation in the number and arrangement of the amino acids. This can be likened to the alphabet and words. The letters (amino acids) can be combined in a multitude of ways to form different words (proteins).

(2) Ten of these amino acids are termed “essential” amino acids. This is due to the fact that these amino acids cannot be synthesized within the body, but must be consumed as food for growth and survival. Other amino acids can be synthesized within the body from other molecules present within the cells. A food protein that contains all the essential amino acids is referred to as a “complete” protein.

e. Fat is primarily an energy source. In addition to its value as an energy source, fat serves as a carrier for the fat-soluble vitamins (A, D, E, and K) and adds flavor to the diet. Fats are emulsified in the intestine and split into fatty acids and glycerol for absorption. If not used as immediate energy sources, fatty acids are re-synthesized into body fat and stored in the many fat cells of the body for future use.