Nutrients are chemicals in foods that provide energy for powering life processes; chemicals aiding or enabling life processes; or materials to construct molecules for the normal development, growth, and maintenance of the body. There are six groups of nutrients: carbohydrates, lipids, proteins, vitamins, minerals, and water. All six groups provide raw materials for constructing new molecules, but only carbohydrates, lipids, and proteins provide energy to sustain life processes.

Essential nutrients are those nutrients that the body cannot synthesize and must obtain in food in order to construct other molecules necessary for life. The essential nutrients include certain amino acids, certain fatty acids, most vitamins, minerals, and water. Because the essential nutrients are not all present in any one food, a balanced diet is required. The conversion of raw materials into molecules for life processes is a major role of the liver.

The Institute of Medicine of the U.S. National Academy of Sciences has developed nutritional recommendations, called the Dietary Reference Intake (DRI), to assist in the planning and assessment of nutrient intake. DRI includes the Recommended Daily Allowance (RDA) for each nutrient. The RDA for a nutrient is the average daily intake level that is sufficient to meet the nutritional needs of a healthy person. Most nutritional labels provide RDA but not DRI recommendations. DRI and RDA recommendations can be found through the U.S. Department of Agriculture (USDA) website (fnic.nal.usda.gov).

Energy Foods And Cellular Respiration

Carbohydrates, fats, and proteins are called “energy foods” because they are used in cellular respiration to release the energy in their chemical bonds for ATP production. Cellular respiration includes both anaerobic and aerobic components. Anaerobic respiration, or glycolysis, occurs within the cytosol of a cell, while aerobic respiration occurs within mitochondria, where the enzymes catalyzing the reactions are located. There are two sequential, linked aerobic processes: the citric acid cycle and the electron transport chain. When oxygen is available, a nutrient, such as glucose, is completely degraded to carbon dioxide and water, in order to release energy. Approximately, 40% of the energy is captured in ATP, while the remainder is lost as heat. ATP does not store energy, but it carries it to where it is needed to power life processes. The released heat energy is important in maintaining a normal body temperature.

Follow the cellular respiration of glucose in figure 15.20 . A molecule of glucose (containing 6 carbon atoms) is split during glycolysis to form 2 molecules of pyruvic acid (each containing 3 carbon atoms) and 2 ATP. Each pyruvic acid molecule is converted to acetyl- CoA (each molecule containing 2 carbon atoms), which releases CO2 . Each acetyl-CoA molecule enters the citric acid cycle. With each turn of the cycle, one acetyl-CoA molecule is broken down to release CO2, H+, and high- energy electrons. Substrate reactions associated with the citric acid cycle produce 2 ATP.

The higher-energy electrons pass along the molecular carriers of the electron transport chain from a higher- energy level to a lower-energy level-much like water flowing down a staircase. At each transfer (step) within the chain, energy is released to form ATP and the electrons move to the next lower energy level (next lower step in the staircase). Ultimately, all of the available energy is extracted, and the electrons, H+, and O2 combine to form water (H2O). Electron transfer yields a total of 32 to 34 ATP, depending upon the cell in which cellular respiration occurs. When added to the 2 ATP from glycolysis and the 2 ATP from the citric acid cycle, one molecule of glucose can yield a total of 36 to 38 ATP.

The end products of digestion for lipids (fats) and proteins also may be used in cellular respiration, either directly or after conversion into compatible molecules. Figure 15.20 shows the major points of entry into cellular respiration for these molecules. The number of ATP produced varies with the type of molecule broken down.

Carbohydrates

Nearly all carbohydrates in the diet come from plant foods. Glycogen is the only carbohydrate in animal foods. While there is very little of this polysaccharide in meat, animal liver is an abundant source. Monosaccharides are in honey and fruits; disaccharides are found in table sugar and dairy products; and starch, a polysaccharride, occurs in cereals, vegetables, and legumes (e.g., beans, peas, peanuts).

Cellulose is a polysaccharide that is abundant in plant foods, but it cannot be digested by humans because they lack the necessary digestive enzymes. However, it is an important dietary component, because it provides fiber (roughage) that increases the bulk of the intestinal contents, which aids the function of the large intestine. Evidence suggests that high-fiber diets reduce the risk of certain colon disorders, such as diverticulitis and colon cancer.

Carbohydrates are used mostly as an energy source, with glucose as the primary carbohydrate molecule used in cellular respiration. The hormone insulin plays a crucial role in moving glucose into cells. Most cells can live by obtaining energy from fatty acids or amino acids via cellular respiration, but some cells, notably neurons, are dependent upon a steady supply of glucose. For this reason, the functions of the nervous system decline if the concentration of blood glucose decreases.

The liver, along with the hormones insulin and glucagon, is involved in the regulation of glucose concentration in the blood. In response to insulin, excess glucose is converted into glycogen for storage primarily in the liver but also in skeletal muscles. If excess glucose still remains, it is converted into triglycerides and is stored in adipose tissues. When blood glucose levels decline, glucagon signals the liver to convert glycogen into glucose. If still more glucose is needed, triglycerides are converted into glycerol and fatty acids. Then glycerol may be converted into glucose.

Lipids

Lipids include triglycerides, phospholipids, steroids, and lipid-soluble vitamins, (A, D, E, and K), but triglycerides are the most common lipids in the diet. Triglycerides may be either saturated or unsaturated. Fats and oils contain a mixture of saturated, monounsaturated (one double bond), and polyunsaturated (more than one double bond) fatty acids. Coconut and palm oils, dairy products, and beef fat contain mostly saturated fatty acids. Peanut, olive, and canola oils consist mostly of monounsaturated fatty acids. Safflower, sunflower, and corn oils contain mostly polyunsaturated fatty acids. Cholesterol is present in dairy products, red meats, and egg yolks.

Lipids are essential components of the diet, although excessive amounts are not desirable. Phospholipids form the major portion of plasma membranes and the myelin sheaths of neurons. Triglycerides are important energy sources for many cells, including liver and skeletal muscle cells. Excess triglycerides are stored in adipose tissue, where they form the largest energy reserve in the body.

While cholesterol is not used as an energy source, it forms parts of plasma membranes and is used in the synthesis of bile salts and steroid hormones. The liver helps to regulate the concentration of triglycerides and cholesterol in the blood.

Lipids are hydrophobic and do not dissolve in the aqueous blood plasma. This poses a problem for their transport to and from body cells. To overcome this problem, lipids are combined with alpha and beta globulins to form complexes called lipoproteins. A lipoprotein has a core of triglycerides and cholesterol with a coating of protein, making the complex soluble in blood plasma. Lipoproteins are classified by their density. The greater the proportion of protein, the greater the density of a lipoprotein.

Low-density lipoproteins (LDLs), the “bad” cholesterol, are formed in the liver and transport cholesterol and triglycerides to body cells, including cells of arteries where dangerous plaques can form. High-density lipoproteins (HDLs), the “good” cholesterol, are formed in the liver as almost empty shells of protein. They pick up cholesterol from cells throughout the body as they circulate and return the cholesterol to the liver. The cholesterol is then disposed of through bile, which helps decrease the risk of plaque formation in arteries. Thus LDLs transport cholesterol from the liver to body cells, and HDLs transport cholesterol from body cells to the liver.

Proteins

Prime sources of proteins include red meat, poultry, fish, milk products, eggs, nuts, cereals, and legumes. of the 20 kinds of amino acids composing proteins, eight cannot be synthesized from other amino acids by the liver. These eight amino acids are the essential amino acids. All essential amino acids must be present in the body in order for the body to synthesize proteins necessary for normal growth and maintenance. Animal proteins contain all of the essential amino acids, but plant proteins lack one or more of them. However, if cereals and legumes (e.g., beans, peas) are eaten together, this combination provides all of the essential amino acids.

Amino acids are used by the body primarily to synthesize proteins: plasma proteins, certain hormones, enzymes, and proteins that form structural components of cells. However, they may be used as an energy source in cellular respiration if there is a deficient supply of glucose or fats. If there is an excess of amino acids, they may be converted to glucose or fat.

For amino acids to be used as an energy source or converted to glucose or fat, the liver first removes the amine groups (-NH2) so that the remainders of the amino acid molecules are available for these alternative pathways. The amine groups react to form ammonia, a toxic substance. However, the liver converts ammonia into urea, a less toxic substance that is released into the blood and is excreted by the kidneys in urine (figure 15.20).

Vitamins

Vitamins are organic compounds that are required in minute amounts for the normal functioning of the body. They are not energy sources, but they are essential for the utilization of energy foods. Most vitamins act with enzymes to speed up particular metabolic reactions.

Vitamins are classified according to their solubility: water-soluble or lipid-soluble. Water-soluble vitamins include vitamin C and the B vitamins. They are sensitive to heat and easily destroyed by cooking. Lipid- soluble vitamins are vitamins A, D, E, and K. They are more resistant to heat and are not easily destroyed by cooking.

Vitamin Sources Importance
B1 (thiamine) Meats, eggs, cereals, beans, peas, leafy green vegetables Required for cellular respiration and the synthesis of ribose
B2 (riboflavin) Meats, cereals, leafy green vegetables; formed by colon bacteria Required for cellular respiration
B3 (niacin) Meats, liver, beans, peas, peanuts Required for cellular respiration and synthesis of proteins, nucleic acids, and fats
B5 (pantothenic acid) Meats, fish, cereals, beans, peas, fruits, vegetables, milk; formed by colon bacteria Required for cellular respiration, conversion of amino acids and lipids into glucose, synthesis of cholesterol and steroid hormones
B6 (pyridoxine) Meats, poultry, fish, cereals, beans, peas, peanuts; formed by colon bacteria Required for protein synthesis and formation of antibodies
B7 (biotin) Liver, eggs, beans, peas, peanuts; formed by colon bacteria Required for cellular respiration and synthesis of fatty acids
B9 (folic acid) Liver, cereals, leafy green vegetables; formed by colon bacteria Required for synthesis of DNA, RNA, and normal blood cells
B12 (cyanocobalamin) Meats, poultry, fish, milk, eggs, cheese Required for synthesis of nucleic acids, formation of red blood cells, and cellular respiration of amino acids
C (ascorbic acid) Citrus fruits, cabbage, tomatoes, leafy green vegetables Required for synthesis of steroid hormones, absorption of iron, and formation of connective tissues
A Eggs, milk, butter, liver; green, yellow, and orange vegetables and fruits Required for healthy skin and mucous membranes; for development of visual pigments in rods and cones; for healthy bones and teeth
D Formed by skin when exposed to ultraviolet light; fish liver oils, milk, eggs Required for healthy bones and teeth; promotes absorption of calcium and phosphorus
E Cereals, vegetable oils, fruits, and vegetables Prevents oxidation of fatty acids and vitamin A; helps keep plasma membranes intact
K Synthesized by colon bacteria; leafy green vegetables, cabbage, pork, liver, soybean oil Required for formation of prothrombin, an enzyme involved in the formation of blood clots

Minerals

Minerals are inorganic substances that plants absorb from the soil. They are present in both plant foods and animal foods because animals obtain them by eating plants. Humans need adequate amounts of the seven major minerals noted in table 15.6 but require only trace amounts of other minerals that occur in the body. About 4% of the body weight consists of minerals. Calcium and phosphorus, the most abundant minerals, account for about 75% of the total minerals in the body.

In the body, minerals may be incorporated into organic compounds, deposited as mineral salts (as in bone), or occur as ions in body fluids.

Mineral Sources Importance
Calcium (Ca) Milk, milk products, leafy green vegetables Forms bones and teeth; required for blood clotting, conduction of nerve impulses, and muscle contraction
Phosphorus (P) Meats, cereals, nuts. milk, milk products, legumes Forms bones and teeth; component of proteins, nucleic acids. ATP. and phosphate buffers
Potassium (K) Meats, nuts, potatoes, bananas, cereals Required for conduction of nerve impulses and muscle contractions
Sulfur (S) Meats, milk. eggs, legumes Component of vitamins biotin and thiamine, the hormone insulin, and some amino acids
Sodium (Na) Table salt cured meats, cheese Helps maintain osmotic pressure of body fluids; required for nerve impulse transmission
Chlorine (Cl) Table salt cured meats, cheese Helps maintain osmotic pressure of body fluids; required for formation of HCI in gastric juice
Magnesium (Mg) Milk, milk products, cereals, legumes, nuts, leafy green vegetables Required for normal nerve and muscle functions; involved in ATP-ADP conversions