What is food biochemistry? |

Structure and Functions

Food biochemistry is concerned with the breakdown of food in the cell as a source of energy. Each cell is a factory that converts the
nutrients of the food one eats to energy and other structural components of the body. The amount of energy that these nutrients supply is expressed in Calories (kilocalories). The number of Calories consumed will determine the energy balance of the individual and whether one loses or gains weight. The nutrients come in a variety of forms, but they can be divided into three major categories: carbohydrates, lipids (fats), and proteins. These nutrients are broken down by the cell metabolically to produce energy for cellular processes. Other components are used by the cell and the entire body for structure and transport. Each of these nutrients is essential to a well-balanced diet and good health. Two other components of a successful diet are vitamins and minerals.

Carbohydrates are molecules composed of carbon, hydrogen, and oxygen. They range from the simple
sugars all the way to the complex carbohydrates. The simplest carbohydrates are the monosaccharides (one-sugar molecules), primarily glucose and fructose. The simple monosaccharides are usually joined to form disaccharides (two-sugar molecules), such as sucrose (glucose and fructose, or cane sugar), lactose (glucose and galactose, or milk sugar), and maltose (two glucoses, which is found in grains). The complex carbohydrates are the polysaccharides (multiple sugar molecules), which are composed of many monosaccharides, usually glucose. There are two main types: starch, which is found in plants such as potatoes, and glycogen, in which form humans store carbohydrate energy for the short term (up to twelve hours) in the liver.

The next major group of molecules is the lipids, which are made up of the solid fats and liquid oils. These molecules are primarily composed of carbon and hydrogen. They form three major groups: the triglycerides, phospholipids, and sterols. The triglycerides are composed of three fatty acids attached to a glycerol (a three-carbon molecule); this is the group that makes up the fats and oils. Fats, which are primarily of animal origin, are triglycerides that are solid at room temperature; such triglycerides are saturated, which means that there are no double bonds between their carbon molecules. Oils are liquid at room temperature, primarily of plant origin, and either monounsaturated or polyunsaturated (there are one or more double bonds between the carbon molecules in the chain). This group provides long-term energy in humans and is stored as adipose (fat) tissue. Each gram of fat stores approximately nine Calories of energy per gram, which is about twice that of carbohydrates and proteins (four Calories per gram). The adipose tissue also provides important insulation in maintaining body temperature. Phospholipids are similar to triglycerides in structure, but they have two fatty acids and a phosphate attached to the glycerol molecule. Phospholipids are the building blocks of the cell membranes that form the barrier between the inside and the outside of the cell. Sterols are considered lipids, but they have a completely different structure. This group includes cholesterol, vitamin D, estrogen, and testosterone. The sterols function in the structure of the cell membrane (as cholesterol does) or as hormones (as do testosterone and estrogen).

The last major group of molecules is that of the proteins, which are composed of carbon, hydrogen, oxygen, and nitrogen. Proteins are long chains of amino acids; each protein is composed of varying amounts of the twenty different amino acids. Proteins are used in the body as enzymes, substances that catalyze (generally, speed up) the biochemical reactions that take place in cells. They also function as transport molecules (such as hemoglobin, which transports oxygen) and provide structure (as does keratin, the protein in hair and nails). The human body can synthesize eleven of the twenty amino acids; the other nine are considered essential amino acids because they cannot be synthesized and are required in the diet.

Two other groups of essential compounds are necessary in the diet for the body’s metabolism: vitamins and minerals. The vitamins are organic compounds (made up of carbon) that are required in only milligram or microgram quantities per day. The vitamins are classified into two groups: the water-soluble vitamins (the B vitamins and vitamin C) and the fat-soluble vitamins (vitamins A, D, E, and K). Vitamins are vital components of enzymes.

The
minerals are inorganic nutrients that can be divided into two classes, depending on the amounts needed by the body. The major minerals are required in amounts greater than one hundred milligrams per day; these minerals are calcium, phosphorus, magnesium, sodium, chloride, sulfur, and potassium. The trace elements, those needed in amounts of only a few milligrams per day, are iron, zinc, iodine, fluoride, copper, selenium, chromium, manganese, and molybdenum. Although they are required in small quantities, the minerals play an important role in the human body. Calcium is involved in bone and teeth formation and muscle contractions. Iron is found in hemoglobin and aids in the transportation of oxygen throughout the body. Potassium helps nerves send electrical impulses. Sodium and chloride maintain water balance in tissues and vascular fluid.

An adequate diet is one that supplies the body and cells with sources of energy and building blocks. The first priority of the diet is to supply the bulk nutrients—carbohydrates, fats, and proteins. An average young adult requires between 2,100 and 2,900 Calories per day, taking into account the amount of energy required for rest and work. Carbohydrates, fats, and proteins are taken in during a meal and digested—that is, broken into smaller components. Starch is broken down to glucose, and sucrose is broken down to fructose and glucose and absorbed by the bloodstream. Fats are broken down to triglycerides, and proteins are divided into their separate amino acids, to be absorbed by the bloodstream and transported throughout the body. Each cell then takes up essential nutrients for energy and to use as building parts of the cell.

Once the nutrients enter the cell, they are broken down into energy through a series of metabolic reactions. The first step in the metabolic process is called glycolysis. Glucose is broken down through a series of reactions to produce
adenosine triphosphate (ATP), a molecule used to fuel other biochemical pathways in the cell. Glycolysis gives off a small amount of energy and does not require oxygen. This process can provide the energy for a short sprint; lactic acid buildup in the muscles will lead to fatigue, however, if there is insufficient oxygen.

Long-term energy requires oxygen. Aerobic respiration can metabolize not only the sugars produced by glycolysis but triglycerides and amino acids as well. The molecules enter what is called the Krebs cycle, an aerobic pathway that provides eighteen times more energy than glycolysis. The waste products of this pathway are carbon dioxide and water, which are exhaled.

Disorders and Diseases

Diet plays a major role in the metabolism of the cells. One major problem in diet is the overconsumption of Calories, which can lead to weight gain and eventual obesity. Obesity is defined as being 20 percent over one’s ideal weight for one’s body size. A number of problems are associated with obesity, such as high blood pressure, high levels of cholesterol, increased risk of cancer, heart disease, and early death.

At the other end of the scale is malnutrition. Carbohydrates are the preferred energy source in the form of either blood glucose or glycogen, which is found in the liver and muscles. This source gives a person approximately four to twelve hours of energy. Long-term storage of energy occurs as fat, which constitutes anywhere from 15 percent to 25 percent of body composition. During times of starvation, when the carbohydrate reserve is almost zero, fat will be mobilized for energy. Fat will also be used to make glucose for the blood because the brain requires glucose as its energy source. In extreme starvation, the body will begin to degrade the protein in muscles down to its constituent amino acids in order to produce energy.

Malnutrition can also occur if essential vitamins and minerals are excluded from the diet. Vitamin deficiencies affect the metabolism of the cell since these compounds are often required to aid the enzymes in producing energy. A number of medical problems are associated with vitamin deficiencies. A deficiency in thiamine will result in the metabolic disorder called beriberi. A loss of the thiamine found in wheat and rice can occur in the refinement process, making it more difficult to obtain enough of this vitamin from the diet. Alcoholics have an increased thiamine requirement, to help in the metabolism of alcohol, and usually have a low level of food consumption; thus, they are at risk for developing beriberi. Lack of vitamin A results in night blindness, and lack of vitamin D results in rickets, in which the bones are weakened as a result of poor calcium uptake.

One interesting feature about diet and the metabolic pathways concerns how different molecules are treated. Some people mistakenly believe that it is better to eat fruit sugar than other sugars. Fruit sugar, the simple sugar fructose, is chemically related to glucose. Because glucose and fructose are converted to each other during glycolysis, it does not matter which sugar is eaten. Far more important to proper nutrition is what accompanies the sugar. Table sugar provides only calories, while a piece of fruit contains both fruit sugar and vitamins, minerals, and fiber for a more complete diet.

Many errors in metabolism occur because genes do not carry the proper information. The result may be an enzyme that, although critical to a biochemical pathway, does not function properly or is missing altogether. One disorder of carbohydrate metabolism is called galactosemia. Mother’s milk contains lactose, which is normally broken down into galactose and glucose. With galactosemia, the cells take in the galactose but are unable to convert it to glucose because of the lack of an enzyme. Thus, galactose levels build up in the blood, liver, and other organs, impairing their function. This condition can lead to death in an infant, but the effects of galactosemia are usually detected and the diet modified by use of a milk substitute.

Amino acid metabolism can also be defective, leading to the accumulation of toxic by-products. One of the best-known examples involves the amino acid phenylalanine. About one in ten thousand infants is born with a defective pathway, a disorder called phenylketonuria (PKU). If PKU is not discovered in time, by-products can accumulate, causing poor brain development and severe mental retardation. PKU must be diagnosed early in life, and a special, controlled diet must be given to the infant. Because phenylalanine is an essential amino acid, limited amounts are included in the diet for proper growth, but large amounts need to be excluded. The artificial sweetener aspartame (NutraSweet) poses a problem for those with PKU. Aspartame is composed of two amino acids, phenylalanine and aspartic acid. When aspartame is broken down during digestion, phenylalanine enters the bloodstream. Individuals with PKU should not ingest aspartame; there is a warning to that effect on products containing this chemical.

Other errors of metabolism are noted later in life. One example is lactose intolerance, in which lactose is cleaved by the enzyme lactase into glucose and galactose. The enzyme lactase, found in the digestive tract, is very active in suckling infants, but only northern Europeans and members of some African tribes retain lactase activity into adulthood. Other groups, such as Asian, Arab, Jewish, Indian, and Mediterranean peoples, show little lactase activity as adults. These people cannot digest the lactose in milk products, which then cannot be absorbed in the intestinal tract. A buildup of lactose can lead to diarrhea and colic. Usually in those parts of the world milk is not used by adults as food. In the United States, one can purchase milk that contains lactose which has been partially broken down into galactose and glucose. One can also purchase the lactase enzyme itself and add it to milk.

Another error of metabolism results in diabetes mellitus, which means “excessive excretion of sweet urine.” A telltale sign of this condition is sugar, specifically glucose, in the urine. In normal people, blood glucose levels remain relatively stable. After a meal, when the blood glucose levels rise, the pancreas starts to secrete the hormone insulin. Insulin causes the cells to take in the extra blood glucose and convert it to glycogen or fat, thus storing the extra energy. In diabetics, there is little or no insulin production or release, or the target cells may have faulty receptors. As a result, the blood glucose level remains high. The excess glucose is then excreted in the urine, leading to the symptom of excess thirst. The body is forced to rely much more heavily on fats as an energy source, leading to high levels of circulating fats and cholesterol in the blood. These substances can be deposited in the blood vessels, causing high blood pressure and heart disease. Excess fats, in levels that exceed the body’s
ability to metabolize and burn them, may produce acetone, which gives the breath of diabetics a sweet odor. A buildup of acetone can lead to ketoacidosis, a pathologic condition in which the blood pH drops from 7.4 to 6.8. Complications arising from diabetes also include blindness, kidney disease, and nerve damage. Furthermore, resulting peripheral vascular disease, in which the body’s extremities do not get enough blood, leads to tissue death and gangrene.

Perspective and Prospects

The study of food biochemistry has evolved over the years from a strictly biochemical approach to one in which diet and nutrition play a major role. An understanding of diet and nutrition required vital information about the metabolic processes occurring in the cell, supplied by the field of biochemistry.

This information started to become available in 1898 when Eduard Buchner discovered that the fermentation of glucose to alcohol and carbon dioxide could occur in a cell-free extract. The early twentieth century led to the complete discovery of the glycolytic pathway and the enzymes that were involved in the process. In the 1930s, other pathways of metabolism were elucidated.

In conjunction with Buchner’s discovery, British physician Archibald Garrod in 1909 hypothesized that genes control a person’s appearance through enzymes that catalyze certain metabolic processes in the cell. Garrod thought that some inherited diseases resulted from a patient’s inability to make a particular enzyme, and he called them “inborn errors of metabolism.” One example he gave was a condition called alkaptonuria, in which the urine turns black upon exposure to the air.

Some of the earliest nutritional studies date back to the time of Aristotle, who knew that raw liver contained an ingredient that could cure night blindness. Christiaan Eijkman studied beriberi in the Dutch East Indies and traced the problem to diet. Sir Frederick Hopkins was an English biochemist who conducted pioneering work on vitamins and the essentiality of amino acids in the early twentieth century. Hopkins realized that the type of protein is important in the diet as well as the quantity. Hopkins hypothesized that some trace substance in addition to proteins, fats, and carbohydrates may be required in the diet for growth; this substance was later identified as the vitamin. Hopkins was the first biochemist to explore diet and metabolic function.

Working with this broad base, scientists have made tremendous advances in the study of diet and nutrition based on the biochemistry of the cell. In 1943, the first recommended daily (or dietary) allowances (RDAs) were published to provide standards for diet and good nutrition; the term now used is dietary reference intake (DRI). The DRIs suggest the amounts of protein, fats, carbohydrates, vitamins, and minerals required for adequate nutrient uptake. The major uses of the DRIs are for schools and other institutions in planning menus, obtaining food supplies, and preparing food labels.

Since the 1970s, research has consistently associated nutritional factors with six of the ten leading causes of death in the United States: high blood pressure, heart disease, cancer, cardiovascular disease, chronic liver disease, and non-insulin-dependent diabetes mellitus. This research has led to improvements in the American diet.

Bibliography

Bonci, Leslie. American Dietetic Association Guide to Better Digestion. New York: Wiley, 2003.

Campbell, Neil A., et al. Biology: Concepts and Connections. 6th ed. San Francisco: Pearson/Benjamin Cummings, 2008.

Clark, Nancy. Nancy Clark’s Sports Nutrition Guidebook. 4th ed. Champaign, Ill.: Human Kinetics, 2008.

Duyff, Roberta Larson. American Dietetic Association Complete Food and Nutrition Guide. 3d ed. Hoboken, N.J.: John Wiley & Sons, 2007.

"Malnutrition." MedlinePlus, June 14, 2011.

Margen, Sheldon. Wellness Foods A to Z: An Indispensable Guide for Health-Conscious Food Lovers. New York: Rebus, 2002.

Marlow, Amy. "Phenylketonuria." Health Library, November 26, 2012.

Nasset, Edmund S. Nutrition Handbook. 3d ed. New York: Harper & Row, 1982.

Nelson, David L., and Michael M. Cox. Lehninger Principles of Biochemistry. 5th ed. New York: W. H. Freeman, 2009.

Nieman, David C., Diane E. Butterworth, and Catherine N. Nieman. Nutrition. Rev. ed. Dubuque, Iowa: Wm. C. Brown, 1992.

Wood, Debra. "Lactose Intolerance." Health Library, May 11, 2013.