Metabolism - Real-life applications

Putting Lipids to Use

Derived from living systems of plants, animals, or humans, lipids are essential to good health, not only for humans but also for other animals and even plants. Seeds, for example, contain lipids for the storage of energy. Because fat is a poor conductor of heat, lipids also can function as effective insulators, and for this reason, people living in Arctic zones seek fatty foods such as blubber. Some lipids function as chemical messengers in the body, while others serve as storage areas for chemical energy. There is a good reason why babies are born with "baby fat" and why children entering puberty often tend to become chubby: in both cases, they are building up energy reserves for the great metabolic hurdles that lie ahead, and within a few years, they will have used up those excessive fat stores.

FATS AND OILS.

Fats and oils are both energy-rich compounds that are basic components of the normal diet. Both have essentially the same chemical structure—a mixture of fatty acids combined with glycerol—and are insoluble (do not dissolve) in water. While fats remain solid or at least semisolid at room temperature, however, most oils very quickly become liquid at increased temperatures. Animal fats and oils include butter, lard, tallow, and fish oil. Numerous other oils, such as cottonseed, peanut, and corn oils, are derived from plants.

Fats have two main functions: they provide some of the raw material for synthesizing (creating)

and repairing tissues, and they serve as a concentrated source of fuel energy. Fats, in fact, provide humans with roughly twice as much energy, per unit weight, as carbohydrates and proteins. Fats are not only an important source of day-to-day energy, but they also can be stored indefinitely as adipose (fat) tissue in case of future need. Fats also help by transporting fat-soluble vitamins, such as A and D (see Vitamins), throughout the system. They cushion and form protective pads around delicate organs, such as the heart, liver and kidneys, and the layer of fat under the skin helps insulate the body against too much heat loss. They even add to the flavor of foods that might otherwise be inedible.

NOT ALL FAT IS CREATED EQUAL.

Although normal amounts of certain kinds of fat in the diet are essential to good health, unnecessarily high amounts (especially of unhealthy fats) can lead to various problems. Healthy fats include those from fatty fish, such as salmon, mackerel, or tuna, or from fat-containing vegetables, such as the avocado. In addition, many vegetable oils, particularly olive oil, can be beneficial.

Bad fats, on the other hand, are usually ones that have been tampered with through a process known as hydrogenation. This is a term describing any chemical reaction in which hydrogen atoms are added to fill in chemical bonds between carbon and other atoms, but in the case of fatty foods, hydrogenation involves the saturation of hydrocarbons, organic chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. When they are treated with hydrogen gas, they become "saturated" with hydrogen atoms. Saturated fats, as they are called, are harder and more stable and stand up better to the heat of frying, which makes them more desirable for use in commercial products. For this reason, many foods contain hydrogenated vegetable oil; however, saturated fats have been linked to a rise in blood cholesterol levels—and to an increased risk of heart disease.

Cholesterol is a variety of lipid, and, like other lipids, some of it is essential—but only some and only of the right kind. Most cholesterol is transported through the blood in low-density lipoproteins, or LDLs, which have been nicknamed bad cholesterol. These lipoproteins are received by LDL receptors on the cell membranes, but if there are more LDLs than LDL receptors, the excess LDLs will be deposited in the arteries. Thus, LDLs are not really "bad" unless there are too many of them. On the other hand, "good" cholesterol (HDLs, or high-density lipoproteins) help protect against damage to the artery walls by carrying excess LDLs back to the liver.

HOW MUCH IS TOO MUCH?

A certain amount of excess adipose tissue can be valuable during periods of illness, overactivity, or food shortages. Too much, however, can be unsightly and also can overwork the heart and put added stress on other parts of the body. High levels of certain circulating fats may lead to atherosclerosis, which is a thickening of the artery walls, and they have been linked to various illnesses, including cancer.

With fat, as with many things where the body is concerned, if a little is a good, this does not mean that a lot is better. In the past, nutritionists considered a diet that obtained 40% of its calories from fats a reasonable one; today, however, they recommend that no more than 30% of all calories (and preferably an even smaller percentage) come from fat. Agreement on this point, however, is far from universal. Some physicians and scientists maintain that dietary fat does not contribute as much to body fat as do carbohydrates. Carbohydrates are good for someone who needs a boost of energy that can be consumed easily by the body, such as an athlete going into competition. But for in active people—and this includes a large portion of Americans—carbohydrates simply are stored as fat.

Experts do not even agree on the answer to a question much simpler than "How much is too much fat in the diet?"—the question "How much is too much fat on the body?" Some doctors classify a person as obese whose weight is at least 20% more than the recommended weight for his or her height, but others say that standard height-and-weight charts are misleading. After all, muscle weighs more than fat, and it is conceivable that a very muscular athlete with very little body fat might qualify as "overweight" compared with the recommended weight for his or her height.

BODY FAT, THE SEXES, AND NATURE.

Because of the complexity of the issue, many experts contend that the proportion of fat to muscle, measured by the skinfold "pinch" test, is a better measure of obesity. (Being obese is not the same as being overweight: the muscular athlete described in the last paragraph is overweight but not obese, a term that implies an excess of body fat.) In healthy adults, fat typically should account for about 18-25% of the body weight in females and 15-20% in males.

The reason for the difference between men and women is that fat naturally accumulates in a woman's buttocks and thighs, because nature "assumes" that she will bear children, in which case such excess fat will be useful. This is why women over the age of about 25 often complain that when they and their husbands or boyfriends embark on a fitness program together, the men usually see results faster. The reason is that there is no genetic or evolutionary benefit to be gained from a man having fat around his waist, which is where men usually gain. If anything—since our genetic codes and makeup have changed little since prehistory—the well-being and propagation of the human species are best served by a lean, muscular male capable of killing animals to feed and protect his family. All of this means, of course, that men should not gloat if they see better results from a regular workout program; instead, they should just recognize that nature is at work in their wives' or girlfriends' bodies as in their own.

Metabolic Disorders

Enzymes, as we noted earlier, are critical participants in metabolic reactions. They are like relay runners in a race, in this case a race along the metabolic pathways whereby nutrients are turned into energy or new bodily material. Therefore, if an enzyme is missing or does not function as it should, it can create a serious metabolic disorder. An example is phenylketonuria (PKU), caused by the lack of an enzyme known as phenylalanine hydroxylase. This enzyme is responsible for converting the amino acid phenylalanine to a second amino acid, tyrosine; when this does not happen, phenylalanine builds up in the body. It is converted to a compound called phenylpyruvate, which impairs normal brain development, resulting in severe mental retardation.

Other examples of metabolic disorders include alkaptonuria, thalassemia, porphyria, Tay-Sachs disease, Hurler syndrome, Gaucher disease, galactosemia, Cushing syndrome, diabetes

mellitus, hyperthyroidism, and hypothyroidism. Most of these conditions affect a small population; however, diabetes mellitus (discussed in Noninfectious Diseases) is one of the leading killers in America. At present, no cures for metabolic disorders exist. The best approach is to diagnose such conditions as early as possible and then arrange a person's diet to deal as effectively as possible with that disorder.

Eating Disorders

Eating disorders are a different matter, because they are psychological rather than physiological conditions. No one is sure what causes eating disorders, but researchers think that family dynamics, biochemical abnormalities, and modern American society's preoccupation with thinness all may contribute. Eating disorders are virtually unknown in parts of the world where food is scarce, but in wealthy lands, such as the United States, problems of overeating, self-induced starvation, or forced purging have gained considerable attention.

Anorexia nervosa, bulimia, and obesity are the most well known types of eating disorder. The word anorexia comes from the Greek for "lack of appetite," but the problem for people with anorexia is not that they are not hungry. On the contrary, they are starving, but unlike poor people in the Third World, they are not starving as the result of a shortage of food but because they are denying themselves nutrition. They do this because they fear gaining weight, even when they are so severely underweight that they look like skeletons.

The name of a related condition, bulimia, literally means "hungry as an ox." People with this problem go on eating binges, often gorging on junk food. Then they force their bodies to get rid of the food, either by vomiting or by taking large amounts of laxatives. A third type of eating disorder, obesity, also is characterized by uncontrollable overeating, but in this case the person does not force the body to eject the food that has been consumed. That, at least, makes obesity more healthy than bulimia, but there is nothing healthy about accumulating vast amounts of body fat, as severely obese people do.

ANOREXIA AND BULIMIA.

Young people are more likely than older people to suffer anorexia or bulimia, conditions that typically become apparent at about the age of 20 years. Although both men and women can experience the problem, in fact, only about 5% of people with these eating disorders are male. And though anorexia and bulimia are closely related—particularly inasmuch as they are psychological in origin but can exact a heavy biological toll—there are several important differences.

People who have anorexia or bulemia often come from families with overprotective parents who have unrealistically high expectations of their children. Frequently, high expectations go hand in hand with a wealthy background, and certainly anorexia and bulimia are not conditions that typically affect the poor. Anorexia and bulimia often seem to develop after some stressful experience, such as moving to a new town, changing schools, or going through puberty. Low self-esteem, fear of losing control, and fear of growing up are common characteristics of people with these conditions. Their need for approval manifests in a quest to meet or exceed our culture's idealized concept of extreme thinness. This quest is a part of our popular culture, promoted by waiflike models whose sunken eyes stare out of fashion magazines.

Like anorexia, bulimia results in starvation, but there are behavioral, physical, and psychological differences between the two. Bulimia is both less and more dangerous: on the one hand, people who have it tend to be of normal weight or are overweight, and unlike those with anorexia, they are aware of the fact that they have a problem. On the other hand, because the effects of their behavior are not so readily apparent, it is easier for a person with bulimia to persist in the pattern of bingeing and purging for much longer.

Approximately one in five persons with bulimia has a problem with drug or alcohol use, and they pursue their binges in a way not unlike that of a guilty addict or alcoholic hiding the spent needles or empty bottles from family members. They may go from restaurant to restaurant to avoid being seen eating too much in any one place, or they may pretend to be shopping for a large dinner party when, in fact, they intend to eat all the food themselves. Because of the expense of consuming so much food, some resort to shoplifting.

During a binge, people suffering from bulimia favor high-carbohydrate foods, such as doughnuts, candy, ice cream, soft drinks, cookies, cereal, cake, popcorn, and bread, and they consume many times the number of calories they would normally consume in one day. No matter what their normal eating habits, they tend to eat quickly and messily during a binge, stuffing the food into their mouths and gulping it down, sometimes without even tasting it. Some say they get a feeling of euphoria during binges, similar to the "runner's high" that some people get from exercise. Then, when they have gorged themselves, they force the food back out, either by causing themselves to vomit or by taking large quantities of laxatives.

Regular self-induced vomiting can cause all sorts of physical problems, such as damage to the stomach and esophagus, chronic heartburn, burst blood vessels in the eyes, throat irritation, and erosion of tooth enamel from the acid in vomit. Excessive use of laxatives can induce muscle cramps, stomach pains, digestive problems, dehydration, and even poisoning, while bulimia, in general, brings about vitamin deficiencies and imbalances of critical body fluids, which, in turn, can lead to seizures and kidney failure.

The self-imposed starvation of people with anorexia likewise takes a heavy toll on the body. The skin becomes dry and flaky, muscles begin to waste away, bones stop growing and may become brittle, and the heart weakens. Seeking to protect itself in the absence of proper insulation from fat, the body sprouts downy hair on the face, back, and arms in response to lower body temperature. In women, menstruation stops, and permanent infertility may result. Muscle cramps, dizziness, fatigue, and even brain damage as well as kidney and heart failure are possible. An estimated 10% to 20% of people with anorexia die either as a direct result of starvation or by suicide.

To save people with anorexia, force-feeding may be necessary. Some 70% of anorexia patients who are treated for about six months return to normal body weight, but about 15-20% can be expected to relapse. Bulimia is not as likely as anorexia to reach life-threatening stages, so hospitalization typically is not necessary. Treatment generally calls for psychotherapy and sometimes the administration of antidepressant drugs. Unlike people with anorexia, those with bulimia usually admit they have a problem and want help overcoming it.

OBESITY.

Unlike anorexia or bulimia, obesity is more of a problem among people from lower-income backgrounds. This probably relates to a lack of education concerning nutrition, combined with the fact that healthier food is more expensive; by contrast, unhealthy items, such as white sugar, corn meal, and fatty cuts of pork and other meats can fill or overfill a person's stomach inexpensively. In addition, though men and women both tend to gain weight as they age, women are almost twice as likely as men to be obese.

Some cases of obesity relate to metabolic problems, while others stem from compulsive eating, which is psychologically motivated. Some studies suggest that obese people are much more likely than others to eat in response to stress, loneliness, or depression. And just as emotional pain can lead to obesity, obesity can lead to psychological scars. From childhood on, obese people are taunted and shunned, and throughout life they may face discrimination in school and on the job.

Physically, obesity is a killer, especially for those who are morbidly obese—that is, people whose obesity endangers their health. Obesity is

a risk factor for diabetes, high blood pressure, arteriosclerosis, angina pectoralis (chest pains due to inadequate blood flow to the heart), varicose veins, cirrhosis of the liver, and kidney disease. Obese people are about 1.5 times more likely to have heart attacks than are other people, and the overall death rate among people ages 20-64 is 50% higher for the obese than for people of ordinary weight.

Hibernation

Having looked at several unnatural ways in which people alter their metabolisms, let us close with an example of a very natural way that animals sometimes temporarily change theirs. This is hibernation, a state of inactivity in which an animal's heart rate, body temperature, and breathing rate are decreased as a way to conserve energy through the cold months of winter. A similar state, known as estivation, is adopted by some desert animals during the dry months of summer.

Hibernation is a technique that animals have developed, as a result of natural selection over the generations (see Evolution), to adapt to harsh environmental conditions. When food is scarce, a nonhibernating animal would be like a business operating at a loss—that is, using more energy maintaining its body temperature and searching for food than it would receive from consuming the food. Hibernating animals use 70-100 times less energy than when they are active, allowing them to survive until food is once again plentiful.

CONTRAST WITH SLEEP.

Many animals sleep more often when food is scarce, but only a few truly hibernate. Bears, which many people think of as the classic hibernating animal, are actually just deep sleepers. By contrast, true hibernation occurs only in small mammals, such as bats and woodchucks and a few birds, among them nighthawks. Some insects also practice a form of hibernation. Hibernation differs from sleep, in that a hibernating animal shows a drastic reduction in metabolism and then awakes relatively slowly, whereas a sleeping animal decreases its metabolism only slightly and can wake up almost instantly if disturbed. Also, hibernating animals do not show periods of rapid eye movement (REM), the stage of sleep associated with dreaming in humans.

THE PROCESS OF HIBERNATION.

Animals prepare for hibernation in the fall by storing food; usually this storage is internal, in the form of fat reserves. A woodchuck in early summer may have only about 5% body fat, but as fall approaches, changes in the animal's brain chemistry cause it to feel hungry and to eat constantly. As a result, the woodchuck's body fat increases to about 15% of its total weight. In other animals, such as the dormouse, fat may constitute as much as 50% of the animal's weight by the time hibernation begins. A short period of fasting follows the feeding frenzy, to ensure that the digestive tract is emptied completely before hibernation begins.

Going into hibernation is a gradual process. Over a period of days, an animal's heart rate and breathing rate drop slowly, eventually reaching rates of just a few beats or breaths per minute. Their body temperatures also drop from levels of about 100°F (38°C) to about 60°F (15°C). The lowered body temperature makes fewer demands on metabolism and food stores. Electric activity in the brain ceases almost completely during hibernation, although some areas—those that respond to external stimuli, such as light, temperature, and noise—remain active. Thus, the hibernating animal can be aroused under extreme conditions.

Periodically—perhaps every two weeks or so—the hibernating animal awakes and takes a few deep breaths to refresh its air supply. If the weather is particularly mild, some animals may venture from their lairs. An increase in heart rate signals that the time for arousal, or ending hibernation, is near. Blood vessels dilate, particularly around the heart, lungs, and brain, and this leads to an increased breathing rate. Eventually, the increase in circulation and metabolic activity spreads throughout the body, and the animal resumes a normal waking state.

WHERE TO LEARN MORE

Bouchard, Claude. Physical Activity and Obesity. Champaign, IL: Human Kinetics, 2000.

"KEGG Metabolic Pathways ." KEGG: Kyoto Encyclopedia of Genes and Genomes—GenomeNet, Bioinformatics Center, Institute for Chemical Research, Kyoto University (Web site). <http://www.genome.ad.jp/kegg/metabolism.html> .

Medline Plus: Food, Nutrition, and Metabolism Topics. Medline, National Library of Medicine, National Insti tutes of Health (Web site). <http://www.nlm.nih.gov/medlineplus/foodnutritionandmetabolism.html> .

Metabolic Pathways of Biochemistry. George Washington University (Web site). <http://www.gwu.edu/~mpb/> .

Metabolism (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/M/Metabolism.html> .

Michal, Gerhard. Biochemical Pathways: An Atlas of Bio chemistry and Molecular Biology. New York: Wiley, 1999.

Pasternak, Charles A. The Molecules Within Us: Our Body in Health and Disease. New York: Plenum, 1998.

Pathophysiology of the Digestive System (Web site). <http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/> .

Spallholz, Julian E. Nutrition, Chemistry, and Biology. Englewood Cliffs, NJ: Prentice-Hall, 1989.

Wolinsky, Ira. Nutrition in Exercise and Sport. 3d ed. Boca Raton, FL: CRC Press, 1998.