Outline and chapter notes to accompany chapter 8 NUTRITION AND HEALTH Jan., 2001 A. DIGESTION PROCESSES FOOD INTO SUBSTANCES THAT THE BODY CAN ABSORB CHEMICAL AND MECHANICAL PROCESSES IN DIGESTION Chemical digestion breaks down foods chemically, but only on exposed surfaces. Mechanical digestion exposes new surfaces by breaking up larger particles and by removing surface layers of material. THE DIGESTIVE SYSTEM MOUTH: Food is moistened and chewed; some starch is digested by the enzyme salivary amylase STOMACH: Food is churned; proteins are broken into smaller fragments (peptides) by pepsin, an enzyme that works best under acid pH; HCl is secreted to keep pH low; mucus helps protect lining against self-digestion SMALL INTESTINE: Fats are emulsified (Fig. 8.2) in the duodenum by bile salts secreted by the liver and stored by the gall bladder for release when signalled by a digestive hormone. Emulsification works because bile salts have both polar (water-soluble) and non-polar (fat-soluble) portions. Additional digestive enzymes are secreted by the intestinal lining and by the pancreas: - proteases break down proteins and peptidases complete this breakdown; - pancreatic amylase breaks down starch; - sugar-digesting enzymes break down larger sugars into simple sugars like glucose; - fat-digesting enzymes break fats into glycerol and fatty acids. In the final portion of the small intestine, breakdown products of food are absorbed. ABSORPTION Mechanisms for absorption of molecules from the small intestine into the cells lining the intestine are the same mechanisms that bring molecules into many other types of cells also. These mechanisms (Fig. 8.3) include: Passive diffusion (down a concentration gradient) Facilitated diffusion Active transport Endocytosis: without receptors receptor-mediated LARGE INTESTINE: Water is absorbed. Symbiotic bacteria are present. These bacteria digest cellulose in certain other animals. In humans, intestinal bacteria do not digest cellulose but help synthesize vitamin B12. B. ABSORBED NUTRIENTS CIRCULATE THROUGHOUT THE BODY CIRCULATORY SYSTEM The circulatory system carries blood throughout the body. Arteries carry blood away from the heart, to the capillaries. Capillaries have enormous surface area and thin walls, permitting the exchange of materials: Nutrients and dissolved oxygen move into the body's cells. Waste products (including carbon dioxide) move out of the body's cells into the blood through capillary walls. In the alveoli of the lungs, oxygen enters the blood, and carbon dioxide moves into the alveoli, from which it is exhaled. Veins carry blood back toward the heart. THE HEART On the right side of the heart, oxygen-poor blood moves from the right atrium into the right ventricle and out the pulmonary artery. On the left side of the heart, oxygen-rich blood returning from the lungs moves from the left atrium to the left ventricle, then out the aorta, whose many branches distribute it to the body's organs. C. ALL HUMANS HAVE CERTAIN DIETARY REQUIREMENTS MACRONUTRIENTS include carbohydrates, proteins, and lipids. CALORIC NEEDS (Table 8.1), measured in kilocalories. Caloric needs depend on an individual's basal metabolism, which is a measure of their level of metabolic activity when resting. Caloric needs also depend on level of activity and on other factors. Caloric needs are met largely by carbohydrates in most diets. Some diets derive substantial calories from fats, but this is unhealthy. Chemical energy conversion at the cellular level (Box 8.3): Chemical groups from all three macronutrient groups enter the citric acid cycle (Krebs cycle) located in mitochondria. The number of mitochondria per cell depends on the energy requirements of the cell. These chemical groups are converted into either: high energy chemical bonds in ATP; or reducing equivalents in NADH NADH donates its reducing equivalents to the electron transport chain, producing a concentration gradient of H+ ions (proton gradient). Gradient formation depends on intact mitochondrial membranes. The gradient is another form of cellular energy storage; this energy is used to make more ATP. CARBOHYDRATES (Fig. 8.4) include: Monosaccharides-- simple (single) sugars like glucose, fructose, and galactose Disaccharides-- double sugars like sucrose (table sugar, broken down into glucose and fructose) or lactose (milk sugar, broken down into glucose plus galactose). Dietary sucrose contributes to tooth decay (Box 8.2). Polysaccharides-- starch (digestible into glucose units) & cellulose (also built of glucose units but not digestible by humans) Diets rich in complex carbohydrates are usually rich in other nutrients and usually healthy. LIPIDS (Fig. 8.5) are constituents of cell membranes (Box 8.1). Lipids in the diet are mostly fats (solid at room temperature) and oils (liquid at room temp.) Both fats and oils are broken down into glycerol and fatty acids. Diets with too much fat are unhealthy and are associated with obesity, heart disease, atherosclerosis, and several cancers. Most North American diets contain too much fat. Cholesterol is a lipid that clogs arteries; it comes from animal sources only. Our bodies need cholesterol to maintain the fluidity of cell membranes, but we can make all we need; we don't need to get it from our diet. Saturated fats (with single bonds only) decrease the fluidity of cell membranes by packing too tightly together. They are also more readily converted into cholesterol. Unsaturated fats (with double bonds) keep cell membranes more fluid (more flexible) and are less easily converted into cholesterol. Lipids are insoluble in water, and so their transport through the blood requires transport particles such as low-density lipoproteins (LDLs or "bad cholesterol") and high-density lipoproteins (HDLs of "good cholesterol"). PROTEINS (Fig. 8.6) are broken down into individual amino acids. The body uses 20 amino acids, but only 8 are essential in adult human diets because the others can be made from these 8. An amino acid that is present in small quantities and is used up before other amino acids is called a limiting amino acid. Complete proteins, containing all 8 essential amino acids, are found in soybeans and most animal proteins (meat, fish, eggs, dairy products, etc.) Many foods rich in animal proteins also contain lots of fat. Proteins from plants (and also fish and other seafood) usually have less fat associated. Most plant proteins, however, are incomplete because they lack one or more essential amino acids (those essential in human nutrition). You can get around this problem by combining proteins from several sources (Box 8.4). FIBER "Fiber" includes all material which passes through the digestive tract without being either digested or absorbed. Insoluble fiber (mostly cellulose from plants) is needed for good digestive health-- it stimulates digestive movements (peristalsis) and stimulates protective mucus to form. Soluble fiber lowers cholesterol levels. Diets high in both types of fiber are associated with lower risks of cancer and heart disease. VITAMINS (Table 8.2) Vitamins are organic nutrients needed only in very small quantities (micronutrients). Many vitamins function as parts of enzymes or as enzyme cofactors (coenzymes). Fat-soluble vitamins include A, D, E, and K. Vitamin A is important for vision, D for bone formation, E for reproductive health, and K for blood clotting. In addition, vitamins A, C, and E are anti-oxidants that prevent certain unhealthy oxidation reactions from taking place. (Oxidation reactions are those in which electrons are removed from atoms or molecules.) The body accumulates fat-soluble vitamins, so overdoses are possible. Water-soluble vitamins include C and the various B vitamins. Vitamin C is an anti-oxidant needed to maintain and repair healthy mucous membranes. Thiamine (B1), riboflavin (B2), niacin (B3), pyridoxine (B6), cyanocobalamine (B12), and pantothenic acid are B-vitamins that are essential parts of enzymes needed to carry out important metabolic processes. Excess amounts of water-soluble vitamins are easily excreted through the urine; the limits of excretion are reached only rarely. MINERALS (inorganic micronutrients) are often parts of coenzymes. Calcium is needed to build (and rebuild) bones and teeth, also for muscle contraction. Iron is needed to build the oxygen-carrying protein hemoglobin and also various enzymes. Sodium and potassium (also called electrolytes) are needed in balanced amounts to regulate fluid concentrations and to maintain the electrical properties of cell membranes. High sodium concentrations are often associated with high blood pressure (hypertension). Copper, zinc, magnesium, and iodine are needed in much smaller quantities. Fluorine in small quantities makes tooth enamel more resistant to decay. D. HUMAN DIETS VARY AMONG POPULATIONS LINKS BETWEEN DIET AND CHRONIC DISEASES Before 1900, many people with inadequate diets suffered from vitamin deficiencies. In industrial countries nowadays, vitamin deficiencies are rare. Attention has shifted to heart disease, stroke, cancer, to the chronic conditions (like obesity and high blood pressure) that lead to these fatal diseases, and to the maintenance of good health habits. Epidemiological evidence (derived from studies on whole populations) suggests many links between diets and the incidence of various diseases: - coronary artery diseases and strokes are more common in countries (like the U.S.) where meat consumption is high; - populations eating lower amounts of saturated fats have lower rates of heart disease and strokes; this is especially true if fish consumption is high. - Japanese people in Japan have much lower rates for heart disease and stroke than do Japanese-Americans living in Hawaii or California; this suggests that genetic differences are less important than environmental differences such as diet. NATURAL SELECTION AND LACTOSE INTOLERANCE Most of the world's people are lactose intolerant (they cannot digest the sugars in milk). In central and northern Europe, however, the need for vitamin D has favored people who are lactose tolerant and able to digest all dairy products (see also chapter 5). Surplus milk from the U.S. was once shipped to Brazil with good intentions but many Brazilians became sick because they were lactose intolerant. REGIONAL VARIATIONS IN NUTRITION The same amount of farm land can support a larger population if the people eat plant products than if they eat animals that consume those plant products. This is because each energy conversion in the food chain converts only about 15% of the energy of the previous level, so the same land area that supports a certain quantity of meat animls could instead support more than seven times as many calories if it were used for grain. Soils vary in their nutrient content. Populations dependent on food supplies from one place only may therefore suffer nutritional deficiencies if the soil is nutrient-poor. This is no longer a problem if foods are obtained from multiple sources by trade, because soils from different places are not likely to have the same deficiencies. Also, if agriculture is more diversified, different plants extract different levels of important minerals from the soil, so nutritional deficiencies are less pronounced. Most countries, even poor ones, have enough arable land to raise enough grains and vegetables to keep their human populations healthy, provided their resources are properly managed. In some places, land used to raise meat or cash crops for export could support a much healthier population if grain for local consumption were raised instead. Ecological factors, however, limit agriculture in certain places (as explained later). E. MALNUTRITION CONTRIBUTES TO POOR HEALTH EATING DISORDERS Anorexia nervosa is a self-imposed starvation or a "relentness pursuit of thinness" resulting from a misperception of body size and a failure to respond to normal hunger cues. Bulimia is a related disease that includes food binges alternating with self-induced vomiting or purging. Both disorders produce severe nutritional deficiencies; both can be fatal. PROTEIN DEFICIENCIES Inadequate protein intake with sufficient calories produces kwashiorkor, a condition in which the loss of muscle mass is concealed by tissue swelling (edema) and inflated abdomens. Inadequate protein intake with insufficient calories produces marasmus, a fatal disease in which body tissue wastes away. ECOLOGICAL FACTORS CONTRIBUTING TO POOR DIETS In much of Africa, the climate is either too dry or too wet for agriculture, and tse-tse flies make additonal large tracts of land unsuitable for domestic animals that might pull plows or supply meat. As a result, Africa suffers from chronic food shortages that are made worse by war and other conflicts. EFFECTS OF POVERTY AND WAR ON HEALTH Poor maternal nutrition can lead to higher rates of infant mortality and premature births. Disruption of food supplies by poverty or war conditions can have devastating results on childhood nutrition. Most evidence on this point comes from retrospective studies in which data is gathered about changes that have already taken place in the past. Childhood depression can also result in a failure to thrive. The effects of inadequate childhood nutrition are often permanent, and cannot be made up by supplying adequate food later in life. MICRONUTRIENT MALNUTRITION Depression, old age, cancer chemotherapy, and alcoholism are among many conditions in which people lose interest in eating properly. Many people with these conditions suffer from chronic vitamin and mineral deficiencies. In many such cases, adequate nutrition is easily restored, but the underlying problems must also be addressed. NUTRITION, HEALTH, AND THE IMMUNE SYSTEM Inadequate nutrition depresses the immune system, increasing the frequency and severity of many diseases. This is especially true in people whose immune systems are already weak from old age, stress, or other causes. ---------------------------------- Jan., 2001