The cell-filled liquid that circulates through the heart, arteries, and veins. A 70 kg adult has a blood volume of 5 liters. Blood is regarded as a tissue in which red and white cells are suspended in a liquid (plasma) in the ratio of 45 parts cells to 55 parts plasma. This vital fluid performs many tasks. Blood supplies all tissues with nutrients and oxygen, and it transports waste such as urea to the kidneys, and carbon dioxide to the lungs, for disposal. Blood is the medium for integration and coordination of tissues of the body through hormonal regulation. It maintains the chemical equilibrium of the body in terms of electrolytes (ionic substances) and polyelectrolytes (serum proteins), which in turn regulate water distribution in blood versus tissues. Blood pH is buffered to maintain a very narrow range at 7.35 to 7.45. Circulating antibodies, gamma globulin, represent blood aspects of the immune system and thus are the first line of defense against foreign substances. Blood also contains special cells, platelets, and protein clotting factors to form clots and thus limit blood loss. Fats, cholesterol, and fat-soluble vitamins are transported in the blood by specialized structures (lipoproteins). Nutrients like vitamin A, iron, and copper are carried by their own transport proteins. In terms of mechanical function, the bloodstream assures an even temperature for all regions of the body.

Most cells in blood are red blood cells (erythrocytes), which are specifically designed to transport oxygen, white blood cells (leukocytes) represent a much smaller fraction; as part of the immune system they protect against infection. Lymphocytes, which represent 20 percent to 50 percent of white cells, are derived from either bone marrow or from the thymus gland. They mount a cellular defense against foreign cells and materials. Plasma, the fluid remaining once cells are removed, contains fibrinogen. This inactive protein can be activated to form fibrin clots to plug holes in blood vessels. The fluid remaining after blood has clotted is called serum which lacks cells and clotting factors, but contains glucose and minerals like potassium, sodium, and chloride, the most common electrolytes. These ions help maintain the appropriate ionic strength, pH, and fluid balance of the body. Serum contains albumin and other proteins that help maintain ion concentrations in the blood, and it contains transport proteins, such as very low-density lipoproteins (VLDL) for carrying fat and low-density lipoproteins (LDL) and high-density lipoproteins (HDL) to transport cholesterol.

Several types of nutrients support the circulatory system. For example, vitamin k and calcium support the blood clotting mechanism, zinc, iron, manganese, magnesium, vitamin B₆, folic acid, vitamin B₁₂, and other nutrients support erythrocyte and leukocyte production. Vitamin C maintains strength and elasticity of capillaries. (See also blood clotting; endocrine system; hemoglobin; immune system.)

Blood-brain barrier

A structural barrier that limits the passage of a variety of substances, including certain drugs and nutrients, from blood vessels into the brain and the central nervous system. This barrier consists of cells lining capillaries (endothelial cells). The attachments between these cells are called “tight junctions.” However, the nature of the physical barriers and biochemical mechanisms for transporting materials across the barrier are complex and are not completely understood. Very small molecules like water and oxygen simply diffuse through cells and capillaries, glucose, the major fuel of the brain, is an example of a substance that can penetrate the blood-brain barrier, passing freely across the barrier, though other sugars do not. During starvation or crash dieting, ketone bodies, small acidic compounds that accumulate in the blood during excessive fat degradation (a condition known as ketosis), can cross through capillary linings, pass into the brain and be burned for energy. In contrast, long-chain fatty acids that make up fat cannot penetrate the blood-brain barrier, and consequently fat cannot supply the brain with energy. Some nutrients rely on transport systems embedded in cell membranes to actively transport substances into the brain. Thus amino acids enter by specific, energy-dependent processes (active transport) tyrosine, phenylalanine, leucine, isoleucene, valine, and tryptophan compete for the same transport sites. Different sites are specific for other types of amino acids. (See also food; neurotransmitter.)

Blood pressure

The pressure maintained in arteries and veins by the heart. Blood pressure usually refers to an indirect measurement of pressure of large arteries at the height of the pulse. Blood pressure reflects the resistance of blood flow in the capillary bed and arterioles as well as the elasticity of arteries themselves. The heart exerts pressure throughout the circulatory system. Ventricles of the heart contract (the systolic phase of the heartbeat), creating systolic pressure in the cycle of heart pumping. Ventricular relaxation between heartbeats creates the lowest pressure between heartbeats, the diastolic pressure. Like a barometer for measuring air pressure, blood pressure is measured in units equivalent to the height of a column of mercury. A pressure of 120/80 represents a systolic pressure equivalent to 120 mm of mercury and a diastolic pressure of 80 mm of mercury. A systolic pressure persistently greater than 140 and a diastolic pressure persistently greater than 100 indicate stages of hypertension (high blood pressure), a potentially serious condition.

The following factors are linked to increased blood pressure: overweight, age, emotional stress, physical activity, and male gender. Quiet sleep and female gender are linked to with decreased blood pressure.

Dietary factors and heredity are risk factors for susceptible individuals. Approximately 20 percent of adults will be adversely affected by overconsumption of sodium. Unfortunately, these salt-sensitive individuals cannot be readily identified. Eating a large meal can lower blood pressure quickly in older people when the stomach fills with food, and experiments show that such people may feel faint or have an angina attack unless they lie down. (See also heart disease.)

Blood clotting

The formation of a semi-solid mass from blood constituents. Exposure of blood to air, to foreign substances or to substances released from injured tissues (thromboplastin) stimulates blood clotting. Blood clotting is a complex process requiring the sequential activation of a series of clotting factors, which are protein modifying (proteolytic) enzymes. It culminates in the activation of thrombin, the terminal enzyme that catalyzes the conversion of fibrinogen, a soluble blood protein, to insoluble fibrin. Fibrin forms fibers that create a sticky mass that enmeshes blood platelets, a very small type of white blood cell, and red blood cells. This mass of fibers and cells forms a plug that covers the injured region of a capillary. The platelets fragment and release serotonin, a compound that causes the capillary to contract and the blood clot to retract. The net result is that the hole is patched and blood flow is reduced at the site of injury.

Nutrition status affects blood clotting. The maturation of prothrombin, the parent molecule of thrombin, and of other blood clotting factors (proenzymes) further up the clotting sequence of reactions, requires vitamin k and calcium. A calcium deficiency effectively slows the activation of clotting enzymes because a calcium-prothrombin complex must first form in order to be activated to thrombin. Vitamin K deficiency slows clotting because prothrombin cannot be modified to bind calcium, starvation and protein malnutrition reduce clotting because the liver synthesizes lesser amounts of the protein clotting factors and fibrinogen.

Blood sugar

The level of glucose in the blood. red blood cells and most of the nervous system, including the brain, rely on this fuel to meet most of their energy requirements. The body strives to maintain blood sugar at a constant level. This reflects hormonal regulation and a delicate balance between diverse processes: carbohydrate digestion and assimilation; tissue uptake of glucose; and release of glucose by the liver. During the fasting state, blood sugar levels remain relatively constant for an individual; the normal range of fasting blood sugar is 60 to 100 mg per 100 ml.

What Happens After a Meal?

Typically, blood sugar levels rise an hour or so after a meal containing carbohydrate, as the glucose produced by digestion of starch and complex sugars is absorbed by the intestine. Elevated blood sugar after a carbohydrate meal signals the endocrine pancreas to release insulin. This hormone lowers blood sugar by stimulating most tissues to take up glucose and metabolize it. The absorbed glucose is either stored as glycogen in muscle and liver, or it is converted to fat by the adipose tissue and the liver. As a result, blood sugar levels return to base line values several hours after eating.

What Happens Between Meals (Fasting)?

Glucose is constantly being consumed by the brain and other tissues. In response to a drop in blood sugar or to stress, the adrenal glands release cortisol and epinephrine and the pancreas releases glucagon. These hormones signal the release of glucose from glycogen stores in the liver, and the synthesis of glucose from amino acids, by the liver, raising blood sugar levels to base line values.

Hypoglycemia refers to a sustained, abnormally low blood glucose level. If blood sugar drops too low, the brain does not function normally. This condition creates mood changes, irritability, fainting, and fatigue. Reactive hypoglycemia refers to a drop in blood sugar levels that can occur several hours after eating. This is usually due to the abnormal functioning of insulin (dysinsulism). Severe hypoglycemia due to profound metabolic imbalances can lead to coma.

Hyperglycemia (elevated blood sugar) is at the other extreme and is characterized by sustained, elevated blood glucose as observed in diabetes mellitus. Chronic high blood glucose, frequent in uncontrolled diabetes, has many unfortunate ramifications. It can lead to the destruction of peripheral nerves and eye damage; lowered resistance to infections; toxemia during pregnancy; and heart and kidney disease. The excretion of excess sugar in the urine causes dehydration.

Lifestyle choices can help stabilize blood sugar levels and thus minimize wide swings in the changes brought about by the over- or underproduction of hormones. The body responds more efficiently to insulin with reduced intake of refined carbohydrates, coffee, and alcohol. Specific nutrients may help the body regulate blood sugar. Dietary chromium can help insulin work more effectively; stress, coffee, and sugar consumption deplete the body of chromium. High levels of biotin seem to assist the liver with carbohydrate and fat metabolism, niacinamide, niacin, vitamin C, vitamin B₆, manganese, magnesium, zinc, and selenium have been shown to improve glucose tolerance in some instances.

Eating frequent, light meals that are high in protein often helps to avoid swings in blood sugar levels, and balanced meals with whole foods high in starch and fiber are effective time-released sources of glucose. Stress reduction with meditation, yoga or biofeedback, regular exercise, and maintaining optimal body weight, also helps minimize blood sugar imbalances. (See also dieting; gluconeogbnesis; glycogbnolysis.)

Gold, Paul E. “Role of Glucose in Regulating the Brain and Cognition,” American Journal of Clinical Nutrition, 61:supplement (1995): 987S-995S.

A compound that prevents or retards the oxidation of sensitive molecules found in the body or in foods. Antioxidants occur in many foods naturally as nutrients or non-nutrients, or as synthetic additives. Antioxidants typically block oxidation by preventing damage caused by free radicals, extremely reactive forms of oxygen and other molecules that lack an electron and tear electrons from molecules they meet. In the body, likely targets are DNA, proteins, and lipids (unsaturated fatty acids).

Free radicals form in the body by normal cellular processes. These include phagocytosis (engulfing viruses and bacteria) by immune cells; incomplete reduction of oxygen as mitochondria burn fuels; production of hydrogen peroxide by the breakdown of fatty acids and the generation of nitric oxide, a free radical that functions as a localized vasodilator, a defensive chemical and as neurotransmitter. Free radicals and reactive forms of oxygen occur by chemical modification of pollutants and toxic substances within the liver. Free radical damage may contribute to cancer, cardiovascular disease, and aging; consequently, antioxidants are a current focus of extensive medical research. It is intriguing that certain antioxidants are both anticancer nutrients and antiaging nutrients. Because there is such a large variety of reactive molecules and free radicals, the body requires a wide range of antioxidant defenses. A “pecking order” exists among antioxidants; some are more readily oxidized than others and will be consumed rapidly unless replenished or recycled in the body. Certain antioxidants are “preventive inhibitors,” that is, they block the initiation of free radical attack. Preventive inhibitors include defensive enzymes like catalase and glutathione peroxidase (destroy hydrogen peroxide and lipid peroxides) and superoxide dismutase (destroys superoxide), chelating agents like citric acid that lock up metal ions, proteins that bind metal ions, including albumin, transferrin, and ferritin. Other antioxidants, “chain breakers,” convert free radicals to stable (safe) products, vitamin e and vitamin c are essential chain-breaking antioxidants. It is worth remembering that under certain conditions, an antioxidant may become an oxidant. If the antioxidant becomes a free radical, then it, too, must be disarmed and regenerated.

Antioxidants as Nutrients

Vitamin A, beta-carotene, vitamin C, vitamin E, and selenium are key antioxidant nutrients.

Carotenoids, including beta-carotene, trap free radicals, while vitamin A helps guide normal tissue development. Inadequate carotenoid intake increases the risk of cancers of the lung, bladder, esophagus, stomach, colon, rectum, prostate, and skin. Studies indicate that when used alone, beta-carotene does not prevent cancer or heart disease. Indeed, there are hints that unless beta-carotene is protected by another antioxidant, such as vitamin E, it may actually increase damage. A multitude of studies indicates that the consumption of foods rich in carotenoids protects against cancer, cataracts, and cardiovascular diseases.

Vitamin C destroys water-soluble free radicals and protects against cancer. It is needed for a healthy immune system and it also speeds wound healing. Vitamin C also protects low-density lipoprotein (LDL) cholesterol from oxidation. Evidence for the role of vitamin C in reducing the risk of coronary heart disease is weak. Ongoing clinical trials may help decide whether vitamin C supplementation is beneficial for preventing heart disease. There is some evidence that high dietary vitamin C may lower the risk of several cancers, such as breast cancer and stomach cancer. Evidence does not indicate that high doses of vitamin C decrease cancer risk, however.

Vitamin E acts as a fat-soluble, free radical trap that seems to protect the brain from free radical damage and to partially reverse age-related decline of the immune system in experimental animals. In addition, vitamin E promotes the normal function of smooth muscle cells and reduces platelet adhesion to arterial cells, factors which could reduce the risk of atherosclerosis. Many population studies have found a reduced risk of coronary heart disease with increased intake of vitamin E. However, most clinical studies of vitamin E supplementation for several years found no benefit in reducing heart disease risk.

Selenium works together with vitamin E by helping an enzyme system (glutathione peroxidase) block free radical attack and to disarm reactive lipids. Selenium is also required for a healthy immune system. Selenium deficiency increases the risk of cancer of the esophagus, stomach, and rectum.

Antioxidants as Nonnutrients in Food

In addition to vitamins, trace minerals, fiber, and carotenoids, vegetables and fruits provide many other ingredients important for long-term health. Vegetables and fruits contain orange-red and yellow pigments called carotenoids. They include carotenes such as beta-carotene and lycopene (from tomatoes) and xanthophylls, oxygen-containing derivatives such as zeazanthin and lutein. Xanthophylls occur at high levels in dark green leafy vegetables. Though relatively few carotenoids serve as sources of vitamin A, they help protect the body as versatile antioxidants, and they enhance the immune system, complementing the actions of beta-carotene. Fruits, vegetables, seasoning, spices, and herbs (tea) possess a wide range of complex-molecules called polyphenols (flavonoids) and phenolic acids that are complex ring structures. Flavonoids include isoflavones (soybean), flavones (such as quercetin from tea, berries, fruits) and flavonones (such as naringenin and hesperidin from citrus), flavanonols (such as catechins, condensed and hydrolyzable tannins), anthocyanins (purple, red, and blue pigments of fruits and berries), coumarins (from citrus), ellagic acid (from grapes), and others. In general, flavonoids possess multiple properties; thus they can quench free radicals, inhibit inflammation, strengthen capillary walls, and reduce oxidative damage to serum cholesterol. The optimal intake of flavonoids and carotenoids is not known and the long-term effects of supplementation with large amounts of phytochemicals has not been studied. It should be pointed out that beta-carotene, vitamin C, and even vitamin E under the appropriate conditions can become oxidants (prooxidants). Certain flavonoids also exhibit prooxidant properties. Chelated (complexed) iron in the presence of vitamin C can generate free radicals spontaneously in the test tube and this could be a potential problem in the body with iron overload diseases. Finally, certain flavonoids can specifically block the thyroid hormone-generating enzyme in thyroid cells. As with many dietary constituents, a little may be beneficial, while a lot could be harmful.

Foods rich in vitamin A and beta-carotene and related carotenoids include orange-colored vegetables like carrots and squash plus dark green leafy vegetables like chard, kale, and spinach. Fresh fruit, frozen juice concentrate, and vegetables like green pepper and broccoli supply vitamin C. Vegetable oil, wheat germ, and nuts provide vitamin E. Selenium occurs in whole grains, seafood, cabbage, onions, and garlic. Fruits and vegetables also provide flavonoids.

Antioxidants Made by the Body

Glutathione is a sulfur-containing antioxidant present in very large amounts in the cytoplasm. Besides helping to keep proteins reduced, it assists amino acid transport, helps regulate the internal oxidation state of the cell, maintains vitamin E in a reduced state, and detoxifies potentially harmful substances.

Coenzyme Q assists mitochondria to burn fat and carbohydrate for energy and it functions as a lipid soluble membrane antioxidant together with vitamin E, which it protects. Coenzyme Q production declines with age and the heart may become deficient in this nutrient.

Uric acid is found in the blood. It is a nitrogen-containing waste product from the breakdown of DNA and RNA.

Citric acid, succinic acid, and other complex organic acids generated by metabolism can bind iron and copper, preventing them from catalyzing of free radical-generating reactions.

Melatonin, a hormone produced by the pineal gland, possesses strong antioxidant properties.

Bilirubin, a breakdown product of hemoglobin, acts as an antioxidant in blood.

Antioxidants as Food Additives

Antioxidants are extensively utilized to prevent or retard deterioration that produces off-flavors or color changes in foods, making them less appetizing or less nutritious. Oxidation can also be promoted by enzymes in foods when exposed to air. This explains why apples, bananas, pears, peaches, and potatoes darken after being sliced. The food industry often employs synthetic antioxidants, particularly butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), ethylene diaminetetracetic acid (EDTA), and propyl gallate, as well as vitamin C, as preservatives to extend the shelf life of processed foods by preventing free radical damage.

Spontaneous oxidation of fats and oils in the presence of oxygen, sunlight, and metal ions causes rancidity unless blocked by antioxidants. (BHA) and BHT are used to prevent rancidity in fats and oils, particularly in baked goods like crackers and cookies. Their safety has been questioned. EDTA is a common additive in salad dressings, margarine, mayonnaise, sandwich spreads, pureed fruits, and vegetables, as well as cured shellfish, beer, and soft drinks. EDTA is judged to be a safe food additive. Propyl gallate retards spoilage of fats and oils and is often used with BHA and BHT to maximize their antioxidant effects. Several studies with experimental animals suggest that propyl gallate may cause tumors. A close relative of vitamin C, erythroboric acid, is a common antioxidant used in the preservation of processed meats such as bologna, frankfurters, and bacon, sulfites are used as antioxidants to prevent discoloration of fruit and vegetables. Spices and herbs, including thyme, rosemary, and sage, are sometimes used as food additives to retard spoilage. (See also atherosclerosis.)

Fairfield, K. M., and R. H. Fletcher. “Vitamins for Chronic Disease Prevention in Adults.” Journal of the American Medical Association 23, no. 287 (June 19, 2002): 3,116-3,126.

Singh, Ram B. et al. “Effect of Antioxidant Rich Foods on Plasma Ascorbic Acid, Cardiac Enzyme and Lipid Peroxide Levels in Patients Hospitalized with Acute Myocardial Infarction,” Journal of the American Dietetic Association 95, no. 7 (July 1995): 775-780.

Chemicals that destroy or prevent growth of microorganisms including bacteria, molds, and fungi. Natural or synthetic compounds are used extensively as antibiotics to treat infectious diseases in animals and plants as well as in humans. Sulfanilamides, penicillins, and erythromycins are examples of major families of these drugs. Antibiotics impact human health in several ways. The prolonged use of broad-spectrum antibiotics in treating disease drastically alters the intestinal microflora by destroying beneficial bacteria. The loss of beneficial bacteria can permit less desirable, opportunistic microorganisms like yeast to flourish, cause intestinal inflammation, and decrease production of nutrients important in maintaining health of the colon. Antibiotics can affect specific vitamin requirements; chloramphenicol blocks riboflavin and vitamin B₆ and B₁₂, for example. Penicillin increases potassium requirements. Antibiotics can decrease nutrient absorption in general by altering the intestinal lining. Neomycin interferes with the uptake of fat, amino acids, carbohydrate, water-soluble and fat-soluble vitamins, calcium, iron, and vitamin K. Tetracycline decreases absorption of fat, amino acids, calcium, iron, magnesium, and zinc, while increasing the rate of urinary excretion of riboflavin, folic acid, and vitamin C.

Antibiotics can have a direct impact on the food supply. Half the antibiotics produced in the United States are applied to livestock. The benefits are more rapid growth and healthier animals. On the other hand the potential exists for generating drug-resistant pathogenic bacteria and persistent antibiotic residues in meat and dairy products. The application of antibiotics in animal husbandry and the permissible levels of antibiotic residues in animal products are regulated by the U.S. FDA. The following examples illustrate the dimensions of this food safety issue.

Chloramphenicol

This drug can cause anemia in humans due to damage to bone marrow. Though banned from use with food-producing animals, periodic spot inspections showed it was widely used in cattle and hogs in the 1980s. The degree to which chloramphenicol continues to contaminate meat through illegal application, and the degree to which such a contamination affects health, are unknown.

Penicillin

This common antibiotic is used to treat dairy herds, among others. The allowable penicillin level in milk is 0.01 units per milliliter (about 20 drops) of milk, but spot checks have found 10 times this level in commercial milk. Such high levels can cause allergic reactions in susceptible individuals.

Sulfamethazine

This sulfa drug is a widespread contaminant in meat, poultry, and milk. One-fourth of milk sampled in the late 1980s was contaminated, despite the U.S. FDA ban on this drug in milk. Sulfamethazine is suspected of being a carcinogen. (See also acidophilus; meat contaminants; pesticides.)

(Tyr, L-tyrosine)

A nonessential amino acid required to build proteins and other important compounds. Tyrosine is readily synthesized by the body from the essential amino acid phenylalanine. Tyrosine in the diet decreases the requirement of phenylalanine because less needs to be shunted to tyrosine synthesis. In addition to serving as a raw material for proteins, tyrosine is converted by the nervous system into a family of neurotransmitters, chemicals that help transmit nerve impulses between cells. These “catecholamines” include dopamine, norepinephrine, and epinephrine. They play a role in sensitivity to pain and in feeling alert. In addition, the adrenal glands convert tyrosine to epinephrine (adrenaline) and norepinephrine, which are released as hormones to gear up the body to respond to stress. The thyroid gland combines iodine with tyrosine to form the thyroid hormone thyroxine, which regulates basal metabolism. Tyrosine also forms melanin, the pigment of skin. Normal transformations of tyrosine require niacin, copper, and vitamin C.

In humans, tyrosine appears to increase performance under stress. Under stress nerve cells release norepinephrine more rapidly. Tyrosine supplements could theoretically increase norepinephrine formation and indirectly help maintain vigilance. Tyrosine may help reduce depression and fatigue associated with premenstrual syndrome (PMS). tryptophan and tyrosine have been used to minimize cravings and depression during withdrawal from addictive substances. In lab animals, administration of tyrosine can increase brain uptake and increase brain production of neurotransmitters. One hypothesis of schizophrenia proposes that dopamine is increased in certain individuals. Some antipsychotic medications are designed to block the conversion of tyrosine to dopamine. Tyrosine seems to be most effective for nerve cells that fire frequently, therefore supplements can affect an aspect of physiology differently depending on the conditions. For example, certain brain cells fire rapidly in hypertension (high blood pressure); thus, giving tyrosine to hypertensive rats lowers the blood pressure. With low blood pressure, sympathetic nerves outside the brain are being triggered in order to raise blood pressure. Administration of tyrosine in this case helps raise blood pressure. Liver cirrhosis can lead to a brain condition called encephalopathy (hepatic encephalopathy). Unusually high blood levels of phenylalanine and tyrosine could increase brain formation of tyramine, and this could disrupt normal nerve function. In any event, cirrhosis alters tyrosine formation from phenylalanine as well as altering tyrosine degradation.

Tyrosine supplements can either raise or lower blood pressure depending on the individual, and migraine headaches can worsen, depending on the conditions. Tyrosine and phenylalanine supplements should not be taken at the same time as anti-depressant drugs containing monoamine oxidase inhibitors. Taken together they can raise blood pressure to fatal levels. (See also amino acid metabolism.)

(Trp, L-tryptophan)

A dietary essential amino acid and a building block of proteins. Tryptophan must be consumed daily because the body cannot synthesize it. In addition to serving as a raw material for proteins, tryptophan is required for the synthesis of serotonin, a neurotransmitter that helps conduct nerve impulses between cells. Serotonin is also released by mast cells, defensive cells embedded in tissues, and by blood platelets. Serotonin causes smooth muscle to contract; it functions as a vasoconstrictor to constrict blood vessels. Dietary sources of tryptophan include pineapple, turkey, chicken, yogurt, unripened cheese, and bananas. The usual diet provides 1 to 2 g daily spread out over time, rather than as a single dose.

Tryptophan supplementation has been used with the following conditions:

Depression Depression is a facet of many diseases and tryptophan, together with painkillers, may help unresponsive depression in some cases. Tryptophan supplements in combination with vitamin B₆ have been tested as antidepressants. The effects of tryptophan on mania and aggressive behavior have also been examined clinically. Its use has been criticized as being less effective than standard drugs.

Food Cravings Administering a combination of tryptophan and tyrosine has been used to help with depression and craving during withdrawal from addictive substances.

Difficulty in Sleeping The effectiveness of tryptophan for inducing sleep is well established. Tryptophan has been used for many years in Great Britain to treat insomnia. For certain people, it enhances relaxation and sleepiness. Tryptophan seems more effective when taken before bedtime with a carbohydrate-rich food.

Side Effects from Oral Contraceptives Women taking oral contraceptives may metabolize tryptophan abnormally, which may be helped by taking vitamin B₆.

Chronic Pain Research suggests that tryptophan partially reduces sensitivity to pain. Administration of small doses of tryptophan throughout the day with a high carbohydrate, low-fat, low-protein diet and appropriate medication can decrease symptoms in some people. More research in this area is needed.

Toxicity and Side Effects

Amino acids are powerful agents in the body. Amino acid research in humans is still considered preliminary, and the long-term effects and safe dosages are not known for many situations. The Centers for Disease Control linked large doses of tryptophan supplements to a rare, painful blood disorder called eosinophilia-myalgia syndrome (EMS) that became an epidemic in 1989. Subsequently, the U.S. FDA issued a nationwide recall of all tryptophan products in which this amino acid is the sole or the major constituent. The only products containing tryptophan not recalled include certain protein supplements, infant formula, and special dietary foods that contain small amounts of tryptophan for nutrient fortification. No cases of EMS have been reported from this use. EMS is marked by severe muscle pain, joint pain, difficulty in breathing, swollen limbs, and fever. In severe cases there are signs of congestive heart failure and paralysis. Most cases were associated with a tryptophan from a single manufacturer; it was contaminated with a related product. Moderate doses of tryptophan (as low as 1 g/day in certain individuals) may cause liver abnormalities. When used with antidepressants, side effects may worsen.

Undigested plant residues that represent the outer coating or plant wall material of vegetables, grains, seeds and fruits. Fiber is considered essential to maintain a healthy digestive tract. A low-fiber diet has been associated with constipation, colon cancer, spastic colon, hiatus hernia, varicose veins, hemorrhoids, heart disease, hypertension, gallstones, diabetes, obesity, colitis, and Crohn’s disease.

Fiber can help curb appetite because the stomach feels full. Fiber-rich foods require longer chewing, which stimulates digestive juices and helps digestion. Fiber displaces fat calories and slows fat digestion and absorption by binding bile salts and, therefore, it may help control obesity.

Fiber also helps fight heart disease. In a study of more than 40,000 U.S. middle-aged men, researchers have found that heart attacks were 41 percent less common among men who consumed at least 28 g fiber daily compared with men who ate less than 13 g daily as typical of the usual U.S. diet. It is estimated that fiber-deprived men reduced their risk of heart attack by 20 percent to 44 percent for each 10 g of additional fiber, without lowering their fat intake. The largest reduction was among men who ate wheat bran, which represents mainly insoluble fiber.

Recent studies have confirmed that a high-fiber diet does not reduce risk of colon cancer because it seems to have no effect on the growth of precancerous colon polyps. In one study researchers at the National Cancer Institute (NCI) put one group of people on a high-fiber diet and told another group to simply eat what they usually eat. All the participants had had at least one precancerous polyp removed from their colon in the six months before the study. Four years later the researchers found that the risk of developing another polyp was the same in both groups.

In the other study researchers at the Arizona Cancer Center asked one group of people to eat half an ounce of wheat fiber daily and gave another group a tenth of an ounce. Again, three years later the risk of developing a precancerous polyp was the same in both groups. Bacterial degradation of dietary fiber releases organic acids, such as butyrate and acetate, which promote a healthy colon. A high intake of whole grains, vegetables and fruits, and derived fiber seems to reduce the risk of cancers of the upper digestive tract and ovarian cancer. In women, increased fiber intake is associated with a reduced risk of cardiovascular disease and heart attacks, according to 2002 research.

It is estimated that Americans consume only one-third to one-half the optimal amount of fiber daily. The National Cancer Institute recommends doubling or tripling fiber intake from 12 g to 20 to 35 g per day. At this level, there is minimal interference with nutrient absorption, while assuring normal maintenance of intestinal function. However, there is as yet no formal recommended dietary allowance for fiber because the exact amount required for health has not been established.

“Dietary fiber” refers to the fiber content in plant foods that resists digestion by enzymes of the gastrointestinal tract, and dietary fiber is the usual designation for fiber content in foods. Lab methods for measuring dietary fiber are more gentle than those for crude fiber, which measure only material that resists strong acid treatment. Values for dietary fiber can be up to four times higher than for crude fiber contents.

Food labels are required to list the amount of dietary fiber as a percentage of daily value, based upon the needs of a 2,000 calories per day diet.

Listing the amount of soluble and insoluble fiber is voluntary.

Major Types of Fiber

Fiber consists of a mixture of very different non-starch complex carbohydrate and noncarbohydrate materials. The major classes of dietary fiber are cellulose, hemicelluloses, pectin, mucilage, gums, algal materials, and lignin.

• Cellulose is a linear chain of glucose units.

• Hemicelluloses are highly branched structures found in plant cell walls. They are a diverse group with varying sugar compositions, including the simple sugars mannose, glucose, galactose, xylose, arabinose, and an acid derived from glucose called glucouronic acid. Some hemicelluloses are water-soluble.

• Pectins function as a glue that holds plant tissues together, and they contain an acidic sugar derived from galactose, galacturonic acid, in the primary chains, with side chains or branches containing less common sugars: arabinose, fucose, and xylose.

• Gums are plant secretions and contain acidic sugars.

• Algal polysaccharides are products of edible seaweed.

• Lignin is a noncarbohydrate insoluble material and a principal structural material of wood.

Insoluble Fiber

It is convenient to break down fibers according to whether they are insoluble or water-soluble. Insoluble fiber includes cellulose and lignin. This type of fiber swells in water, increases stool weight and stool frequency, and helps prevent constipation, colonic inflammation (diverticulitis) and hemorrhoids, by softening stools and speeding up the movement of waste through the intestine. Cellulose is not digested, although colon bacteria break down 40 percent to 80 percent of cellulose. Lignin is not degraded and passes through the digestive tract unchanged. Lignin softens stools, increases regularity and may lower blood cholesterol.

Bran is the most common source of insoluble fiber, derived from the outer husk of kernels of wheat and other cereal grains. It contains cellulose and other cell wall materials and slows the rise in blood sugar after a meal. Bran can protect against heart disease in middle-aged men; oatmeal is also effective, though more may need to be consumed to get the same effect as wheat bran.

Soluble Fiber

This type of plant fiber swells in water and forms glue-like gels. Soluble fiber is made up of noncellulose carbohydrates, including pectins, gums, algal polysaccharides and some types of hemicellulose. Soluble fiber has important physiologic effects. It becomes viscous, thus softening stools and slowing the rate of stomach emptying. Soluble fiber also slows starch digestion and glucose uptake, in turn lowering the amount of insulin needed to process blood glucose after a meal, and it may help diabetics. Eating oat bran (2 oz.) regularly each day may effectively lower blood sugar levels. Despite the popularity of oat bran as a source of water-soluble fiber, it provides both water-soluble and insoluble fibers.

The water-soluble fibers are completely degraded by intestinal microorganisms to short-chain fatty acids, which are used as fuels to help maintain the intestinal lining.

Soluble fiber also seems to lower blood cholesterol. Regular consumption of oat bran and legumes may lower blood cholesterol. Fiber binds bile salts in the intestine, which could reduce their resorption, thus forcing the liver to remove more cholesterol from the blood to make more bile. Pectin, guar gum, and locust bean gum have been reported as effective in this regard. Because it believes that long-term health benefits are not yet proven, the U.S. FDA does not permit health claims linking fiber consumption with the prevention of either heart disease or certain types of cancer.

Good sources of fiber include dried beans, lentils, lima beans, pinto beans, sweet potatoes, broccoli, brussels sprouts, spinach, almonds, corn, wheat, oat bran, and fruit (blackberries, pears, apples). In general, the less processed a food is, the more fiber it has. Therefore, eating whole, minimally processed foods assures a mixture of soluble fiber and insoluble fiber; both kinds are needed for health.

Sources of insoluble fiber include skins of vegetables and fruits, whole grains (not white flour), high-fiber cereals, dried beans, broccoli, and bulgur wheat. Bran is a common and inexpensive source of insoluble fiber. Cold bran cereals have more insoluble fiber than hot cereals. Bran is lost in preparing white flour and it is not replenished by enrichment.

Good sources of soluble fiber are fruits, cooked dried beans, chickpeas, barley, lentils, navy beans, vegetables such as squash and carrots, plus barley, oat, and rice bran, guar gum, glucomannan, and pectin are common soluble fiber supplements. Fructo oligo saccharides are also considered to be a form of fiber.

When increasing fiber consumption, the recommendation is to begin gradually because excessive fiber intake causes bloating, gas, cramps, nausea, and diarrhea. A month may be required to adapt to a high-fiber diet. Initial steps can be eating whole fruit instead of juices; popcorn instead of potato chips; whole wheat instead of white bread; and a baked potato with its skin instead of mashed potatoes. Patients should consult a physician when planning to take fiber supplements if they have a serious digestive disease, or if they plan a daily consumption over 35 g of fiber. Following the food guide pyramid, which specifies eating six to 11 servings of grain-based foods and two to four servings of fruits and three to five servings of vegetables daily, will satisfy the recommended fiber intake.

Fiber Supplements

Supplements are a popular way of increasing dietary fiber intake, although the National Cancer Institute recommends increasing fiber intake through whole, fiber-rich foods. Fiber supplements often contain bran, guar gum, pectin, or psyllium. Since they swell in water and help create a feeling of being full, fiber supplements have been used in weight-reduction programs to control appetite, ft is not clear that they help with permanent weight loss, however. Fiber from psyllium seeds is the primary ingredient of several popular brands of non-chemical laxatives.

Patients with serious intestinal disorders such as diverticulitis, ulcerative colitis, or Crohn’s disease should avoid taking fiber supplements without medical supervision. Patients should consult a physician before consuming more than 35 g of fiber supplements a day; high levels of fiber can block the uptake of iron, calcium, zinc, copper, and other minerals and cause calcium losses. Patients should also avoid fiber supplements that contain appetite suppressants like phenylpropanolamine, which can cause side effects.

Ornish, D. et al. “High Fiber Diet and Colorectal Adenomas,” New England Journal of Medicine 343 (September 7, 2001): 736-738.