N 451 N-acetylcysteine (NAC) A derivative of the sulfur-containing amino acid CYSTEINE that occurs naturally in the body, N-acetylcysteine is used to produce the powerful cell ANTIOXIDANT GLU- TATHIONE. This sulfur compound delivers cysteine to cells. It has been used medicinally to reduce mucus buildup in people suffering from respiratory ailments such as chronic bronchitis and asthma, and adult respiratory distress syndrome. NAC may help reduce the risk of heart attack in patients with unstable chest pain. It is also helpful as an IV treat- ment in ridding the body of toxic levels of the nonsteroidal anti-inflammatory drug (NSAID) acetaminophen. Now also available as a DIETARY SUPPLEMENT,N- acetylcysteine is promoted as an aid in boosting the body’s immune system, preventing heart disease and cancer and slowing the progression of Parkin- son’s disease and multiple sclerosis. Initial studies indicate N-acetylcysteine shows promise in treating some or all of the conditions. However, additional research is needed to confirm these preliminary studies. Because N-acetylcysteine is sold as a dietary supplement for treating these conditions and not as a drug, its safety and efficacy has not been tested by the U.S. Food and Drug Administra- tion (FDA) or any other government agency. Breast-feeding women should avoid NAC. Preg- nant women should use it only with a doctor’s pre- scription. Grandjean, E. M., P. Berthet, R. Ruffmann, et al. “Efficacy of Oral Long-Term N-acetylcysteine in Chronic Bron- chopulmonary Disease: A Meta-Analysis of Published Double-Blind, Placebo-Controlled Clinical Trials,” Clinical Therapy 22 (2000): 209–221. naphthoquinone See VITAMIN K. narcotics See ADDICTION. National Cholesterol Education Program See CHOLESTEROL. National Marine Fisheries Service Organization that is part of the National Oceanic and Atmos- pheric Association (NOAA), responsible for over- seeing fisheries management in the United States. Under the federal Agricultural Marketing Act of 1946, NOAA administers a voluntary Seafood Inspection Program to ensure that seafood sold in the United States complies with applicable food regulations, including the Hazard Analysis and Critical Control Point (HACCP, pronounced “has- sip”) regulations implemented by the U.S. Food and Drug Administration (FDA) in 1997. This pro- gram requires seafood processors, repackers, and warehouses that supply seafood for sale in the United States to have in place a food safety pro- gram to identify and eliminate sources of food- borne illnesses. The aim of the HACCP system is to prevent problems before they start by conducting spot checks of manufacturing processes and ran- dom testing of seafood products. Retailers are exempt from HACCP requirements. National Research Council (NRC) The branch of the National Academy of Sciences promotes the effective use of scientific information and advises the federal government on scientific and technical matters. The NRC provides services for following policies set by both the National Academy of Sci- ences and the National Academy of Engineering in meeting the needs of government, the public, and scientists. It is administered by these academies, as well as by the Institute of Medicine. Among the many responsibilities of the NRC is the development and evaluation of nutrient stan- dards for good health. In 1941, the Food and Nutrition Board of the NRC published the Recom- mended Dietary Allowances (RDAs), the bench- mark of nutritional adequacy in the United States, with revised editions generally appearing every five years. The 10th edition was published in 1989. Since then the Food and Nutrition Board, in coop- eration with Health Canada, has been gradually replacing the RDAs with DIETARY REFERENCE INTAKES (DRIs), which incorporate more recent science that has broadened our understanding of the role of nutrients in human health. (See also DIETARY GUIDELINES FOR AMERICANS ; NUTRIENT; VITAMIN.) National School Lunch Program See SCHOOL LUNCH PROGRAM . natural flavors See FLAVORS. natural food Foods that have been minimally processed and grown without the use of synthetic fertilizers and pesticides. Other than for meat and poultry, the term natural has not been defined legally and therefore has many interpretations. The “natural” food label is a strong selling point, and food producers have capitalized on consumer inter- est. Thus the word natural may be used to describe PROCESSED FOODS and synthetic food and beverages, which sometimes are highly processed and contain preservatives and artificial coloring. A beverage labeled “natural” might be completely synthetic, with only the lemon flavor as a nonsynthetic ingre- dient; a product labeled “Natural Juice Apple Pie” may indicate only the juice is natural. Natural food also has the connotation of being free from addi- tives like PRESERVATIVES, EMULSIFIERS, and thicken- ers. Under current U.S. FDA regulations, an entire food cannot be labeled natural if it contains syn- thetic ingredients, ARTIFICIAL FLAVORS, or ARTIFICIAL FOOD COLORS . Natural Meat According to the USDA, MEAT or poultry labeled “natural” has not been processed more than in usual kitchen practice. This refers to procedures that can be performed in the kitchen: washing, shredding, chipping, grating, grinding, chopping, mixing, and cooking. Natural meat must come from animals raised without hormones or at least not exposed to feedlot growth stimulants (such as antibiotics) to fatten livestock later than 60 days before slaughter. The “natural beef” label doesn’t guarantee the meat is absolutely free of antibiotics and growth factors because some producers feed hormones and antibiotics up to 60 days before slaughter. This time may be inadequate to com- pletely clear chemical residues from some animals. (See also FOOD ADDITIVES; ORGANIC FOODS .) natural sweeteners Naturally occurring CARBO- HYDRATES or compounds related to SUGARS that are used as sweeteners. Carbohydrate sweeteners con- tribute four CALORIES per gram (less than half as much as fat). The most common are the simple sugars or materials that are primarily sugars: FRUC- TOSE and high fructose corn syrup; GLUCOSE (known in the food industry as DEXTROSE); CORN SYRUP ; various forms of SUCROSE, including MOLASSES, brown sugar, table sugar, turbinado sugar, and cane sugar; BARLEY MALT; HONEY; fruit concentrates; and ground dates (date sugar). Amasake is a sweetener prepared from fermented rice. Each provides carbohydrate with only a few other nutrients; therefore the content of VITAMINS, minerals, and FIBER, as compared to that found in whole foods, is low ( EMPTY CALORIES). Sugar alcohols taste sweet and are not as easily metabolized as sugars. MANNITOL, related to the sugar mannose, is poorly utilized. XYLITOL, derived from the sugar xylose, while degraded, contributes fewer calories than sugar. SORBITOL, related to sor- bose, is the most widely used sugar alcohol and is only slowly absorbed by the intestine. Relative to table sugar, fructose is 1.7 times as sweet; glucose, 0.7; mannitol, 0.7; and sorbitol, 0.6. Potential Problems with High Sugar Consumption Undernutrition Sugars supply empty calories because they provide low or negligible amounts of key nutrients. CARBOHYDRATE METABOLISM requires B vitamins like THIAMIN and minerals like ZINC, CHROMIUM, and MAGNESIUM. Excessive concen- trated sweeteners displace more nutritious food 452 National School Lunch Program and increase the body’s burden for these nutrients. Few people relying on a junk food diet increase their consumption of high-quality foods to make up the difference. Overnutrition Excessive consumption of sugar and refined carbohydrate represents excessive calo- ries. Surplus calories are converted to FAT, regard- less of their source. It should be noted that starches and sugars are not in themselves inherently fatten- ing unless they represent excess calories. Blood Sugar Imbalances The excessive con- sumption of simple sugars and sucrose can cause BLOOD SUGAR levels to fluctuate. In contrast, STARCHES in vegetables and legumes are often slowly digested and glucose is absorbed slowly, per- mitting a more balanced control of blood sugar. Cavities Excessive sugar consumption leads to more cavities and contributes to soaring dental costs. U.S. DIETARY GUIDELINES FOR AMERICANS call for eating less sugar and more complex carbohydrate, starch, and fiber (whole foods) as vegetables, fruits, and legumes. The FOOD GUIDE PYRAMID, the most recent meal planning guideline, specifies eating sweets sparingly. (See also ADDICTION AND SUGAR; INVERT SUGAR; MAPLE SYRUP; NUTRIENT DENSITY; TEETH.) Anderson, G. H. “Sugars, Sweetness and Food Intake,” American Journal of Clinical Nutrition 62, supp. 1 (1995): 195S–202S. Wolever, T. M., and J. B. Miller. “Sugars and Blood Glu- cose Control,” American Journal of Clinical Nutrition 62, supp. 1 (1995): 212S–221S. natural vitamins See VITAMIN. nectar A FRUIT drink made with fruit juice and fruit pulp. Typical nectars are made from APRICOT, GUAVA, MANGO, PEACH, and other fruit. Ingredients are specified by the STANDARDS OF IDENTITY estab- lished by the U.S. FDA. These are standard recipes long used by the food industry. Nectars include var- ious combinations of fruit juice, SUGAR, and other natural sweeteners, together with fruit puree, fruit pulp, VITAMIN C, and CITRIC ACID. The term nectar also refers to a sweet, syrupy fluid secreted by flow- ers and collected by bees to produce HONEY. nectarine (Prunus persica) A subspecies of PEACH with a smooth skin and a distinctive flavor. The close similarity between nectarines and peaches is demonstrated by the fact that nectarines occasion- ally mutate to peaches and vice versa. Nectarines and peaches are believed to have originated in China. There are more than 150 varieties of nec- tarine, differing slightly in taste, shape, size, and skin coloring. Fantasia, Summer Grand, Royal Giant and May Grand are popular varieties. The United States is a major producer, and California produces most of the domestic crop. Hard nec- tarines can be softened by storing them in a paper bag at room temperature for several days. The mature fruit does not get very much sweeter, but it can become softer and juicier. Nutrient contents of one raw nectarine, without pit, is 67 calories; protein, 1.3 g; carbohydrate, 16 g; fiber, 3.13 g; fat, 0.6 g; potassium, 288 mg; vitamin A, 100 retinol equivalents; vitamin C, 7 mg; thiamin, 0.02 mg; riboflavin, 0.06 mg; niacin, 1.35 mg. nephron The functional unit of kidneys that fil- ters the blood. The nephron further modifies fil- tered fluid from the blood to produce urine. Nephrons are composed of two structures: renal tubules and the glomerulus, a cluster of CAPILLAR- IES. WATER , MINERALS , GLUCOSE, VITAMINS, AMINO ACIDS , very small proteins, nitrogenous wastes, and small molecules migrate from the capillaries of the glomerulus and enter the renal tubule, which draws urine from the kidneys. As the filtrate moves down the tubules, nearly 99 percent of the water is reabsorbed into the blood. Only 1.5 liters (1 percent leaves as urine daily. By reabsorbing most of the materials, the body can retain essential nutrients, while disposing of wastes like UREA. For example, normally all of the glucose filtered by the glomerulus is reab- sorbed, thus, maintaining blood glucose levels. So much liquid is filtered by the kidney that if nutri- ents were not reabsorbed back into the blood- stream, the blood would be drained of these materials within half an hour. The region of the tubule adjacent to the capsule (proximal tubule) reabsorbs glucose, amino acids, SODIUM, CHLORIDE, BICARBONATE, POTASSIUM, and water. Farther down the tubule, sodium, chloride, urea, and water are reabsorbed. Sodium and water are controlled by the hormone ALDOSTERONE. The nephron 453 kidney releases potassium and AMMONIA to main- tain ELECTROLYTE balance and water balance of the blood. In order to maintain blood pH close to 7.4 (neutral pH) and to maintain acid-base balance, sodium is reabsorbed while hydrogen ions are released. The kidney plays a role in VITAMIN D metabolism as well. Tubules of the kidney activate vitamin D by converting a partially modified form (called hydroxycholecalciferol) to the fully active hor- mone, called calciferol. (See also GLOMERULAR FIL- TRATION .) nervous system A primary communication sys- tem consisting of the brain, spinal cord, and nerve fibers radiating to all parts of the body. The brain and spinal cord occupy a central position in the body and are called the “central nervous system” (CNS). The nervous system transmits information via electrical nerve impulses conducted between nerve cells. Impulses are generated by changes in the body’s internal or external environment. Specialized cells of the nervous system are called neurons and neuroglia. Neuroglia are connective tissue cells supporting the neurons, while neurons are specialized cells of the nervous system that con- duct nerve impulses. The nervous system is com- posed of billions of neurons. With age, neurons are progressively and irreversibly lost. Sensory neurons transmit impulses to the spinal cord and brain from all regions of the body, while motoneurons transmit impulses away from the brain and spinal cord to muscles and glands. Interneurons lie within the gray matter of either the brain or spinal cord and conduct impulses from sensory neurons to motoneurons. A fatty layer called MYELIN insulates nerve fibers, bundles of neurons. Neurons contain a cell body with nucleus, CYTO- PLASM and MITOCHONDRIA. Each neuron possesses many dendrites, highly branched extensions of the cell body that are the “receiving” parts of neurons and conduct a nerve impulse toward the cell body; and an AXON, a single, long cytoplasmic extension that conducts nerve impulses away from the cell body to another cell. Axons contain mitochondria but no protein synthesizing machinery; they vary in length from a few millimeters up to a meter or more. The ends of axons terminate in bulb-like structures that store NEUROTRANSMITTERS. These chemicals determine whether an impulse will pass onto the next cell. Nerve fiber refers to any process projecting from the cell body. Typically, sensory neurons transmit impulses from the skin, sense organs, muscle joints and viscera to the spinal cord and the brain. Motoneurons convey impulses from the brain and spinal cord to either muscles or glands. Connecting neurons (intraneurons) carry impulses from sensory neurons to motor neurons and occur in the brain and spinal cord. Many nutrients support nerve function, and the following minerals are critical. CALCIUM is required for nerve transmission; when blood calcium levels fall, nerves become hypersensitive, and the result is frozen muscles (tetany). High calcium concen- trations depress nerve irritability. The amount of neurotransmitters (serotonin, acetylcholine, nor- epinephrine) released is proportional to the cal- cium concentration in the nerve terminus. POTASSIUM and SODIUM are important in nerve transmission. Stimulation of a nerve causes potas- sium to migrate out of the neuron and sodium to migrate inward, altering the membrane potential (electrical current) and conducting a nerve impulse down the axon. Individual nerve cells communicate at special- ized gaps called synapses by means of chemical messages called neurotransmitters. When a nerve impulse reaches the end of an axon it triggers the release of neurotransmitters that migrate across the gap and stimulate a nerve impulse or other response in the receiving cell. Neurotransmitters are often derived from AMINO ACIDS. Dopamine and norepinephrine are derived from the amino acid TYROSINE, while TRYPTOPHAN supplies SERO- TONIN. Cholinergic nerve fibers release the neuro- transmitter ACETYLCHOLINE and are derived from the nitrogen-containing nutrient CHOLINE. The peripheral nervous system contains sensory nerves. These carry information from organs like the skin to the brain. Motor nerve fibers carry impulses from the brain to specific regions such as skeletal muscles. The automatic nervous system adapts the body to change. It consists of two functional parts. The sympathetic nervous system responds to stress, 454 nervous system and adaptations include dilation of bronchi and increased heart rate. The PARASYMPATHETIC NERVOUS SYSTEM returns the body to the normal, unstressed state, counterbalancing the sympathetic nervous system. It restores energy during rest and recupera- tion. The average human brain weighs about three lbs (1.4 kg) and contains 11 billion cells. Although it represents only 2 percent of the total body weight, it consumes 20 percent to 30 percent of the GLUCOSE from digestion and CARBOHYDRATE METAB- OLISM. The thalamus of the forebrain is an egg- shaped mass that relays sensory information to other regions of the brain. It seems to help with concentration. Lying beneath the thalamus is the HYPOTHALAMUS, which regulates eating, drinking, sexual behavior, heat production, and emotions. It also controls the ENDOCRINE SYSTEM , a system of hormone-secreting glands located throughout the body. Twelve pairs of nerves are attached at the under surface of the brain. Several are important in regu- lating eating and digestion. Their fibers conduct pulses between the brain and the head, neck, tho- rax, and abdominal cavity. The olfactory nerve is responsible for the sense of SMELL. The trigeminal nerve functions in the sensations involving the face, teeth, and chewing. The facial nerve is responsible for TASTE. The glossopharyngeal nerve controls taste, swallowing, salivation, and sensa- tions in the throat region. The vagus nerve also regulates swallowing, sensations of throat and lar- ynx, and of the abdominal regions; for example, it regulates PERISTALSIS. The hypoglossal nerve regu- lates tongue movements (swallowing). It is now known that brain peptides, such as ENDORPHINS, are synthesized in the intestine, while hormones bind to targets in the brain and affect its function. The nervous system can activate the immune system, while chemicals released by the immune response can affect the central nervous system. The gastrointestinal tract is surrounded by an elaborate network of nerves so that the stomach and intestines are directly linked to the brain. The ability of the digestive system to monitor food has important ramifications for WEIGHT MANAGEMENT, SATIETY, and hunger. With the discovery that the hormonal system (endocrine system) as well as the immune system can affect the brain and alter behavior and mood, and that the nervous system can alter the immune response and hormone bal- ance, has come a new appreciation of the interde- pendency of these three systems in maintaining a balance among all systems of the body ( HOMEOSTA- SIS). This synthesis has spawned a new scientific discipline, psychoneuroimmunology. In addition to supplying energy, primarily as glucose, food can alter the nervous system in important ways. One focus has been in the forma- tion of neurotransmitters, chemicals manufactured by nerves to help transit nerve impulses. Two amino acids that serve as the raw materials for the manufacture of neurotransmitters are tryptophan and tyrosine. Tryptophan forms the neurotransmit- ter serotonin, used to regulate relaxation and sleepiness. Low blood levels of tryptophan are linked to depression, and some clinical studies sug- gest that tryptophan supplementation can lessen depression. Tryptophan tends to make normal peo- ple drowsy, and it has been sold as a mild sleep inducer. However, the U.S. FDA has banned its sale because of deaths associated with contaminated tryptophan. Foods low in protein and rich in car- bohydrate can raise brain levels of tryptophan, probably because they stimulate the release of insulin, which lowers the blood levels of most amino acids, except for tryptophan. According to one hypothesis, tryptophan in the blood competes with more abundant, branched chain amino acids, which also possess lipid-like properties, for entry into the brain. The amount of tryptophan reaching the brain increases after eating starchy or sweet foods. On the other hand the more numerous amino acids can displace tryptophan for entry into the brain after eating a protein-rich meal. As a result, brain tryptophan levels may drop. The observation is that normal healthy adults can feel relaxed, sleepy, or more calm or lethargic within two hours following a high-carbohydrate meal. Another possibility is that glucose from carbohy- drate digestion can affect the brain’s activity. The amino acid tyrosine is converted to the neurotransmitter norepinephrine, which helps maintain mental alertness. The brain level of norep- inephrine drops with stress. Tyrosine supplements may help combat fatigue and improve the ability to nervous system 455 perform mental tasks. A light lunch or snack increases mental functioning and decreases the error rate in tasks, and seems to sharpen the mind. Certain B vitamins help in the manufacture of serotonin and norepinephrine: VITAMIN B 6 , VITAMIN B 12 , and FOLIC ACID. Deficiencies of any of these vit- amins can promote depression, senility, and decreased ability to concentrate. Correction of the B vitamin deficiency can ease these symptoms. Even in healthy elderly people without obvious deficiency symptoms, a lower level of folacin and vitamin B 12 has corresponded to lower scores on reasoning tests. Surveys have shown that between 20 percent and 66 percent of healthy people over the age of 65 possess low levels of vitamin B 6 , vit- amin B 12 , and folic acid. (See also ADDICTION; DIGES- TIVE TRACT; FOOD AND THE NERVOUS SYSTEM; MOUTH.) Duthie, S. J. et al. “Homocystine, Vitamin B Status, and Cognitive Function in the Elderly,” American Journal of Clinical Nutrition 75, no. 5 (2002): 908–913. net protein utilization (NPU) An index used to estimate the ability of a food PROTEIN to supply indispensable AMINO ACIDS. Only retained nitrogen is measured. The NPU is based on the ratio of the amount of dietary protein converted to body pro- tein divided by the amount of protein eaten. Thus, a poorly digested protein would have a low NPU score. In this respect, it differs from the BIOLOGICAL VALU E , which does not account for digestibility of food. Frequently, proteins are measured for their growth-promoting effect on young animals or for their ability to maintain nitrogen equilibrium, a balance between protein nitrogen consumed and the amount lost daily ( NITROGEN BALANCE). Since only retained nitrogen is measured, NPU does not specifically assess the inefficiency of digestion. Thus, a poorly digested protein would have a low NPU score. (See also CHEMICAL SCORE.) neural tube defects (NTD) Congenital disorders related to the incomplete development of the neural tube. Neural tube defects contribute to infant mortality throughout the world. Anen- cephalus is the absence of brain and spinal cord. In spina bifida, the spinal cord is pushed through the wall of the spinal canal between vertebrae. Encephalocele refers to the protrusion of the brain through a cranial opening. Both genetic and envi- ronmental factors are implicated. Several studies have confirmed that women who take the B vita- min FOLIC ACID before becoming pregnant and dur- ing the first two months of pregnancy greatly reduce the risk of bearing a child with a neural tube defect. Consequently, the Centers for Disease Con- trol and Prevention (CDC) recommends that all women of childbearing age take a multivitamin that contains at least 400 mcg of folic acid daily. In 1998 the U.S. FDA began requiring all enriched grain products, including cereals, breads, pasta, and rice, to be fortified with folic acid at the rate of 140 mcg per 100 g of grain. This action was taken based on studies that showed that only about 25 percent of women of childbearing age in the United States regularly consumed enough folic acid. In 2001 the CDC reported that the number of children born with these defects had dropped by 19 percent since the enrichment program began. (See also BIRTH DEFECTS ; HOMOCYSTEINE; VITAMIN.) Honein, Margaret A. “Impact of Folic Acid Fortification in the U.S. Food Supply on the Occurrence of Neural Tube Defects,” JAMA 285 (2001): 2,981–2,986. neuron See NERVOUS SYSTEM. neuropathy, peripheral Disease of the peripheral nerves (the nerves feeding into the spinal column and brain). Nutritional deficiencies can cause peripheral nerve degeneration. Severe deficiency of the B vitamin THIAMIN, a disease whose chief characteristic is BERIBERI, also leads to neuropathy. Certain toxic chemicals like plasticizers and several medications such as Isoniazid also cause neuropa- thy. Diseases affecting the entire body like diabetes, certain AUTOIMMUNE DISEASES like Guillain-Barre syndrome, and a variety of rare hereditary diseases are associated with peripheral neuropathy. Neu- ropathy can be caused by HEPATITIS, infectious mononucleosis, ALCOHOLISM, and lead poisoning. (See also DEFICIENCY DISEASE; HEAVY METALS; NER- VOUS SYSTEM.) McLeod, J. G. “Investigation of Peripheral Neuropathy,” Journal of Neurological and Neurosurgical Psychiatry 58 (1995): 274–283. 456 net protein utilization neurotransmitter A chemical required to transmit nerve impulses between nerve cells. Unlike an elec- trical network, nerves are not continuous; the end of a nerve cell does not touch its target. Instead, individual nerve cells are separated by a microscopic gap called a synapse. In response to a nerve impulse traveling down the nerve cell, the nerve ending releases neurotransmitter molecules, which then diffuse across the gap and bind to the adjacent cell. Depending on the nature of the receiving cells, this contact can stimulate or inhibit the target cell. At least three classes of neurotransmitters respond to diet: Catecholamines ( DOPAMINE and norepinephrine) come from the AMINO ACID, TYRO- SINE. Eating a protein-rich meal can increase blood tyrosine levels and increase levels of this amino acid in the brain. Elevated brain tyrosine increases dopamine and norepinephrine production, pro- moting wakefulness and alertness. On the other hand, SEROTONIN tends to induce relaxation and sleep. Serotonin comes from the essential amino acid- TRYPTOPHAN. Eating a meal high in carbohydrates is believed to increase indi- rectly brain tryptophan levels. Elevated blood sugar stimulates insulin release; this hormone stimulates the uptake by muscle of amino acids that compete with tryptophan for uptake by the brain, allowing more tryptophan into the brain. This in turn leads to higher serotonin formation in the brain, pro- moting a state of relaxation. Amino acid research in humans is still considered preliminary. ACETYLCHOLINE is manufactured from CHOLINE,a nitrogen-containing nutrient and raw material for neurotransmitter synthesis. The body can synthe- size choline, and it comes also from dietary lipids, particularly LECITHIN, a common phosphate-bearing lipid found in most foods. Acetylcholine partici- pates in many brain functions, including memory. Victims of ALZHEIMER’S DISEASE have low levels of acetylcholine in their brains. (See also ENDORPHINS; NERVOUS SYSTEM.) neutral fats See FAT; VEGETABLE OIL. NHANES (National Health and Nutrition Examina- tion Survey) In 1956 Congress passed the National Health Survey Act, which authorized the federal government to conduct a continuing survey that would produce statistics on the amount, distri- bution, and effects of illness and disability in the United States. During the next decades nutrition began to play a greater role in the understanding of sickness and disability. In 1970 a special task force reporting to the Department of Health, Education, and Welfare rec- ommended that future surveys include clinical observation and professional assessment as well as recording of dietary intake patterns. Since then the U.S. Center for Health Statistics has been conduct- ing regular National Health and Nutrition Exami- nation Surveys. NHANES I provided statistics between 1971 and 1975; NHANES II between 1976 and 1980; and NHANES III between 1988 and 1994. A Hispanic Health and Nutrition Examination survey was con- ducted from 1982 to 1984. Since 1999 NHANES has been conducted annu- ally. Beginning in 2002 it merged with the U.S. Department of Agriculture’s Continuing Survey of Food Intakes (CSFII), to produce the National Food and Nutrition Survey (NFNS). This integrated sur- vey provides comprehensive information on the health and nutrition intakes of the U.S. population. (See also EATING PATTERNS .) Kleges, R. C., L. H. Eck, and J. W. Ray. “Who Underre- ports Dietary Intake in Dietary Recall? Evidence from the Second National Health and Nutrition Examina- tion Survey,” Journal of Consultative and Clinical Psychol- ogy 63, no. 3 (1995): 438–444. niacin (nicotinic acid, vitamin B 3 ) A heat-stable member of the B VITAMIN complex needed by the body to extract ENERGY from FAT, CARBOHYDRATE, and PROTEIN. Tissues convert niacin to two closely related COENZYMES (enzyme helpers): NICOTINAMIDE ADENINE DINUCLEOTIDE (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). NAD functions as an oxidizing agent used in processes that “burn” fuels to produce energy in the cell. These include the oxidation of GLUCOSE (GLYCOLYSIS) and of FATTY ACIDS. NAD transfers electrons to the mitochondrial ELECTRON TRANSPORT CHAIN, which ultimately reduces OXYGEN to water. This process liberates vast amounts of chemical energy trapped as ATP as well as releasing heat. NADP is generated from the oxidation of glucose (pentose phosphate niacin 457 pathway). The reduced form of NADP, NADPH, is used as a reducing agent in biosynthetic reactions rather than for energy production. CHOLESTEROL, fatty acids, and other important compounds require NADPH for their synthesis. NADPH is also the basis of a powerful antioxidant system ( GLUTATHIONE) to protect most cells from the damaging effects of highly reactive molecules called FREE RADICALS. NADPH also functions in the synthesis of DEOXYRI- BOSE, the sugar building block of DNA. Possible Roles of Niacin in Disease PELLAGRA is a severe niacin deficiency disease that mimics schizophrenia. Skin rashes, DIARRHEA, and mouth sores are prevalent in pellagra. At the turn of the century, pellagra was epidemic in the south- ern United States among people whose diet was based on corn. By 1937, research demonstrated that niacin cured pellagra-like symptoms in dogs and, shortly thereafter, that niacin treatment cured pellagra. Niacin has been used in food fortification since 1941. Niacin is effective in the treatment of elevated (hyperlipidemia) blood lipids that does not respond to dietary intervention alone, and it has been approved by the USFDA for this treatment. It may also reduce the risk of a second heart attack in men. Niacin is effective in lowering high blood triglyc- erides and elevated LDL-cholesterol, the “bad” form, and raising HDL-cholesterol, the beneficial form. Niacin is generally considered an adjunct therapy, used in conjunction with cholesterol- lowering drugs, such as statins and bile binding resins. As an example, niacin in combination with bile-binding agents may effectively slow the pro- gression of atherosclerosis in men with existing car- diovascular disease. The use of niacin to treat schizophrenia and DEPRESSION is controversial. Generally negative results have been reported for patients with long- standing schizophrenia. Some clinicians reported improvement in patients with schizophrenia using megavitamin treatment with niacin or niaci- namide, VITAMIN B 6 and VITAMIN C and in patients with depression using niacinamide and trypto- phan, together with taking steps to improve the diet. The American Psychological Association dis- approved the use of niacin in the treatment of mental disorders in 1979. Niacin when taken in combination with choles- terol-lowering “statin” drugs may prevent HEART ATTACKS . Niacin seems to increase the production of PROSTAGLANDINS. These hormone-like chemicals are produced locally within tissues to help control many physiologic processes such as BLOOD CLOTTING and INFLAMMATION. Large doses of niacin may alleviate noninflam- matory ARTHRITIS, while low doses of niacin can relieve migraine headaches. Niacin supplements may help normalize blood sugar in patients with hypoglycemia. Large doses of niacin may prevent harmful effects of chemical pollutants, drugs, and alcohol, and may help during recovery and drug rehabilitation (the mechanism is not known). This is a promising area of research, but niacin cannot be claimed to be a broad-spectrum detoxification agent. Sources Good sources of niacin are EGGS, MEAT (especially liver), fish, POULTRY, and unprocessed food, includ- ing peanuts and potatoes. COFFEE provides about 3 mg per cup. Niacin is one of the commonly fortified nutrients; consequently, enriched flour and cereals are good sources. MILK and CHEESE are good sources because they contain large amounts of the essential AMINO ACID , TRYPTOPHAN, which is partially con- verted by the body to niacin. Protein-deficient diets are often related to pellagra. This explains why diets incorporating milk can prevent or cure pellagra. The form available from animal foods is NIACI- NAMIDE, a derivative of niacin; plant foods provide niacin itself. Niacin is one of the most stable of the B vitamins: It resists most cooking procedures and can be stored in the dry state indefinitely without loss. Canning, DEHYDRATION, and exposure to air or light cause little destruction. Up to 70 percent of the niacin in most cereal grains, including corn, is present as a bound form called niacytin. It is not released during digestion and therefore is poorly absorbed. Traditionally, corn tortillas are prepared from corn pretreated with lime water to improve dough consistency, a treatment that also frees niacin. Pellagra is uncom- mon in Mexico, Central America, and South Amer- ica, where corn is soaked in lime. Niacin deficiency and pellagra are common only in certain regions of 458 niacin Africa and Asia, where corn is a major source of protein. Because niacin is so prevalent in high- quality protein, only alcoholics and heavy drinkers are likely to be deficient in the United States. Symptoms of mild deficiency include apathy, headache, irritability, and memory loss. Requirements The RECOMMENDED DIETARY ALLOWANCE for health adult men is 19 mg; for nonpregnant, nonlactating women it is 15 mg. The daily niacin requirement varies with the number of calories burned daily (the energy expenditure) and the protein intake. One niacin equivalent equals 1 mg of preformed niacin or 60 mg of dietary tryptophan. Niacin can be synthesized in the body from the essential amino acid tryptophan. Tissues can form an average of 1 mg of niacin from 60 mg of trypto- phan provided by dietary protein. Consequently, a high-quality protein diet supplies substantial tryp- tophan for niacin synthesis. On the other hand, a protein-deficient diet or a diet relying on low tryp- tophan protein increases the dietary requirement for niacin. Most animal protein contains 1.4 per- cent tryptophan; vegetable protein, 1 percent; and corn protein, only 0.6 percent. Safety Niacin (but not closely related niacinamide) expands CAPILLARIES and can lead to itching and flushing at doses commonly used in multivitamin supplements (100 mg or more). While nausea, diar- rhea, and flushing may accompany niacin usage, these side effects are not considered dangerous. Excessive niacin can cause irregular heartbeat, cramps, headache, and liver inflammation. “Sus- tained release” niacin may cause less flushing, but some preparations may be more toxic to the liver. Other forms, such as inositol hexaniacinate, may improve niacin therapy. Large doses of niacin can increase BLOOD SUGAR in diabetics, increase the risk of GOUT, and aggravate ulcers. Niacin supplements are not recommended for those with PEPTIC ULCERS, COLITIS, ASTHMA, liver disease, GOUT, or erratic heart- beat. Using niacin therapeutically (1,000 mg or more per day) requires medical supervision; liver function and blood sugar need to be monitored. (See also CARBOHYDRATE METABOLISM; CHOLESTEROL- LOWERING DRUGS; FAT METABOLISM; FORTIFICATION.) Brown, B. G. et al. “Simvastatin and Niacin, Antioxidant Vitamins, or the Combination for the Prevention of Coronary Disease,” New England Journal of Medicine 345, no. 22 (2001): 1,583–1,592. niacinamide (nicotinamide) A derivative of the water-soluble vitamin NIACIN. Niacinamide is read- ily formed in the body from niacin and incorpo- rated into two important COENZYMES (enzyme helpers): NICOTINAMIDE ADENINE DINUCLEOTIDE (NAD) and its phosphate-containing analog, nicoti- namide adenine dinucleotide phosphate (NADP). By assisting in oxidation-reduction reactions, NAD performs an essential role in energy production from nutrients. NAD is required by all cells in many aspects of metabolism, including CARBOHYDRATE, FATTY ACID, and AMINO ACID degradation. The re- duced form of NADP, NADPH, participates in reductions required for biosynthesis. Niacinamide is the most prevalent form of this vitamin in animal products and MEAT, although niacin is more prevalent in plant foods. Niacina- mide lacks the capillary-expanding (skin-flushing) activity of niacin and is somewhat safer than niacin; hence it is more often used when large amounts of the vitamin are required. Side effects of niacinamide include nausea, headache, fatigue, sore mouth. Large amounts can injure the liver (jaundice). (See also ANABOLISM; B COMPLEX; DETOX- IFICATION; GLUTATHIONE .) nickel A possible TRACE MINERAL nutrient. Nickel is now known to be an essential trace mineral nutrient of all higher plants, including cereal grains and legumes, which require nickel for seeds to grow. Nickel helps plants liberate nitrogen from soil and absorb IRON. Nickel also seems to be a trace mineral nutrient for some animal species, though its function is unknown. Diets that exclude nickel slow growth in sheep, goats, cows, and rats. Nickel deficiency decreases red blood cell production in these animals as well. Nickel-deficient chickens develop abnormally. Human requirements, if any, are unknown. A typical American diet supplies an estimated 0.3 to 0.6 mg of nickel daily. Like many trace minerals, minute amounts may be essential nutrients, though high-level exposure is hazardous. Because nickel is an industrial waste, it nickel 459 has emerged as an environmental pollutant. The toxicity of high doses of nickel is well documented; for example, nickel carbonyl is a hazardous indus- trial chemical, and exposure in the workplace is reg- ulated. Nickel allergies are linked to jewelry; once a person is sensitized, nickel allergies are long-lasting. nicotinamide See NIACINAMIDE. nicotinamide adenine dinucleotide (NAD, NADH) An enzyme helper ( COENZYME) that func- tions in oxidation/reduction reactions of cells. NAD contains the B vitamin NIACIN. NAD assists dehy- drogenases, a family of enzymes that remove hydrogen atoms from substances (oxidation). Lac- tate dehydrogenase is a typical dehydrogenase: This enzyme oxidizes LACTIC ACID to PYRUVIC ACID as an intermediate step in the formation of GLUCOSE, the dominant sugar used by the body. Other dehy- drogenases oxidize fatty acids and perform oxida- tion steps in the KREB’S CYCLE, the central energy yielding pathway of the cell. Nicotinamide Adenine Dinucleotide Phosphate (NADP, NADPH) NADP closely resembles NAD in structure and function. As the second coenzyme based on niacin it contains an additional phosphate group. The reduced form, NADPH, is used in biosynthetic pathways, including fatty acid and cholesterol syn- thesis. NADPH is produced by the oxidation of glu- cose (the pentose phosphate pathway). NADPH supports GLUTATHIONE PEROXIDASE, an important ANTIOXIDANT system for neutralizing oxidative dam- age to lipids and membranes. (See also B COMPLEX; CARBOHYDRATE METABOLISM; FATTY ACID; OXIDATIVE PHOSPHORYLATION .) nicotinic acid See NIACIN. night blindness (nyctalopia) A condition result- ing from a chronic deficiency of VITAMIN A. Vitamin A is required to form “visual purple,” the pigment required for vision in dim light. With vitamin A deficiency, inadequate pigment causes an abnor- mally slow adaptation in going from vision in strong light to dim light. Hereditary factors may contribute to night blindness as well. nightshade family A plant family that includes TOMATOES, POTATOES, EGGPLANT, PEPPERS, and tobacco. Anecdotal reports have been interpreted by some to suggest that eating vegetables from this family may promote joint inflammation in suscep- tible people. However, no carefully controlled stud- ies have been conducted, and the hyptothesis remains unproven. Arthritic symptoms may dimin- ish by avoiding contact with foods that cause reac- tions. (See also ARTHRITIS; FOOD SENSITIVITY.) nitrate An inorganic, nitrogen-containing ion used as a FOOD ADDITIVE for processed MEAT and meat products. Meat can legally contain 91 mg added sodium or potassium nitrate per pound. Chopped meat can contain 778 mg per pound, and dry, cured meat can contain 991 mg per pound. Its role in meat is not clear, though nitrate seems to provide a reservoir for bacterial conversion to nitrite, which acts as an antimicrobial agent. Nitrite can be converted to a potential cancer-causing agent called NITROSOAMINE. Nitrate occurs naturally in food. Green VEGETA- BLES are a major source of nitrates and BEETS, SPINACH, and LETTUCE are likely sources. The level depends on the plant species and variety, the part of the plant consumed, the stage of maturity, levels of fertilizers in the soil, and the rate of plant growth. The estimated-typical daily intake of nitrate is 6 mg from vegetables; 9.4 mg from cured meats; 2 mg from bread; and 1.4 mg from FRUIT and fruit juices. Drinking water supplies an average of 0.7 mg daily. High levels of nitrate can occur in well water in rural areas, due to contamination from feed lots and run-off from fertilizer-affected fields. A limit of 10 mg of nitrate per liter has been set by the U.S. Public Health Service. Nitrate in well water used to prepare formula can be a hazard for infants. Up to six months after birth, stomach acid production in infants is low, which permits bacteria in the intestine to convert nitrate to nitrite. Nitrite can disable infants’ hemo- globin and induce a condition called methenoglo- binemia, in which nitrite-modified hemoglobin cannot transport adequate oxygen. In older chil- dren, nitrate is absorbed in the stomach before it can reach intestinal bacteria that could reduce it to nitrite. Infants drink proportionately much more water (in formula) than adults do, and their hemo- 460 nicotinamide [...]... hydrogen, carbon, and OXYGEN Nitrogen is a key constituents of AMINES, alkaline substances found in cells In all cells, nitrogen appears in the amino acids, which are incorporated into protein, including enzymes; in purines and pyrimidines, the building blocks of DNA and RNA, the information molecules of the cell; in PHOSPHOLIPIDS, the structural elements of mem- 462 nitrogen balance branes; in certain... HORMONES; and in brain chemicals (NEUROTRANSMITTERS) synthesized by nerve cells to carry nerve impulses Nitrogen contributes to the function of HEME (the oxygen-binding pigment of the oxygen-transport protein, HEMOGLOBIN); THIAMIN (vitamin B1); RIBOFLAVIN (vitamin B2); NIACIN (vitamin B3); VITAMIN B6; and VITAMIN B12 The nitrogen-containing waste product of protein metabolism is urea, excreted in the. .. Both DNA and RNA are assembled from building blocks: a simple sugar, DEOXYRIBOSE, phosphate, and nitrogen-containing cyclic bases called purines and pyrimidines The overall composition of RNA is similar to DNA Each contains a phosphate, a simple sugar— ribose in RNA and deoxyribose in DNA RNA and DNA contain four nitrogen-containing ring compounds Three of these are identical: ADENINE, GUANINE, and cytosine... action on nitrate in saliva and in the intestine can produce several milligrams of nitrite daily Safety Nitrite-containing bacon, when fried, yields NITROSOAMINES, a family of nitrogen-containing, cancercausing chemicals Furthermore, significant levels of nitrosoamines appear in cooked SAUSAGE and cured meats Specific nitrosoamines have caused CANCER in all animals tested Under conditions present in the. .. that can be used by the body for growth and maintenance of health Foods are mixtures of nutrients, and the relative 466 nutrient density amounts of the various nutrients in a given food depend on conditions of growth, storage, degree of processing, and method of cooking DIGESTION releases nutrients from foods, which can then be absorbed by the intestinal tract Certain nutrients must be supplied by the. .. minerals, including CHROMIUM, MANGANESE, ZINC, and SELENIUM Typically, nuts contain up to seven times the amounts of these minerals as found in other unprocessed foods Nuts also provide CHOLINE, a nutrient required for fat transport and nerve function (See also AFLATOXIN; COMPLETE PROTEIN; PROTEIN QUALITY.) Kris-Etherton, P M et al “Nuts and Their Bioactive Constituents: Effects on Serum Lipids and. .. strong antioxidants, linked with cancer prevention, increased immunity, and lowered risk of cardiovascular disease Common sources include red wine, apples, onions, and tea ISOFLAVONES (such as genistein) from soy possess weak estrogen activity and apparently lower the risk of breast cancer Organosulfur compounds in GARLIC and ONION, including allicin, dially disulfide, and ajoene, in a addition to saponins... much of the essential nutrients, as defined by the U.S recommended daily allowances (USRDA), as the basic food Thus a food can be considered nutritionally equivalent if it is not nutritionally inferior There are important limitations to the U.S FDA’s definition of nutritional equivalency A product can be declared nutritionally equivalent though it contains excessive amounts of any of the 20 nutrients... Nutrient recommendations rely upon biochemical research, clinical case histories of nutrient-deficient individuals, human experimentation with suboptimal diets, animal growth experiments and examination of nutrient intake of healthy people Reduced nutrient intake as well as excessive nutrient consumption can lead to MALNUTRITION A BALANCED DIET supplies all nutrients in adequate amounts to maintain health. .. process They convert atmospheric nitrogen to the simplest nitrogen compound, AMMONIA (NH3), via a process called nitrogen fixation Alternatively, lightning bolts energize nitrogen to react with oxygen in the air to create NITRATE Plants incorporate both ammonia and nitrate into AMINO ACIDS, which form PROTEIN When ingested by animals, plant proteins are broken down to amino acids and incorporated into animal . by the Institute of Medicine. Among the many responsibilities of the NRC is the development and evaluation of nutrient stan- dards for good health. In 1941, the Food and Nutrition Board of the. National Health and Nutrition Exami- nation Surveys. NHANES I provided statistics between 1971 and 1975; NHANES II between 1976 and 1980; and NHANES III between 1988 and 1994. A Hispanic Health and Nutrition. brain and spinal cord to either muscles or glands. Connecting neurons (intraneurons) carry impulses from sensory neurons to motor neurons and occur in the brain and spinal cord. Many nutrients