U 634 ubiquinone See COENZYME Q. ulcerative colitis See COLITIS. ulcers See DUODENAL ULCER; PEPTIC ULCER. ultraviolet light (UV light) A form of radiant energy that lies between visible light and X rays in the electromagnetic spectrum. Ultraviolet light has a shorter wavelength than visible light and there- fore has more energy than visible light. In fact, UV light can induce chemical reactions in the body and in manufacturing processes. UV light from the sun converts a cholesterol product in the skin called 7- dehydrocholesterol to vitamin D 1 . Commercial treatment of the plant STEROL, ERGOSTEROL, with UV light produces vitamin D 2 . UV light is used to sterilize milk and foods and has other industrial uses because it can destroy viruses and bacteria. UV light is potentially harmful to eyes and SKIN. Lifelong exposure increases the risk of CATARACTS due to damage to the lens of the eye. UV exposure is also implicated in damage to the cornea and the retina. Sunburn is an immediate symptom of the dangers of overexposure to UV light. Prolonged exposure to UV light in sunlight can cause prema- ture aging of the skin; loss of the skin’s elasticity causes wrinkles. Long-term exposure to sunlight can cause rough, brownish, scaly growth on the skin, a precancerous condition called actinic keratosis. Tan- ning increases the risk of skin cancer, the most com- mon type of cancer; malignant melanoma is the most serious form. A second type of skin cancer, basal cell carcinoma, does not spread to other parts of the body and is more prevalent. Above 1 million new cases were diagnosed in 2001. Those at greatest risk are fair-skinned people with light-colored eyes; those who have a high incidence of moles or who have a family history of cancer; and people who spend long periods of time out of doors. Preventing skin cancer involves protection from direct exposure to sunlight. Specialists recommend wearing UV-absorbing sunglasses and using appro- priate sunscreen lotion whenever individuals are out in the sun. Nutrition seems to play an impor- tant role in limiting the effects of long-term UV exposure. The reason for this lies in the nature of the damage induced by UV light. UV light causes the formation of free radicals, highly reactive chemical fragments that randomly attack cell com- ponents like protein. When free radicals attack DNA, they can cause mutations, which can be a prelude to cancer. Nutritional ANTIOXIDANTS are agents that counter the effects of free radicals by either squelching them or by preventing their for- mation. Included in the family of antioxidants are VITAMIN C, VITAMIN E, and BETA-CAROTENE. Certain trace minerals like selenium and zinc can also effectively protect cells against oxidative damage. (See also MACULAR DEGENERATION; CARCINOGEN.) undernutrition See MALNUTRITION. unsaturated fat FAT that contains a relatively high percentage of FATTY ACIDS containing chains of carbon atoms that are deficient in hydrogen atoms and possess double bonds. Each double bond lacks two hydrogen atoms: Fish oils and most vegetable oils are unsaturates. In contrast, animal fat and shortening are saturated fats, enriched in saturated fatty acids whose carbon atom chains are filled up with hydrogen atoms. These important chemical differences govern the physical and chemical properties of fats. Unsatu- rated fats are liquids at room temperature and are sensitive to attack by OXYGEN, which can lead to RANCIDITY (decomposition), while saturated fats are solid at room temperature and are more stable to heat and oxidation. Unsaturated fats fall into two groups. MONOUN- SATURATED OILS are rich in the monounsaturated fatty acid, OLEIC ACID; this building block lacks a single pair of hydrogen atoms. Such oils are called “monounsaturates” or “oleic-rich” oils; OLIVE OIL is a typical example. Monounsaturates seem to lower the risk of HEART DISEASE by lowering blood CHO- LESTEROL (a desirable effect) without lowering HIGH-DENSITY LIPOPROTEIN (HDL), the beneficial form of cholesterol. Monounsaturates are not usu- ally partially hydrogenated. Polyunsaturates are the second type of unsatu- rated fats. These oils are rich in polyunsaturated fatty acids, which lack many hydrogen atoms. Because they are less saturated, polyunsaturated are more susceptible to oxidation. Vitamin E pro- tects polyunsaturates against oxidation, and the requirement for vitamin E increases with increased consumption of polyunsaturates. An industrial process called hydrogenation is used to harden unsaturated fats by adding hy- drogen atoms and making the fatty acids more saturated. Partially hydrogenated vegetable oils maintain some of their unsaturated character and possess some double bonds; such oils are less sus- ceptible to rancidity and have a longer shelf life than untreated oils. Polyunsaturates provide varying amounts of ESSENTIAL FATTY ACIDS, required in the diet because they cannot be manufactured by the body. Most vegetable oils like soybean oil, corn oil, and saf- flower oil provide the essential fatty acid, LINOLEIC ACID , a polyunsaturated fatty acid of the omega-6 family, the most prevalent in the diet. These polyun- saturated oils seem to lower blood cholesterol (a desirable effect), but in large amounts they also lower the beneficial HDL. Polyunsaturates lower the risk of HEART ATTACK and STROKE because they block the formation of dangerous blood clots. On the other hand, in excess they may increase the risk of CANCER and lower immunity, at least in lab animals. Certain seed oils like FLAXSEED OIL and fish oil supply the second essential fatty acid, ALPHA LINOLENIC ACID , and other omega-3 polyunsatu- rated fatty acids. Fish oils supply large polyunsatu- rated fatty acids that may help prevent or decrease the symptoms of heart disease and HYPERTENSION and of inflammatory conditions like RHEUMATOID ARTHRITIS . Unsaturated fats produce essentially the same number of calories as saturated fats when burned for energy, nine calories per gram—more than twice the amount derived from carbohydrate or protein. Both saturated and unsaturated fats when consumed in excess can lead to weight gain. Cur- rent dietary guidelines call for selecting a diet that is lower in total fat (less than 30 percent of total daily calories, with saturated fat representing less than 10 percent of calories) in order to lower the risk of cancer and heart disease. (See also FAT METABOLISM .) Judd, J. T. et al. “Effects of Margarine Compared with Those of Butter on Blood Lipid Profiles Related to Car- diovascular Disease Risk Factors in Normolipidemic Adults Fed Controlled Diets,” American Journal of Clin- ical Nutrition 68, no. 4. (1998): 768–777. urea The major nitrogen-containing waste pro- duced by the body. Urea production is the body’s way of safely disposing of ammonia, the potentially dangerous product released when AMINO ACIDS are broken down. Urea is nontoxic; it accounts for 60 percent to 90 percent of total excreted nitrogen. Urea production occurs in the LIVER as part of the body’s detoxification function; the amount pro- duced is proportional to the amount of protein consumed. The liver uses a group of specialized enzymes (the UREA CYCLE) to produce urea. Urea is released into the bloodstream and transported to the kidney for excretion in the urine. Normally 20 to 35 g of urea are excreted each day. Blood urea nitrogen (BUN) refers to the measurement of blood urea levels and is used as a diagnostic tool to assess liver and kidney function. The amount of urea nitrogen in a 24-hour urine sample can be multi- urea 635 plied by 6.25 to estimate the amount of protein used per day. (See also AMINO ACID METABOLISM.) urea cycle A collection of enzymes responsible for transforming AMMONIA to UREA. Ammonia is a toxic by-product of amino acid breakdown ( AMINO ACID METABOLISM ) that is released into the bloodstream. The brain is extremely sensitive to ammonia; there- fore, its disposal is important. To detoxify this sub- stance and simplify its elimination from the body, ammonia is efficiently removed from the blood by the liver, where it is combined with bicarbonate to create a safe product, urea, to be excreted by the kid- neys. The liver also disposes of ammonia produced by gut bacteria and absorbed by the intestine. ORNITHINE , a nonprotein amino acid that accepts ammonia, is regenerated when ammonia is released. It is worth noting that the urea cycle produces the amino acid ARGININE. However, the production of this amino acid can be limited during illness and periods of rapid growth, and dietary sources are important. (See also GLUTAMINE; KREB’S CYCLE.) urethane A cancer-causing agent in alcoholic beverages. Urethane has been found to form spon- taneously in WINE, BEER, and hard liquors. Bourbon whiskey, fruit brandies, and some desserts and table wines sold in the United States may contain high levels, exceeding levels permitted in a number of countries, including Canada. Research is under- way to lessen urethane contamination of alcoholic beverages. Because the PRESERVATIVE DEPC (die- thylpyrocarbonate) spontaneously breaks down into urethane, it was banned by the FDA in 1972. (See also CANCER.) uric acid A waste product form the degradation of RNA and DNA. Uric acid is a nitrogen-containing carbon compound derived from PURINES, which are building blocks of DNA, the genetic material of the cell, and of RNA, which directs the manufacture of proteins. Because the body cannot decompose the purine ring system, enzymes convert purines to uric acid instead. Between 0.5 and 1.0 g of uric acid is secreted in the urine daily. A high blood level of uric acid is associated with GOUT, a form of ARTHRITIS that is characterized by deposits of uric acid in joints. The risk of gout increases with a high protein diet, a family history of gout and an acidic blood pH. Uric acid can form kidney stones, particularly in patients with gout. People with gout should minimize their consump- tion of foods with a high purine content: liver and organ meats; fish like anchovies, MACKEREL, and SARDINES; and yeast. Excessive alcohol can raise blood uric acid levels and increase the risk of kid- ney stones and gout. Extra fluid, 10 to 12 glasses of nonalcoholic, noncaffeinated beverages daily, can minimize uric acid deposits. Uric acid performs a useful function for the body: It is a powerful ANTIOXIDANT that can destroy dangerous free radicals, which are linked to degen- erative diseases and AGING. Evolution may have favored humans’ ability to accumulate relatively high blood levels of uric acid in order to assure the presence of an antioxidant that is unrelated to diet. (See also VITAMIN C; VITAMIN E.) urinary tract A system made up of two kidneys, two ureters (tubes leading to the urinary bladder) and a single urethra, the duct that conveys urine to outside the body. The primary function of the uri- nary tract is to assist the body in maintaining a con- stant internal environment (homeostasis) by controlling the composition and volume of the BLOOD. Kidneys aid in eliminating toxic materials, drugs, normal waste products, and even excesses of essential nutrients as urine. Kidneys assume a role in red blood cell production by secreting erythro- poietin, stimulatory protein. They help regulate blood pH and form RENIN, a hormone to regulate BLOOD PRESSURE . Finally, the kidneys activate VITA- MIN D to its hormone form, dihydroxycalciferol (calcitriol). The two kidneys are located in the back just above the waistline. Usually each kidney is encased in a heavy cushion of fat for protection. An extremely thin person may lack this fatty sup- port, and the kidneys may drop, placing a kink in the ureter. The rate of blood flow through the kid- ney is extremely high; about 10 percent of the total blood pumped each minute enters the kid- neys. This high volume, together with a normal blood pressure, is required to filter the blood and to form urine. 636 urea cycle Each kidney contains more than a million microscopic functional units called NEPHRONS.A nephron resembles a tiny funnel with an extremely long, twisted stem. Nephrons filter blood: some materials, like water, pass into the tubules, small tubes that drain the kidney. Others stay in the blood. Useful nutrients like GLUCOSE and minerals ( ELECTROLYTES) are reabsorbed into the blood. The sum of these activities is urine. Filtration usually occurs at a rate of 7,500 ml per hour, equivalent to 190 quarts of fluid filtered per day. Normally between 97 percent and 99 percent of the water is reabsorbed, together with all of the glucose because this nutrient, the primary brain fuel, is too precious to waste. However, in DIABETES MELLITUS the blood glucose concentration may ex- ceed the rate at which it can be reabsorbed by kidney cells and is excreted in urine. Glucose in the urine (glucosuria) is a sign of uncontrolled diabetes. Sodium reabsorption by the kidneys varies with blood pressure: When the sodium concentration in the blood is low, blood pressure drops and renin is released. This enzyme activates ANGIOTENSIN , a hor- mone that constricts minute arteries located at the nephron, increasing blood pressure. Angiotensin further stimulates the ADRENAL GLAND to release ALDOSTERONE, which increases sodium and water reabsorption by the tubules. Conditions that limit urine output are inadequate fluid consumption, stress, kidney disease, and CARDIOVASCULAR DISEASE. Usually urine is sterile; the closer it gets to the urethra, the greater the possibility of bacterial con- tamination. The daily flow of urine out of the body and the immune defenses are usually adequate to assure a sterile urinary tract. However, bladder infections are common among women. Because over half of these infections involve the kidneys, they can be serious. Partial blockage of the urinary tract, for example by stone formation or scarring, can reduce urine flow and favor infection. Cran- berry juice contains materials that prevent bacteria from sticking to the walls of the bladder or urethra, thus inhibiting bacterial infection. Garlic and onions also contain substances that limit bacterial growth. Sugar and sweetened juice can help pro- mote bacterial growth, however. Drinking plenty of water (5 pints daily) is recommended. Stone formation is a second frequent problem of the urinary tract; 10 percent of men and 5 percent of women in the United States will experience stone formation. Calcium as calcium carbonate or calcium oxalate can become insoluble and form crystalline deposits when water consumption is inadequate. Alternatively, uric acid can form stones when certain anti-cancer drugs are taken, or when excessive purines (yeast sardines, organ meats like liver, caviar) are part of the diet. Stone formation correlates with inadequate water intake and fiber consumption; high intake of foods like cocoa, spinach, parsley, rhubarb, and certain nuts contain- ing oxalate, can favor stone formation in the uri- nary tract. (See also AMINO ACID METABOLISM; ERYTHROPOIESIS; KIDNEY STONES; UREA CYCLE.) urticaria The medical term for hives, welts that form in response to irritation or allergy. Urticaria is typical of an immediate allergy reaction (immedi- ate hypersensitivity) and is often accompanied by ASTHMA, itchy eyes, and runny nose. Animal dan- der, pollen, house dust, and certain foods can cause urticaria in susceptible people. (See also ALLERGY, IMMEDIATE.) USDA (U.S. Department of Agriculture) A fed- eral agency responsible for regulating food quality and safety. The USDA operates under the authority of the Federal Meat Inspection Act and the Poultry Products Act, which assign the USDA the responsi- bility for assuring the safety of MEAT and POULTRY. The USDA inspects meat and meat products, as well as poultry and poultry products destined for human consumption. It inspects slaughtering and processing practices. State laws for meat and poul- try inspection must equal USDA standards. The USDA sets guidelines for meat labeling and grades beef according to eating quality for food companies that pay for this service. Programs for health and nutrition, nutrition education and research are carried out by the eight divisions of the USDA. These divisions are the Agricultural Research Service; Consumer and Marketing Ser- vice; Cooperative State Research Service; Annual and Plant Health Inspection Service; Federal Exten- sion Service; Cooperative State Research Service; Food and Nutrition Service; Veterinary Services; USDA 637 Labeling and Registration Section. (See also PROCESSED FOOD.) USRDA (U.S. recommended daily allowances) A standard for nutrient intake for healthy people, designed by the U.S. Department of Agriculture ( USDA) for use on food labels through 1994. The USRDAs represent the highest RECOMMENDED DIETARY ALLOWANCES (RDAs), which were devel- oped by the Food and Nutrition Board of the National Academy of Sciences for several nutrients in 1968. The USRDAs are not to be confused with the RDAs, which are not used on food labels. The list of RDAs is much more extensive now, and sev- eral values differ from those specified in 1968. The USRDA was replaced in 1994 by the RDI or REFERENCE DAILY INTAKE (RDI) on food labels. For the time being, the RDI is identical to the USRDA. Individual USRDAs were condensed into just four population groupings: adults and children over four years; infants up to one year; children under four years; and pregnant or lactating women. Gen- erally the highest values for a given age group were used. Thus the USRDAs for adults and children over four years generally were the dietary allowances recommended for a teenage male. When nutritional claims about a food were made on a food label, or if the food was labeled “enriched,” or if extra nutrients were added, the manufacturer was required to list the content of several nutrients as percentages of the USRDAs. The values food manufacturers used for the most common category, adults and children over four, are shown in the accompanying table. Nutrient USRDA Vitamin A 1,000 mcg Vitamin C 60 mg Calcium 1,000 mg Iron 18 mg Vitamin D 10 mcg Vitamin E 30 IU Thiamin 1.5 mg Riboflavin 1.7 mg Niacin 20 mg Vitamin B 6 2.0 mg Folate 400 mcg Vitamin B 12 6 mcg Biotin 300 mcg Pantothenic Acid 10 mg Phosphorus 1,000 mg Magnesium 400 mg Zinc 15 mg Iodine 150 mcg Selenium N/A Copper 2 mg For this category the protein value is 45 g of high-quality protein (equivalent to milk protein, CASEIN), or 65 g of lower quality protein (less than casein). (See also FOOD LABELING.) National Research Council. Recommended Dietary Allowances, 10th ed. Washington, D.C.: 1989. 638 USRDA . nitrogen. Urea production occurs in the LIVER as part of the body’s detoxification function; the amount pro- duced is proportional to the amount of protein consumed. The liver uses a group of specialized enzymes. (BUN) refers to the measurement of blood urea levels and is used as a diagnostic tool to assess liver and kidney function. The amount of urea nitrogen in a 24-hour urine sample can be multi- urea. disease, and CARDIOVASCULAR DISEASE. Usually urine is sterile; the closer it gets to the urethra, the greater the possibility of bacterial con- tamination. The daily flow of urine out of the body and