Micrograms of other vitamin A precursor carotenoids

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Until the RDA for vitamin A was published in 2001, vitamin A was measured in retinol equivalents (REs). Food composition tables and nutrient analysis software sometimes use REs instead of RAEs. RAE and RE values are the same for retinol and preformed vitamin A, but the RAE value for carotenoids is about half the RE value.

Deficiency and toxicity. Vitamin A has been identified as a nutrient that is consumed by American adults in amounts low enough to be of concern. Low intakes of vitamin A tend to reflect low intakes of fruits and vegetables.

Vitamin A deficiency is of most concern in developing countries, where it affects the health of many children and adults, causing night blindness, blindness, poor growth, and other problems. Up to 500,000 children worldwide go blind each year because of vitamin A deficiency. Signs of deficiency include night blindness, dry skin, dry hair, broken fingernails, and decreased resistance to infections. In the United States, vitamin A deficiency is sometimes seen in the elderly, the poor, and preschool children. In children, a mild degree of vitamin A deficiency may increase the risk of developing respiratory and diarrheal infections, decrease the growth rate, slow bone development, and decrease the likelihood of survival from a serious illness.

Prolonged use of high doses of preformed vitamin A (the Tolerable Upper Intake Level is 3000 micrograms/day) may cause symptoms of hypervitaminosis A such as hair loss, bone pain and damage, fatigue, skin problems, liver damage, nausea, and vomiting. High doses are particularly dangerous for pregnant women (they may cause birth defects) and the elderly (they can cause joint pain, nausea, muscle soreness, itching, hair loss, and liver and bone damage). Overconsump-tion of beta-carotene supplements can be quite harmful as well.

II Vitamin D

Vitamin D differs from most other vitamins in that it can be made in the body. When ultraviolet rays shine on your skin, a cholesterol-like compound is converted into a precursor of vitamin D and absorbed into the blood. Over the next one and a half to three days, the precursor is converted to vitamin D3, an inactive form that also is called cholecalciferol. Vitamin D3 is converted into its active form by enzymes in the liver and then the kidney.

Of course, if you are not in the sun much or if the ultraviolet rays are cut off by heavy clothing, clouds, smog, fog, sunscreen (SPF of 8 or higher), or window

  • RICKETS A childhood disease in which bones do not grow normally, resulting in bowed legs and knock knees; it is generally caused by a vitamin D deficiency.
  • OSTEOMALACIA A disease of vitamin D deficiency in adults in which the leg and spinal bones soften and may bend.

glass, less vitamin D will be produced. On the positive side, a light-skinned person needs only about 15 minutes of sun on the face, hands, and arms two to three times per week to make enough vitamin D. A dark-skinned person needs more time in the sun because melanin (dark brown to black pigments in the skin) acts like a sunscreen. Several months' supply of vitamin D can be stored in the body; this is helpful during winter months when the sun is not as strong in northern climates and you need to wear more clothing. That's why you need to get an adequate amount of exposure to sunlight in the spring, summer, and fall to get you through the winter. As you get older, your body makes less active vitamin D.

Functions. In its active form, vitamin D functions more as a hormone than as a vitamin. Hormones are substances secreted into the bloodstream that travel to one or more organs. Once the hormone reaches what is called the target organ(s), it affects something that that organ does. The active form of vitamin D travels through the bloodstream to increase calcium (and also phosphorus) absorption in the intestine, decreases the amount of calcium excreted by the kidney, and pulls calcium out of the bones when necessary. Blood calcium levels must be kept high so that enough calcium is present to build bones and teeth, contract and relax muscles, and transmit nerve impulses. Vitamin D works with other nutrients and hormones to make and maintain bone.

Evidence is growing that vitamin D may have subtle but important effects on regulating cell growth and on the cardiovascular and immune systems. Studies show an association between sunlight exposure and increased blood levels of vitamin D and a decreased risk of some common cancers: colon, breast, prostate, and ovarian.

Food sources. Significant food sources of vitamin D include vitamin D-fortified milk and cereals. If you drink 2 cups of milk each day, you will get about half the RDA of vitamin D (the rest comes from sun exposure and other foods). No vitamin D is added to milk products such as yogurt and cheese. Vitamin D-fortified butter and margarine contain some vitamin D.

Vitamin D was previously measured in international units, and so most nutrient composition tables use IU. The current AI for vitamin D is expressed in micrograms of cholecalciferol. The relationship between IU and micrograms of cholecalciferol is as follows:

1 IU = 0.025 microgram cholecalciferol

The AI for vitamin D assumes that you are not getting any vitamin D from exposure to the sun.

Deficiency and toxicity. Vitamin D deficiency in children causes rickets, a disease in which bones do not grow normally, resulting in soft bones and bowed legs. Rickets is rarely seen.

Vitamin D deficiency in adults causes osteomalacia, a disease in which bones become soft and hurt. In the elderly, there is an increased risk among

OSTEOPOROSIS The most common bone disease, characterized by loss of bone density and strength; it is associated with debilitating fractures, especially in people 45 years and older, due to a tremendous loss of bone tissue in midlife.

people who tend to remain indoors and avoid milk because of lactose intolerance. Vitamin D deficiency is more prevalent in colder climates where exposure to the sun is more limited.

Vitamin D deficiency can contribute to and worsen bone disease called osteoporosis. Osteoporosis is characterized by fragile bones, which are more likely to fracture. Having normal storage levels of vitamin D in your body is one of several steps needed to keep your bones strong. Osteoporosis is a major public health threat for an estimated 44 million Americans, or 55 percent of people 50 years of age and older. In 2005, 10 million individuals were estimated to already have the disease and almost 34 million more were estimated to have low bone mass, placing them at increased risk.

Vitamin D supplements are often recommended for exclusively breast-fed infants because human milk may not contain adequate supplies. Mothers of infants who are exclusively breast-fed and have limited sun exposure should consult a pediatrician on this issue. Since infant formulas are routinely fortified with vitamin D, formula-fed infants usually have adequate dietary intake of vitamin D.

Vitamin D, when taken in excess of the AI, is the most toxic of all the vitamins. All you need is about four to five times the AI to start feeling symptoms of nausea, vomiting, diarrhea, fatigue, and thirst. It can lead to calcium deposits in the heart, blood vessels, and kidneys that can cause severe health problems and even death. Young children and infants are especially susceptible to the toxic effects of too much vitamin D.

■ ALPHA-TOCOPHEROL The most active form of vitamin E in humans; also a powerful antioxidant.

H Vitamin E

Vitamin E exists in eight different forms. Each form has its own biological activity, which is the measure of how powerful it is in the body. Alpha-tocopherol is the name of the most active form of vitamin E in humans. It is also a powerful biological antioxidant.

Functions. Vitamin E has an important function in the body as an antioxidant in the cell membrane and other parts of the cell. Vitamin E is of particular importance to cell membranes at the highest risk of oxidation, which includes cells in the lungs, red blood cells, and brain. Vitamin E even protects vitamin A from oxidation. Vitamin E is also important for a healthy immune system and nerve tissue. Studies are under way to determine whether vitamin E, through its ability to limit the production of free radicals, helps prevent or delay the development of heart disease and cancer.

Food sources. Vitamin E is widely distributed in plant foods (Figures 6-2 and 6-3). Rich sources include vegetable oils, margarine and shortening made from vegetable oils, salad dressings made from vegetable oils, nuts, seeds, leafy green vegetables, whole grains, and fortified ready-to-eat cereals. In oils, vitamin E acts like an antioxidant, thereby preventing the oil from going rancid or bad. Unfortunately, vitamin E is easily destroyed by heat and oxygen.

figure 6-2 FOOD SOURCES OF VITAMIN E

meats, poultry, fish, and alternates

Food and Serving Size

Milligrams

Adult RDA

Seeds, sunflower, 1/4 cup

8.3

15

Nuts, almonds, 1/2 oz. (12 nuts)

3.7

15

Peanut butter, 2 Tbsp.

2.9

15

Peanuts, 1 oz. (28 nuts)

2.2

15

Chicken breast, cooked, 1/2 breast

1.5

15

fats and oils

Food and Serving Size

Milligrams

Adult RDA

Oil, sunflower, 1 Tbsp.

5.6

15

Oil, safflower, 1 Tbsp.

4.6

15

Oil, olive, 1 Tbsp.

1.9

15

Margarine, 1 Tbsp.

1.3

15

Oil, canola, 1 Tbsp.

1.2

15

French dressing, 1 Tbsp.

0.8

15

Mayonnaise, 1 Tbsp.

0.7

15

grains

Food and Serving Size

Milligrams

Adult RDA

Special K, 1 cup

4.7

15

Whole-wheat bread, 1 slice

0.2

15

White bread, 1 slice

0.0

15

fruits and vegetables

Food and Serving Size

Milligrams

Adult RDA

Spinach, boiled, 1/2 cup

3.4

15

Tomato sauce, 1/2 cup

2.6

15

Sweet potato, canned, 1/2 cup

1.3

15

Broccoli, boiled, 1/2 cup

1.2

15

Mangoes, raw, 1/2 cup

0.9

15

Raspberries, frozen, 1/2 cup

0.9

15

dairy

Food and Serving Size

Milligrams

Adult RDA

Soy milk, fluid, 1/2 cup

3.3

15

Source: U.S. Department of Agriculture, Agricultural Research Service, 2004. USDA Nutrient Database for Standard Reference, Release 17. Nutrient Data Laboratory Home Page, http://www.nal. usda.gov/fnic/foodcomp.

The RDA for vitamin E is expressed in milligrams ofalpha-tocopherol (the most active form of vitamin E). Food composition tables usually measure vitamin E in milligrams of tocopherol equivalents, which includes forms of vitamin E in addition to alpha-tocopherol. If you know the number of tocopherol equivalents in a food, multiply that number by 0.8 to come up with the alpha-tocopherol content.

FIGURE 6-3 Good sources of vitamin E (vegetable oils, salad dressings, nuts, seeds, and margarine).

Photo by Frank Pronesti.

FIGURE 6-3 Good sources of vitamin E (vegetable oils, salad dressings, nuts, seeds, and margarine).

Photo by Frank Pronesti.

Deficiency and toxicity. Most Americans do not typically consume toods that are especially rich in vitamin E on a daily basis. Although salad dressings, mayonnaise, and oils provide the greatest amount oi vitamin E in American diets overall, the oil most commonly used in these products is soybean oils, which is not an especially rich source of vitamin E. Oils containing higher amounts of vitamin E, such as sunflower and safflower oils, are less commonly consumed. The same is true for nuts; almonds and hazelnuts are relatively rich in vitamin E, but peanuts and peanut butter, with lower levels of vitamin E, represent the majority oi all nut consumption in the United States. Toxicity oi vitamin E is rare and can cause bleeding problems.

Vitamin K

Vitamin K plays an essential role in the activation of a number of blood-clotting factors, such as prothrombin. Blood clotting prevents excessive blood loss when the skin is broken. Vitamin K is also needed to make an important protein used to form bone.

Excellent sources of vitamin K include green leafy vegetables such as kale, collards, spinach, broccoli, brussels sprouts, scallions, cabbage, and iceberg lettuce, as well as oils and margarine.

Vitamin K is also made in the body. Billions of bacteria normally live in the intestines, and some of them make a form of vitamin K. It is thought that the amount of vitamin K produced by bacteria is significant and may meet about half your needs. An infant is normally given this vitamin after birth to prevent bleeding because the intestine does not yet have the bacteria that produce vitamin K. Food sources of vitamin K provide the balance needed. Vitamin K deficiency is rare but can occur if you have problems absorbing fat or are taking certain drugs, such as antibiotics, that can destroy the bacteria in your intestines that make vitamin K.

Toxicity is normally not a problem. No Tolerable Upper Intake Level has been set for vitamin K.

mini-summary

  1. Fat-soluble vitamins (A, D, E, K) generally occur in foods containing fats and are stored in the body either in the liver or in adipose (fatty) tissue until they are needed. They are absorbed and transported around the body in the same manner as other fats.
  2. If anything interferes with normal fat digestion and absorption, these vitamins may not be absorbed.
  3. Dietary intake of vitamin E is low enough to be of concern for both adults and children. Low intake of vitamin A is also a concern for adults.
  4. Although it is convenient to be able to store these vitamins so that you can survive periods of poor intake, excessive vitamin intake (higher than the Tolerable Upper Intake Level) causes large amounts of vitamins A and D to be stored and may lead to undesirable symptoms.
  5. Figure 6-4 summarizes the recommended intake, functions, and sources of the fat-soluble vitamins.

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