Vitamin D

What Is Vitamin D?

Vitamin D is a fat-soluble vitamin and is somewhat unique in relation to the other vitamins because the body can produce it in adequate amounts with the assistance of the sun. In fact, many researchers feel that since certain cells can produce vitamin D and because it then circulates and affects tissue throughout the body, it might be better classified as a hormone than a vitamin. However, the body's ability to make vitamin D

relies upon exposure to sunlight (ultraviolet B light), and not everyone receives adequate exposure. Furthermore, direct exposure to sunlight is not recommended due to the increased risk of skin cancers. For this reason, vitamin D will maintain its position as a vitamin.

What Foods Provide Vitamin D?

There are two possible ways of supplying the body with vitamin D: through diet and exposure to the sunlight. In the human diet, the richest sources of vitamin D are vitamin D-fortified milk and milk products, tuna, salmon, margarine (vitamin D fortified), herring, and vitamin D-fortified cereals (Table 9.10). Vitamin D in foods appears fairly stable in various cooking and storing procedures. Vitamin is available in nutrition supplements in the form of cholecalciferol. International Units are often used to express vitamin D levels on packaging whereby 1 microgram is equal to 40 IU of vitamin D. Thus 10 micrograms would be equal to 400 IU which is commonly used in supplements and provides 100 percent of the Daily Value (DV).

How Much Vitamin D Do We Need?

The RDA for adults 50 years of age and younger as well as pregnant women is 5 micrograms (200 IU) of vitamin D daily. One microgram is the equivalent of 40 IU of vitamin D. For adults over the age of 50 and 70 the RDA increases to 10 and 15 micrograms (400 IU) daily. See Table 3.2 for recommended levels for children and teens.

How Much Sunlight Is Required to Make Vitamin D?

People with lighter skin color require as little as 10 minutes of sun exposure to make adequate amounts of vitamin D. However this requires direct sunlight exposure to skin during midday. However, sunscreen with SPF 8 or higher significantly reduces the process. Also the necessary

Table 9.10 Vitamin D Content of Select Foods

Food

Vitamin D (^g)

Food

Vitamin D (^g)

Milk

Fish and seafood

Milk (1 cup)

2.5

Salmon (3 ounces)

8.5

Meats

Tuna (3 ounces)

3.7

Beef liver (3 ounces)

1.0

Shrimp (3 ounces)

3.1

Eggs

Egg (1)

0.7

exposure is increased for people with darker skin color and in a manner relative to the degree of darkness. This also means that a person will make less and less vitamin D as they tan longer or over several days, such as vacationing at the beach. This helps to protect people from potentially making too much vitamin D. Interestingly, the ability to make vitamin D appears to be stronger during youth and decreases as humans get older. For this reason, the need for vitamin D from food and or supplements increases as we get older (50+).

What Processes Are Involved in Making Vitamin D?

The process of making vitamin D can be simplified to three primary locations within the body. First, within the skin a derivative of cholesterol called 7-dehydrocholesterol is converted to another substance called cholecalciferol. In order for this to occur, 7-dehydrocholesterol must be exposed to ultraviolet radiation from the sun or other sources such as tanning beds. As mentioned, the efficiency of this conversion appears to decrease as ultraviolet light exposure time increases.

Once cholecalciferol has been produced it leaves the skin and circulates in the blood with the help of a transport protein called vitamin D binding protein (DBP). In the cholecalciferol form, vitamin D is only minimally active in the body. The activity of vitamin D depends on its ability to be recognized by vitamin D receptors in specific cells. In order for cholecalciferol to become more attractive to the vitamin D receptor, it must undergo more changes in its molecular design. The first change takes place in the liver as circulating cholecalciferol is removed and modified to become 25-hydroxycholecalciferol. This form of vitamin D is released by the liver and re-enters the blood. This form of vitamin D is a little more attractive to vitamin D receptors and some of the effects of vitamin D are realized. 25-Dihydroxycholecalciferol can circulate to the kidneys and be modified further to the most potent form of vitamin D called 1,25-dihydroxycholecalciferol or calcitriol, which is released back into circulation (Figure 9.4). In this form, vitamin D is exceptionally attractive to vitamin D receptors strategically located within certain cells in the body.

Does the Vitamin D in Foods Need to Be Processed in the Body?

The vitamin D in foods is fairly well absorbed across the wall of the small intestine. Because this form of vitamin D is fat soluble (water insoluble), it will require the same digestive and absorptive assistance as other lipid substances. This includes the presence of bile and the incorporation into chylomicrons. This vitamin D will eventually make it to the liver and must also undergo the same modifications in the liver and kidney as did the vitamin D made from cholesterol in the skin.

What Does Vitamin D Do in the Body?

In order for a cell to be influenced by vitamin D it must possess the vitamin D receptor which is located in the nucleus. This further strengthens the argument that vitamin D is more like a hormone than a vitamin. Remember that a hormone must bind with a specific receptor in order to be active. While researchers continue to discover vitamin D receptors in various tissues throughout the body, most of the attention has centered on the bone, kidneys, and intestines. Vitamin D is classically recognized as being principally involved in bone and calcium metabolism although newer functions of vitamin D are being added to the list.

Vitamin D functions include:

• Calcium balance—Vitamin D is principally involved in maintaining blood calcium levels. About 99 percent of the body's calcium is found in bone, it serves as a reservoir for blood calcium, the concentration of which is tightly regulated. When blood calcium levels begin to fall below normal levels, the parathyroid gland releases parathyroid hormone (PTH) into circulation. PTH is dedicated to re-establishing normal blood calcium levels. One of its activities is to increase the conversion of vitamin D in the kidneys to its most active form. Vitamin D can then work to promote an increase in calcium absorption from the digestive tract and to also decrease the amount of calcium lost from the body in urine. Researchers believe that vitamin D promotes the production and activity of proteins that help transport calcium across the wall of the small intestine.

Vitamin D is crucial to bone health by increasing calcium absorption as well reducing calcium loss in the urine.

  • Normal cell development—All cells are derived from reproduction of existing cells. This occurs daily and is ramped up during growth, pregnancy and wound healing. However these cells are immature and lack the final, specialized design and function to do the job they are intended to do. Vitamin D is pivotal in the proper development of cells to their mature and productive form.
  • Immunity—The active form of vitamin D is a potent stimulator of the immune system. In fact, several cells involved in immune responses including T cells have vitamin D receptors. In addition, other cells involved in immune functions produce the enzyme necessary for conversion of vitamin D to its most active form.

Figure 9.4 Sunlight (ultraviolet light) can convert a cholesterol derivative (7-dehydrocholesterol) to cholecalciferol. Cholecalcifierol (from food or sunlight exposure) can be converted to active vitamin D by conversion in our liver and then kidneys. Vitamin D will increase available calcium in our body by increasing absorption from our diet and decreasing urine losses. Also vitamin D can promote the mobilization of calcium from our bone, which can become significant when dietary calcium is lacking.

Figure 9.4 Sunlight (ultraviolet light) can convert a cholesterol derivative (7-dehydrocholesterol) to cholecalciferol. Cholecalcifierol (from food or sunlight exposure) can be converted to active vitamin D by conversion in our liver and then kidneys. Vitamin D will increase available calcium in our body by increasing absorption from our diet and decreasing urine losses. Also vitamin D can promote the mobilization of calcium from our bone, which can become significant when dietary calcium is lacking.

What Happens If Too Little Vitamin D Is Consumed?

Deficiency of vitamin D can occur when a combination of factors is present. If vitamin D intake and/or absorption is low and an individual does not receive adequate exposure to sunlight, the potential for a vitamin D deficiency is present. Depending on the stage in life, vitamin D deficiency can results in:

  • Rickets—In children, vitamin D deficiency results in rickets, a condition wherein bones are not properly formed and mineralized. Thin, pliable bones of the legs bow under the weight of a child's body. Bowed legs are often accompanied by an enlarged head, rib cage, and joints, which are considered the classic signs of rickets. It is important to remember that milk, whether it is from a human or from another mammal (such as cow or goat), is not a naturally rich source of vitamin D. However, most milk bought in a store is fortified with vitamin D thereby making it a good food source. Infants will be at greater risk of developing vitamin D deficiency if they do not receive periodical exposure to the sun and/or infant foods or a supplement containing vitamin D.
  • Osteomalacia—The adult form of rickets is medically referred to as osteomalacia. This name literally means "bad bones." In osteomalacia bones gradually lose their mineral content, become less dense and physically weaker, and are more susceptible to fracture. The underlying cause of osteomalacia may be related directly to a lack of dietary vitamin D as well as a lack of exposure to sunlight. Or it may be related to internal disease in a vitamin D-metabolizing organ, such as the liver and/or kidneys, or organs involved in digestion and absorption of vitamin D, such as the pancreas, gallbladder, liver, and small intestine. Osteomalacia and a seemingly similar disorder (osteoporosis) are discussed in Chapter 12.

Can Too Much Vitamin D Be Consumed?

Of the vitamins, vitamin D has one of the lowest levels of intakes above recommendations that could give rise to side effects. Many of the manifestations appear to be related to vitamin D's calcium absorption, which in turn results in too much calcium in the blood. Prolonged hyper-calcemia (elevated blood calcium) can affect muscle cell activity, which includes the heart, and can induce nausea, vomiting, mental confusion, and lead to calcium deposition in various tissues throughout the body. While the Tolerable Upper Limit has been set at five times the AI for adults, more recent research suggests that the threshold for potential side effects of excessive intake could indeed be at double that level.

Luckily, as exposure to sunlight increases the body's ability to make vitamin D decreases. In addition, as the level of active vitamin D increases, kidney cells produce less and less of the converting enzyme needed to make more active vitamin D. These mechanisms attempt to decrease the potential for toxicity. That's because we may be more sensitive to vitamin D toxicity than other vitamins when looking at the intake level associated with signs and symptoms.

Vitamin E

What Is Vitamin E?

Similar to many other vitamins, vitamin E is not necessarily a single molecule but is a class of similar molecules accomplishing related activities. There are about eight or so vitamin E molecules that can be subdivided into two major classes, tocopherols and tocotrienols, which themselves can be subdivided and given the Greek descriptors a, P, 8, or y).

What Foods Are Good Sources of Vitamin E?

Good food sources of vitamin E include plant oils, margarine, and some fruits and vegetables, such as peaches and asparagus (Table 9.11). The average adult intake of vitamin E approximates the RDA, which is 15 a-TE daily (see below for an explanation of TE). More common supplement forms of vitamin E include a-tocopherol succinate and a-tocopherol acetate. In addition, a-tocopherol phosphate, which has the same nutritional value of the succinate and acetate forms, is also available, as are mixed tocopherols and Y-tocopherol versions. Furthermore, a-tocopherol supplements from natural sources (often labeled dl-a-tocopherol) will have more of the usable form of vitamin E than synthetic vitamin E, which can contain forms of a-tocopherol that our body can't use.

Table 9.11 Vitamin E (a-TE) Content of Select Foods

Food

Vitamin E

Food

Vitamin E

(mg)

(mg)

Oils/fats

Vegetables

Oil (1 tablespoon)

6.7

Sweet potato (% cup)

6.9

Mayonnaise (1 tablespoon)

3.4

Collard greens (% cup)

3.1

Margarine (1 tablespoon)

2.7

Asparagus (% cup)

2.1

Nuts and seeds

Spinach, raw (1 cup)

1.5

Sunflower seeds (% cup)

27.1

Grains

Almonds (% cup)

12.7

Wheat germ

2.1

Peanuts (% cup)

4.9

(1 tablespoon)

Cashews (% cup)

0.7

Bread, whole wheat

2.4

Seafood

(1 slice)

Crab (3 ounces)

4.5

Bread, white (1 slice)

1.2

Shrimp (3 ounces)

3.7

Fish (3 ounces)

2.4

a-TE = a-tocopherol equivalents.

a-TE = a-tocopherol equivalents.

How Is Vitamin E Handled in the Body?

Vitamin E shows a fair absorption (25 to 50 percent) from the small intestine. Factors such as an increased need or low stores of vitamin E may certainly increase the absorption percentage. Like other fat-soluble vitamins, vitamin E needs the assistance of lipid digestive and absorptive processes (for example, chylomicrons). Much of the absorbed vitamin E will end up in the liver as chylomicron remnants are removed from the blood. The liver can then add vitamin E to VLDLs, which are then released into circulation where they can be delivered to most cells. By and large this is vitamin E in the form of a-tocopherol which means that it is the most significant form found in the blood as well as throughout our body.

Vitamin E is a powerful antioxidant and most of its activity in the body comes from a-tocopherol.

Because vitamin E is not very water soluble, very little is lost in urine; however, large intakes of vitamin E will result in a proportionate increase in urinary losses. The primary means for vitamin E loss from the body appears to be through the feces. The liver incorporates vitamin E into bile, which is dumped into the digestive tract. Some of this vitamin E, along with vitamin E from dietary sources, is not absorbed and becomes part of feces.

What Does Vitamin E Do in the Body?

By and large vitamin E functions as an antioxidant protecting cells from free radicals and most of its activity is attributable to a-tocopherol. As vitamin E is a lipid-soluble molecule it is logical to think that vitamin E would be most active in lipid-rich areas of our cells. This appears to be the case, as vitamin E's antioxidant activities are recognized mostly in regard to protecting the lipid-rich cell membranes. Cell membranes contain a tremendous amount of phospholipids, each of which contain two fatty acids. Furthermore, double bonds within some of these fatty acids seem to be very vulnerable to free-radical attack. Vitamin E appears to protect fatty acids by donating one of its own electrons to a free radical. This pacifies the free radical and also spares the fatty acids in cell membranes.

Since lipoproteins provide a primary means of shuttling vitamin E throughout the body, researchers have speculated that vitamin E may be involved in the prevention of heart disease. Some evidence suggests that vitamin E helps protect LDL from oxidation. Oxidized LDL is believed to be a strong risk factor for atherosclerosis. This is discussed in more detail in Chapter 13.

How Much Vitamin E Do We Need Daily?

The RDA for men and women of all ages is 15 milligrams (or 22.5 IU) of vitamin E daily. This is also the recommended level during pregnancy while the RDA is increased to 19 milligrams during lactation. See Table 3.2 for recommended levels for children and teens.

What Are a-Tocopherol Equivalents ?

Among the vitamin E molecules, a-tocopherol is the most prevalent, popular, and probably potent in the body. For this reason the RDA for vitamin E is provided in a-tocopherol equivalents (a-TE). Here, 1 a-TE unit has the activity of 1 milligram of a-tocopherol. Since other forms of vitamin E are not as potent, the a-TE unit amount bestowed to a food is based on the amount of a-tocopherol as well as the potential vitamin E activity contributions made by the other forms. For example, if a food contained 25 milligrams of a-tocopherol and 50 milligrams of another form of vitamin E which is only 50 percent as potent as a-tocopherol, the food is said to contain 50 a-TE (25 milligrams of a-tocopherol + 25 milligrams [50 percent of 50 milligrams] of other vitamin E form).

What Happens If Too Little Vitamin E Is Consumed?

Vitamin E deficiency is somewhat rare in adults with the exception of those who have medical conditions that impact the normal digestion of lipids. Any situation in which normal fat digestion and absorption are hindered can ultimately reduce the amount of vitamin E absorbed from the digestive tract. A deficiency may take many months or years to show itself through medical symptoms such as red blood cell fragility and neurological abnormalities. Usually the medical condition is treated long before vitamin E deficiency signs are recognized. However, children with cystic fibrosis are a special concern, as the pancreas produces inadequate amounts of digestive enzymes in those with this disease.

Can Vitamin E Become Toxic?

Compared with the fat-soluble vitamins discussed so far, vitamin E is relatively nontoxic. However, studies on people eating fifty to one hundred times the RDA have demonstrated that these amounts can result in nausea, diarrhea, and headaches, while some individuals complained of general weakness and fatigue. It should be recognized that excessive vitamin E supplementation may interfere with vitamin K's activity in blood clotting.

230 Vitamins Are Vital Molecules in Food Vitamin K

What Is Vitamin K?

Vitamin K is a general name for a few related compounds that possess vitamin K activity. Phylloquinone is the form of vitamin K found naturally in plants; menaquinones are the form of vitamin K derived from bacteria; and menadione, which is not natural, is the synthetic (laboratory derived) form of vitamin K.

What Foods Provide Vitamin K?

Humans receive vitamin K not only from various foods but also from bacteria in the colon. Good sources of vitamin K include broccoli, spinach, cabbage, Brussels sprouts, turnip greens, cauliflower, beef liver, and asparagus. Foods lower in vitamin K such as cheeses, eggs, corn oil, sunflower oil, and butter also make a respectable contribution to our vitamin K intake because of their frequency of consumption.

How Much Vitamin K Do We Need?

The RDA for men and women is 120 and 90 micrograms of vitamin K daily. The RDA for pregnant and lactating women over the age of 18 is the same as non-pregnant adult women, however, for pregnant females 18 and younger the recommendation is only 75 micrograms daily. See Table 3.2 for recommended levels for children and teens.

It has been estimated that as much as one-half of the vitamin K absorbed from the digestive tract was originally made by intestinal bacteria. Being a fat-soluble substance, vitamin K relies somewhat upon the activities of normal lipid digestion for optimal absorption. Vitamin K must also be transported from the intestines by way of chylomicrons, which ultimately reach the liver. Once in the liver, vitamin K can be packaged into VLDL and carried throughout the body.

What Does Vitamin K Do in the Body?

For years the only recognized activity of vitamin K was its involvement in proper normal blood clotting. In fact, rumor has it that vitamin K was so named by Danish researchers with respect to blood coagulation, a word spelled with a "K" in Danish. The liver is responsible for making the proteins, or clotting factors, that circulate in the blood. These proteins are activated when there is a hemorrhage and allow blood to clot at that site.

When clotting factors are initially made by liver cells, but before they are released into circulation, several of these proteins are modified by vitamin K. The modification occurs only in few amino acids; however, it changes the design and function of the proteins significantly. With this slightly modified design, these and other clotting factors are released into circulation. Once in circulation, these proteins await the signal to initiate clot formation. The signal is a tear in a blood vessel wall producing a hemorrhage. In light of vitamin K's involvement with blood clotting, the vitamin K status of a patient is typically determined prior to any surgical procedure.

Vitamin K also seems to be active in other tissue besides the liver. In bone, muscles, and kidneys, vitamin K appears to be necessary for activities similar to those in the liver. At least two proteins in bone and one in the kidneys have been identified as needing modification by vitamin K to function properly.

Can Too Little or Too Much Vitamin K Be Consumed?

Unlike other fat-soluble vitamins, vitamin K is not stored very well in the body and appreciable amounts are lost in urine and feces every day. This certainly presents the opportunity for a more rapid onset to deficiency. However, since vitamin K is abundant in the human diet and vitamin K is produced by bacteria in the digestive tract, vitamin K deficiency is uncommon in adults. The typical American adult may eat five to six times the RDA daily.

Opportunities for vitamin K deficiency do arise during infancy. There does not seem to be an appreciable transfer of vitamin K from the mother to the infant prior to birth. Thus newborns enter the world with very limited stores of vitamin K. Furthermore, a newborn's digestive tract is sterile and will not develop a mature bacterial population for a couple of months. Further, maternal breast milk is not a good source of vitamin K. All of these factors place infants at greater risk for developing vitamin K deficiency, which can lead to poor blood clotting and hemorrhage, among other considerations. With these concerns in mind, newborns are commonly provided with vitamin K shortly after birth.

One other situation may raise concern regarding the development of a vitamin K deficiency. People using antibiotics for long periods of time are at a greater risk for vitamin K deficiency. Certain antibiotics can reduce the number of vitamin K-producing bacteria from the colon which puts someone at a greater risk of deficiency, especially if a person eats a low vitamin K diet and/or is experiencing problems with lipid digestion. But the combination of these factors is indeed rare.

Vitamin K is relatively nontoxic in natural forms; however, there have been situations of toxicity from chronic use of excessive vitamin K in the synthetic menadione form.

FAQ Highlight

0 0

Post a comment