Vitamin B

What Is Vitamin B6?

Vitamin B6 is the general term for the six compounds, namely pyridoxal (PL), pyridoxine (PN), pyridoxamine (PM), and their phosphate derivatives including pyridoxal 5'-phosphate (PLP), pyridoxine 5'-phosphate (PNP), and pyridoxamine 5'-phosphate (PMP). It is the PLP form that is the most significant in human operations. Although long recognized for its pivotal role in the processing of amino acids, vitamin B6 has received attention for its role in homocysteine metabolism and in reducing cardiovascular disease risk.

What Foods Provide Vitamin B6 and What Form Is Found in Supplements?

One form of vitamin B6 is pyridoxine, which is mostly found in plant foods, with good sources being bananas, navy beans, and walnuts. The remaining four forms of vitamin B6 are found mostly in animal foods with good sources being meats, fish, and poultry (Table 9.6). Vitamin B6 is fairly well absorbed from the small intestine but vitamin B6 from animal sources may be better absorbed than B6 from plant sources. In addition, vitamin B6 is fairly stable in cooking processes; however, some losses are experienced with prolonged exposure to heat, light, or alkaline conditions. Vitamin B6 is available primarily as pyridoxine hydro-chloride in multivitamin, vitamin B-complex, and vitamin B6 supplements.

Vitamin B6 is crucial for amino acid metabolism and how much a person needs can be based on their protein intake.

Table 9.6 Vitamin B6 Content of Select Foods

Foods

Vitamin B6

Foods

Vitamin B6

(mg)

(mg)

Meats

Legumes

Liver (3 ounces)

0.8

Split peas (% cup)

0.6

Salmon (3 ounces)

0.7

Beans, cooked (% cup)

0.4

Chicken (3 ounces)

0.4

Fruits

Ham (3 ounces)

0.4

Banana (1)

0.6

Hamburger (3 ounces)

0.4

Avocado (%)

0.4

Veal (3 ounces)

0.4

Watermelon (1 cup)

0.3

Pork (3 ounces)

0.3

Vegetables

Beef (3 ounces)

0.2

Brussels sprouts (% cup)

0.4

Eggs

Potato (1)

0.2

Egg (1)

0.3

Sweet potato (% cup)

0.2

Carrots (% cup)

0.2

Peas (% cup)

0.1

What Does Vitamin B6 Do in the Body?

Vitamin B6 can be found in nearly if not all cells throughout the body with higher concentrations found in muscle and liver tissue. Similar to most of its water-soluble vitamin siblings, vitamin B6 is primarily lost from the body in urine. Once inside the cells, vitamin B6 forms can be converted to the active forms of vitamin B6, PLP (pyridoxal phosphate) and PMP (pyridoxamine phosphate). PLP and PMP are key participants in many cell reactions. By and large the most significant roles of vitamin B6 are as follows.

  • Amino acid metabolism—Vitamin B6 is crucial for the processing of amino acids including the production of nonessential amino acids made from other amino acids. During this process, the nitrogen-containing amine portion of an amino acid is transferred to a specific molecule (Figure 9.2), which creates a nonessential amino acid. In fact, if an individual developed a vitamin B6 deficiency, most of the nonessential amino acids would actually become dietary essentials.
  • Glycogen breakdown—Glycogen breakdown in muscle requires vitamin B6. Glycogen is stored glucose and the breakdown of this complex provides invaluable fuel during exercise and work.
  • Neurotransmitter production—Vitamin B6 is also necessary to convert certain amino acids into the neurotransmitters gamma-amino-butyric acid (GABA) and serotonin.
  • Hemoglobin—Vitamin B6 is crucial for the normal production of hemoglobin, the oxygen-carrying protein found in RBCs.
  • Immunity—In addition, vitamin B6 is essential in the formation of hemoglobin and white blood cells. Finally, vitamin B6 also seems to be necessary to break down glycogen stores during exercise and fasting.
Figure 9.2 Some nonessential amino acids can be made by transferring the nitrogen contain amine group from an existing amino acid to another molecule thereby creating a nonessential amino acid.

What Are the Recommended Intake Levels of Vitamin B6?

The adult RDA is 1.3 milligrams of vitamin B6 for women and men 18 to 50 years of age. After 50 the recommendation increases to 1.5 and 1.7 milligrams daily for women and men respectively. During pregnancy and lactation the recommendation increases for women to 1.9 and 2.0 milligrams of vitamin B6 respectively. The bottom line is that the metabolism of every amino acid at some point or another will encounter a chemical reaction requiring vitamin B6 as a coenzyme. In fact, vitamin B6 is so deeply rooted in the metabolism of amino acids that the RDA is based on the typical protein content of the American diet. Approximately 0.016 milligrams of vitamin B6 is apportioned per gram of protein in our diet. Therefore, since the typical daily protein intake of an American adult has been estimated to be 100 to 125 grams of protein, this translates to about 1.6 to 2 milligrams of vitamin B6. For athletes consuming more protein and with higher glycogen stores, more vitamin B6 is warranted and accounted for if vitamin B6 intake is based on grams of protein intake. See Table 3.2 for recommended levels of vitamin B6 for people of all ages.

What Happens If Too Little Vitamin B6 Is Consumed?

Deficiency of vitamin B6 is unlikely due to the popularity of meat, fish, and poultry as components of the American diet. However, if a deficiency occurred, amino acid metabolism would be greatly restrained, leading to poor protein synthesis. The production of hemoglobin, white blood cells, and many neurotransmitters would also be greatly hindered. Therefore the signs of a vitamin B6 deficiency would significantly affect human body functions at many levels, including growth, immunity, and reproduction.

Can Vitamin B6 Be Toxic?

The Tolerable Upper Limit has been set at 100 milligrams daily for both men and women with lower levels for children and during pregnancy and early lactation. If vitamin B6 is consumed in gram doses (2 to 6 grams) over many months, it can affect nervous function and possibly lead to irreversible damage to nervous tissue. At one time, vitamin B6 was considered a possible treatment for premenstrual syndrome (PMS), but this concept has since been abandoned and should not be pursued due to lack of promising supportive research and the potential for toxicity.

212 Vitamins Are Vital Molecules in Food Folate (Folic Acid)

What Is Folate?

The name folate, as well as the other names associated with this vitamin (folacin and folic acid), suggests its food sources. Folium is Latin for foliage or forage.

What Foods in the Diet Contribute Folate?

As its name suggests, good food sources of folate include green, leafy vegetables such as spinach, turnip greens, and asparagus (Table 9.7). Other vegetables and many fruits, juices, and organ meats also are good contributors of folate. Folate's molecular structure is somewhat unstable when it is heated, making fresh, uncooked fruits and vegetables better sources than cooked foods. The RDA for adults is 400 micrograms of folate daily.

What Does Folate Do in the Body?

Earlier we mentioned that when most molecules are made in the body they are constructed from smaller molecules or parts of other molecules. Folate, functioning as a coenzyme, is dedicated to transferring small, single carbon atom-containing molecules to the processes involved in making some pretty special molecules (Figure 9.3). Key roles for folate include:

• DNA production—Before cells can reproduce they must make a copy of their DNA. The necessity of folate is particularly realized in cells that rapidly reproduce. This includes cells associated with the body

Table 9.7 Folate Content of Select Foods

Food

Folate

Food

Folate

(vg)

(vg)

Vegetables

Fruits

Asparagus (V cup)

120

Cantaloupe (%)

100

Brussels sprouts (V cup)

116

Orange juice (1 cup)

87

Black-eyed peas (V cup)

102

Orange (1)

59

Spinach, cooked (V cup)

99

Grains

Lettuce, romaine (1 cup)

86

Oatmeal (V cup)

97

Lima beans (V cup)

71

Wild rice (V cup)

37

Peas (V cup)

70

Wheat germ (1 tablespoon)

20

Sweet potato (V cup)

43

Broccoli (V cup)

43

surfaces (skin, hair, and digestive, urinary, and reproductive tracts) as well as blood cells and certain liver cells. Cells of these tissues must constantly be replaced or turned over to guarantee proper function and integrity. However, in order for these cells to reproduce they must first make a duplicate copy of their DNA so that when the cell divides into two cells, both will get a complete set of DNA.

• Amino acid metabolism—Folate is also involved in transferring single-carbon molecules in the metabolism of certain amino acids as well. For instance, folate helps convert homocysteine to methionine.

Homocysteine

Homocysteine

Figure 9.3 Folate passes a single carbon building block to the construction of various molecules such as nucleic acids. In the process folate is converted to an nonreusable form. Vitamin B12 can convert folate back to an active form.

• Homocysteine metabolism—Recently a link has been made between homocysteine levels and heart disease. When folate transfers a carbon molecule to homocysteine it is converted to methionine (see Figure 9.3). The conversion requires the help of vitamin B12 as well. Therefore, a deficiency of folate and/or vitamin B12 can allow for homocysteine levels to become elevated. Vitamin B6 is also important because it helps folate pick up the carbon unit that will be added to homocysteine to form methionine.

Folate is needed for DNA production as well as the metabolism of homocysteine, which has been linked to heart disease.

How Important Is Folate During Pregnancy?

Because folate is fundamentally involved in DNA production and thus the reproduction of cells, periods of life when rapid growth occurs demand a higher folate intake. During pregnancy a woman's diet must include extra folate to assist in the rapid reproduction of cells of the unborn infant and herself (for example, blood cells, placenta). Chapter 12 provides more details to what can happen if folate status is inadequate during pregnancy. Most prenatal vitamin supplements include folate to help meet a pregnant woman's increased needs.

What Happens If Too Little Folate Is Consumed?

Folate deficiency can result in several problems including anemia. Red blood cells (RBCs) have a life span of about 4 months and are constantly reproducing (two million RBCs per second) in bone marrow to compensate for their normal destruction. Although RBCs do not contain a nucleus (with its DNA), there is a time in its development when each new RBC is created from the division of another cell. Before that cell divided into two new cells, it needed to copy its DNA. During folate deficiency, the original cell cannot properly copy its DNA because folate is not present to help construct the building blocks of DNA. This results in the development of large and immature RBCs, which then enter the blood and are readily noticeable with a microscope. Furthermore, fewer and fewer normal RBCs are produced, resulting in anemia. Anemia is a significant reduction in the level of hemoglobin in the blood. Remember: hemoglobin is found in RBCs, so a reduction in RBC concentration in our blood results in less hemoglobin. The anemia that results from folate deficiency is clinically referred to as macrocytic megaloblastic anemia. Macrocytic means big cell and megaloblast is the name for the pre-RBC form, which still has its nucleus. These changes in RBCs can be observed as early as a few months after consuming a folate-deficient diet.

Can Folate Be Toxic?

Folate toxicity is rare for two principal reasons. First, it is difficult to consume too much folate through normal consumption of foods. Second, the folate content of nutrition supplements is limited by the government. The limitation in supplements is due to an overlap between folate metabolism and vitamin B12 function. Vitamin B12 is fundamentally involved in folate metabolism in cells as it keeps folate in a form that can be used over and over again in cells (folate recycling). This means that a deficiency in vitamin B12 can in turn decrease folate recycling, resulting in the development of the anemia mentioned previously. Therefore, signs of a folate deficiency can actually help physicians identify a vitamin B12 deficiency. By taking higher dosages of folate (supplements) we can overcome the need for vitamin B12 in the recycling of existing folate in the cells. This is good for folate, but the vitamin B12 deficiency still remains and may go undetected. Thus, folate supplementation has eliminated an early warning sign (anemia) of vitamin B12 deficiency. If the vitamin B12 deficiency progresses it can lead to paralysis and death.

Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Post a comment