10.7 Vitamin B6

Vitamin B6

Vitamin B₆ is one of two B vitamins that isn’t usually referred to by its chemical name. This is due to the fact that it is composed of three compounds instead of just one: pyridoxine, pyridoxal, and pyridoxamine. Like all B vitamins, it is water-soluble.

All three forms can be activated by being phosphorylated (having a phosphate group added). In animal products, vitamin B6 is found in its phosphorylated forms, pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP). PMP is less common than PLP, however. In plants, vitamin B6 is primarily found as pyridoxine, with up to 75% being pyridoxine glucoside, which is believed to be the plant storage form1.

Vitamin B₆ is found in a wide variety of foods. The richest sources of vitamin B₆ include fish, beef liver and other organ meats, potatoes and other starchy vegetables, and fruit (other than citrus). In the United States, adults obtain most of their dietary vitamin B₆ from fortified cereals, beef, poultry, starchy vegetables, and some non-citrus fruits. About 75% of vitamin B₆ from a mixed diet is bioavailable2.

Vitamin B₆ is well absorbed from foods (~75%) through passive diffusion. PLP and PMP from animal products are dephosphorylated before uptake into the enterocyte. The pyridoxamine glucoside from plants is cleaved to form free pyridoxine, but some pyridoxine glucoside is absorbed intact. Pyridoxine glucoside absorption is lower (~50%) than pyridoxine alone. The primary circulating forms of vitamin B₆ are pyridoxal and PLP.

Vitamin B₆ is primarily excreted in the urine, and like many other B vitamins, vitamin B₆ is destroyed during cooking or heating1.

For more information on Vitamin B₆, see the Required Web Link below.

Required Web Link:Vitamin B6 Dietary Supplement Fact Sheet

Subsections:

• 10.71 Vitamin B₆ Functions
• 10.72 Vitamin B₆ Deficiency & Toxicity

References & Links

• Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
• Vitamin B6 Dietary Supplement Fact Sheet. https://ods.od.nih.gov/factsheets/VitaminB6- HealthProfessional/

Vitamin B6 Functions

PLP is a cofactor for over 100 different enzymes, most are involved in amino acid metabolism. In fact, without PLP, all amino acids would be essential amino acids because we would not be able to synthesize nonessential amino acids. Functions of PLP and PMP include1:

• Transaminases (enzymes required for transamination; Section 6.41, Chapter 6) require PLP or PMP.

• Some deaminases (enzymes required for deamination; Section 6.41, Chapter 6) require PLP.
• Glycogen phosphorylase (enzyme required for glycogenolysis; Section 6.22, Chapter 6) requires PLP.
• PLP is required for decarboxylase enzymes that are involved in the synthesis of the neurotransmitters GABA, serotonin, histamine, and dopamine.
• PLP is also required by the enzyme involved in heme synthesis. Heme will be discussed in more detail in the iron section.
• PLP is also used in one of the multiple reactions that occurs between kynurenine and niacin in its synthesis from tryptophan that we saw in Figure 10.56.
• In addition, PLP is also involved in:
• Carnitine Synthesis
• 1-Carbon Metabolism

References & Links

1. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.

Vitamin B6 Deficiency & Toxicity

Vitamin B₆ deficiency is rare, but symptoms include:

• Skin or scalp ailments (seborrheic dermatitis)
• Microcytic hypochromic anemia (small cells, low color)
• Convulsions
• Depression
• Confusion

Given what we know about the functions of vitamin B₆ most of these symptoms make sense.

The microcytic hypochromic anemia is a result of decreased heme synthesis. The neurological symptoms are due to the decreased production of neurotransmitters1.

Vitamin B₆, unlike many of the other B vitamins, can produce toxicity. High doses of vitamin B₆, taken for an extended period of time, can lead to neurological damage1. There are, however, some potential uses of vitamin B₆ supplementation. In these cases, it is important that the supplementation be done with consultation with a physician.

One of the conditions that people may take vitamin B₆ for is carpal tunnel syndrome. While the evidence is not conclusive, it appears that vitamin B₆ supplementation may be beneficial, and may be used alone, or in combination with other complementary treatments, before surgery is undertaken2,3.

Morning sickness that occurs early in pregnancy is another condition where vitamin B₆ supplementation is sometimes utilized. The evidence again is not clear on whether it is beneficial4,5, but the American College of Obstetricians and Gynecologists suggests that vitamin B₆ may be tried to treat nausea and vomiting during pregnancy6. In 2013, the FDA approved doxylamine-pyridoxine (Diclegis) for use in pregnancy7. It is not known exactly what causes morning sickness, but it is believed that lower circulating vitamin B₆ levels are associated with increased morning sickness severity8.

The last condition for which vitamin B₆ is commonly supplemented is premenstrual syndrome (PMS). However, a systematic literature review found that it is inconclusive whether vitamin B₆ supplementation is beneficial in managing PMS9.

References & Links

• Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
• Ryan-Harshman M, Aldoori W. (2007) Carpal tunnel syndrome and vitamin B6. Canadian Family Physician 53(7): 1161-1162.
• Aufiero E, Stitik T, Foye P, Chen B. (2004) Pyridoxine hydrochloride treatment of carpal tunnel syndrome: A review. Nutr Rev 62(3): 96-104.
• Koren G, Maltepe C. (2006) Preventing recurrence of severe morning sickness. Canadian Family Physician 52(12): 1545-1546.
• Tan P, Yow C, Omar S. (2009) A placebo-controlled trial of oral pyridoxine in hyperemesis gravidarum. Gynecol Obstet Invest 67(3): 151-157.
• http://www.acog.org/Patients/FAQs/Morning-Sickness-Nausea-and-Vomiting-of-Pregnancy
• Slaughter SR, Hearns-Stokes R, van der Vlugt T, Joffe HV. (2014) FDA approval of doxylamine- pyridoxine therapy for use in pregnancy. N Engl J Med. 370: 1081-1083.
• Wibowo N, Purwosunu Y, Sekizawa A, Farina A, Tambunan V, Bardosono S. (2012) Vitamin B6 supplementation in pregnant women with nausea and vomiting. Int J Gynaecol Obstet. 116: 206-210.
• Whelan A, Jurgens T, Naylor H. (2009) Herbs, vitamins and minerals in the treatment of premenstrual syndrome: A systematic review. The Canadian Journal of Clinical Pharmacology 16(3): e407-e429.