Vitamin B6 is a water-soluble vitamin that is naturally present in many foods, added to others, and available as a dietary supplement. It is the generic name for six compounds (vitamers) with vitamin B6 activity: pyridoxine, an alcohol; pyridoxal, an aldehyde; and pyridoxamine, which contains an amino group; and their respective 5’-phosphate esters. Pyridoxal 5’ phosphate (PLP) and pyridoxamine 5’ phosphate (PMP) are the active coenzyme forms of vitamin B6 [1,2]. Substantial proportions of the naturally occurring pyridoxine in fruits, vegetables, and grains exist in glycosylated forms that exhibit reduced bioavailability .
Vitamin B6 in coenzyme forms performs a wide variety of functions in the body and is extremely versatile, with involvement in more than 100 enzyme reactions, mostly concerned with protein metabolism . Both PLP and PMP are involved in amino acid metabolism, and PLP is also involved in the metabolism of one-carbon units, carbohydrates, and lipids . Vitamin B6 also plays a role in cognitive development through the biosynthesis of neurotransmitters and in maintaining normal levels of homocysteine, an amino acid in the blood . Vitamin B6 is involved in gluconeogenesis and glycogenolysis, immune function (for example, it promotes lymphocyte and interleukin-2 production), and hemoglobin formation .
The human body absorbs vitamin B6 in the jejunum. Phosphorylated forms of the vitamin are dephosphorylated, and the pool of free vitamin B6 is absorbed by passive diffusion .
Vitamin B6 concentrations can be measured directly by assessing concentrations of PLP; other vitamers; or total vitamin B6 in plasma, erythrocytes, or urine . Vitamin B6 concentrations can also be measured indirectly by assessing either erythrocyte aminotransferase saturation by PLP or tryptophan metabolites. Plasma PLP is the most common measure of vitamin B6 status.
PLP concentrations of more than 30 nmol/L have been traditional indicators of adequate vitamin B6 status in adults . However, the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies (formerly National Academy of Sciences) used a plasma PLP level of 20 nmol/L as the major indicator of adequacy to calculate the Recommended Dietary Allowances (RDAs) for adults [1,3].
Sources of Vitamin B6
Vitamin B6 is found in a wide variety of foods [1,3,4]. 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 [1,3,5]. About 75% of vitamin B6 from a mixed diet is bioavailable .
The table of selected food sources of vitamin B6 suggests many dietary sources of vitamin B6.
|Table 2: Selected Food Sources of Vitamin B6 |
|Food||Milligrams (mg) per serving||Percent DV*|
|Chickpeas, canned, 1 cup||1.1||55|
|Beef liver, pan fried, 3 ounces||0.9||45|
|Tuna, yellowfin, fresh, cooked, 3 ounces||0.9||45|
|Salmon, sockeye, cooked, 3 ounces||0.6||30|
|Chicken breast, roasted, 3 ounces||0.5||25|
|Breakfast cereals, fortified with 25% of the DV for vitamin B6||0.5||25|
|Potatoes, boiled, 1 cup||0.4||20|
|Turkey, meat only, roasted, 3 ounces||0.4||20|
|Banana, 1 medium||0.4||20|
|Marinara (spaghetti) sauce, ready to serve, 1 cup||0.4||20|
|Ground beef, patty, 85% lean, broiled, 3 ounces||0.3||15|
|Waffles, plain, ready to heat, toasted, 1 waffle||0.3||15|
|Bulgur, cooked, 1 cup||0.2||10|
|Cottage cheese, 1% low-fat, 1 cup||0.2||10|
|Squash, winter, baked, ½ cup||0.2||10|
|Rice, white, long-grain, enriched, cooked, 1 cup||0.1||5|
|Nuts, mixed, dry-roasted, 1 ounce||0.1||5|
|Raisins, seedless, ½ cup||0.1||5|
|Onions, chopped, ½ cup||0.1||5|
|Spinach, frozen, chopped, boiled, ½ cup||0.1||5|
|Tofu, raw, firm, prepared with calcium sulfate, ½ cup||0.1||5|
|Watermelon, raw, 1 cup||0.1||5|
*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet. The DV for vitamin B6 is 2 mg for adults and children age 4 and older. However, the FDA does not require food labels to list vitamin B6 content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient.
The U.S. Department of Agriculture’s (USDA’s) Nutrient Database lists the nutrient content of many foods and provides a comprehensive list of foods containing vitamin B6 arranged by nutrient content and by food name.