The importance of vitamin B6 is often overlooked in conventional medicine, except for cases of overt B6 deficiency which doctors believe to be rare.1 Although not widely recognized, poor vitamin B6 status may be relatively common in individuals eating a Standard American Diet.2 In the United States, a remarkably high number of adults—ninety percent of women and seventy-one percent of men—consume diets that are deficient in vitamin B6 using the Recommended Dietary Allowance (RDA) as a measure.3 Furthermore, according to recent national health data, many individuals have inadequate vitamin B6 status despite meeting the RDA of B6 from their diets.4
Individuals particularly at risk include women of reproductive age, especially those who are using or have used oral contraceptives, teenagers, and men and women over age sixty-five.4 Children and the elderly are more likely to have a deficiency of B6 than of any other B vitamin.1 It is important to emphasize that for many, simply consuming an intake of B6 that meets the RDA will not ensure blood levels of vitamin B6 in a healthy range.
The causes behind poor vitamin B6 status are varied, including exposure to pollutants and use of medications that deplete vitamin B6, increased needs during rapid periods of growth and development, and chronic digestive disorders.1 But a probable common cause for a poor status is a low intake of foods rich in highly bioavailable vitamin B6. This article will briefly summarize the biochemistry, metabolism, and dietary bioavailability of vitamin B6. We will explore the causes and effects of poor intakes and how a nourishing traditional diet can supply ample amounts of readily usable vitamin B6. Clinical applications for supplemental vitamin B6 will be explored, as well as the often unmet needs of expectant and lactating mothers.
FORMS OF B6 AND BIOAVAILABILITY
Vitamin B6 is a water-soluble vitamin that naturally exists in three main forms: pyridoxine (PN), pyridoxal (PL), and pyridoxamine (PM), often collectively termed “pyridoxine.” It is more accurate to use “vitamin B6” or “B6” as the generic term, as will be done in this article.5 The abbreviations PN, PL, and PM will be used when referring specifically to one of the three main forms.
The overall bioavailability of vitamin B6 from an omnivorous mixed diet averages 75 percent, with a range estimated between 60 and 90 percent.6 Vitamin B6 bioavailability is a function of the degree of entrapment in the nondigestible portion of the diet, the proportion of the glycosylated form—PNG—in the overall diet, and the body’s metabolic conversion of B6 to functional coenzyme forms.5,7 PNG, or pyridoxine β-glucoside, is found almost exclusively in plant foods, comprising 5-80 percent of the total B6 content of plant foods.8 PL and PM are found primarily in animal foods in non-conjugated forms (no glucosides) but usually with a phosphate group attached, then referred to as PLP and PMP.9 An omnivorous diet contains about 15 percent of B6 in the form of PNG, with the proportion of PNG in vegetarian diets higher, up to 50 percent of the total B6 content.8,10
Vitamin B6 is unstable to alkaline conditions, light, oxygen, and high temperatures. Therefore, much of the B6 originally present in foods can be lost in processing: prolonged or high heating, canning and pressure cooking, milling of grains into refined flour, sterilization and freezing. This is especially true for the less stable PL/PLP and PM/PMP in animal foods.5 Significant losses can also occur rapidly in milk when it is exposed to light, even diffuse daylight or fluorescent light.11
PNG in plant foods is the form most stable to food processing, but it is also the least bioavailable. Animal foods provide an overall higher bioavailability of vitamin B6—estimates range from 10 to 100 percent higher than plant foods—and will serve as superior sources of B6 when they are eaten raw or cooked at lower temperatures and served in their own juices. Cooking in an acid medium (such as vinegar, lemon or tomato-based sauce) can help decrease the heat degradation of B6.12
Losses from the cooking of meat can be considerable: 30-40 percent is destroyed from boiling (more is lost if the broth is not consumed), and between 15-50 percent from roasting or broiling. Vitamin B6 in fish seems to withstand the cooking processes better with losses of only between 10-20 percent, possibly due to reduced cooking times. Again, fish juices should be consumed with the flesh for best retention.
Most losses for cooked vegetables occur due to the leaching into the cooking liquid; this wasteful practice can increase the losses for vegetables from 5-15 percent to as high as 60 percent (the highest losses are attributed to the conversion of some of the PN in some vegetables to the more labile PL or PM).13 Baked vegetables fare very well, such as potatoes and sweet potatoes, which only lose about 5 percent of their vitamin B6.12 Canning of foods results in losses of 20-30 percent with higher reports for meat, fish, and dairy; the milling of wheat results in losses of 80-90 percent and baking an additional 17 percent.14 The drying of meats retains a large majority of the B6, according to one published report.15
Most published research concludes that milk does not lose a significant amount of vitamin B6 during the conventional pasteurization or homogenization processes, citing only about a 10 percent loss.14,16,17 However, in contrast to the more common microbiological assays (utilizing yeast) that have shown losses of B6 from pasteurization to be inconsequential, bioassays in rats have shown deleterious effects from milk pasteurization.
In the Randleigh Farm studies carried out between 1935 and 1940, rats were fed either pasteurized or unpasteurized milk alone. The rats fed pasteurized milk exhibited poor development and acrodynia compared to the rats that were fed raw milk and grew normally with healthy coats.50 Acrodynia is a pellagra-like skin disorder that causes hair loss in rats when their diets are deficient in B6. In fact, vitamin B6 in a yeast extract was first described as the active “rat pellagra prevention factor” by Paul György in 1934.18 The rat acrodynia cure test was later replaced by the rat growth test, and then by microbiological assays.19 Currently, microbiological assays and high-performance liquid chromatography remain in use for determining the vitamin B6 content of foods.5
There is disagreement on the applicability of results obtained from rat bioassays to humans, as unlike humans, rats can absorb vitamin B6 synthesized by gut bacteria.20 On the other hand, for reasons that are not completely clear, microbiological assays may not adequately assess B6 bioavailability in humans. This could be due to the interrelationships of vitamin B6 with other dietary factors such as heated proteins, along with the impact of soluble sugars on intestinal flora.21 In any case, it is widely acknowledged that the heat sterilization of milk results in B6 losses of one-half to two-thirds, with additional losses during the subsequent storage period.14,21 During the 1950s, several infants developed convulsions after being fed a commercially sterilized infant formula. The convulsions were resolved by the administration of vitamin B6.21 It is very important to avoid consuming evaporated milk and UHT, or ultra-high temperature processed milk as these products are sterilized and often have an extended shelf time.
PLANT SOURCES OF B6
Researchers found that women who consumed mainly plant foods as their source of B6 (PNG) excreted more B6 in their stools, and experienced a decrease in B6 status indicators after only eighteen days. These researchers recommended that “during the determination of Recommended Dietary Allowances, the reduced bioavailability of PNG and its presence in higher amounts in some diets should be considered,” referring to vegetarian-type diets which were becoming popular at the time.22 (Emphasis added.) Because a portion of dietary PNG is absorbed intact, and once absorbed exerts a weak opposing action on the other forms of B6 in the body, this may have contributed to the worsening B6 status in the women whose diets contained higher amounts of PNG.10 The extent to which PNG opposes the utilization of the non-glycosylated forms is not well quantified and may be under-appreciated. Another study found that vegetarians consuming an equivalent amount of B6 from their diets had blood levels of PLP (active phosphorylated form) 22 percent lower than non-vegetarians.23
ABSORPTION AND UTILIZATION OF B6
Plant foods vary considerably in the proportion of pyridoxine present in the free versus the glycosylated form. A few plant foods, such as banana and avocado, are low in glucosides (see Figure 1, page 28), and hence are better plant sources of B6.24, 25 Note that raw broccoli and cauliflower have very low levels of glycoside as compared to frozen; it is thought that raw enzyme activity may release B6 from glucosides, activity that is destroyed during processing.25 All the more reason to eat your cruciferous vegetables lacto-fermented!
Vitamin B6 is absorbed in the small intestine, mostly in the free forms after the glycoside or phosphate groups are removed. As mentioned above, a portion of PNG is absorbed intact, and when consumed in high concentrations, the phosphorylated forms can also be absorbed intact. After absorption, some of the free forms are taken up by the red blood cells and bound to hemoglobin,9 but the majority of B6 goes to the liver for conversion into the active form, pyridoxal-5-phosphate, or PLP (also called P- 5-P). PLP is then released into the bloodstream, traveling to the rest of the body largely bound to the protein albumin. A small amount of PLP is retained by the liver for storage.9 Finally, B6 enters the target tissues after removal of the phosphate group from the circulating PLP; muscle tissues store the majority of the body’s B6.9 Because most vitamin B6 metabolism occurs in the liver, individuals with liver disease are at high risk for B6 deficiency.26
It is important to emphasize the fact that the absorption and utilization of B6 can vary greatly among individuals.8 There are a few factors that likely explain this variability. First, the activity of intestinal enzymes that split off glucosides from PNG (plant form) in order to facilitate absorption has been shown to vary with the intake of PNG.27 Second, the intestinal absorption of both PNP and PMP (phosphorylated animal forms), as well as the final tissue uptake of circulating PLP requires a different enzyme that has zinc as a co-factor. Third, riboflavin, or vitamin B2, is needed for the phosphorylation of all forms of absorbed B6 to active PLP in the liver. Thus, poor intakes of zinc and/or riboflavin may exacerbate or lead to B6 deficiencies. Diets that restrict meat, seafood, poultry, dairy or eggs, or diets that largely rely on improperly prepared grains, legumes and nuts that are high in mineral-binding phytates present a three-fold problem: they are low in riboflavin, low in bioavailable zinc, and they contain vitamin B6 mainly in a form that is more difficult for the body to absorb and utilize.9
B6 METABOLISM: ROLES IN THE BODY
Vitamin B6’s chief function is to act on amino acids, the building blocks of protein, mostly to remove or transfer nitrogen, sulfur, or carbon-containing molecules. Overall, B6 is important for protein metabolism, growth, and the nervous, endocrine, and immune systems. More than one hundred vitamin B6-dependent enzymes have been identified, mostly involved in amino acid metabolism: for oxygen transport via hemoglobin synthesis; in blood sugar regulation via conversion of stored carbohydrate to energy; in the development of the myelin sheath surrounding nerve cells; in the conversion of alphalinoleic acid to the essential long-chain fatty acid DHA;28 and in the synthesis of neurotransmitters, phospholipids and sphingolipids, the vitamin niacin from tryptophan, and other vital metabolites.5 In addition to its role in enzyme reactions, B6 appears to moderate the action of some steroid hormones such as the glucocorticoid hormones, which in turn influence the metabolism of protein, carbohydrate and lipids.5,9 B6 also is a potent antioxidant, rivaling carotenoids and vitamin E in its ability to quench reactive oxidants in the body.29
EFFECTS OF B6 DEFICIENCY
Because vitamin B6 plays several important roles in the body, a suboptimal intake or status will have many detrimental effects on health. The classically recognized effects of a vitamin B6 deficiency are microcytic hypochromic anemia (small red blood cells with low levels of hemoglobin), dermatitis, glossitis (sore, inflamed tongue), depression, confusion, and convulsions. Hyperhomocysteinemia, or elevated levels of homocysteine in the blood, can also be due to marginal B6 status, often seen in conjunction with low folate and/or vitamin B12 intake.5,9 Hyperhomocysteinemia is thought to be a risk factor for cardiovascular disease and stroke. Yet studies have produced conflicting findings as to whether using supplements to lower homocysteine levels provides protection against these diseases. Blood vitamin B6 levels are significantly decreased in diabetics and B6 supplements have been shown to help with problems related to glucose intolerance.18 A recent population study showed that lower blood levels of B6 are associated with higher levels of inflammation, oxidative stress, and poor blood sugar control.30
FIGURE 1: Vitamin B6 and percentage of glycoside form in selected plant foods.24,25
|Plant Food||Portion||B6 in MG||Percent glycoside|
|Banana||1 cup||0.55||3 – 16%|
|Broccoli, raw||3-1/2 oz||0.17||Not detectable|
|Broccoli, frozen||3-1/2 oz||0.12||65%|
|Cauliflower, raw||3-1/2 oz||0.16||5%|
|Cauliflower, frozen||3-1/2 oz||0.084||82%|
TRADITIONAL DIETS AND B6
Considering the widespread frequency of marginal intake of vitamin B6, one has to wonder which limitations of the Western diet are behind this phenomenon. We have already discussed the roles of zinc and riboflavin and the limited bioavailability of B6 from the majority of plant-based foods. Compounding that situation is the fact that B6 is heat sensitive and is destroyed by either long cooking times, high temperatures, or both. As the best dietary sources of B6 are animal foods, it is easy to surmise what is happening. Most, if not all, of the animal foods eaten today are heated thoroughly prior to consumption, mostly out of food safety concerns; many are consumed in a highly processed state—frozen, canned, or packaged for convenience. Traditional cultures always ate at least a portion of their animal foods in the raw or fermented state, and milk was rarely if ever heated beyond the relatively low temperatures needed to make cheese, yogurt, and other fermented milk products.
PREGNANT AND BREASTFEEDING WOMEN
For women who follow conventional dietary advice, pregnancy is often accompanied by the risk of several vitamin and mineral deficiencies, including vitamin B6 and iron. Both are critical nutrients for the developing fetus; vitamin B6 is essential for the proper development of the central nervous system.31 Most women will be counseled to increase their intake of iron-rich foods and/or given a prescription for an iron supplement to avoid or treat the anemia of pregnancy, but rarely is the same advice given concerning vitamin B6. This oversight is unfortunate since during the third trimester the mother’s iron and B6 levels often fall sharply, creating a situation where iron-deficiency anemia can be exacerbated by a concurrent vitamin B6 deficiency anemia. Vitamin B6 deficiency anemia can also occur in the absence of an iron deficiency.
The levels of PLP (active B6) in umbilical cord blood are determined by the intake of B6 by the mother during pregnancy.31 In a study of 56 pregnant women diagnosed with anemia,23 women who experienced no improvement in their anemia following iron supplementation improved with vitamin B6 supplementation.32 Anemia is a serious condition in pregnancy because it can adversely affect the mental development of the baby. Unfortunately, iron deficiency anemia is indistinguishable from vitamin B6 deficiency anemia in the standard blood tests, as both are microcytic, hypochromic forms of anemia. The authors of this study noted that levels of protein in the blood were low in women with vitamin B6 deficiency, suggesting that levels of serum albumin could help characterize this deficiency. They also observed that women with multiple pregnancies had a greater risk for vitamin B6 deficiency.
The content of vitamin B6 in breast milk is likewise affected by the vitamin B6 status of the mother. It appears that the body does not have a way to regulate the B6 content of the milk when the mother’s intake is low (as it does for some nutrients such as calcium), so mothers who do not eat sufficient B6-rich foods and do not make up the shortfall by supplementing with B6 will produce breast milk with inadequate levels of B6 for their infants.33 One group of researchers concludes that a minimum of 3.5 to 4.9 mg of vitamin B6 equivalents (from diet and supplements) are needed to maintain saturated levels of B6 in the mothers’ breast milk, about double the RDA.34 Irritability in the infant may be a sign of less than adequate vitamin B6 status.35
|SIDEBAR: VITAMIN B6: THE RECOMMENDED DAILY ALLOWANCESVitamin B6 requirements are thought to be dependent on the amount of protein in the diet. A number of studies show that the higher the protein intake, the higher the need for B6.45,46 In fact, the previous RDAs set in 1989 were based on protein intake (0.016 mg vitamin B6 per gram of protein). Thus the popular trend for high-protein diets is a dangerous one, potentially depleting levels of B6 in the body, among other nutrients. All types of liver and poultry giblets are excellent sources of B6, further evidence that reinforces the inclusion of organ meats and seafood in the diet as a healthful traditional practice.
The current RDAs for adults are 1.3 mg per day for ages 19-51, 1.5 mg per day for females and 1.7 mg per day for males 51 and older, 1.9 mg per day for pregnant women, and 2.0 mg per day for lactating women.47 The RDAs for B6 are often inadequate to meet the body’s needs.4 For example, the combined results from five different studies strongly indicate that the RDA for young women should be closer to 1.7 mg per day.48 A group of researchers from Tufts University, the USDA and the National Institutes of Health recently concluded that “Vitamin B6 intakes of 3 to 4.9 mg per day appear consistent with the definition of a Recommended Dietary Allowance for most Americans.”4 Unfortunately, menu plans that follow the Dietary Guidelines for Americans typically provide less than 2.0 mg per day of B6.6,49
On a personal note, during my training with neonatal intensive care dietitians, they stated that they never counseled the mothers of premature infants on what to eat while pumping their breast milk for their precious babies. This is because, to paraphrase these dietitians, “A mother’s milk is not affected by her diet; we just want to make sure they eat enough. They are under enough stress already.” These poor babies—whose mothers aren’t told that the quality of their milk is directly related to the quality of their diets—will suffer because conventional health practitioners continue to ignore this scientifically supported fact.
As stated earlier, many women of reproductive age in the United States have suboptimal B6 status.4 Other investigations have yielded similar results. Women of reproductive age by and large are not even meeting the RDA for vitamin B6, and the RDA itself may not even be adequate for their babies’ health or their own. A symptom of low vitamin B6 status during pregnancy may be excessive morning sickness, which has been shown to be ameliorated by short-term vitamin B6 supplementation.36 However, caution must be exercised when considering dietary supplements and vitamins in levels above what is contained in nourishing traditional foods.
For women eating a traditional diet, foods such as liver, red meat, and other organ meats are sources of both iron and B6 in highly usable forms for the body. These wise women will prepare for pregnancy and will space their children at least three years apart. These traditional practices can prevent deficiencies of vitamin B6 and other nutrients during pregnancy and lactation, making the experience easier for the mother and giving each baby the optimal start on life.
Considering the myriad biochemical reactions that B6 facilitates in the body, it is not surprising that nutritionally oriented practitioners have reported significant improvements with B6 supplementation in cases of premenstrual syndrome, estrogen overload, adverse effects of oral contraceptives, nausea and vomiting in pregnancy, depression, carpal tunnel syndrome, and asthma. Other possible uses for vitamin B6 are treatment of acne, alcoholism, attention deficit disorder, autism spectrum disorder, MSG sensitivity reactions, general inflammation, and depression and other neurological disorders.1 Treatment of these conditions with higher doses of supplemental B6 is not recognized by conventional medicine,37 and should be done under a qualified practitioner’s care. Supplementation with doses above the upper tolerable intake level of 100 mg per day can lead to nerve damage in the arms and legs and possibly the spinal cord, usually reversible when supplementation is stopped.9 In addition, there is research suggesting that high doses of B6 can generate toxic photo-products as a result of UV irradiation.38
Pyridoxine hydrochloride (PNHCl) is the most common form of B6 available in supplements. When compared with pyridoxal-5-phosphate (P-5-P) supplements, PNHCl requires an additional enzymatic step in the liver to be converted to the form utilizable by the body.5,39 Many practitioners recommend P-5-P supplements over PNHCl supplements for this reason1 and because there have been more reports of adverse effects with use of high doses of pyridoxine.40 However, in healthy individuals, bypassing this step using large doses of P-5-P may not be wise, as the liver serves as an important control to prevent excessive production of P-5-P.5
AUTISM SPECTRUM DISORDER
Individuals with Autism Spectrum Disorder (ASD) often do well with higher intakes of vitamin B6 and other nutrients. According to William Shaw, PhD, one reason for this are the high levels of candida often found in their guts. The sugar arabinose, a byproduct of candida metabolism, competes with B6 for the same enzyme sites, effectively blocking the action of this vitamin. Another reason is that high levels of compounds called kryptopyrroles might actually deplete B6 in these individuals. Dr. Shaw feels pyridoxine supplements are more effective in these cases because to absorb P-5-P supplements in the gut, the body must remove the phosphate group. Some of the benefits for B6 supplementation in autism are probably related to its action on neurotransmitter production and its ability to reduce oxalate levels, which often rise to high levels in ASD. Parents have reported many benefits from B6 supplements including improvements in sleep, focus and concentration, language skills, and interest in learning, and reductions in hyperactivity, irritability, aggressive and self-injurious behaviors. Dr. Shaw also recommends supplementing with magnesium, as it is a cofactor for most of the biochemical actions of the vitamin B6 coenzyme.41
Note that blood tests can be hard to interpret, as levels of total vitamin B6 will be elevated and yet levels of PLP will be depressed in individuals with ASD, reflecting a low activity of the enzyme that converts B6 to the active PLP.42 Again, supplementation should only be implemented under the supervision of a qualified practitioner.
TESTING FOR B6 STATUS
To determine whether an individual has sufficient vitamin B6 stores, a urine test measuring xanthurenic acid excretion following an oral dose of tryptophan is commonly used.9 A blood test for plasma PLP is considered the single best indicator of tissue B6 stores; levels of 20nmol/L are generally considered adequate (DRIs).47 All tests must be interpreted by a qualified practitioner as other factors besides B6 deficiency can affect the results, including pregnancy and liver and bone disease.43, 44 Elevated levels of homocysteine in the blood may point to a B6 deficiency as well, but the status of vitamin B12 and folate must be considered when evaluating this laboratory test.
Vitamin B6 is an important vitamin that gets very little attention by most health practitioners. While the United States Department of Agriculture requires the enrichment of foods made from refined grains with vitamins B1, B2, B3, folic acid, and iron, there is no such requirement for vitamin B6. The general public is thereby lured into a false sense of security in the belief that they can get all their vitamins from a government-recommended “balanced diet.” Unfortunately this same diet is often high in processed foods, high in dietary phytates, excessive in protein that requires higher intakes of vitamin B6, high in plant foods containing hard-to-absorb glycosylated B6, and low in the full range of animal foods and properly processed plant foods that would supply adequate vitamin B6. A true balanced diet would be based on the traditional foods that nourished humans for generations—ones that are naturally rich in bioavailable vitamins and minerals essential for good health and successful reproduction—including vitamin B6.
|SIDEBAR||FOODS RICH IN VITAMIN B6|
|TYPE||FOOD||AMOUNT||B6 in MG||TYPE||FOOD||AMOUNT||B6 in MG|
|Animal||Tuna, yellowfin||4 oz||1.17||Plant||Garbanzo beans||1/2 cup||0.57|
|Animal||Beef liver||4 oz||1.16||Plant||Banana, raw||1 medium||0.43|
|Animal||Chicken liver||4 oz||0.85||Plant||Potato, baked, no skin||1 medium||0.34|
|Animal||Pork chops||4 oz||0.79||Plant||Potato, mashed||1/2 cup||0.26|
|Animal||Sockeye Salmon||4 oz||0.78||Plant||Sweet potato||1/2 cup||0.25|
|Animal||Halibut||4 oz||0.71||Plant||Brussel sprouts||1/2 cup||0.22|
|Animal||Beef steak||4 oz||0.71||Plant||Spinach, cooked, drained||1/2 cup||0.22|
|Animal||Turkey giblets||4 oz||0.67||Plant||Avocado, raw, sliced||1/2 cup||0.2|
|Animal||Turkey with skin||4 oz||0.52||Plant||Pinto beans||1/2 cup||0.19|
|Animal||Chicken giblets||4 oz||0.45||Plant||Buckwheat flour, dry||1/4 cup||0.17|
|Animal||Codfish, canned||4 oz||0.32||Plant||Red pepper, sweet, raw||1/2 pepper||0.17|
|Animal||Salami, beef & pork||2 oz||0.26||Plant||Lentils||1/2 cup||0.17|
|Animal||Cheese, feta||1 oz||0.12||Plant||Winter squash||1/2 cup||0.17|
|Animal||Crab, blue, canned||1/2 cup||0.1||Plant||Broccoli||1/2 cup||0.16|
|Animal||Milk, pasteurized||1 cup||0.1||Plant||Sauerkraut||1/2 cup||0.15|
|Animal||Cheese, cottage||1/2 cup||0.1||Plant||Green peas||1/2 cup||0.14|
|Anima||Cheese, Cheddar||2 oz||0.1||Plant||Brown rice||1/2 cup||0.14|
|Plant||Cabbage, Chinese||1/2 cup||0.14|
|Plant||Turnip or collard greens||1/2 cup||0.13|
|Plant||Sunflower seeds, shelled||1/8 cup||0.13|
|Plant||Prunes, stewed||2 oz||0.12|
|Plant||Summer squash||1/2 cup||0.12|
|Plant||Cauliflower, cooked||1/2 cup||0.11|
|SOURCE: USDA National Nutrient Database for Standard Reference, Release 21. Content of Selected Foods per Common Measure, Vitamin B-6 (mg sorted by nutrient content). Available at http://www.nal.usda.gov/fnic/foodcomp/Data/SR21/nutrlist/sr21w415.pdf.|
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This article appeared in Wise Traditions in Food, Farming and the Healing Arts, the quarterly journal of the Weston A. Price Foundation, Spring 2011.