Vitamin B6/ja: Difference between revisions

植物は、太陽光に含まれる[[UV-B/ja|UV-B放射]]から身を守る手段として、また[[chlorophyll/ja|クロロフィル]]の合成に果たす役割のために、ピリドキシンを合成する。動物は様々な形のビタミンを合成することができないため、植物や他の動物の食事から摂取しなければならない。腸内細菌]]によって産生されるビタミンの吸収も多少はあるが、食事による必要量を満たすには十分ではない。成人の場合、各国の食品規制機関が推奨する摂取量は1日あたり1.0～2.0[[:en:milligram|ミリグラム]]（mg）である。これらの機関はまた、摂取量が多すぎることによる悪影響も認識しているため、国によって25 mg/日の低いものから100 mg/日の高いものまで、安全上限値を設定している。乳製品、卵、軟体動物、甲殻類もビタミンB₆を含むが、その量は少ない。多種多様な植物性食品には十分な量が含まれているので、[[vegetarianism/ja|ベジタリアン]]や[[veganism/ja|ビーガン]]の食生活を送っている消費者が欠乏症に陥る危険性はない。

ビタミンB₆は水溶性の[[vitamin/ja|ビタミン]]で、ビタミンB群の一つである。このビタミンは、実際には6つの化学的に関連した化合物、すなわち[[vitamer/ja|ビタマー]]のグループから構成されており、それらはすべてそのコアとして[[pyridine/ja|ピリジン]]環を含んでいる。これらは[[pyridoxine/ja|ピリドキシン]]、[[pyridoxal/ja|ピリドキサール]]、[[pyridoxamine/ja|ピリドキサミン]]、およびそれぞれの[[phosphorylation/ja|リン酸化]]誘導体である[[pyridoxine 5'-phosphate/ja|ピリドキシン5'-リン酸]]、[[Pyridoxal phosphate/ja|ピリドキサール5'-リン酸]]、[[pyridoxamine 5'-phosphate/ja|ピリドキサミン5'-リン酸]]である。ピリドキサール5'-リン酸が最も高い[[biological activity/ja|生物学的活性]]を持つが、他のものはその形に変換可能である。ビタミンB₆は140以上の細胞反応において[[Cofactor (biochemistry)|/ja|補要素]]として機能し、そのほとんどは[[amino acid/ja|アミノ酸]]の生合成と異化に関連するが、[[fatty acid/ja|脂肪酸]]の生合成や他の生理機能にも関与する。

{{Medical uses}}

===Safety===

{{Main|Megavitamin-B6 syndrome}}

Adverse effects have been documented from vitamin B₆ dietary supplements, but never from food sources. Even though it is a water-soluble vitamin and is excreted in the urine, doses of pyridoxine in excess of the dietary upper limit (UL) over long periods cause painful and ultimately irreversible neurological problems. The primary symptoms are pain and numbness of the extremities. In severe cases, motor neuropathy may occur with "slowing of motor conduction velocities, prolonged [[F wave]] latencies, and prolonged sensory latencies in both lower extremities", causing difficulty in walking. Sensory [[neuropathy]] typically develops at doses of pyridoxine in excess of 1,000 mg per day, but adverse effects can occur with much less, so intakes over 200 mg/day are not considered safe. Trials with amounts equal to or less than 200 mg/day established that as a "[[No-observed-adverse-effect level]]", meaning the highest amount at which no adverse effects were observed. This was divided by two to allow for people who might be extra sensitive to the vitamin, referred to as an "uncertainty factor", resulting in the aforementioned adult UL of 100 mg/day.

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===Labeling===

For US food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value. For vitamin B₆ labeling purposes 100% of the Daily Value was 2.0&nbsp;mg, but as of May 27, 2016, it was revised to 1.7&nbsp;mg to bring it into agreement with the adult RDA. A table of the old and new adult daily values is provided at [[Reference Daily Intake]].

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==Sources==

Bacteria residing in the large intestine are known to synthesize B-vitamins, including B₆, but the amounts are not sufficient to meet host requirements, in part because the vitamins are competitively taken up by non-synthesizing bacteria.

Vitamin B₆ is found in a wide variety of foods. In general, meat, fish and fowl are good sources, but dairy foods and eggs are not (table). Crustaceans and mollusks contain about 0.1&nbsp;mg/100 grams. Fruit (apples, oranges, pears) contain less than 0.1&nbsp;mg/100g.

[[Bioavailability]] from a mixed diet (containing animal- and plant-sourced foods) is estimated at being 75% – higher for PLP from meat, fish and fowl, lower from plants, as those are mostly in the form of pyridoxine [[glucoside]], which has approximately half the bioavailability of animal-sourced B₆ because removal of the glucoside by intestinal cells is not 100% efficient. Given lower amounts and lower bioavailability of the vitamin from plants there was a concern that a vegetarian or vegan diet could cause a vitamin deficiency state. However, the results from a population-based survey conducted in the U.S. demonstrated that despite a lower vitamin intake, serum PLP was not significantly different between meat-eaters and vegetarians, suggesting that a vegetarian diet does not pose a risk for vitamin B₆ deficiency.

Cooking, storage, and processing losses vary, and in some foods may be more than 50% depending on the form of vitamin present in the food. Plant foods lose less during processing, as they contain pyridoxine, which is more stable than the pyridoxal or pyridoxamine forms found in animal-sourced foods. For example, milk can lose 30–70% of its vitamin B₆ content when [[dried milk|dried]]. The vitamin is found in the [[Cereal germ|germ]] and [[aleurone]] layer of grains, so there is more in whole wheat bread compared to white bread wheat, and more in brown rice compared to white rice.

!Source

!Amount<br /> (mg per 100 grams)

|[[Whey]] protein concentrate || 1.2

|[[Beef]] liver, pan-fried || 1.0

|[[Tuna]], skipjack, cooked || 1.0

|[[Beef]] steak, grilled || 0.9

|[[Salmon]], Atlantic, cooked || 0.9

|[[Chicken]] breast, grilled || 0.7

|[[Pork]] chop, cooked || 0.6

|[[Turkey]], ground, cooked || 0.6

|[[Banana]] || 0.4

!Source

!Amount<br /> (mg per 100 grams)

|[[Mushroom]], [[Shiitake]], raw || 0.3

|[[Potato]], baked, with skin || 0.3

|[[Sweet potato]] baked || 0.3

|[[Bell pepper]], red || 0.3

|[[Peanut]]s || 0.3

|[[Avocado]] || 0.25

|[[Spinach]] || 0.2

|[[Chickpeas]] || 0.1

|[[Tofu]], firm || 0.1

!Source

!Amount<br /> (mg per 100 grams)

|[[Grits|Corn grits]] || 0.1

|[[Milk]], whole || 0.1 (one cup)

|[[Yogurt]] || 0.1 (one cup)

|[[Almonds]] || 0.1

|[[Bread]], whole wheat/white || 0.2/0.1

|[[Rice]], cooked, brown/white || 0.15/0.02

|[[Bean]]s, baked || 0.1

|[[Bean]]s, green || 0.1

|[[Egg as food|Chicken egg]] || 0.1

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===Fortification===

As of 2019, fourteen countries require food fortification of wheat flour, [[maize]] flour or rice with vitamin B₆ as pyridoxine hydrochloride. Most of these are in southeast Africa or Central America. The amounts stipulated range from 3.0 to 6.5&nbsp;mg/kg. An additional seven countries, including India, have a voluntary fortification program. India stipulates 2.0&nbsp;mg/kg.

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===Dietary supplements===

In the US, multi-vitamin/mineral products typically contain 2 to 4&nbsp;mg of vitamin B₆ per daily serving as pyridoxine hydrochloride, but a few contain more than 25&nbsp;mg. Many US dietary supplement companies also market a B₆-only dietary supplement with 100&nbsp;mg per daily serving. While the [[US National Academy of Medicine]] sets an adult safety UL at 100&nbsp;mg/day, the European Food Safety Authority sets its UL at 25&nbsp;mg/day.