how much vitamin b12 does a body need?!

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Vitamin B12 2.4 mcg Biotin (30 mcg)

Around 2.4 micrograms a day, although it's not something that is necessary daily (although recommended), as your body stores excess b12 in your brain for later use.

You can read more about b12 in ocean sources (seaweed, algae, spirulina, nori, etc) here: http://www.veganhealth.org/b12/plant

Mushrooms also have b12, but a very small amount - not enough to provide a strong enough source of the substance.

Additionally, the drink Kombucha contains naturally occurring b12 in high amounts.

If none of these natural sources work for you, you can always eat fortified foods such as cereal (cheerios, speacial k, wheaties, etc), soy milk and meat analogues and nutritional yeast.

Recommended Dietary Allowances (RDAs) for Vitamin B12
Age
0-6 months* 0.4 mcg
7-12 months* 0.5 mcg
1-3 years 0.9 mcg
4-8 years 1.2 mcg
9-13 years 1.8 mcg
14+ years 2.4 mcg

Nori contains 32 to 78 micrograms of bioavailable B12 per 100g dry weight. This has been shown to increase serum B12 levels in vegans. (WATANABE, (2007). Vitamin B12 Sources and Bioavailability Experimental Biology and Medicine 232:1266-1274).

http://ods.od.nih.gov/factsheets/vitamin…
http://tier-im-fokus.ch/wp-content/uploa…

I take 500mcg's and sometimes if feeling something coming on I take 2...It's safe and I use to eat seaweed when my grandfather was alive as he always shopped for it at the asian store also and that goes back many years..B12 helps cell renewel and is very good for you,also Folic acid is important..I take at least 10 supplements a day..

The amount of vitamin B12 you need each day, or recommended daily allowance, depends on your age. 9 to 13 years old, you need 1.8 micrograms each day. If you are 14 years old or older, you need 2.4 micrograms of vitamin B12 each day.

As for seaweed I am unsure is edible seaweed not called doulis?

Adults need 2.4 mcg daily, although if there is any kind of underlying disorder affecting absorption, more may be needed.

http://ods.od.nih.gov/factsheets/vitamin…

The analogue in seaweed is variable, depending on the type of seaweed, but can be as high as 1.4 ug (micrograms) per 30g of seaweed.

http://www.veganhealth.org/b12/plant

Seaweed does NOT contain B12.

From Vegan Dietitian, Dr. Jack Norris:

"...Contrary to rumors, there are no reliable, unfortified plant sources of vitamin B12, including tempeh, seaweeds, and organic produce. The overwhelming consensus in the mainstream nutrition community, as well as among vegan health professionals, is that plant foods do not provide vitamin B12, and fortified foods or supplements are necessary for the optimal health of vegans, and even vegetarians in many cases..."

He also has recommendations for how much B12 to take at the link. You're very welcome.

ADDED: Oh, and you need to know that Whole Foods has removed Kombucha from their shelves. There's absolutely no proof it contains B12.

http://www.dailyfinance.com/story/whole-…

ADDED 2: Ignore me all you want, but I offered a link to a VEGAN dietitian who backed up what I said about there being no B12 in seaweed. Ignoring it won't change the FACT that B12 is a necessary nutrient and it is NOT available from any plant source. IMO, that's proof that humans are natural meat eaters. :)

From another VEGAN RD:

"All vegans:
Vitamin B12. You can’t get enough by eating unwashed organic produce or mushrooms grown in B12-rich soil. The recommended dose is 25 to 100 micrograms per day or 1,000 micrograms 2-3 times per week. If you have not been taking B12 for a while, start out with 2,000 micrograms daily for several weeks. Or get a blood test to see where you are and whether you might need a more therapeutic dose."

http://www.veganhealth.org/articles/vita…