Can Low Intakes of B12 Cause Problems?
The answer from research is yes.
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The main source of vitamin B 12 is animal products.
As a vegan, the only reliable sources of B12 are B12 supplements, some plant milks, some breakfast cereals and some soy products.
Where does B12 Come From?
Soy
B12 comes from micro-organisms.
Most vegans take insufficient B12 to avoid nervous system damage and anemia.
However, many vegans still suffer from pregnancy complications and heart diseases because they do not take enough B12.
What Should Vegans DO?
So as to access the full advantage of a vegan diet, vegans should:
Eat fortified foods at least thrice or twice a day to get a minimum of three micrograms of B12 for vegans in a day.
Or ingest one B12 supplement every day providing a minimum of 10 micrograms.
Or take a weekly B12 supplement offering a minimum of 2000 micrograms.
Look At Labels . . .
If you depend on fortified foods for B12, ensure you are getting enough B12 by checking the labels. For instance, if a fortified food contains 1 microgram per day of B12 per serving, then consume three servings.
Can You Take Large Amounts?
If you consume vitamin B12 less frequently, then you need more B12.
This is because the vitamin is best taken in small amounts.
There is no harm in taking more than the required B12.
The lack of B12 in your diet will cause symptoms after five years or more. However, some people will show symptoms in one year.
B12 is the only vitamin that is not reliably supplied from varied wholefood, vegetables, fruits and exposure to the sun.
Several herbivorous animals absorb B12 produced by bacteria in their gut. To some extent, B12 is found in soil and plants.
National & International Recommendations
B12 intakes according to national recommendations change from nation to nation.
The US recommends an intake of 2.4 micrograms per day for normal adults and 2.8 micrograms per day for nursing mothers.
Germany recommends 3 micrograms per day.
The recommendation intake is based upon 50% absorption. In order to meet the Germany and US B12 intake standards, you require taking enough B12 to absorb 1.5 micrograms per day. This intake is enough to avoid the initial signs of insufficient B12 intake. These symptoms are such as high levels of MMA and homocysteine. High homocysteine causes neural tube defects in infants, heart diseases in grown-ups and preeclampsia during pregnancy.
How Should You Take the Supplement?
A B12 supplement having 10 micrograms provides the same amount of absorbed B12 for vegans as a 1 microgram fortified food taken three times a day.
The supplement tablet should be chewed and allowed to dissolve in the mouth. This enhances absorption.
These supplement tablets should be kept in an opaque container.
Some people have impaired B12 absorption and they should take in 2000 micrograms per week. This is the best method for these people as it relies not on the normal intrinsic factor in the digestive system.
Testing the status of B12 
Algae and other plants have false B12 that imitate true B12 in blood tests.
This makes testing hard for vegans consuming algae.
Testing of blood homocysteine is reliable. The desired levels of B12 for vegans are 10 micromoles per liter.
The most accurate test is MMA testing (methylmalonic acid). It should be less than 370 mmol/liter in blood and less than 4 mcg/mg creatinine in urine.
There are some technical aspects in this post but I do hope it enlightens you as to the B12 and vegan debate.
