I had heard that spirulina can deplete the Vitamin B12 in your system; and since I recently was told by my doctor that I was low in B12 (this is a 1st in my life), naturally I wondered if there could be a connection since I'd been taking Spirulina for some time.
Spirulina contains a B12 analogue that your body can't use as B12. This analogue can block the absorption when you eat other B12 foods at the same time. So if you are taking spirulina, take it away from B12 supplements or the foods you are eating that are high in B12. If you are vegetarian, the foods highest in B12 are dairy and eggs, possibly bee pollen. If you are vegan you'd have no dietary source of B12, other than possibly the dirt from your unwashed veges. If you eat animals, you get B12 from meat and fish, the highest sources are brain, clam, herring, kidney, liver, oysters, salmon and sardines. The liver stores B12, so it can take quite a long time (several years) for a B12 deficiency to develop. Some people eat sufficient B12 but still have trouble absorbing it.
This happens often in the elderly as they lack intrinsic factor necessary for absorption, and I recently read that smokers can also develop a deficiency because the cobalt contained in the cigarettes competes with the B12, which is a cobalt derivative (cobalamin). Methylcobalamin seems to be the most effective B12 supplement, either taken orally or as an intramuscular injection. Spirulina is a great source of most nutrients, other than vitamin C, B12 and fiber.
B12 analogues in spirulina can indeed inhibit simultaneous active B12 absorption, but if taken separately spirulina's benefits can be had without worry of this negative interaction.
The human colon has bacteria that naturally produces B12 when it is fed. Much in the same way, nurtitional yeast is grown by feeding molasses to bacteria. Nutritional yeast is a 100% vegan, active source of B12, among other things. So, feed your body right and you will create your own B12 to some extent as a vegan. You can always rely on nutritional yeast or supplements otherwise.
Meat and dairy consumers are also susceptible to B12 deficiency, so don't let this put you off the right path when it comes to economics, ethics and health. Animals are routinely supplemented with things like B12 -- that DOESN'T make them any better of a source than fortified plant foods. You can inject a gummy bear with B12 and call it a good source by meat-eater logic.
Natural animal B12 is produced in their guts; NOT the flesh. Somehow I doubt that many people tuck into cow manure for dinner, so it can only really come from injections in a meat based diet. ~SoooOOooo natural -- just like our ancestors did it, right? ;).
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