RDA is only part of the story. Here is an article explaining the histories of the RDA (Recommended Daily Allowance) and how it became DRI (Dietary Reference Intake), as well as other values it gave rise to, i.e. EAR (Estimated Average Requirement), AI (Adequate Intake), and UL (Tolerable Upper Limit)—which for some vitamins & nutrients, including B12, has not been determined.

It's that last one—the UL (Tolerable Upper Limit)—that we should be concerned about with regard to too much of this or that.

Nutrient Requirements Get a Makeover:
The Evolution of the Recommended Dietary Allowances

See also: tolerable upper limits of vitamins and minerals

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