Why is Vitamin B12 Called an Extrinsic Factor?
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4 min read
In the early 20th century, pernicious anemia was a fatal and mysterious disease. It was during the search for its cure that Dr. William B. Castle famously referred to vitamin B12 as the "extrinsic factor," differentiating it from a necessary substance produced within the body. This name highlights its crucial role as a nutrient that must be obtained from external dietary sources.
