Tag: Extrinsic factor

In the 1920s, physician William B. Castle discovered that two separate factors were necessary to prevent pernicious anemia, one found inside the body and one obtained from the diet. This discovery led to the classification of vitamin B12 itself as the “extrinsic factor”.

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.

While animals need it, vitamin B12 is a product of microbial synthesis and is considered the "extrinsic factor" for red blood cell formation, not the intrinsic one. Understanding the complex relationship between this dietary component and the body's own *intrinsic* factor is crucial for proper nutrition and preventing deficiency.