William Bosworth Castle (1897-1990) was a Harvard Medical School graduate who, in 1927 demonstrated the existence of gastric Intrinsic Factor. He then performed a series of experiments in which he gave human gastric juice to Pernicious Anemia patients and observed that it failed to stimulate a red blood cell response. He then tried giving near raw hamburger meat alone to these patients and it too, did not achieve any response. What he then did would, well... make your stomach turn. He consumed the near raw hamburger meat, waited an hour and then regurgitated it and.... fed it to the pernicious anemia patients via a nasogastric tube. Bingo! It worked and he saw a prompt improvement in red blood cell response. He published the results on 10 patients he subjected to this treatment and in his 1929 article, he wrote:

...there is found within the normal stomach during digestion of beef muscle some substance capable of promptly and markedly relieving the anemia of these patients.

By 1930 he was able to conclude

The lack of this particular property (Intrinsic Factor) of the gastric contents in pernicious anemia is probably the essential defect leading to the development of the disease, through a failure of the normal reaction, occurring in these experiments with beef muscle proteins (Extrinsic Factor) and normal gastric juice.