Authorities are respected in science, law, government, and everyday life. We rely on authorities because they are highly trained and knowledgeable in their fields.
Of course, authorities, even if relevant, come under question if they do not agree among themselves and if their opinions change from place to place and time to time. In such cases, we need to inquire why an authority makes a claim and how the claim can be verified by other means.
The main case of citing an authority which is outright mistaken is the ad Verecundiam fallacy. This fallacy occurs when appealing to the testimony of an authority outside of the authority's special field. Of course anyone is free to offer opinions or advice; the fallacy only occurs when the reason for assenting to the conclusion is based on following an improper or irrelevant authority.
The following appeal to authority raises some unique issues. The author states that the italicized sentence is a myth associated with vegetarianism, and the remainder of the passage dispels the myth:
(Carole Raymond, Student's Vegetarian Cookbook (New York: Three Rivers Press, 2003), xvii.)
Health professionals find vegetarian eating a questionable choice. The Physicians' Committee for Responsible Medicine recently called for four new food groups (whole grain, vegetables, fruits, and legumes) and lists dairy products and meat as optional.
As an Argumentum ad Verecundiam, the crucial question involves the relevance of the authority: The Physicians Committee for Responsible Medicine (PCRM).
As it turns out, the PCRM and its president, Dr. Neal Barnard were censured by the American Association in 1992:
(From "The Physician's Committee for Responsible Medicine" Lowcarbezine (17 December 2003) URL=<http://www.holdthetoast.com/httblog/archives/000050.html>)
The AMA continues to marvel at how effectively a fringe organization of questionable repute continues to hoodwink the media with a series of questionable research that fails to enhance public health. Instead, it serves only to advance the agenda of activist groups interested in perverting medical science. The Physicians Committee for Responsible Medicine is an animal 'rights' organization, and, despite its title, represents less than .5 percent of the total U.S. physician population. Its founder, Dr. Neal Barnard, is also the scientific advisor to People for the Ethical Treatment of Animals (PETA), an organization that supports and speaks for the terrorist organization knows as the Animal Liberation Front (ALF).
It seems that Dr. Neal Benard was educated at a psychiatrist, not as a nutritionist or cardiologist. And the organization the Physicians Committee for Responsible Medicine is an animal rights organization not representative of medical authority.
Thus, it appears the ad Vericundiam fallacy occurs because the authority cited is one which is selected as the conclusion of another fallacy--that of Hasty Generalization or Converse Accident: the fallacy of considering certain exceptional cases and generalizing to a rule that fits them alone.
Yes, sometimes authorities disagree--but in this case it is arguable that the PCRM is not a medical authority, but an organization with an entirely different agenda.
A cogent argument for vegetarianism, in this case, is diminished by deceptive reasoning.