>As an easy example, there are many that are more overweight than I am that know nutrition better than I do. That said, it is does seem somewhat logical to question health advice from clearly unhealthy people.

Using your easy example with OP's logic, a 100-pound overweight person trying to lose 100 pounds by throwing up and taking dangerous diet pills should also discount the advice of a physician who've never lost 100 pounds themselves or successfully helped a patient lose at least 100 pounds.