I'll just give a general rule:
Being a celebrity does not make you an expert on anything other than being a celebrity.
Oprah's entertainment media is wildly popular and she is gifted at eliciting and portraying stories of empowerment. What she is not gifted at, apparently, is detecting or preventing the spread of bad medical/health advice. In fact, the opposite seems to be the case.
This recent newsweek article sums it up nicely. Now, please lower your tomatoes until you've read the article.