Bloggers who accept ads participate in marketing. Many of those who successfully lost weight and initially started blogging to share what worked for them, choose to monetize their blogs by advertising weight loss products and services.
Most likely the ads are for the products that pay best publishing rates, not necessarily the ones that had been used by the blog owners.
How important is it for you that the person who renders weight-loss advice to you is struggling with or used to struggle with weigh issue himself?
Do you consider this factor when you assess his credibility and value of the advice received?