This is irrelevant to the author’s argument. We’re talking about meat-based foods specifically, so information about non-meat-based foods wouldn’t make any difference!

This is irrelevant to the author’s argument. It doesn’t matter how many people follow the standard recommendations—whether it’s a lot or just a few, the fact remains that they are more likely than others to contract the diseases in question.

The argument doesn’t require (and therefore doesn’t overlook) any discussion of symptoms. We can assume that the premises are true, meaning that the people we’re told have or have not contracted the diseases have been accurately diagnosed.

If (D) were true, then the author’s argument would be strengthened: we would be able to pinpoint the content of the standard recommendations as “inappropriate” and perhaps “counterproductive”. But we’re looking for a something the argument overlooks, not a strengthener!

This is overlooked by the author’s causation assumption. It’s possible that the recommendations cause disease contraction, but it’s also possible that those who follow the recommendations are a self-selected group who were more likely to contract the diseases to begin with!