To be fair, much of the "research" related to dietary recommendations is written in such a way that fat-free, low-everything is the "healthiest" diet. Explaining precisely what that is, and why it's problematic, would take a few thousand words, but I'll just give one example.
In research, very broadly, we have the "basic science" community (think people looking at cells under microscopes) and the "clinical outcomes" community (research done with human subjects, such as giving 10,000 people diet questionnaires, following them for 5 years to see how many people are hospitalized for heart disease, then analyzing which foods seem to be associated with the best/worst outcomes).
Here's an example of how the "basic science" community can go astray. We want to study whether dietary fat is associated with cardiovascular risk. We decide that our surrogate measure of cardiovascular risk is going to be cholesterol. We feed six mice a "high fat" diet and six mice a "low fat" diet. At the end of our experiment, the mice fed a "high fat" diet are fatter and have higher cholesterol than the mice fed a "low fat" diet. Our conclusion is that a high-fat diet leads to obesity and high cholesterol.
But hold up for a second. What exactly does it mean to feed mice a "high fat" diet or a "low fat" diet? We're not cooking up eggs in coconut oil for the high-fat guys and whole-grain oatmeal for the low-fat guys. It's usually just "mouse chow" versus "mouse chow with canola oil" added, or something like that. Is that translatable to humans eating a dozen different sources of dietary fat? What does that mean about avocado, pork lard, grass-fed butter, or coconut oil? Probably very little; but because this is the long-held research dogma, the conclusion is invariably "fat -> high cholesterol and obesity" - there's no room for nuance. The study gets published in a journal, written up in the news, and RD's (and doctors, for that matter) have their belief reinforced after reading that "A study published this month in the American Journal of Clinical Nutrition confirmed that a high-fat diet leads to higher levels of cholesterol and obesity."
So the problem is not just "skinny women and their stupid beliefs that low-fat everything is the way to go." They're often just repeating what "the research" has shown; my personal opinion is that the research is flawed.
Brick will have even more direct thoughts on the subject from the RD world; I'm merely giving the epidemiologist / researcher's point of view.