My constant cry is that from what I read type of fat is another very important factor.
It is a complex subject.
This is an article that gives an insight into the generalities of how important fats are to body function.
http://www.benbest.com/health/essfat.html
Historically prior to farming / industrialisation fats were a scarce resource in diet.
So for those who think there is some sense in the idea that we have by evolution or design functioned best on what nature provided it is likely a case of BOTH
Balance
And
Limited intake.
Lack of balance which might include lack of omega three, imbalance in threes and sixes, reliance of processed or trans fats etc could from what I read increase risk factors for a range of medical conditions.
Lack of balance plus trans fats etc might be a factor for some of those on low fat diets.
Trans fats (also mineral deficieny, sugar intake,....) are reported as impedeing the ability of the body to convert the mother omega three and six to the long chain derivatives which are so essential to health eg DHA and EPA.
I cannot give definative answers as large enough trials based on hard evidence eg fat levels in the body rather than what you are meant to have ate and BC have not been done. I am also not a professional just somebody who has developed an interest read quite a lot on the subject.
I am afraid you will have to do a little reading yourself, talk to your advisors and come to your own conclusions.
RB