IMO the 40-30-30 ratio is a good starting point, not the optimum. In the second PM I really hash this out... basically if one leans out on the base Zone using the 40-30-30 ratio one will then need to increase fat blocks between 2 and 5x to support activity level. This leaves the ratio at the 5x level at something like 15-25-60. That is obviously a huge difference between the baseline and athletes zone...and it tells me there is not an optimum fuel mixture other than what your individual needs are both with regards to genetics and performance needs. What the Zone does (in both base and AZ versions) is provide adequate protein and fats while heading towards an optimized carb intake. I think the carb intake can be further tweaked to match activity by putting most of the carbs in a post workout meal...but that is being covered in this months PM. So bottom line is the Zone is just "good", not magic.
Back to the question of the fate of dietary fat. Our body is in a constant state of flux. Tissue is rebuilt and turned over and dietary fat is used to build cell membranes and hormones...and it is utilized for energy. In general the body will preferentially utilize whatever fuel is most available. If we eat mainly carbs we will burn mainly carbs. Same story for fat. There are also hormonal consequences of what fuel is predominating in our diet. Too many carbs obviously pose a problem. Too much fat can also...but it's tougher to over do fat in the absence of carbs.
This paper from Nutrition and Metabolism talks about ketogenic diets and performance. The take home points are:
1-It takes time to adapt to a high fat diet.
2-Once adaptation has occurred performance is just as good as a high carb diet except for sprint activities.
3-This makes sense as fats can fuel brief explosive efforts an long slow efforts...but we need glycogen for thing like 400 and 800m runs and CF style metcons.
Here is that paper:
Let me know if this clears things up or makes it muddier!