ubetido:

The problem is that we have nearly unlimited storage capacity for fat, but low storage capacity for glucose (carbs), as glycogen in the muscles and liver -- and even lower storage capacity for fructose, which can only be stored in the liver.  And that storage capacity is dependent on how often we exercise at high intensity: if we never exercise, we'll never deplete any muscle glycogen.

The problem with carbs comes when we eat more of them than we have room to store.  This causes all sorts of metabolic havoc -- imagine a big enzymatic and intermediate product traffic jam in cells all over your body -- because the last-ditch disposal pathways are much slower than glycogenesis.  Result: damaged metabolism.

In contrast, eating too much fat will indeed cause you to store fat...but it won't screw you up metabolically except in combination with too much sugar.  (Caveat: I assume you're eating healthy saturated and monounsaturated fats.  A high-PUFA diet can indeed screw you up metabolically.)

JS

"Guest":

It depends on what sort of exercise you're doing, and how much.  But if you've got your diet relatively dialed otherwise (e.g. you're weight-stable and happy with your performance), and you're trying to lose some fat mass, I've had success with dialing up the protein (in relative terms) and dialing down the fat.  This is because you want to "eat" the fat on your ass instead of the fat in your diet, but you don't want to lose muscle mass or leave yourself glycogen-depleted.

Note that you don't want to eat a low-fat diet!  This will dramatically decrease satiation and cause your food to be digested too quickly...you'll know how little fat is too little because none of your food will taste good anymore.  But if you're eating a high-fat functional paleo diet, there's plenty of room to chop out some calories...usually I've found that removing cheats or borderline cheats is sufficient.

JS

Guest:

No, I don't suggest VLC for people undergoing high-intensity exercise, except as an occasional training tool to increase FAO: my recommendations assume you're already on a high-fat paleo diet with carbs sufficient to maintain your glycogen stores. 

I recommend somewhat higher protein because it's satiating and will help you eat less (to a point), and will help you maintain muscle mass during caloric deficit...and I don't recommend cutting carbs while you're exercising intensely.  That leaves fat, some quantity of which you want to come off your ass instead of from your diet.  Again, I don't recommend a low-fat diet...just that when cutting calories, you should cut more of them from fat.

Remember that protein can be burned directly for energy: it doesn't all have to be converted to glucose.  The problem is that we can't store it: we have to either build tissue, burn it, or convert it to glucose.

JS