Know of any studies that show a performance benefit from this? I understand the theory, though I'm a sceptic - just curious if there is evidence that it works in practice.

While you are obviously correct that you already have way more energy stored than you need, the reason to 'fat load' from what I read is that it will increase the enzymes used to burn that fat so you can burn the stores you have at a higher rate. I am not a nutritionist but this is what I read. Granted it could be wrong.

Exactly. I will expound by saying, the reason I have been posting the Pubmed links, and in particular the Louise Burke "Nail in the Coffin" editorial is because these matters have been extensively investigated by her and others for over 20 years. We can go back to the studies out of Costill and Hargreaves labs in the late 80s and early 90s as the start of this avenue of research specifically with regard to performance, and the follow up work of Burke and her collaborators after that. If anyone had a vested interest in the plausability of "fat loading" as an ergogenic approach, it was Louise Burke as she pursued it as a focused research agenda for about a decade. At the end of it, the conclusion she came up with is, ... there's nothing there. Yes, increasing the ingestion of fat will increase the oxidation of fat, but we've known that for near 100 years, or more (too bad Coggan isn't around anymore, it's in his latest book chapter), and the Randall Glucose-Fatty Acid cycle has been around since the mid-sixties. And yet, nobody has demonstrated a clear performance benefit to fat loading. OTOH, the evidence is overwhelming as to the benefits of carb loading, clear performance benefits. So, no scientific evidence supporting one case, and a wealth of scientific evidence supporting the other, which one will you take? I love lard as much as the next guy, but....... Further, another poster suggested performance of anything longer than a minute requires fat loading,.. but in fact, for efforts of less than 30 min in duration, fat loading will likely exhibit an ergolytic effect. Since work above ~ 70% VO2max cannot be supported without carbohydrates, if the athlete is glycogen depleted and fueled with fat, they will be limited to this intensity, or, in other words, efforts lower than FTP. Also, as Burke pointed out in her editorial, since the other side of the upregulation of fat oxidation coin is a down regulation of PDH, and reduced uptake of the products of glycolysis into the TCA cycle, even if glycogen levels are not depleted, performance at intensities above FTP will likely be inhibited. At this point, I've said my piece and will now Exit Stage Left.

I just listened to a podcast on IM Talk, interviewing a dietician, Bob Seebohar, about metabolic efficiency.

His message is to keep carbs low before and during training (and racing) sessions, in order to prevent any spike in blood sugars. The idea is that the body can efficiently burn fat, even during intensive sessions, as long as there is not an increase in insulin to counter it. Instead of carbo loading pre-race, and eating a big bowl of cereal pre-workout, and eating gels every half an hour during a workout, he reckons stick to less carbohydrate rich foods and you will be able to go for a 3 hour ride without needing any supplementary food source. And that you can increase your efficiency such that only 5-10% of your energy needs come from an external source.

Fascinating stuff, and may be something in it for those unable to drop weight, even when training. I'm a bit dark that only yesterday I placed another order for $140, 2 x 25 gel, and am thinking I may not need it.

Then again, maybe I will? Has anyone else heard the podcast or pursued this concept?

My brain thinks that if I use less oxygen for carbohydrate oxidation that would leave more oxygen for my muscles.

One thing I never see addressed is that fat needs more oxygen than carbohydrate to oxidize as fuel. My brain thinks that if I use less oxygen for carbohydrate oxidation that would leave more oxygen for my muscles.
Just something that I always thought about when I Did low carb training.

What would your muscles be doing with that extra oxygen?

Pretty simple, in competitive endurance sports you want to use as much carbs, as sustainable for the distance and as little fat as necessary.

Marathon:

You need about 3000 cal
Stored as carbs: about 2000 (but only 1600-1800 can be used, the body keeps some reserve)
Stored as fat: unlimited (practically for that purpose)
Uptake: very limited, maybe 200-400 total

So fat needs to contribute roughly 1/3 or 1000 cals. The trick is, to teach the body to burn 1/3 of calories from fat close to threshold. Kenian runners proved beyond any doubt, that this is possible on a high carb diet.

Do you (or does anyone) have data on how much of the ~2000 calories worth of muscle glycogen is stored in the leg muscles versus upper body? Just curious since muscle glycogen is "non-transferable" between muscles and only liver glycogen, a much more limited supply, is available to muscles that run low. It makes sense that there would be more glycogen stored in the lower body since there is more muscle mass there.