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Alun - frankly, the advice given to your friend on the surface seems so stupid, that I honestly can't believe it is the full story. There must be some other context or caveats to what they were saying. Having a "wild metabolism" is just marketing-fluff-talk, but OK, it can be sniffed from a distance.
Perhaps your friend had mentioned some other objectives such as lowering cholesterol, body fat percentage etc, and the 'wild advice' was directed towards that? It's certainly true that not all calories are equal (otherwise we'd all be having pre-marathon beer drinking sessions) (actually, that might numb the pain a little!).
There are times when I might consider doing a long run and backing off the carbo-load beforehand. I'd certainly choose complex carbs over simple sugars before a long run. But a simple, unbounded statement like stick to a non-carb diet is so far off the mark when it comes to marathon preparation. Any ('non-wild') running nutrition book will tell the same story. Carbs aren't optional - they are absolutely essential.
The body is also capable of metabolising fat or catabolising proteins to fuel energy systems, but always after the principal energy system (glycogen) is heading towards depletion. Glycogen is provided through carbohydrates. Any attempt to run post glycogen depletion will be more painful, more slow and less enjoyable. Spread that out over a whole marathon training programme and you will have a weak, demotivated, slow runner who is more prone to injury and probably accidents too (guess what, the brain is primarily fuelled by glycogen!).
Well done for being a good friend Alun and challenging the cr4ppy advice she is being given.
"If this was true then how can body builders get so lean whilst only expending 300 calories in their workouts but eating around 5000 calories per day? The answer lies in our hormonal response to exercise and the effect it has on our metabolism
No,the answer lies in the consumption of steroids - mind you for all the good they did me I might as well have stuck them in my arse.
If she carb loads before the race and her training has progressed ok up until then, she will probably do quite well during the race because she'll have a lot more ready energy than she's used to. However, I still think it's a stupid idea and likely that her training will suffer quite seriously to the point where she won't be able to get enough mileage in before the event.
You can only offer an opinion though, if that's what she's set on doing, let her get on with it.