Is the marathon really an energy issue?

I think you make a big assumption when you say "at 2 mmol you get 50% of energy from fat and 50% from carbs". The research shows that this is highly idiosyncratic to the individual and to the context (time active, pre and peri nutrition, typical macronutrient content of the diet).

However from my reading I tend to agree that it is no longer glycogen depletion that is the limiting factor. I was intrigued by someone who posted on here months ago saying that he thought it was a factor of no longer being able to maintain muscle stiffness. I'm a fan of Noakes' central governor theory and it makes sense to me that as you begin to get material failure your brain has no choice but to decrease the power output.

If you were jumping on a trampoline and you noticed that the springs holding the canvas to the frame were starting to fail, you'd stop bouncing as hard or else risk catastrophic failure.

It is an interesting topic that I do not think has seen enough discussion lately, or at least not in widely published circles.

Another assumption is a steady burn rate. As the race goes on, body temp rises, heart rate goes up, fatigue breaks down form, etc. It takes more energy to get through miles 16-20 than from 0-4. I have always thought that the body starts to shut down the burning of muscle glycogen when it senses that it is running out and that demand for glycogen is increasing. Contrast the marathon with an ultra. In an ultra, you are able to replenish because you never reach a tipping point where your body is quickly running out of glycogen while increasing the demand. The demand holds fairly steady and the depletion is slow enough that you can get caught up by eating skittles and drinking flat coke.

Can some experts join this thr wrote:

-At 2 mmol you get 50% of energy from fat and 50% from carbs

-The body can store up to 2000 calories in carbs in the muscles, liver etc.

1. You will run faster if you get 60% from carbs and 40% from fat. Even faster at 70%/30%. (Glycogen depletion would not happen if you don't push your pace to the limit. See Noakes' "Lore of Running."))

2. Not all glycogen in your muscles is available for running, because some of it is "localized."

This is a great question and I would also enjoy getting some input. I'm more like 6'2" and a lean 90kg. I have run 4 marathons and smashed into the wall each time. I have pretty much given up on the marathon, since these have not been pleasurable experiences...Don't think its a training issue, in my build ups I ran three 20 milers, put in as many as 10mi at MP, and peaked at about 60mpw. For the latest attempt, my half marathon time was 1:22'45" about 6 weeks prior to the marathon. Based on the training, I thought that the 3 hour barrier was a reasonable goal. In each race, I have felt decent at the half point - usually hitting it at around 1:28-29 range, so paced pretty well. But, between miles 18-20 my legs start to fail me and I end up reduced to a walk/run by mile 21 or so....managed to stagger in 15+ min. off goal pace.

I figure it has to be energy consumption issue as the training should be enough to get me through, based on comparing training plans with others.

Anyone have suggestions? Its been long enough since my last marathon attempt (2012) that I'm thinking of a spring marathon, but am hesitating to commit unless I can come up with a new plan to stave off the wall.

Some sensible answers here if you ignore the trolls.

http://www.letsrun.com/forum/flat_read.php?thread=3550511&page=1

Thanks for the useful replies everyone! That thread is really helpful.

@Clyde, In your case it is very easy to say that you need to lose some weight. 90kg is much. It is almost twice as much as the Ethiopians and Kenyans weigh.

Further, only 3 times a 20 miler, and 60 miles/week doesn't build your glycogen stores much and your fat burning system.

From what I recall in grad school (without getting out my notes), you burn glucose at about 80-90%+ for marathon pace (depending on fitness and how well you're adapted to burning fat). So you have about 21-23 miles of stored glycogen before you "hit the wall". That's where the extra glucose from gels/fluids comes into play so you don't hit the wall.