Bad nutrition vs. not enough endurance

Are you taking in nutrition regularly during these longer efforts? 

Most likely it is endurance.  But if you think it might be nutrition, monitor what you are taking in.  Then try taking in more next time.  See how you feel.

Target 300 calories of easily digestible sugar per hour to begin.

IM is an eating contest, so as high as you can get you calories per hour and still digest at your projected intensity level will increase your chances of achieving your goals.

Before recommending a change in what you're doing (nutrition/endurance/volume, etc.), I'd really have to get an idea of your current program in order to diagnose what you might be doing incorrectly.

It's sort of like me asking you "Why was I tired on my ride last weekend?".  

In order to offer anything other than anecdotal advice, I'd have to have more data.

Which part? 

I agree.  BUT, "as high as you can and still digest while racing at the proper intensity" is far different than simply as "high as you can".  I would guess not too many are going to go over 500 cal/hour within the proper context.  I would think the range would be quite narrow.

GI distress from consuming too much would not be adhering to the "and still digest" part of my statement.

"IM as an eating contest" is a piece of wisdom related to me by a pro who won a WTC Ironman event.  GI distress is not allowed on eating contests BTW.

My personal, anecdotal, clue is how the performance drop occurred: if I was flying along, feeling really strong, alert and had a positive outlook and suddenly just couldn't do it any more-as if I had run out of gas, coupled with sudden frustration, discouragement and low energy levels then I bonked. If I struggled to maintain my pace and gradually faded then it was fitness for the given effort.

Am I close?

Well put Bryan. With all the formula's and calculations out there it's easy to get swept into eating too much. The only thing I would add from my own personal experience is that I find it a bit harder to dig out of the hydration hole than the nutrition hole.

I personally think that bad nutrition gets the blame for poor results when the culprit is more likely lack of endurance. This is very obvious when someone has a poor result from a 5k or sprint and the goes on to explain how their nutrition plan was off.

I consistently take in 500 cals/hr on long efforts without GI issues but still have to grind out the last few miles of long runs or rides due to lack of fitness. I get the abrupt weak legs and see my drop in power. I also loose my positive attitude. It is not nutrition but lack of fitness. It is also not a bonk. I have not had a true bonk but have seen it. It is not pretty.

You both seem to be missing the words "still digest" and "projected intensity" in my metaphor of IM as an eating contest. This covers the GI distress issues you think I am missing. You don't eat too much in an eating contest. You eat enough to win and not throw up.

Whatever you do, practice it in training.  It is not a bad idea to start with some numbers, formulas and calculations even, and refine it from there.

I found this article by Alan Couzens interesting, "What does it take to win Kona - a physiologist's perspective"

http://www.endurancecorner.com/Alan_Couzens/win_ironman_hawaii

His view is that it's not exactly an eating contest, but it is a fat oxidization contest. The article is one of a trio of articles on completing IMs at different speeds.

I'm with you. Marketing tells us it's a nutrition issue when in fact its usually an endurance issue. The fact is if you run 3 miles a week, you're going to have a tough time running 26.2 no matter what you eat. Same for all endurance sports.

I've gotten tired, but never bonked. From what I understand (and I could be wrong), the feeling that you're bonking is low blood glucose and low liver glycogen. A true bonk occurs when your muscles nearly run out of glycogen and there's nothing in the blood to supplement. Your muscles barely work, and do so in more of a survival mode. Most AG'ers will never experience this feeling. They just get tired and miserable and ask why they are pushing themselves so hard when people are passing them. I rarely take calories in except for races and race prep. Also, efforts longer than a couple of hours I'll eat an apple (biking) or gel (when running).

bike more, it was fitness related, you had plenty of nutrition.  you could do a 2 hour ride in those conditions with nothing and be fine.  while i would not recommend it, you probably dont even need water.

And a grown man should never use the word "winky."