Again, to echo Angry-
My morning is virtually carb free- just the fruit that ends up in a small shake. Egg whites + two sticks of sausage. I'm fine all the way to lunch.
The only time I crashed on this low carb diet was the first week, when I was basically having carb withdrawls.
Been on the low carb style for almost a decade now, and still feel great. Running like Angry rides.
Calories are not calories- your body uses sugar to make energy. so it takes almost no energy to intake sugar. Next is starch- pretty easy to turn starch to sugar (you beer makers know that). Up the line a lot is fat and protien. yes, if you burn fat to create heat, there's a lot of it there. A lot. And if you are already in excess mode, where you have more than enough sugar, you will likely barely process the fat, and then store it.
OTOH, if you are using your liver to process the fat into sugar- that takes some work, and there's the rub.
IMHO, we really don't fully understand how our bodies process food. Especially when many are currently "saturated" with enough sugar vs. being sugar deprived. From what I can observe in my own body, it's not normal at all for us to be totally flush with either sugar or most grains- especially us northern Europeans.
Fat is bad if it's with sugar or other simple carbs. Fat by itself isn't.
BTW, a lot of the low carb theories are getting into the distance running crowd- there were some interesting articles in Runners World about it. They, too, suggest that during training you train yourself to go without. Which will help your liver get up to optimum performance. Something to consider- for the average distance runner (not elite (lightweights) or beginners (heavier)- you use 100cal/mile. Or just over 2600 cal per marathon. Now, you can't eat and store over 2000 cal of carbs that you can use over a 4 hour window. OTOH, everyone has an extra pound of fat that can be converted to energy, as long as your liver is up to it.
I digress. I'm very happy with the low carb diet. And start back running from a foot injury tonight.