Originally Posted by Hartmann
First, you can't "train" your body to use a particular macro, digestion is autonomic. Second, ingestion of dietary fat has no direct connection to using fat for energy. Carbohydrates don't inhibit fat oxidation if you're at a caloric deficit.
Train was a poor word choice, I wasn't expecting someone to dissect my every word...your body becomes more efficient at fat oxidization. This in turn allows you to delay fatigue and thus perform better. Plenty of studies have been conducted and support this claim. Your body uses carbohydrates for short term moderately high -> high intensity. Carbohydrate stores are limited and their depletion is the primary cause of fatigue during prolonged runs at higher intensities; this part of the reason GU was invented. Take someone who has thrived on a traditional high carbohydrate diet and then remove carbs. It'll be brutal until their body adapts and becomes efficient at utilizing fats for energy. Consider the following from Greg McMillan, http://www.mcmillanelite.com/about.htm
"Finally, (and this is optional) a great way to ensure that you will deplete your carbohydrate stores on these long, steady runs is to not eat any carbohydrates immediately before or during the run. Any carbohydrates ingested will be used by the body for fuel, and we don't want this. We want to deny the body carbohydrates in these runs so that the muscles will become better at sparing the carbohydrate stores, more efficient at burning fat and used to running with lowered blood glucose levels. Now, many people think I'm crazy when I say this, but it works. It takes time to get adjusted to it if you have always been carbing up before and during your long runs, but with time and practice you can do it. I will note, however, that it is important to drink water and electrolytes throughout these runs so that you don't get dehydrated. I also recommend carrying an energy gel with you just in case you run into trouble (like taking a wrong turn, having to run longer than expected and getting a little woozy). Two words of wisdom here. First, I don't recommend withholding carbohydrates for runs lasting longer than three and a half hours. And second, withholding carbohydrates is the "icing on the cake" for the long, steady run. The "cake" is the fact that you are running for over two hours. If you're sent into hypoglycemia by the thought of having no carbohydrates on a long run then by all means, ingest them. You'll still be stressing the body to adapt to these longer runs. I cannot stress enough that if you want to adopt this long run strategy that you very gradually wean yourself off of carbohydrates. Your body is likely used to it so I recommend that you continue with your same breakfast and gradually begin to space apart your intake of carbohydrates during the run. For example, if you take an energy gel every 45 minutes, begin to take them every 50 minutes. On the following long run, extend this to 55 minutes. See how your body responds. Then, gradually begin to reduce the amount of breakfast you have before the long run. Over the course of several weeks and months, you will learn that your body has plenty of energy stored in it for long runs and marathons. You just have to retrain it to access these energy stores and not depend on external sources. My experience has been that in most athletes (there are exceptions), the body and mind can be trained to work more efficiently with fuel use in training so that when more fuel is available during the race, you feel like a million bucks! Another note: I recommend that you do these long, steady runs on a soft, uneven surface like dirt trails. This helps avoid injuries, challenges the accessory muscles and is usually a more enjoyable way to run easily. Take someone along with you as well. I run my long, slow runs first thing in the morning and have nothing to eat before the run. I tuck a Clif Shot into my pocket on my shorts and hit the trails. I'll drink water with electrolytes during the run during hotter months but no carbohydrates. I get my 120-180 minutes of running in, then begin the reloading process described in the nutrition article on this website."
Secondly, look at Effects of high-fat and high-carbohydrate diets on metabolism and performance in cycling. (Rowlands and Hopkins 2002) Endurance capacity and high-intensity exercise performance responses to a high fat diet. (Fleming et all. 2003) Lastly, am I incorrect in thinking that carbohydrates are preferred by the body for energy?