No sports drink during base phase?

Just to add a little to the above (with which I thoroughly agree):

1.  It is widely agreed amongst exercise scientists (as far as I can tell) that the ratio of fat to carbs burned during workout depends largely on intensity, and not on the immediate relative availability of
the two fuels.

2.   A lot of your fat burning happens after exercise, due to higher metabolism. (This point mitigates the first, a bit, since a more intense workout will tend to burn more carbs during the workout, but will also trigger a higher post-workout metabolism.)

3.  If your body wants to burn carbs, it will find them, mostly as stored glycogen, up to the point where you bonk, and bonking is not generally a good thing (though it might be useful,strategically, to experience it so that you know what to avoid and how to avoid it, but that's a different issue).

I'll try to chip in my 2 cents ... something I never really understand or realized until this was taught to me a couple of weeks ago by the owner of a local running and triathlete specialty store. 

He has also been tri'ing for about 7 years now I think and has completed numerous IM's nd HIM's.

He was giving a Tri 101 type class, preparing us first-timers for a sprint race, about transitions, etc... and was talking about eating the night before the race and pre-race meal.  He said many believe "carb loading" such as a big pasta dinner the night before the race is helpful.

He said he is neutral on it.  That what really determines your energy stores and how well you are prepared for an endurance event is mostly based on how well you have trained your muscles and how strong, fit, and efficient they are.

That the body burns fat for energy, and that even the most fit people have plenty of fat stores to get them through the longest event (a full triathlon, an ultra marathon, etc....) that even those with liek 5-8% bodyfat have enough fat stores.

The key to burning that fat is you need a catalyst, an accelerant so to speak - like the lighter fluid you squirt onto your grill to get the charcoals fired up and burning ... and that it is the glycogen in your muscles and bloodstream that do this.

It is like when you exercise or need energy, the body sets on fire the glycogen, then that heats up and is what burns away at the fat.  The body can only store so much glycogen, any excess is what gets converted to fat and stored.

The big pasta dinners, etc ... if not burned up after the meal, will mostly get process, converted, and stored as fat ... which is probably not needed.  What you need to glycogen.  Any time after you eat you have glycogen in your bloodstream for a few hours and as time passes some or most gets transferred into your muscles.

What can't be stored into muscle as pure glycogen gets converted to fat and stored.  That is why triathletes and others will tell you that once you get about 30-45 minutes - maybe an hour, into your training or race (whether it still be the swim portion, the bike, or the run), that you need to start taking in carbs (sugar, sports drinks, gels such as GU, peanut butter and jelly sandwhiches ... whatever trips your trigger)...

... cause you most likely already have plenty of fat to still burn, but you need that catalyst, that glycogen to set on fire that will in turn heat up and burn that fat.  Of course this is an analogy, it is really glycogen being digested and broken down to be used by muscles and the whole ATP process, etc ... and that energy and process is what breaks down fat into usuable glycogen and energy, etc .... but it takes pure glycogen from your bloodstream or muscles to get the whole process started.

Bonking is when you have absolutely no glycogen left and your body is unable to access and breakdown fat for energy.

So, it seems to me if you really want to burn fat, you will definitely need glycogen and to replace that every 45 minutes or so while training ... cause that is what starts the whole fat-burning process.

..

I do 90% of my running without sports drink.  Because it's completely unnecessary for anything but my longer runs.

You can go for up to two hours (give or take) at around marathon pace before hitting the wall.  If you slow down, you can go longer.  If I'm running mostly easy, I definitely don't need anything during, except on the long run.