Honestly though, I can't see a reason for anyone, regardless of their level of experience to physically need anything other than the Gatorade that's on the course.  I ran my first half marathon as undertrained as one can possibly be.  I was still significantly overweight.  I was still chain smoking and, indeed, went looking for the nearest store to buy a pack of cigarettes and a lighter immediately after the race.  I had been running a grand total of two months.  My longest run ever had been 7 miles and I'd only done that once.  I did it the Wednesday before Saturday's race.  I had no idea that such things as energy gels existed.  So, I just drank the Gatorade at the aid stations.  I didn't even know you could walk through the aid stations so I grabbed and gulped as best as I could.  Hell, it was my first race ever.

This was a race with 35,000 people and corrals A-Z.  I started in Corral S.  It took 25 minutes from the gun before I crossed the start line.  I walked for the first two miles because there was no room to run.  Then, I ran the rest of the way.  I picked up the pace significantly after Mile 10 and held that pace to the finish.

Now, don't get me wrong.  I don't know that I have ever hurt that badly in my life.  There was a picture of me crossing the finish line and it looks like I'm celebrating but I'm really screaming in pain.  That has nothing to do with glycogen depletion and everything to do with being woefully unprepared to run 11.1 miles while finishing the last three at a clip roughly a minute per mile faster than you finished the first 9.1.  I was out there for 2:28x (don't feel like looking it up) and add 25 minutes to that if you want to include how long it took me to cross the start line.

That's my personal experience.  Now, I'm going to pick up my copy of "Lore of Running" and see what Noakes has to say about the subject.

My bias is confirmed.

Untrained runners on an average diet of 45% of calories from carbohydrates will store about 380 grams of carbohydrates in both muscles and liver.  There are 4 calories per gram of carbohydrates so 380 x 4 = 1,520 calories stored.  Not all of those are available to the exercising muscles.  Glycogen stored in arm muscles, for example, is unavailable to the leg muscles.  You could exhaust your leg muscle and liver glycogen and still have glycogen in your arms but it wouldn't be "cannibalized" for your legs.

If an untrained runner was smart and spent the week prior to a race eating a diet in which 74% of his or her calories came from carbohydrates (and let me tell you, that is damned difficult to do on a typical Western diet) the runner can store about 490 grams of carbohydrates.  490 x 4 = 1,960 calories of carbohydrates stored.

In a 13.1 mile race, a 170 pound person who completed the race in 2:30, would burn 1,684 calories.  Of course, not all of those calories would be from glycogen.  At an intensity above 80% of VO2max, almost all of the calories would be from glycogen.  However, there's not a chance in the world that an untrained runner can run 13.1 miles at 80% of VO2max.  The untrained runner will be more in the range of 50-60% of VO2max.  At that intensity, about 25% of the calories would be from fat and 75% from glycogen.  The 50/50 crossover is around 35% of VO2max.  1,684 x 0.75 = 1,263 calories of glycogen burned during the race.

Moreover, if you're out there longer than 2:30, or finish quicker than that, it doesn't really matter.  You'll still burn a total of 1,684 calories if your body weight is 170.  The only variables that matter are how much glycogen you have in your body when the race starts and the percentage of VO2max at which you race.

Looking at a GU nutrition label, I notice 25 grams of carbs for a total of 100 calories.  Not all of this will be sent to your legs.  Moreover, if you take one closer than about 45 minutes from the finish, it won't even be useful to your body until after you cross the finish line.  That's why a Gatorade stop at Mile 12 is pointless unless it's just to slake your thirst but in that case, you'd be better off with water.

I fucking love science.

Too. Much. Math. Math makes my brain hurt. Trying to decipher LTH's post. So....basically.... I don't "need" to take any fuel at all.

ALSO! I just was looking at the race website. I just noticed this for the first time:

Gatorade:

Gatorade Endurance Formula (Lemon-Lime flavor) will be served at each aid station along the course of this year’s race. Gatorade Endurance Formula is a specialized sports drink that contains approximately twice the amount of sodium (200 mg per 8 ounces) of Gatorade Thirst Quencher, along with chloride, potassium, magnesium and calcium to help sustain hydration by more fully replacing what is lost in sweat. Gatorade Endurance Formula also contains a 6% carbohydrate solution (14 grams per 8 ounces), just like Gatorade Thirst Quencher, which research shows is optimal to help speed fluid back into the body and fuel working muscles.

I am not sure what other races I have taken Gatorade during have had, but I want to say it was just regular Gatorade. What do people think about this Endurance Formula? Is the extra sodium increasing potential for tummy trouble? Or extremely helpful?

Too. Much. Math. Math makes my brain hurt. Trying to decipher LTH's post. So....basically.... I don't "need" to take any fuel at all.

 

No - you likely don't need as much as you think you do, but the point really isn't to not take anything if you feel like you need/want it.  The fact that you felt better after taking it is more important to you as the individual runner, over what anyone else thinks or does.

Also - the benefit of taking in carbs during exercise is well-established.  Here's a recent review if anyone really wants to read up on the current research.  http://link.springer.com/article/10.1007/s40279-013-0079-0   "As a result, it has been well-established that carbohydrate ingestion during prolonged (>2 h) moderate-to-high intensity exercise can significantly improve endurance performance....Currently, for prolonged exercise lasting 2–3 h, athletes are advised to ingest carbohydrates at a rate of 60 g·h−1 (~1.0–1.1 g·min−1) to allow for maximal exogenous glucose oxidation rates. "

blah, blah, blah at the science Brad explained... So, was I right or was I wrong?  That's all I want to know. 

LTH, how much does a person's weight alter the equation? My weight is closer to 135. Also, what effects do heat and humidity have? I have been told that it is more important to fuel with a GU during the summer.

My last 13 mile run was a month ago. I ran it in 3:05. It was 84 degrees outside, with a dewpoint of 74. Dailymile.com says I burned 2243 calories on that run. That doesn't seem to line up with your math, unless you are differentiating between "regular" calories and glycogen calories.

 

Seems that more runners think in terms of miles though rather than time.  Since the timing clock is not always found on the course but rather the mile markers are, people know that they will cover a mile in X minutes so a 10 minute/mile racer knows they will hit the 45 minute mark somewhere between mile marker 4 and 5 so they plan to look for the aide station around that point to take a gel going in to the aide station.  I'd rather have a plan to take a gel every 5 miles at 5, 10, 15, etc than be thinking in my head alright 45 minutes, 1:30, 2:15 oh wait was the last gel at 1:20 or 1:40.

 

Keep it simple.  Run more, think less.

Not all races have mile markers or aid stations. Just using a watch makes it easier to figure out when to do things. Better yet, just go by feel.

The marathon I did on Sat is relatively flat for first 8 mi, then climbs about 1800ft in next 5 miles. On Sat, it was partly frozen and snowy, so a little slick and a little slower than planned. The pace charts for this course have a variation of about 50% per mile across the 26.2 mi. That course actually does have aid stations and accurate mile markers, but is probably the only race I do where that's true.

Oh, and sometimes the advertised aid stations vary from where they should be.

Keep it simple. Listen to your body and know what it's saying.

The question becomes, "at what distance is it beneficial?"  

"carbohydrate ingestion during long-duration exercise lasting 2 h or more nearly always delays the onset of fatigue and improves performance. Carbohydrate may also be beneficial during more intense continuous exercise lasting about 1 h and during intermittent high-intensity exercise. "

From this paper by Asker Jeukendrup

The same paper suggests that at least 16-22g of carbs per hour are necessary to get a benefit-- about the amount in one gel/GU.  Taking in two during an event you expect to last at least 2 hours is not at all unreasonable.