The need for carbs during running comes down to math to me
(Ignore this
@Keels 
). Do I have enough carbs in storage in my muscles to run my pace for this distance?
Current body Weight (in kg) X Distance (in km) = Total kcals burned
In total, fat and carbs equal out the total kcals burned. The faster you run (relative to your fitness level) the more carbs you burn. I'll use my numbers as an example.
Weight = 72.7 kg
Distance = 42.195 km
kcals = 3068.7
Now my calculator makes two assumptions.
1. You have an average weight to leg muscle ratio. Outside of a leg muscle biopsy, it is near impossible to know where a person falls on the scale of leg muscle. The size of the leg muscle partially determines the capability of carb storage.
2. You have an average metabolic efficiency profile. Your use of carbs and fat the faster you run (relative to your fitness level) is average. Again, outside of a lab metabolic efficiency test, it is near impossible to tell whether someone is average or not. The faster you run the more carbs you use. Some people might be 60% carbs 40% fat at 65% VO2max and others at 80% VO2max (this is where the metabolic efficiency profile would be helpful).
If we
assume (and it is a leap), that a person is average for both of these parameters then we can make some calculations based on needs.
Leg Muscle Mass = 15.3
Carb storage = 1221.8 kcals
Current VO2max = 64
My desired pace is 7:33 min/mile and based on some other calculations that makes it 67% of my VO2max. Based on an average metabolic efficiency ratio I would need 1902 kcals carbs to run 42.195 km, at my weight, at 67% VO2max. This gives me a deficit of 680 kcals carbs. I can pull some from the liver and thus my final deficit is 580 kcals carbs (which equals 175.7 kcals carbs per hour which equals 43.9 g carbs per hour). This is where a traditional gel strategy would kick in. Ok I need 43.9 g carbs per hour to run a marathon at my weight and current fitness level at my goal pace. Done!
BUT.... I want to get rid of using gels or carbs during racing. How can I do that?
It's a two-step process.
1. Teach the body to increase carb storage capacity.
2. Store more than 100% carb capacity immediately prior to race day.
Step 1 can be accomplished with something called Glycogen Depletion training.
WARNING! THIS CAN BE DANGEROUS AND SHOULD NOT BE ATTEMPTED LIGHTLY! These glycogen depletion training runs occur at least 8-12 weeks prior to race day and can be no more than once every 3-4 weeks. You plan to do your normal long run in duration between 90-150 minutes. You eat no breakfast prior and you use no fuel during the run. Your body becomes starved for carbs during the run, but there is nothing available. You typically fade hard at the end of these runs and as others have described learn what it feels like to be at near 0. When you get home you consume a large load of carbs within 15 minutes of finishing. Preferably mostly in liquid form as it can digest more readily. Your body will quickly store as many carbs as it can. The body adapts to the training by saying to itself, hey this guy is crazy and needs more carbs. Let's store more carbs in his legs for the next time he tries to do this to us. Thus, this increases your ability to store above 100% of your physical carb storage capabilities based on the leg muscle mass calculations.
Step 2 is accomplished through a sophisticated pre-race day carb loading strategy. There are a few out there, but I prefer the Western Australian method. Over the past year, I have been training my body to be able to tolerate the carb loading procedure. According to research, it can allow you to maximally store as much as 190% carb capacity (although results varied in the study and I believe the variation in the results is due to Step 1). So if I could maximally train my body to store 190% carb capacity, then it means I could store 2321 kcals of carbs. Remember how much I needed to run my pace, my distance, my weight... 1902. So now I've created a surplus of carbs stored in the muscles in the day or days (research by others suggest that carb loading could be done as many as 5 days prior to the event) ahead and can run the distance without carbs being a limiting factor. In fact, if I were physically capable, then carb depletion wouldn't become an issue at my weight and VO2max until I tried to run a 2:44:49 (81% VO2max). Yea, there are plenty of other reasons why I can't currently run that pace and one of them is not carbs. Here's the Western Australia plan based on my weight (as you can see this is not your typical spaghetti dinner):
View attachment 187154
What's the drawback to this plan? There is no feedback prior to race day on how many carbs you've stored. I could calculate to the cows come home that I'll store 190%, but there's not really any way to know outside of a muscle biopsy the day prior whether I've done it successfully. So the plan takes a bit of leap of faith, but I know based on the science it works. The gels are easier from a standpoint that I can pre-calculate to eat this many gels at a certain rate with a certain water intake to determine I'll have sufficient carbs to finish. This however doesn't eliminate the need for electrolyte replacement (but if you drink some gatorade or electrolyte tabs on course, then you'll be fine).
I put my intervals back to 90/30 the other day and... wooops. Now I'm back around 12:30.
