Max bodyfat you can burn in an hour is roughly 1 gram per 10 pounds, or in calories, 9 times your weight in pounds.

Anything else is food, muscle glycogen, or actual muscle tissue. Glycogen max is about 4% lean muscle mass, which usually is enough for 90 mins, plus or minus. Food is whatever is in your gut, though exercise slows digestion.

Bonk is when you have used up all food, and your glycogen stores, effectively exercising while fasted.

Bonk power is the max sustained energy ouput when you are fasted/bonked. This is fat burn, and muscle breakdown, combines.

Average watts is roughly 1/4 your calories per hour.

Me as an example
I’m 280 pounds, and bonk power for me is 133 watts, which is about 520 calories per hour. Doing that pretty much guarantees cramps from muscle breakdown.

Biking, I tend to burn 750-850 calories per hour, but I can peak at over 1000 in some instances (beginning, well fed, well rested, very driven).

That’s a big gap, because being big, I get more wind drag, which is 50% of your energy above 15mph. I also take more energy to climb a hill.

Downhill is faster, so less benefit (less time spent going downhill), and often waste the energy by riding brakes so as to not plow through others.

I do best consuming 600 calories per hour while riding more than 90 mins.

So, I have to eat the equivalent of a meal every hour to keep up, and reduce cramp risks. Most of that needs to be carbs that break down in less than an hour. Also, I don’t want to have a bathroom break every hour.

Ride Fuel
Sugary colas have phosphate, glucose, and fructose – all good for refueling. Cookies, sandwiches, etc usually are low roughage, good energy density, and include salt. M&Ms were actually designed to be endurance fuel for the army, and they hold up pretty well in a plastic bag.

Basically, all the things that are bad for you normally make great endurance fuel.

As to proper “race fuel”, honestly, it’s too low calorie for someone my size. Some people only need 200 calories an hour to stay fueled, so half banana, a 2″ square of granola, and a swig of gatorade is fine. For me, that would be a whole bunch bananas, and two quarts of gatorade. Just too much bulk.

Impediments
Add to all that the need for oxygen to build ATP (actual muscle energy chemical). It takes 35% more oxygen to burn blood glucose than intra-muscular glycogen. Fat takes twice as much oxygen as glucose. High heat, humidity, low pressure, altitude, carbonation, and alcohol all reduce oxygen availability. Transport of glucose into the cell takes ATP. Digestion of food takes ATP.

Diabetes, Insulin Resistance, and Metabolic Syndrome
The key there is to not eat much carbohydrate outside of the exercise times. When glycogen reserves are full (muscles recovered), and adipose cells are replete, then why would you need more fuel? That’s the practical wording of the physiology here, despite the perception of a faulty hunger mechanism for the obese, or lack of islets for type I, or the defective signalling in Type II without obesity triggers.

Exercise induced glucose uptake is normal in diabetic muscle cells:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4315445/
https://www.ncbi.nlm.nih.gov/pubmed/3899806
http://www.medscape.org/viewarticle/438374

Exercise may increase glucose sensitivity:
https://www.ncbi.nlm.nih.gov/pubmed/3899806

While glycogen is being replenished, glucose uptake by muscles in normal. GLUT4 is transported to the membrane during exercise, even in absence of insulin.
http://diabetes.diabetesjournals.org/content/62/2/572
http://physiologyonline.physiology.org/content/20/4/260.full
http://www.sciencedirect.com/science/article/pii/S1550413107000678