I do not do runs, walking sure, so I am just really curious how much it takes for the human body to do this type of run
Runners can drink anything they want as long as it doesn't violate doping rules. The elites all have customized carbohydrate mixes.
They're worried about ingesting carbs to supplement what they have stored via carb-loading over the previous day(s) before the marathon.
Kipchoge has used Maurten supplements (see https://www.maurten.com/ ) for a previous race.
The commentators during the Ineos159 broadcast also referred to his use of Maurten supplements every 5km during the Ineos159 run.
Why Maurten? One of the supposed benefits of Maurten GEL is the use of two forms of carbs (maltodextrin and fructose).
Early carb supplements (Gatorade, etc) typically used one type of sugar to supply carbs to the body. The rate of carb absorption by the body was thought to be 1 gram per minute (60g/hr).
Researchers discovered that fructose was ingested via a different pathway than other sugars and when combined with another sugar could increase the absorption rate of carbs up to 90g carbs/hr. see https://www.torqfitness.co.uk/news/maltodextrinfructose for more info.
Kipchoge was taking drinks every 5km, at target pace of 21.1km/hr - 21.1/5 = ~4 times/hr
Each serving of Maurten GEL has 25g. So, assuming each drink is close to one serving of GEL, he'd get ~100g carbs/hr which his body would absorb close to 90%.
The support team also believed that the cause of Kipchoge's slowdown in the previous Breaking2 attempt was that Kipchoge wasn't supplementing enough and hit the wall near the end of the run. To reduce the chance of that happening, they gathered the used drink bottle after Kipchoge was finished and analyzed how much supplement was remaining. Not sure what they would do if they found he wasn't drinking enough other than verbal encouragement.
This race didn't follow the rules so he could do whatever he wanted. Under official rules, you can have a custom drink placed on a table, which you run by and pick up yourself. Most marathons with an elite wave do this for elite runners. Nobody can hand you your drink.
However, it is not common to have very calorie-dense drinks because it is just not necessary over that distance. You want a little bit of carbohydrate so that you don't deplete your glycogen stores or get low blood sugar. That is about it. So most runners will consume maybe 200-300 calories during a marathon. Elite runners will typically consume less, again since they are more efficient.
In addition, you can only digest and metabolize a certain number of calories per time unit. This limits how many carbohydrates you can consume during the race.
The idea behind carbo-loading [1] is to pack as much glycogen as possible into your muscles and liver before the race. You then top up those stores just before the race with more carbs, and then during the race you take on as many additional carbs as your body allows.
The faster you run, the more quickly you use up glycogen, similar to driving a car more quickly.
One thing that training does is to make you a more efficient/economic runner. [2] Your body can run faster using less fuel. I don't think we know exactly how all this works. Some of running economy comes from improved form, and indeed, Kipchoge has perfect running form. As well, your body's vascular system improves. You improve your mitochondrial volume. And so on and so forth. [3]
But at the end of the day, you eventually run out of glycogen and slow down. You just hope you've picked a pace not too fast for your ability so that you cross the finish line before that happens.
This is largely why ultra runners run at a significantly slower pace.
Lastly, metabolic factors[5] are only one many things that ultimately limit marathon performance.
1. https://en.wikipedia.org/wiki/Carbohydrate_loading
2. https://runscribe.com/improving-run-efficiency-what-research...
3. https://www.issaonline.com/blog/index.cfm/2018/mitochondrial...