So why can't I eat a month's worth of calories in one go and then fast for the rest of the month with water and multivitamins?
So the main problem would be nutrient absorption, and there would be a number of problems that would impact it, from literal surface area of your digestive system, to current bacterial populations etc. etc. These problems matter less in a 24 hour window when you are far less likely to hit the same thresholds eating a reasonable amount of food. But basically, you could not effectively break down and absorb all the nutrients quickly enough, and you would have pretty nasty shits/vomiting.
But, let's say somehow you could, and somehow your body ended up storing a bunch of fat. In general, the second half of your statement is totally doable because the human body absolutely kicks ass at energy storage and expenditure. https://www.sciencealert.com/the-true-story-of-a-man-who-sur...