I am still trying to "dial in" bowel cleansing.
In order to ride long distances in hot temperatures, it is essential to make sure a person has plenty, and I mean "a lot" of hydration to allow for the major loss through cooling. If the temperature is below 70, it doesn't take much; 70-85, increasing amounts; above 85? huge quantities.
But in order to get "huge quantities" of hydration, not only must it "between the lips and past the gums", the water has to wend it way through the stomach, through the 24' of small intestines, and , finally, to the absorption area in the large intestines.
Now, for long rides, the "fuel factory" tends to be in full usage. It is a bit "persnickety" about the processing of supplies to create fuel. Yes, the "supplies" need to be dissolved down to an osmotic level acceptable to the factory, but the processing takes time.
To add insult, if the fuel factory is still processing high fiber, complex things, or even "high volume" things from the previous day, the first half may absorb the fuel, but the hydration then languishes in the middle as the second 12' is stopped up with "stuff".
Bottom line: Breakfast.