Rowan

I've done a full imperial century on just water. It proved I could do it, and it gave me an idea of what I could expect to do if I ran out of food on a randonnee, for example. But apart from that, it's not something I would repeat unless I had to.

The takeout from Al's experience probably is the need to match the ability to digest what's been eaten, especially when it's something that takes much longer to process than say, carbs, with the output at the pedals.

While 12mph might be training speed for some, effort to achieve that might be significant depending on terrain. There seem to be a few climbs and undulating terrain in the ride, which translates to some intensity... which means the protein is going to sit in the stomach stewing rather than being converted to energy.

FWIW, I think protein is an important part of food intake on rides, but in much smaller proportions compared with carbs. But if someone is on a low-carb diet, I think they just have to accept what others have said -- their intensity is going to be lower than others when it comes to longer distance rides.