Yeah the protein shake thing reminds me of my buddy who does landscaping. He used to swear by those big breakfast burritos, like two of them, before a full day of hauling mulch. One time he tried just a shake to see if it was lighter, and he said by ten AM he was so hungry his hands were shaking. Went right back to the burritos the next day, I mean it just didn't stick for that kind of work for him.
What exactly was in those breakfast burritos that made the difference? Like, were they just eggs and cheese, or did they have potatoes or sausage too? I'm wondering if it's the mix of protein and slower carbs that does it, not just the protein alone. Because a shake is just liquid protein, right? Maybe your buddy's body needed the actual food to break down over hours.