One of my buddies and I used to train for and compete in ultra distance cycling, 400k, 600k, 1200k events. Not unusual to ride 24hrs straight before a quick 3-4 hr nap and back on the bike. We had to resort to liquid meal replacement due to the calories we were burning.
Many whey concentrates are supposedly "lactose free"; however, their definition of lactose-free meant very low levels of residual lactose. That's okay for folks who drink a glass a day; however, when you consume 5,000+ calories/day of whey and carbs as your meal replacement, then that lactose can compound into a real issue. My buddy had mild lactose intolerance, but ended up with big digestive issues from high consumption of whey based product.
My buddy fixed his whey/lactose issue by switching to powdered egg protein, mixed with a powdered orange flavored carbohydrate supplement. He loved it and it worked great for him.
JL