You can almost think of fat free milk as juice with protein. both lactose and fructose require the liver to metabolize to glucose. The fat free milk, has some p

You can almost think of fat free milk as juice with protein. both lactose and fructose require the liver to metabolize to glucose. The fat free milk, has some protein to help slow down the absorbtion of the lactose, so it’ll be a slower carb than pure juice. The fat slows down the carb even more since the glycemic index of a food tends to be anti correlated with the proportion of carbs to other things (including non soluble fiber). More fat slower carb. Generally the slower the carb the slower the insulin response as well. And it’s insulin that triggers cells to ingest glucose. Which means the faster the carb the faster the blood glucose rollercoaster. And the more frequent the urge to eat, potentially leading to obesity (and increasing issues with insulin resistance). Now having said all that I personally suspect whole milk is the way to go, but I don’t know the actual proportion. I’d expect whole milk to be correct for calfs, and if you really wanted to be correct for children, you’d look at the fat content of breast milk.