Measuring utilization of nutrients.

There are several threads going in which posters talk about the utilization of nutrients. I’ve always wondered how researchers know what, and how much of, these nutrients are utilized. As in “The calcium in cow’s milk is well utilized”.
Surely they don’t analyze poop! Do they?
Please please please say they do! :eek:

There’s been a bit of research into this issue. Dig in.

Whoa! and that’s current to five years ago.
I was hoping for something a little more, ah, brief?