Fat absorbed by vegetables when cooked with meat
One of my favorite simple dishes to make is to season a whole chicken, put it in a roasting pan, add root vegetables (some combination of potato, sweet potato, rutabaga, parsnip, carrot, and/or onion) and then roast it covered for 90 minutes.
My wife suggested yesterday that the vegetables should be roasted separately because they soak up "a lot" of fat from the chicken when roasted in the same pan. I don't make gravy or pour any extra "juice" over the vegetables, but they always have a thin coating of drippings - a coating that gives them a great flavor I'd hate to part with (unless I found out it was really fatty).
Is there a good rule of thumb for estimating how much fat the vegetables take in when roasted with the meat?