trt,
If you're talking about liquids like Boost, Ensure or Carnation Instant Breakfast, I can assure you, from very personal experience that they do NOT get absorbed.

When you are curled up in a fetal position on the bathroom floor and throwing up, you'll know that you're probably constipated.

Milk of magnesia is not recommended for "maintenance" and are recommended only "to get things going" and will typically cause diarrhea, which can lead to dehydration.

If you are on opioids, Senna and stool softeners are what are recommended.

Colace is good - don't exceed 3 tablets per day, you can get it in liquid form. Colace is mainly a stool softener and not effective, by itself, for management of constipation.

Source: "Living Well With Cancer", Katen Moore and Libby Schmais (Katen is an oncology nurse and Libby is a researcher)

Preparation H suppositories may come in handy as well.

The constipation issues were the worst part of my post Tx experience. I felt that the doctors could have done a better job managing it.