Hi Zack,
You do not say how tall you are. I am a reasonably skinny male - 6.1' 59kg and have had no probs with UT - in fact I gained a couple of kg.
I would suggest also maybe trying the UE's (enemas) as retention. Many of the nutrients can be better assimilated (no stomach acid). In any case the UE's are much easier taken than ANY other fluid in that respect.
As others have noted, the massaging is vital in long fasts - though these are usually used by people facing specific, serious conditions.
Otherwise fasting cycles can also be very effective; after 2-4 days the body starts to cannibalise, but the great thing is that only damaged (white blood) cells are 'eaten' first. Then when food is again consumed new cells are created.
In this way a much improved immune system/body can result.