I believe the response from Soylent is that fiber itself is not necessary; it's only needed to move along solid food through the digestive system.

If you're only drinking Soylent, you should need far less fiber, if any at all.

I am seeing a lot of criticism of this online [1], however.

[1] http://discourse.soylent.me/t/reduced-fiber-in-1-5/21706/8

I think the insanity inherent in drinking only soylent would set in far before malnutrition. I don't even think rob rhineheart drinks the stuff for 100% of his intake.

I'm guessing fiber is one of those things that comes easily with the impulse to eat "something solid"

As a lot of people have mentioned in previous discussions, Soylent isn't about being the best, it's about tradeoffs like most things in life. 60% daily value of fiber isn't optimal, but it could be more than you are getting on average now.

More or less, No.

Yes, it is a survivable diet.

No, it isn't an optimal diet.