I was wondering: why aren't the Fremen worried about getting "degenerated" by taking the melange drug so as to the point do they become like the guild of navigators? At least in the first book.
In the original Frank Herbert books, the Guild Navigators had been mutated over millennia by living and breathing in supersaturated spice gas, seemingly over many generations - in fact, it seemed they subsisted on nothing but the spice gas.
The prequel/sequel series by Frank's son Brian (and Kevin J Anderson) introduced the idea that a human could become a navigator, given enough spice (possibly to parallel what happens to Leto II in Children of Dune).
The Fremen, on the other hand, are only incidentally exposed to trace amounts of spice in the air and food they eat - it took a huge amount of spice to be force-fed for Leto II to be able to become one with the worms and eventually mutate.
Most humans across the galaxy only manage a light sprinkle - just enough to promote health and longevity, but with no real side effects (other than the blue on blue eyes).
This is based entirely on Frank Herbert’s Dune and sequels:
Guild Navigators have been living in vats full of spice for years (possibly centuries). The spice in large quantities alters physical qualities and life span. This explains their ‘alien’ appearance.
Although Fremen interact with the spice they simply do not interact with it in the same capacity.
I would also theorize the Navigators use more concentrated spice as they have more technology at their fingertips.
Metaphorically speaking, Fremen chew cocaine leaves habitually in Columbia, Navigators have been smoking crack/ using cocaine in Hollywood for years.