I remember reading a sci-fi novel as a child, at least 30 years ago but cannot remember the title or even most of the story. The one part that I remember is the man and woman eating "Tubers" and then turning into some kind of aliens or maybe even alien cattle.
I suspect you're thinking of Protector by Larry Niven. The details don't quite match up, but allowing for fuzzy memory they may be close enough.
- Tree-of-life root ("tuber") turns humans into Protectors:
It was humanoid, as much so as a stick-figure drawing is humanoid. It was all knobs. Elbows, knees, shoulders, cheekbones, they stuck out like marbles or grapefruit or bowling balls. The bald head swelled and rose behind like hydrocephalus.
- The second part of the book involves Roy Truesdale and Alice Jordan, who are held captive for a time in an Earth-like artificial environment. Roy is later turned into a Protector, though not by eating tree-of-life.
(There are also a number of other novels by Larry Niven involving Protectors, so it might instead be one of those.)
This sounds like a fragmentary memory of part of the Ringworld series, possibly "Return to Ringworld"?. There is a female character, Teela, who is not terribly intelligent but is incredibly lucky (somehow this is genetic). She discovers a cache of these tubers and by eating them becomes a "Protector", a super strong, super-intelligent organism. The man might have been a surviving member of the species that populated Ringworld, since Teela had been left when the surviving explorers returned to "Known Space". and the world would have been Ringworld.