Rosemary's Baby is a fascinating exploration of paranoia, alienation and identity. There is also a hell of a lot of eating in the film. And if you include the sense of smell into the mix, then you've got nearly 50% of the film taken up by characters eating and smelling or talking about eating and smelling. The food becomes so intertwined with plot that they are bound together, possibly through Christian mythology.
The first time we encounter food within the film is when the happy young couple Guy and Rosemary Woodhouse have dinner with their soon to be former landlord Hutch. Food comes into play again when the Woodhouses befriend Roman and Minnie Castevets. The Castevets have them over for dinner that consists of tough meat and revolting cake (according to Rosemary anyway, Guy seems to eat it up - literally and figuratively). Then, of course, there is the chocolate mousse that Rosemary swears has a chalky taste to it which eventually knocks her out allowing the Satanic cult that lives next door to raise the Devil and allow him to rape her. Rosemary then suffers from her pregnancy feeling only pain and drinks only coffee. In a moment of confrontation between Rosemary and Guy when she demands real medical attention for her pain, the pain miraculously dissipates. We then see her eat an extemely raw piece of meat which turns her mouth red as the blood and juice drips out. Minnie still plies her with cake and drinks.When Rosemary eventually makes her daring escape only to be caught by Guy and her Satanic doctor and returned to their apartment where she gives birth. She is told the baby died but that she and Guy can try again once they move to LA. She is fed plain broth and toast as she recovers. But lo, she hears a baby crying and... well, I think (or I hope) we all know what happens from there.
The relationship between gender and food is a complicated one. Women are generally associated with being nurturers and mothers often meaning making and consuming family meals. When Rosemary's Baby was released in 1968, the feminism movement was coming to a head. Gender roles were changing within the social structure. Rosemary is an interesting example of an emerging feminist. She is beginning to question Guy and her marriage but ultimately values her role as a wife and homemaker (the montage of her fixing up the apartment is particularly apt at showing this). There is an interesting dichotomy between Rosemary and Minnie with Minnie taking over many of the cooking duties (making dinner, preparing snacks) so when Rosemary decides to cook, she must assert herself to do so.
One of the big debates that was emerging just prior to this period was the notion that nurturing and reproduction were "forces of human labor" (Source) which expanded the traditional Western notion of the value of production and labor. The production of food is also heavily linked to family dynamics. Minnie is already inserting herself into the fold of the Woodhouse's family by infantalizing Rosemary and rendering herself a faux-mother figure which ultimately seeks to subvert their family.
In analysis of food on film, food (the production of it anyway) is generally associated with a divine grace (Source) so Rosemary's Baby offers the perversion of that. (similarly to the inverted cross representing the Devil) When the Woodhouses initially eat Minnie's food Rosemary is disgusted by the tough steak and revolting cake. Even the chocolate mousse which is initially delicious comes with an unwanted aftertaste. Even the supposed decadence of the food (cake, steak etc) seems to be in direct opposition in the taking of sacrament in Christianity. Though food is our body's fuel- our sustenance- Rosemary's Baby reveals the temptation provided by the Castevets. While Guy is offered fame and fortune, Rosemary is offered the proverbial apple. Even though she rejects it, it is forced upon her which in my opinion illuminates the true nature of fear within Rosemary's Baby; the notion that even if we reject evil it can still be forced upon us by playing to our weaknesses. Rosemary still values being a mother, and therefore even after bringing the Anitchrist into the world, she cannot reject them. She still has no real agency or choice no matter how hard she fights.
But at least she spits in Guy's face.