The uncle is in the wrong here. He acts as if he is threatened by your diet choices and success in academics. That He feels the need to attack you directly is nasty and says alot about who he is.
If he cared that you might somehow harm yourself with your doet he could gently suggest that you make sure you are getting enough protien or something like that. But to ridicule is mean. If you had a close relationship, there might be room for some two way jetting. I had a friend who ate vegetarian for a time and we cmwould tease back and forth calling each other carnivore or omnivore and herbivore and silly things like that bit it was acceptance of differences not ridicule. We would discuss what he was learning about cooking and eating more legumes amd fermenting and things like that. I would tease him for eating fake meat like patties and such while I was bbqing.
Your uncle also likely feels threatened by the University and academic success. Again if he cared and knew you he might give a parental warning about losing yourself or loosing values or something like that not attack you directly.
If you don't know your uncle these comments should not hurt anymore than the rantings if a crazy acting stranger walking on the sidewalk. If the stranger said the same things they would not sting because there is no expectation of a relationship and being known or respected or loved.
Your uncle has shown you who he is and you should believe him. Your further relationship with him should be adjusted accordingly. Not saying you should not interact, but rather adjust your expectations of the relationship as well as limiting the fuel you pour on that fire. This is not someone I would trust with details about myself, and I would not give him any new material. I realize he might get it second hand.
The same consideration should be made of the information you trust other family members with.
I don't see that this has to do with conservative or being a farmer. I am very conservative and own two farm properties. I have a vegetarian living one with her kids to farm organic veggies and lease the other to an organic grass fed cattleman. I just moved the vegitarian there from across the state to start fresh after her work fell apart.
Your uncle is just rude and mean regardless of his politics or career etc.
Trust your feelings. You feel tense and nervous because you feel the wrongness of the abusive interactions.
On a website called outofthefog.net there is a resource called 100 Traits. It's sort of a list of all sorts of ways people can be abusive or manipulative. I found it useful to read through it and identify the various firms of abuse that were in my life and relationships. Then to learn tools and strategies to reduce it. Some were mine to own up as poor coping mechanisms I had adopted when dealing or when acting from a place if weakness. Many were easily identified as abuse I was suffering by those I let into my life ir by relatives.
Congrats on your academic success and being emotionally mature to understand that this was wrong of the uncle to say.
