This question already has an answer here:
Because we're in such close proximity and we share a kitchen, he'll see that I do things a little differently to him. (one example would be that I drink almond milk instead of cow's milk).
Which it seems is a trigger for questions, which then moves onto debate, within these debates, he seems to want to emphasise how "I'm being brainwashed" or how his brother went on a vegan diet and that's all he ever spoke about (which I can understand, some vegetarians/vegans can be very passionate), plus other things out the scope of the question.
But, I have no way indicated any interest in said questions (unprovoked), nor' have I spoke about my diet previously, except with the action of cooking in front of him (I guess?). He's just "giving me his honest opinion" apparently.
Anyway, the most recent debate he got quite offensive without realising it and I would like to cut the conversations about my diet off before it begins from now on. I want to do it politely regardless of any offence caused, we're good friends and will continue to be so.
What can I say to him if he brings anything up again?
Note: When writing an answer, I'm really not looking for your opinion on dietary requirements (whether you praise it, or criticise it.) I would very much just like to emphasise to my housemate that I'm not looking to answer questions on it. It just seems this time my diet is the topic of conversation.
Also, I'm normally happy to answer questions about my diet (if someone is curious) as much as someone who goes to the gym often would. But, I now know where these questions lead with this particular individual.