Dear Miss Frazzled,
Porcupines are hard to deal with. I should know; I am one, on occasion. Let's see if I can frame my answer in a way that will be helpful to you.
1: People will always have opinions where they have no business opining. The truth is, there is such a vast spread of ways to live your life that any one person's ideas are bound to conflict with another's. The choices that you make are based on your best understanding of the situation, and the choices others make are based on theirs. The trouble comes when we get up in arms about someone else's way of doing things because we assume that ours is automatically better. You've probably already discovered that no matter what you do or believe, someone will call you crazy for doing or believing it.
I get angry when I see people sticking their fingers where they don't belong - when they presume to have opinions about someone else's personal choices that don't affect them - but it's a hypocritical anger, because I do it too.
"I can't believe they named their baby Zebra!"
"Yet another person on a gluten-free diet - doesn't he know that's just a fad?"
"Why on earth would she dye her hair that color?"
Kudos to those who have mastered the impulse to judge. A few of you have, and you are among the most Christlike people I know. But really - MOST OF US FALL INTO THIS TRAP. And not just occasionally, either. ALL THE TIME.
2: We can't always change those opinions. You may have all kinds of reasons for eating the way you do and all kinds of evidence that it's important, and they may have reasons that you shouldn't and evidence that it's not. You won't change many minds by arguing about it.
3: Given 1 and 2, we have to learn to live with and love people with different opinions. Unfortunately, life is bound to be pretty miserable if we only associate with those who approve of all of our actions and beliefs. Therefore, we have to accept that people will judge, and we have to find a way to not let it bother us.
4: Yes, you actually can achieve 3. Letting go of what other people think about us is truly an epic battle. I'm not saying you should totally disregard others' feelings or intentionally flout social norms to make a statement. But it is so important to be able to shrug at censure when you have done the research and the soul searching required to make a personal decision.
4.1: Make your position clear to others. To directly answer your question, you can start by explaining your reasoning to those around you, including your porcupine roommate. Don't make a ward announcement or anything, but explain it to your friends and to people you associate with regularly. It's a topic that frequently comes up in the routine course of conversation, especially if they witness some of your eating choices.
This does not entail arguing, mind you, but simply letting them know why you do what you do. Then you can be sure that you've given it your best shot - if they don't believe you or accept your explanations, the problem is on them. Acknowledge their doubts and let them know you're okay with their differences in opinion: "I know you think it's silly, and you have a good point that _________. I've thought about it a lot, though, and the reason I still choose this course of action is _______. But I'm not offended that you don't agree with me."
4.2: Seek for integrity and consistency in your actions. By integrity, I don't so much mean "doing what's right even if no one is watching," but rather being sure enough that what you're doing is right that you're willing to move forward regardless of criticism. It means prayerfully and thoughtfully considering all of your options before deciding on a course of action. It means researching and giving due attention to all sides of the argument. In some cases, the right answer isn't obvious even AFTER you've done all the research - but you should still learn all you can about the subject so that you know you can justify your decision (especially to yourself) when others cast doubt on it. Knowing that you've done your best to come to the correct conclusion will give you confidence to defy the consensus.
4.3: Be a good person. Even if you make eating choices that people think are dumb or lack scientific validation, it won't actually matter if they enjoy being around you. Be aware of the needs of others, look for opportunities to serve, show real interest in people, and be thoughtful, humble, and optimistic. People will want to be around you just for being you, and while they might still think you're silly for your diet choices, they will hold you in high regard for the things that really matter.
Example 1: One of my cousins gave his first child a name I don't approve of. I still shake my head when I think about it - but I don't think less of him. It's just a quirky part of his personality.
Example 2: Another of my cousins is on a diet that I don't approve of. I won't be adopting it myself, but she is my best friend and one of the most incredible people I know. It doesn't really matter to me that she's making weird eating choices.
4.4: Develop a strong relationship with your Heavenly Father. This goes along with 4.2. If you know you're acting according to the best information and inspiration you have, you know that God is satisfied with what you're doing. You can always act in confidence when you know that he loves you and is on your side.
5: Keep in mind that everyone has different ideas about what constitutes healthy eating, and that just because your roommate and this ward member judge you for your choices doesn't mean that everyone else does, too. Even if gossip has somehow spread, most people don't care. They won't even give it a second thought.
6: Don't stress about it! (I know, easier said than done, right?) Life will go on regardless of what happens here or what anyone thinks about you. Keep up the things that you're doing right and strive to be a better person, and when you look back on this time of your life, you'll remember that more than anything else.
7: Just a quick P.S. It's also probably a good idea to head off situations like this before they happen by always making sure the people in your living room know you're there. I have a roommate who sometimes cuddles up under a blanket by the couch, and it doesn't look like a person - she usually stretches or yawns or something when people come in so that they're aware she's there and don't do anything... incriminating.