I very much agree with RitzyFritz on all her points. I'll likely repeat some of them!
It does sound from your account that you expect your MIL to center her attention around food, so some simple items to mention before arriving might include, for instance, how you've come to enjoy the challenge of scratch cooking or substituting for certain trickier foods. You can then educate her about the condiments, sauces, etc. that a person might not at first realize are off your list. You might go on to those foods you now enjoy so much more of! Maybe then you can elicit from her personally anything she may want to try, similar to your present favourites. Vette her ideas, to give her some sense of security about learning your new standards. And of course, recognize her care and creativity, as she might otherwise feel she no longer has much of those to offer you!
If your doctor has ever said anything that led you to this diet, relate it, as well as the way sticking to this diet has improved your life. A doctor's professional advice can be pure gold, when long-separated, possibly insecure in-laws aren't sure of your mindset. And if you can tie your life improvements back to long-term benefits for your husband, in-laws should be able to infer how helping you stay on your plan is as direct a show of love to their son, as is feeding him his childhood specialties.
Geographically distant in-laws can become worried about future sources of alienation, so you'll want to clear up any confusion. When you're there and have time, make sure they see that you care about the credibility of authorities you looked up before adopting this new "lifestyle", that you're not interested in switching to the latest popular program whenever the mood strikes you, and that you're not testing anyone to see how far they'll bend for you. Not that you should spell this out, just talk about your commitments to health and balance, that preclude all these. I don't know your husband's family, of course - they could well be immune to such concerns! I only speak from my own experience, having found with my in-laws that it always paid to think through the many possible sources of misapprehension.
Remember that it took time for you to learn your new diet, and the change can be a sudden shock to your husband's family, who thought they already knew you. Just soften the transition for them, describe clearly how easily they can make your trip a real joy, and show them how you don't relish causing any present or future awkwardness. Let them see how much more wonderful you are, after your change in diet. Then, if they care about you the way you obviously care about them, you will have a mutually marvellous vacation!
Please let us know how well you enjoyed yourself, after you come back? I'm looking forward to your postings!