Learner1, that's what I read in Afrin's book and on some sites, so while it may look odd, I wasn't the one who came up with it! Here is something you might want to read about D3:
https://www.mastattack.org/2016/06/effect-of-vitamin-d-on-mast-cells/#:~:text=Vitamin D3 can also induce,inflammation resulting from IgE activation.
Loratidine is a second gen H1. Cornstarch may indeed cause you problems but with MCAS, nearly anything can be a trigger and it's all individual. For me, without major gut issues, I try to eat a lot of different food items, though I avoid meat of all kinds for ethical reasons (I do consume some gelatin in some of the supplements I take, though). It's quite obvious when there's an issue. For instance, an all natural lemon "Italian Ices" dessert makes my nose run profusely, like nothing else I eat, so I try to avoid citrus and citric acid as much as is reasonably possible.
The first thing I took was some Antivert I had from a few years earlier, and within 24 hours my "head fog" was gone, but most of the other symptoms remained (I think the sensitivity to heat was relieved quickly too, and I slept more soundly). I switched over to loratidine within about a week and also started taking about 600 mg of aspirin throughout the course of a day. I then came upon Luteolin,, and purchased Mexican oregano and then celery seed (I make a tea with those and other things like marjoram, green tea, black cherry concentrate, etc.). After a week or so of that, I started to see some major relief, including: breathlessness upon minor exertion, leg weakness/pain, sleep apnea type issues, swallowing issues, all kinds of rash problems, blood pressure drop upon standing or walking up steps, etc. The only thing that didn't get relief were my eye issues, but Zaditor works great (it has ketotifen in it); it seems that when I read too much the eyes get irritated and I need to use those drops. I will be taking another DEXA exam some time next year, so that's when I'll see where my osteoporosis is at.
I should also mention that I have other medical conditions apparently not related to MCAS, so I can't play sports any longer or do anything too strenuous, but even standing up for too long leads to a recurrence of the leg weakness/pain, for example. When I was younger, this kind of thing would occur after I did too much physically, and it would take days or even about a week to go away. Now the weakness/pain can be gone within a few hours, just so long as I keep to the treatment protocol and rest. My point here is that it's certainly not clear to me that people with this condition can do too much with their bodies. Taking a short walk around the block may be too much, and I think a lot of people are seeking a 100% cure, which may not be realistic. I try to do just a little walking each day to prevent deconditioning, but I don't think more is possible. I may be able to take more of this or that, but I don't want to take the risk, because my other conditions prevent me from doing things like a lot of walking anyway. If anyone else can speak to this, I'd be curious to hear about your experiences!