It took me about 10 years, more or less. The first pieces of data were jokes about rushing home with a sour stomach from yet another sushi date night. A scary optical aura and migraine was the event that got me taking notes and ruling things out. I had a huge dose of soy and my vision kinda went out for 30 minutes. Scary.

If I eat less soy, like say eating someone's homemade bread made with soy vegetable oil (which is the most likely one), I'll get brain fog, and about 5 days later eczema.

Multiple times I have deliberately eaten soy and logged the effects. I'm about 90% confident it's soy at this point. I'm a skeptic and still feel it could be some other factor but I am highly confident.

Avoiding soy is difficult. Especially for someone that loves chocolate.