gibby, thank you for your reply.
To answer your question, yes I have had a wide range of blood tests - both at public hospitals and private clinics.
For the most part, everything is fine. The private clinic had lower baseline ranges, which led me to purchase supplements in an attempt to improve those results.
I have yet to have a comprehensive GI stool analysis (is this the same as the Great Smokies 'Comprehensive Digestive Stool Analysis' test?). It may not be available here in Japan, so I would need to look into sending my samples to the US.
I too have eaten oats for years - typically as oatmeal (porridge) or muesli (dried fruits, nuts, oats).
I never experienced any problems in my teens or early 20s, but understand that the body's sensitivity to food can change as we mature. A test in the UK ruled-out Celiacs, however I am unsure if this was IGG or IGE, or possibly something else. Regardless, I stopped eating oats too, but didn't experience an improvement in my symptoms.
Thank you once again for your suggestions.