Wild. So how exactly did cutting gluten out of his diet mask his symptoms that effectively? Does gluten somehow feed/provoke the giardia? Or were the dietary changes mostly irrelevant?
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Celiac is more well known than fodmap issues, and you'd have apparent causality. Eat bread, feel bad. Don't eat bread, feel better.
But not perfect, as many other foods are high fodmap.
Anyway it turns out those with giardisis are sensitive to high fodmap foods so perhaps that could explain it. (Surprisingly also sensitive to low fodmap foods)