I think they are looking in the wrong place. This is related to intestinal health and microbiome. So it is closely related to diet. But, I also suspect it is habitual. Some creatures in the body get "drunk" by the chemicals released when the body is in the state of anxiety. One such chemical may be histamine. This is the reason I believe why such trivial things as choosing train seats can cause anxiety. These internal creature can "paint" a perfectly normal process as catastrophic to get their feed. These are guesses from observation of what happens in myself. But I'm sure anxiety is caused by issues in the gut.