Cutting out foods

[Hastings]

Mr Hastings has been having GI issues increasingly over the last few months, so we've decided to try some weeks of cutting out various types of food to see if we can narrow down what's causing them.

We're trying a no dairy week this week, with plans to try no wheat and no meat in future weeks. We're keeping a diary to chart what he eats and what... goes on.

I have a question about this plan.
Is one week long enough to make a difference, or should we try for longer?

Also, are there any other things we should consider cutting out for this trial?

Thanks for any and all advice.