Constipation following fast days

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Only on week 2 but anyone else getting this? I would have thought all the low cal veg on fast days would help but I think my body is missing its daily dose of weetabix.

It seems quite common from what I've read, possibly a less in less out scenario. Unfortunately I get it the other way

I had it for a while about 6 weeks in but it went after I took a couple of senna tablets and cut down on fibre a little.