Fibre has been found to have a lot of detrimental effects, and can be hard on the digestive system, so caution is advised.
One of those things that sounds logical but in practice doesn't work as expected.
What works for me is magnesium chloride. This is really just concentrated seawater but high in magnesium. I take 5-10ml a day in lots of water.
I now take enough to keep me regular bot not too loose either.
The beauty of magnesium is that it has a lot of health benefits, is a vital mineral, and most are deficient in it.
Lots of info on the web.
Once you catch up to the body's needed level, you start to get loose bowels, so that's the time to reduce the dose a little.
The tablets you can get aren't nearly as good, but the powder is OK.