I buy prunes at Costco. Three every morning at breakfast. No problem! Ever!
I eat virtually no fiber at all. No problems at all. Fat lubricating the tube is better for keeping things flowing than fiber.
Fiber works with lots of water, but many older people in particular are underhydrated and fiber without plenty of fluid aggravates constipation.
I eat plums. Same difference.
It usually doesn’t help. I have obstipation.