These materials are incapable of supporting a grab bar, but it might be possible to add reinforcement behind these surfaces. There are grab bars available that clip on to the side of a bathtub or toilet, and don't require structural support from a wall. Be very certain they are firmly installed, check the attachments regularly to keep them tight, and then test carefully in advance by applying force in several directions before relying on them for support. While there are standards which advise specific grab bar heights, optimal height and position for grab bars depend on your exact capabilities and size. A grab bar that is standard height might be too high for some people because of their height or because the muscles they would need to use are weak or unavailable.
Some people benefit from supports immediately next to the toilet, functioning as armrests.
For others, these can be obstructions during transfer from a wheelchair. Some products are hinged from the wall, so can be raised out of the way during a transfer, and brought down for safety and stability while using the toilet. Handholds can include a shelf, countertop, or a ledge around the bathtub. This kind of thinking might mean you reinforce the support of the sink, for instance. The selection of access products for the bathroom has expanded.