Here's an article that may assist you with your parents:
https://www.globallymealliance.org/blog/how-to-respond-to-but-you-dont-look-sick
Have you read about the "spoon therapy" and chronic illness?

The "Spoon Theory" is a famous metaphor created by Christine Miserandino, a lupus patient, to explain the daily reality of living with a chronic illness or invisible disability. It uses "spoons" as a visual unit of energy to show how every daily task requires a calculated, limited amount of effort.

While healthy people generally wake up with an unlimited supply of spoons to do whatever they want, people with chronic conditions start their day with a strictly counted, much smaller batch (e.g., 12 spoons).

The Math of Daily Life: Every basic task costs a spoon. Getting out of bed might cost one, taking a shower costs two, and getting dressed costs another. By the time a "spoonie" has prepared breakfast and brushed their teeth, they may have already used up a significant chunk of their daily energy.

The "Invisible" Struggle: The phrase "but you don't look sick" is hurtful because it ignores the exhausting mental math required to survive the day. Someone may look perfectly fine or put-together on the outside, but they have already made painful sacrifices—like choosing between eating dinner or taking a shower—just to get through the day.

No Reserve Tank: When your spoons are gone, they are gone. Pushing through means "borrowing" energy from the next day, which usually results in extreme fatigue or a severe flare-up of symptoms the following morning.

The theory provides a powerful way to reframe how people view disability, shifting the focus away from assumptions of laziness and toward the careful, necessary rationing of energy.

To read the original essay that started the community, check out The Spoon Theory by Christine Miserandino.