I presume they still eat some food and don’t just consume water for 40 days.
Yeah, some people fast. Sometimes I fast. There are different types of fasting. I’ve seen a lot of Muslims have to work all day while doing the Ramadan fast. I don’t think that’s a great idea as it’s hard for them to work when they are hungry.
A good fast is somewhat difficult but also healthy. A fast that is either not difficult or not healthy is not helpful.
Most Catholic “fasts” for me are not very difficult because I often don’t eat. Catholic churches like a “2 small meal and one big meal and no meat” diet, which is often my regular menu, or a “bread and soup” meal which for me is a normal lunch, or a “bread and water” day where the bread is quite enough for me and i have to buy bread i dont like to make it a sacrifice.
By contrast, some activities such as just drinking water for a protracted time (while not restricting your eating, you do not drink any other beverage including spraking water), or removing all sugar from your diet while still eating a balanced menu, are quite difficult and very healthy