I nearly started a thread like this a few days ago, but I think your OP is a bit clouded by the connection to anorexia @Tomlettegregg - that's a whole different issue and, as pp have said, a severe disease.
It's not the same as the MN posters who function on a really low calorie intake on a daily basis, though there is probably some overlap and the undereating threads feel very pro-ana. But I always want to know how women are managing their daily lives - work, family, exercise, socialising - on eg one meal a day as I frequently read, or on extremely long fasts or eating 1k calories and no more.
For me, long fasts or a very low calorie intake makes me fatigued, confused, weak and unable to focus, dizzy, shaking and irritable. I always want to know how some women keep going on so little and where they get their energy from, how their hair and nails keep growing, how they exercise and how they function. But I read it on here all the time, people going for months and months on under 1000 calories, or doing fasts that last for days or living off one meal a day. I couldn't do it; I would be impossible to live with and I wouldn't make the gym or be able to do my job.