OMAD is typically an hour window. A lot of people find it difficult to get all their nutrients into a single meal, even if it were prepared all at once. Don’t be a hero, if 2MAD is better for you, do that.
For me, the most I can get into a single meal is about 900 calories, because I eat higher protein and I get full really quickly that way. Essentially my protein requirement is too high to eat in one meal.
YMMV.
I consider myself an OMADer, but the eating window varies (I don’t worry as much about specific labels defining my lifestyle). Sometimes my meal only takes 30 min to eat (or less), sometimes it’s a multi-hour, multi-course dinner. It all balances out in the end, my only rule being that I sit down for one meal every day.
It sounds like you are trying to fit all your normal meals into a 4-hour window. That’s not what IF, or OMAD is about. OMAD is about cutting out your other meals and just eating your normal meal in your window. So if your normal diet is breakfast - 400 cal, snack 200 cal, lunch 700 cal, snack 200 cal, dinner 800 cal, snack 200 cal. Then OMAD is dinner 800 cal. Not Dinner 2500 cals.
Imo, a meal should last 30-60 minutes. So, if you’re doing OMAD, then you should finish it in an hour.
I can’t get all the calories in that fast. So minimum I like 4-5 hours for 2 meals.
But honestly, I prefer 3 meals a day. So I do ADF. 40:8. I eat 3 meals in 8 hours every other day.
I’m considering moving up to 44:4 for 2MAD (every other day). We’ll see.
If you’re underweight why are you doing omad? Or even fasting? You should take it one step at a time.
Fix the underweight issue, then after that is fixed, start fasting. Otherwise I think it’s too much to handle