Do those who eat Once Meal a Day (OMAD) or follow a very low calorie diet (1200) have to fast?  How would this be done without becoming ill?

Do non-meat eaters have to abstain from something else since they already don't eat meat?  Wondering about the very vague fasting and abstinence laws.  Not a lot of guidance.

The Roman fasting rules are pretty clear. One main meal, and up to two smaller meals (snacks) that together are less than the main meal. Liquids allowed any time. (What constitutes fasting is the same in the modern Roman rite, as far as I know, but they only have 2 fasting days required.)

If you have a medically required diet, you are probably excused from the fasting rules to the extent they would conflict with the required diet.

If you're following a vegan diet, you don't need to do something else on abstinence days. Though you could, if you wanted.