I think it's an exaggeration to think that eating 2 days of ramen and that it wijld have an significant effect on your sodium or energy levels. If anything, I think it's moetly the possibly low caloric value that I would attribute to feeling lethargic.

I've myself eaten a soylent type drink for a month straight, at 1200kcal daily intake and I've had nothing bad to say. My basic metabolic rate is somewhere around 3500kcal and even with a huge deficit, I haven't been tired or anything.